MEDICINE AND SURGERY "F" Course of LABORATORY MEDICINE Respiratory gases and the blood pH; the hemogas analysis HEMOGAS ANALYSIS THE REGISTRATION OF ATTENDANCE TO THIS ON-LINE LECTURE IS ACTIVE!       To register your attendance please type in your surname and matricola number. Surname: Matricola: Notice that your attendance will be registered only if you completed the reading, questions, and audios, and that you cannot interrupt and resume the session (but you can repeat it as many times as you like). Remember to press the [send] button before leaving this page! A confirmation message will appear at the end of this page.       A comment section has been added at the end of this lecture. Adding a comment or question does not require registration with your matricola number, feel free to comment whenever you like.       Notice: this lecture requires previous attendance to the lecture on water and electrolyte balance. and renal excretion. RESPIRATORY GASES AND BLOOD BUFFERS       The function of the lung is gas exchange; respiratory gases are O2 and CO2. N2 is inhaled and expired with air, and is dissolved in all body fluids, but it does not take part in cellular respiration and is completely inhert in our body. H2O in the vapour phase is inhaled and expired, and respiration causes water loss (approx. 200-300 mL/day). The study of respiratory gases in the expired air and dissolved in the blood is important to assess respiratory function. Moreover, CO2, together with bicarbonate, is one among the most important blood buffer and participates to determine the blood pH; thus, the study of respiratory gases is essential for the diagnosis of acid-base imbalances. The concentration of bicarbonate in our blood is controlled by: (i) conversion to and into CO2 and lung excretion of the latter; and (ii) direct urinary excretion. As a consequence a complete study of the acid-base balance requires also the assessment of kidney excretion of bicarbonate. ASSESSMENT OF LUNG FUNCTION: THE RESPIRATORY GASES       The essential laboratory test for respiratory gases is the hemogas analysis, carried out on a sample of arterial blood taken in a gas tight syringe to avoid contamination with air. In babies venous blood may be used becauze of the difficulty of puncturing an artery. The hemogas analysis evaluates the respiratory function of lungs and blood and the blood's pH. The respiratory and buffer activities of blood and lung are strictly correlated, but it is important to discuss them separately, to improve clarity. Hemogas anaysis: reference values for arterial blood PO2 75-100 mmHg (age dependent) Hemoglobin O2 saturation 96-100% Total O2 content 20 mL/dL or 8-8.5 mmol/L (depends on Hb content; reduced under anemia) PCO2 35-45 mmHg (1.1-1.3 mM) HCO3- 24-27 mM (plasma) total CO2 (bicarbonate+CO2+carbamates) 26-30 mM (plasma); 20-22 mM (blood) pH 7.4-7.44       The main function of the lungs is the exchange of respiratory gases O2 and CO2. Since CO2 is also a blood buffer, the lung is the principal organ controlling the blood pH (but kidney is also extremely important because of the excretion of bicarbonate). An accessory function of the lung is the participation to the regulation of blood pressure via the renin-angiotensin system. Lung diseases are often revealed by altered blood gases in the hemogas analysis (reduced PO2 and increased PCO2, with respiratory acidosis). Diseases of the lung may compromise its mechanical ventilatory capacities (e.g. emphysema, coma, paralysis of respiratory muscles) or its ability to exchange gases (e.g. interstitial pneumonia), or both.       During respiration O2 and CO2 are exchanged in comparable amounts; the ratio CO2 excreted : O2 absorbed is called the respiratory quotient and varies between 0.7 and 1.0 depending on the nutrient we are utilizing, 0.7 being the respiratory quotient for triglycerides and 1.0 for glucose.       The two respiratory gases have very different solubility in water and this has profound effects on the mechanisms of their transport; some relevant differences are reported in the table below.   O2 CO2 total content in arterial blood 20 mL (gas)/dL or 8.5 mMoles/L 20.5 mMoles/L (serum concentration, 26 mM, is higher than erythrocyte concentration) Partial pressure in arterial blood 95-100 mmHg 40 mmHg total content in venous blood 15 mL (gas)/dL or 6.5 mMoles/L 22.5 mMoles/L (serum concentration, 26 mM, is higher than erythrocyte concentration) Partial pressure in venous blood 40 mmHg 45 mmHg physical status 97% bound to hemoglobin in the erythrocytes; 3% physically dissolved in erythrocytes and plasma 90% as bicarbonate; 5% as CO2; 5% as protein carbamates artero-venous difference 2 mMoles/L ; 60 mmHg 1.7 mMoles/L ; 5 mmHg       Three important differences between the two gases are: (i) O2 is mostly Hb-bound, CO2 is mostly present as dissolved bicarbonate. Anemias may cause a very significant reduction of O2 content, while affecting CO2 content to a negligible extent (the amount of Hb-carbamates accounting for only 5% of the total CO2). Severe anemias may cause hyperpnea and hypocapnic respiratory alkalosis. (ii) The O2 capacity of blood is saturable and follows the O2 binding curve of Hb; the capacity of CO2 is non-saturable (except that at very high levels the patients enters a comatous state). (iii) O2 has higher artero-venous differential pressures than CO2 and higher diffusion coefficient. The arterovenous gradient of PCO2 (normal values under rest for arterial and venous PCO2, respectively: 45 mmHg - 40 mmHg) is lower than that of PO2 (normal values under rest: 100 mmHg - 40 mmHg); by contrast the O2 content of arterial blood (8-8.5 mMoles/L) is much lower than the total CO2 content (28 mMoles/L in the plasma, 22 moles/L in whole blood, due to the lower bicarbonate content in the erythrocyte cytoplasm).       Alterations of blood gases and other parameters of the hemogas analysis may be observed under conditions where the airways present partial obstruction (e.g. severe asthma; bronchitis; Chronic Obstructive Pulmonary Disease, COPD) or the gas exchange in the lung alveoli is compromised (e.g. pneumonia, interstitial pneumonia, emphysema); however this effect may not occur to the same extent for the two respiratory gases, and dissociations in the alterations of PO2 and PCO2 are quite common. Indeed, while one may expect that pulmonary lesions should impair O2 and CO2 to a similar extent, and thus the decrease of arterial PO2 should parallel the increase of PCO2, this is often not the case and either gas is more compromised than the other. Two opposite cases should be considered: (i) type 1 respiratory failure (hypoxemic respiratory failure): PCO2 is not significantly increased and may even be decreased, while PO2 is severely reduced. This may be observed, ofr example, in acute interstitial pneumonia. The main reason why PCO2 is not significantly increased is hyperventilation. Hyperventilation (dyspnoea; air hunger) causes the values of partial pressure of arterial gases to move in the direction of their partial pressures in air: PO2, air = 150 mmHg; PCO2, air < 1 mmHg. Since the normal value of arterial PO2 is 90-100 mmHg there is little room for it to increase due to hyperventilation, whereas there is large room for PCO2 to decrease. Thus hyperventilation has greater effect in reducing PCO2 than in increasing PO2. If O2 is administered, it may occur that hyperventilation is diminished and the PCO2 increases. (ii) type 2 respiratory failure (hypercapnic respiratory failure): PCO2 is severely increased; PO2 is reduced to a variable extent. This may be observed in conditions in which ventilation is severely impaired, whereas gas exchanges in the pulmonary capillaries are not significantly affected (e.g. paralysis of respiratory muscles, coma, emphysema). Under chronic conditions renal retention of bicarbonate occurs and partially restores the normal blood pH.       The gas diffusion properties of O2 and CO2 differ, thus pulmonary lesions that cause impaired gas diffusion may yield results that are difficult to predict. On the one side, the CO2 content of blood is greater than O2, and the presence of carbonic anhydrase in the lung facilitates conversion of bicarbonate to CO2, improving its diffusion. Moreover PCO2 in the external air is very low, leading to a significant gardient. All these factors facilitate the diffusion of CO2. On the other side the much higher arterio-venous gradient of O2 gives a powerful contribution to its diffusion. As a general rule hyperventilation favors CO2 excretion over O2 uptake, and the opposite applies to hypoventilation. Impaired gas diffusion as observed in interstitial pneumonia or lung fibrosis impairs O2 uptake to a greater extent than CO2 excretion, especially if compensatory hyperventilation is present. Reduced PO2 may cause reduction of Hb O2 saturation (in arterial blood Hb is almost completely O2 saturated and close to the upper asymptote of the O2 binding isotherm; thus a significant decrease of PO2 is required to observe desaturation). Low Hb O2 saturation causes cyanosis, i.e. a purplish o bluish discoloration of the skin. Cyanosis in the absence of reduced PO2 is suggestive of diseases that do not involve directly the lung (e.g. hemoglobinopathies, heart failure). Significant alterations of both PO2 and Hb O2 saturation in the absence of lung diseases may be observed in congenital artero-venous shunts (e.g. atrial septum defects, Fallot tetralogy). Indeed an important consideration is that a cause of the incomplete O2 saturation of Hb is the (minor) physiological mixing of arterial and venous blood occurring in the anastomoses between pulmonary and bronchial veins.       Hypoxemia, hypoxia and the relationships between O2 and hemoglobin       The condition in which the O2 content of arterial blood is reduced is called hypoxemia. It may have several causes and the laboratory analyses are essential to establish a correct diagnosis. The principal symptoms of hypoxemia are hyperpnea (increased respiration rate), and tachycardia; cyanosis, a purplish discoloration of the extremities due to the purplish colour of deoxygenated Hb, may be present. Myocardial infarction, arrhythmias and ischemic damage of tissues are possible complications.       The normal value of the O2 content of blood is 20 mL O2 measured in the gas phase under standard conditions per dL of blood; corresponds to 8-8.5 mmoles/L. The plasma content of O2 is practically negligible (approx. 2-3% of the total blood O2) and 97-98% of the blood O2 is bound to Hb with a maximum stoichiometry of 4 O2 per tetrameric molecule of Hb.       Some definitions are in order: hypoxemia indicates a reduced oxygen "level" in the blood. Level is a generic term that some authors interpret as "reduced PO2", other authors as "reduced O2 content". As shown in the table below the former definition would exclude anemias and hemoglobinopathies, the latter definition would include these conditions. ischemia is the condition in which obstruction of the artery that carries blood to an organ reduces or abolishes the oxygen supply to that organ (this condition may cause an infarction, i.e. the ischemic death of the tissue) hypoxia refers to the condition of insufficient oxygenation of the tissues. Hypoxia occurs when the O2 content of the blood is reduced (as in the second definition of hypoxemia), in which case all organs are affected, or when an arterial obstruction causes ischemia, in which case the condition is limited to an organ.       The O2 content of blood depends on the following factors: (i) partial pressure of O2 (PO2; normal range for arterial blood 90-100 mmHg); depends on the partial pressure of O2 in inspired air (reduced at high altitude) and on the gas exchange function of the lung (reduced in several lung diseases: pneumonia, interstitial pneumonia, emphysema, fibrosis, etc.); (ii) blood hemoglobin content (normal value 14 g/dL, corresponding to 2.2 mmoles Hb tetramers/L; reduced in anemias); (iii) hemoglobin saturation (SaO2; normal range 94-98% for arterial blood); reduced if PO2 is reduced, or in the case of low O2 affinity Hb mutants (see the lecture on hemoglobinopathies), or in the presence of (usually congenital) artero-venous shunts (e.g. defects of the atrial septum). Reduced O2 saturation is associated to cyanosis. laboratory findings associated to reduced blood O2 content disease PO2 SaO2 PCO2 other laboratory findings lung diseases decreased decreased increased acute or chronic respiratory acidosis high altitude; mountain sickness decreased decreased decreased respiratory alkalosis anemias normal normal normal or decreased reduced Hb content low O2 affinity hemoglobinopathies and methemoglobinemias normal decreased normal abnormal electrophoresis of Hb; mutations on Hb genes right-to-left (congenital) arterovenous shunts decreased decreased increased ecographic evidence of shunt       Clinical example: severe viral pneumonia in a elderly subject: pH = 7.39 PO2 = 60 mmHg SaO2 = 92% PCO2 = 43 mmHg HCO3- = 25 mM Comment: the patient has impaired gas exchange due to interstitial pneumonia, and significant compensatory hyperventilation (air hunger); this helps keeping PCO2 at nearly normal levels, but is not sufficient to restore the PO2. Bicarbonate and pH are unremarkable.       Complementary analyses       An important complement to hemogas analysis is the determination of respiratory volumes by spirometry.       Pulse oximetry can be carried out at the patient's side and may be very useful. A pulse oximeter uses red and infrared light to measure the arterial Hb O2 saturation, thus it provides only one of the different parameters of the hemogas analysis; however the simplicity and affordability of pulse oximeters allows the physician to carry out this single measurement very efficiently.       Measurement of pulmonary blood flow and ventilation perfusion ratio       The measurement of of pulmonary blood flow (PBF) and ventilation perfusion ratio provides important information on lung function and disease. It is conceptually similar to the measurement of glomerular filtration rate (GFR) in that it uses the partition of a tracer (oxygen for PBF, creatinine for GFR) among two fluids (air and blood for PBF, urine and blood for GFR). The analysis is carried out as follows: - the patient breathes in a spirometer capable of measuring volume, pressure and O2 content of inspired and expired air; - samples of arterial and venous blood are taken and their O2 content is determined.       Typical results are as in the following table: inspired air 5 L/min at 1 atm, T=25oC O2 content = 20% (12 respiratory acts/min of 400 mL each)   total O2 in inspired air n = 5 x 1 x 0,2 / 0,082 x 298 K 0,041 moles/min expired air 5 L/min at 1 atm, T=25oC O2 content = 15% total O2 in expired air n = 5 x 1 x 0.15 / 0,082 x 298 K 0,031 moles/min differential O2 content, air   10 mmoles/min O2 content in arterial blood Hb=14 g/dL and saturation 97% 8.2 mmoles/L O2 content in venous blood Hb=14 g/dL and saturation 75% 6.4 mmoles/L differential O2 content, blood   1.8 mmoles/L pulmonary blood flow O2 absorbed from air, per min / differential O2 content per liter of blood 10 moles/min / 1.8 mmoles/L = 5.5 L/min ventilation/perfusion ratio   5 L/min / 5.5 L/min = 0.91       Normal values of ventilation/perfusionn ratio are 0.9-1.1 variations may occur as a result of heart insufficiency, impaired ventilation, etc. ACIDOSIS AND ALKALOSIS       The alterations of the blood pH are called acidoses (if arterial pH < 7.35) and alkaloses (if arterial pH > 7.45). They are due to abnormal production or excretion of acidic or basic solutes in the serum, and of course are counteracted by the blood buffers. An interesting and well-written tutorial may be found at this link; a very useful article on the subject may be read at this link. BLOOD BUFFERS Buffer Concentration (ionizable group equivalents) pK Production/excretion Hemoglobin circa 45 mEq/L (see text) produced during the red blood cell differentiation; degraded by macrophages CO2 / bicarbonate CO2:1.2 mM - 40 mmHg bicarbonate: 26-28 mM 6.1 Krebs cycle; excreted by the lung (CO2) and the kidney (bicarbonate) Phosphate 1.2 mM 7.0 in equilibrium with calcium phosphate in the bone matrix; excreted in the urine Audio: Blood buffers       The blood buffers behave in quite a complicated manner; a mathematical model of the functioning of blood buffers is provided at this link. The most concentrated buffer system is provided by the titratable aminoacid residues located on the surface of proteins, most notably hemoglobin. The concentration of proteins in the blood presents interindividual variability; morover each exposes several titratable aminoacid residues on its surface, each with its own pKa. Thus it is not convenient to describe their action analytically and the functional concept of buffer capacity is used instead. This concept was introduced into clinical use by D.D. Van Slyke in 1922; it is defined as the amount of strong acid (or strong base) required to change the pH of 1 L of blood by one unit; thus it is measured in mEq/L. Buffer capacity of blood whole blood 38 mEq/L plasma 16 mEq/L       As shown in the Table the buffer capacity of plasma, due to plasma proteins, phosphate and bicarbonate/CO2, is less than half that of whole blood. One should take into account that the concentration of bicarbonate and CO2 in the erythrocyte is only slightly lower than in the plasma; thus one may assume that in the whole blood over 22 mEq/L of buffer capacity are provided by hemoglobin.       In order to better understand the concept of buffer capacity, we may consider the case of a simple buffer made up by a monoprotic weak acid and its sodium salt, e.g. acetic acid and sodium acetate. We call the total concentration of this buffer the sum of the concentrations of its components, namely: Ctot=Ca+Cs. The pH of this buffer is given by Henderson and Hasselbalch law: pH = pKa + log (Cs / Ca)       We now select the two pH values pH1=pKa-0.5 and pH2=pKa+0.5, such that the two are equally spaced on the pH scale on the left and on the right of pKa, and their difference is 1 pH unit. We can easily calculated that to raise the buffer pH form pH1 to pH2 we need an amount of strong base (e.g. NaOH) corresponding approximately to 0.52xCtot (the actual value for the maximum buffer capacity measured on a smaller pH interval is 0.57xCtot). For example, if the total buffer concentration were 0.1 M we would need to add 0.052 Eq/L of a strong base to raise the pH from pH1 to pH2.       Unfortunately, the buffer capacity depends not only on the total buffer concentration, but also on the buffer pH and reaches its maximum when pH=pKa. The buffer capacity measured at pH=pKa+1 (or pH=pKa-1) is only 0.2xCtot. Thus, the buffer capacity of 22 mM bicarbonate/CO2, with pKa=6.1, at the physiological pH=7.4 accounts at most for one tenth of the total buffer capacity of blood.       The buffer capacity of a mixture of different buffers is additive: each buffer contribute its own, and the total buffer capacity is the sum of the buffer capacities of each buffer in the system.       Which is the most important blood buffer? Answering this simple question is complex and not unequivocal. From a static view point, i.e. if we take a sample of arterial blood and measure its buffer capacity, we observe that over 80% is due to proteins, predominantly hemoglobin, and the rest to inorganic components (mainly bicarbonate and phosphate). If we add a dynamic dimension to our analysis we observe that in one hour the amount of CO2/bicarbonate produced by our metabolism exceeds that of all other buffers contained in the whole extracellular fluid and in the erythrocytes. Moreover, although hemoglobin is the most important buffer component of blood, it is not the most informative; indeed, except in cases of severe anemia, hemoglobin is scarcely influenced by dynamic conditions (e.g. diseases). Much more relevant from the diagnostic view point is bicarbonate, because it is continuosly produced by our metabolism (as CO2), and continuously excreted by the lung and the kidney, the principal organs involved in the regulation of buffer concentration and blood pH, given that they excrete CO2 and bicarbonate (and the kidney excretes phosphate as well). The lung alone eliminates some 20 moles CO2 / day, a massive amount. Thus from the view point of the clinical laboratory, bicarbonate is , if not the most important, certainly the most informative buffer in our blood. A simplified scheme of the gas exchanges in relation to blood pH is depicted in the following figure. Audio: Bicarbonate buffer       The reactions and Henderson Hasselbalch equation for bicarbonate are as follows: CO2 + 2 H2O <=> [ H2CO3 + H2O ] <=> HCO3- + H3O+ pH = pKa + log ([HCO3-]/0.031 PCO2) Since carbonic acid is scarcely populated (its concentration being 700-times lower than that of CO2), we can ignore its contribution, and assume CO2 as the acidic component of the buffer. Under this assumption the apparent pKa of CO2 at 37o C is 6.1. [HCO3-] is measured in mMol/L and PCO2 is measured in mmHg; 0.031 mM/mmHg is the solubility coefficient of CO2. Audio: The Henderson and Hasselbalch equation of bicarbonate buffer       Production and excretion of CO2 and bicarbonate: nutrition provides us an apport of energy, in the form of reduced carbon in food. Oxidation of nutrients with oxygen obtained from respiration develops this energy. In the case of glucose, the overall reaction is as follows: C6H12O6 + 6 O2 --> 6 CO2 + 6 H2O + 720 kcal /mole of glucose       As an example of the oxidation of fats, we can take the case of palmitic acid, the principla fatty acid produced by our own biosynthesis: C16H32O2 + 23 O2 --> 16 CO2 + 16 H2O + 2,380 kcal/mol The caloric equivalent of O2 is very similar in the two cases (fat produces more energy but requires more oxygen to be oxidized); for glucose we calculate 720/6=120 kcal/mol O2; for palmitic acid 2380/23=104 kcal/mol. An average value of 117 kcal /mole is usually accepted, and one can easily calculate that a basal metabolism of 1600 kcal/die requires approx. 14 moles of O2 and produces almost as much CO2. The amount of O2 consumed and CO2 produced are approx. 9.7 and 8.4 mMoles/min. respectively. With a pulmonary blood flow of 5 L/min. this implies that the amount of O2 uploaded and CO2 excreted by the lung are approx. 2 mMol/L and 1.7 mMol/L. In the case of O2 this amount is accounted for by the increase of hemoglobin saturation from 75% in venous blood to 98% in arterial blood, but for CO2 the amount of gas excreted exceeds the blood content (1.2 mMol/L). This is made possible by conversion of bicarbonate to CO2 (catalyzed by carbonic anhydrase), but the reaction requires hydrogen ions, provided by the other blood buffers (essentially hemoglobin); in the absence of this contribution respiration would cause a massive pH increase (see the figure above). total O2 content of arterial blood circa 8.5 mMoles/L total O2 content of venous blood circa 6.5 mMoles/L artero-venous difference in O2 content circa 2 mMoles/L total CO2 content of arterial blood circa 22 mMoles/L total CO2 content of venous blood circa 23.7 mMoles/L artero-venous difference in CO2 content circa 1.7 mMoles/L       As already stated, respiratory excretion of CO2 requires the conversion of bicarbonate to CO2, according to the chemical reaction: HX+ + HCO3- <=> X + H2O + CO2 where HX+represent the protonated component of the protein buffers (mainly due His residues), and X the deprotonated component of the same buffers. One may approximate the behaviour of blood buffers to that of a system composed by only two buffers: bicarbonate and protein histidine residues. We remark that the sum of the deprotonated components of the two buffers (Sdp) is constant, i.e. the system converts HX+ to X and bicarbonate to CO2 transferring one hydrogen ion from X to bicarbonate or vice versa. Thus we can write the following relationships: [HX+] + [X] = [Xtot] (constant) [X] + [HCO3-] = Sdp (constant) pH = pKCO2 + log [HCO3-]/0.03 PCO2 = pKX + log [X]/[HX+] KCO2 x 0.03 PCO2 / (Sdp - [X]) = KX ([Xtot] - [X]) / [X] This equation can be solved to yield [X], and all other relevant parameters (e.g. pH, bicarbonate, buffer capacity, etc.)       Bicarbonate can be excreted or retained by the kidney in an exchange with chloride, or together with sodium ions. However the interplay between the kidney and lung is quite complex and can be summarized as follows: (i) the tissuse produce CO2. (ii) CO2 is released in the blood and converted to bicarbonate, thanks to the hidden buffers (hemoglobin, serum albumin, phosphate, etc.) in the following reaction X + CO2 + H2O ↔ HX+ + HCO3- (iii) CO2 and bicarbonate are carried by the blood to the lungs where the opposite reaction takes place and allows CO2 to be eliminated. (iv) Thus the lungs, the blood and the tissues reversibly convert CO2 to bicarbonate and vice versa, but cannot separate these two compounds. (v) The kidney produces ammonia from glutamine thanks to the enzymes glutaminase and glutamate dehydrogenase and secrete ammonia in the urine (average excretion 30-40 mmol/day; up to 200 mmol/day in metabolic acidosis). Ammonia is converted to ammonium ion by the dissociation of CO2 (assisted by carbonic anhydrase), according to the following reaction: NH3 + CO2 + H2O → NH4+ + HCO3- (vi) bicarbonate is mainly reabsorbed in the blood, whereas ammonium ion is eliminated in the urine (together with chloride); thus the kidney can effectively uncouple bicarbonate from CO2. The most obvious example of the functioning of this mechainism is the renal compensation of respiratory acidosis.       In practice, because of the other buffers, the blood pH varies relatively little (at most between 7.0 and 7.6, below and above these values the patient entering in a comatous state that, if untreated, leads to death). Thus, excretion of CO2 is always associated to its reformation at the expense of bicarbonate and vice versa; yet the respiratory excretion of CO2 consumes hydrogen ions and raises (slightly) the blood pH, while renal excretion of bicarbonate acts on the blood pH mainly via the alteration of its ratio to CO2. It is important that, being pH almost constant and bicarbonate concentration much higher than CO2, changes in CO2 are easily (if not completely) compensated by bicarbonate, whereas the opposite is not true. Regulation of bicarbonate/CO2 and pH organ action / defect effect lung hyperventilation increased excretion of CO2 - pH increases (alkalosis); PCO2 decreases lung reduced ventilation, emphysema, reduced gas exchange decreased excretion of CO2 - pH decreases (acidosis); PCO2 increases kidney increased excretion of bicarbonate pH decreases; bicarbonate and total CO2 decrease; Cl- increases kidney increased reabsorption of bicarbonate pH increases; bicarbonate and total CO2 increase; Cl- decreases       Disturbances of the blood pH and buffer concentrations are called respiratory if caused by altered functioning of the lungs, metabolic otherwise. It is important to stress that dysfunction of the lung can be to some extent compensated by the kidney and vice versa; thus each organ tends to oppose the disfunction of the other. Since the concentration of protein buffers is relatively constant, the only buffer which can adapt to pH changes is bicarbonate, whose concentration can be rapidly varied; this is why plasma bicarbonate from the diagnostic view point is the most important blood buffer. The key features of the different forms of acidosis and alkalosis are as follows: TYPES OF ACIDOSIS AND ALKALOSIS Abnormality possible cause(s) Blood pH (normal values: venous 7.36; arterial 7.4) Pressure of blood CO2 (normal values: venous 45 mmHg; arterial 40 mmHg) total CO2 (i.e. CO2 + plasma bicarbonate; normal values 27-30 mM) Metabolic alkalosis acid loss (renal; vomiting; etc.) increased normal to moderately increased increased Metabolic acidosis production of abnormal acidic substances (e.g. ketoacid, lactic acid); renal loss of bicarbonate decreased decreased (compensatory hyerventilation) decreased Acute (uncompensated) respiratory alkalosis hyperpnea (lesions of the respiratory centers in the brain) strongly increased decreased (primary) slightly decreased Chronic (compensated) respiratory alkalosis hyperpnea (high altitude; lesions of the respiratory centers in the brain; anemia) slightly increased decreased (primary) strongly decreased Acute (uncompensated) respiratory acidosis reduced ventilation; impaired gas exchange(e.g. interstitial pneumonia); emphisema strongly decreased increased (primary) slightly increased Chronic (compensated) respiratory acidosis same as acute respiratory acidosis slightly decreased increased (primary) strongly increased (compensatory renal retention of bicarbonate) Audio: Acidoses and alkaloses       The following remarks will help explaining the above table: (i) pressure of CO2 in the blood indicates the concentration of pure CO2, i.e. it does not include bicarbonate; total CO2 is dominated essentially by bicarbonate ion concentration (the ratio [HCO3-] / [CO2] being approximately 20). In lung diseases the increase of PCO2 tells us which gradient of the gas is required in order to excrete the CO2 produced by metabolism, and monitors the severity of the patient's condition. (ii) The metabolism produces mainly acids (CO2 and lactic acid, acetoacetic acid, etc.); however our organism is also better equipped to eliminate acids (CO2 by respiration; organic acids by liver conversion to other products, e.g. lactic acid to glucose) than bases. Respiratory acidosis is frequent, varied and may be severe. Respiratory alkalosis is usually due to anemia (anemia causes hyperventilation). (iii) Acids can be volatile (CO2), excreted by the lung at a very fast rate, and non-volatile (lactic acid, acetoacetic acid), metabolized by the liver or excreted by the kidney at a slower rate. (iv) Metabolic alkalosis is most often due to loss of acids (e.g. vomiting forces the gastric mucosa to replace the gastric juice, whose HCl content is obtained by a mnechanism that increases the serum bicarbonate and causes alkalosis), excess intake of alkaline substances (e.g. gastric antiacids, bicarbonate), and diuretics; it is corrected mainly by the kidney that excretes the excess bicarbonate. (v) Metabolic acidosis, caused by overproduction of non-volatile acids (e.g. diabetic ketoacidosis) or by their impaired renal excretion, stimulates respiration that excretes the volatile acid (CO2): hence compensatory hyperpnea and hypocapnia (reduced P CO2; Kussmaul breathing). It is interesting to remark that pulmonary correction of metabolic acidosis is more effective than of metabolic alkalosis because respiration frequency can be increased to a more significant extent than it can be decreased. (vi) Respiratory alkalosis is a consequence of hyperventilation (loss of CO2); this occurs when the oxygen content of blood is reduced (e.g. because of anemias), or under unusual environmental conditions (e.g. muscular effort at high altitude, where atmospheric P O2 is decreased - air hunger); moreover it may be due to (usually ishemic) lesions of the respiratory centers in the central nervous system. (vii) Respiratory acidosis is a common consequence of impaired gas exchanges (e.g. depression of respiratory centers in the CNS, insufficient mechanical ventilation in polyomyelitis or tuberculosis, or ventilation perfusion imbalance in chronic obstructive pulmonary disease, emphysema, etc.). Audio: The hemogas analysis       The blood gas analysis is the measurement of the pH and the concentrations and partial pressures of O2 and CO2 in a sample of the patient's blood drawn in a gas tight syringe. The measure is usually effected by means of potentiometric methods, using gas-specific electrodes.       As a general rule, a hemogas analysis will indicate if an abnormality is present and will give some indication of its possible cause; the fundamental indications are as follows: Diminished pH and diminished total CO2 = acute or chronic metabolic acidosis with respiratory compensation (e.g. diabetic ketoacidosis). Strongly diminished pH, strongly increased PCO2 and slightly increased total CO2 = acute respiratory acidosis. The exchange of CO2 in the lung is impaired (e.g. because of acute viral pneumonia); this leads to increased arterial PCO2 and decreased arterial pH. Metabolic compensation is scarce or absent because it requires several days to become operative; this causes bicarbonate and total CO2 to increase only slightly. Normal to diminished pH and strongly increased total CO2 = chronic respiratory acidosis. If the impaired gas exchange in the lung lasts long enough for the kidney to retain bicarbonate, metabolic compensation occurs (e.g. chronic obstructive pulmonary disease). Retention of bicarbonate (partially) restores the arterial pH but causes a strong increase in bicarbonate concentration and total CO2. Increased pH and increased total CO2 = metabolic alkalosis with respiratory compensation (e.g. vomiting). Increased pH and decreased PCO2 with moderately increased bicarbonate = acute respiratory alkalosis with minimal metabolic compensation (uncommon; e.g. neurological hyperpnea). Normal to increased pH and decreased total CO2 = chronic respiratory alkalosis with metabolic compensation (e.g. anemia; life at high altitude).       The above set of rules allows one to interpret simple deviations from the healthy conditions, i.e. those conditions where one disease (either respiratory or metabolic) is present and compensation is respiratory. These conditions are typical of young patients suffering of acute acid-base imbalance (notice that respiratory compensation of metabolic conditions is almost immediate, whereas metabolic compensation requires time). Notice that two parameters (pH and total CO2) are necessary even in the least complicated cases. Audio: Classification of blood pH disturbances Audio: Acute respiratory acidosis Audio: Chronic respiratory acidosis Audio: Metabolic acidosis Audio: Alkaloses       The anion gap, is the difference between the concentrations of (sodium + potassium) and (chloride + bicarbonate). The blood, as any other mixture, has zero net charge; thus the anion gap estimates the amount of non-measured negative charges (e.g. proteins). The normal value for plasma is around 15 mEq/L. An increased anion gap may provide a gross indication of the presence of excess unmeasured negatively charged ions usually derived from carboxylic acids (e.g. lactate or acetoacetate). Accordingly there are two types of metabolic acidosis, either with normal anion gap (NAGMA: Normal Anion Gap Metabolic Acidosis) or with increased anion gap (HAGMA: High Anion Gap Metabolic Acidosis). The expected anion gap is calculated if albumin concentration is lower than normal. This correction is important in hypoalbuminemia because albumin provides an important contribution to the non-measured anions. The correction is as follows: expected anion gap = 15 - 2.5 x (normal albumin - actual albumin) where albumin is expressed in g/dL and normal albumin = 4 g/dL. If the patient has hypoalbuminemia his/her anion gap is to be interpreted with reference to the expected value, rather than the usual normal value.       HAGMA is usually characterized by normal chloride and reduced bicarbonate, and indicates the presence of abnormally high non-measured anions (e.g. acetoacetate in diabetic ketoacidosis). An important component of bicarbonate loss is its conversion to CO2 and increased respiratory elimination. NAGMA occurs because increased renal loss of bicarbonate is associated to chloride retention (thus chloride is increased and the sum Cl- + HCO3- is constant). common causes of HAGMA common causes of NAGMA Renal failure Ketoacidosis Lactic acidosis Several types of poisoning Some inherited defect of metabolism Gastrointestinal loss of bicarbonate Renal tubular acidosis Carbonic anhidrase inhibitors       A very useful representation of the results of a hemogas analysis is a plot of bicarbonate concentration versus PCO2. Because of the Henderson-Hasselbalch equation each couple of these parameters identifies a pH value, and couples corresponding to the same pH appears as lines in this graph.       The graph identifies regions of the (PCO2,[HCO3-]) space corresponding to the main categories listed above: acute respiratory acidosis and alkalosis, chronic respiratory acidosis and alkalosis, metabolic acidosis and alkalosis. The main reason why respiratory conditions require the distinction between acute and chronic is that renal compensation requires several days, thus acute respiratory conditions are poorly compensated or uncompensated, whereas chronic respiratory conditions are usually well compensated. Metabolic conditions require respiratory compensation, which is established rapidly: thus they are usually well compensated irrespective of their onset being acute or chronic.       We remark that chronic, compensated respiratory conditions may be compatible with a normal blood pH of 7.4, but are revealed by grossly altered values of PCO2 and bicarbonate. Basic 4-parameter diagnostic interpretation of the arterial hemogas analysis.       1) The first parameter to analyze is the blood's pH. This defines the conditions of acidosis and alkalosis, even though in the presence of effective compensation the alteration of the blood's pH may be minimal or even absent.       2) PCO2 is the second most important parameter, which distinguishes hypocapnic from hypercapnic conditions. An important increase of PCO2 is diagnostic of lung diseases (acute or chronic); a moderate increase of PCO2 may reflect the pulmonary compensation of metabolic alkalosis. A decrease of PCO2 may reflect either pulmonary disease (type 1 respiratory failure, with hyperventilation due to hypoxemia, and consequent hypocapnia) or respiratory compensation. Changes in PCO2 due to pulmonary disease affect strongly the blood pH, unless metabolic compensation (with retention of bicarbonate) intervenes.       3) Total CO2 is essentially determined by bicarbonate concentration. Bicarbonate is controlled by both the kidney and the lung, and important changes of its concentration reflect metabolic disease or compensation. It is important to consider that while the kidney has great freedom in increasing or decreasing the excretion of bicarbonate, the lung has this freedom in one direction only, i.e. it can only increase the elimination of CO2/bicarbonate via hyperventilation; it cannot significantly decrease elimination of bicarbonate via hypoventilation because this would also entail reduced uptake of O2, and hypoxemia is a strong stimulus to increase ventilation.       4) The anion gap is required to distinguish HAGMA from NAGMA.       The above considerations are summarized in a diagram due to Arbus, which the student is invited to consider as a usueful help to the interpretation of simple clinical cases: Audio: Chronic respiratory acidosis Some typical examples.       1) Chronic obstructive pulmonary disease (COPD), diffuse interstitial pneumonia, etc. reduce the efficiency of gas exchanges. A typical hemogas analysis may be as follows: oxygen saturation 78% on room air (normal value > 90%) arterial pH 7.25 (normal value 7.44) PCO2 70 mmHg (normal value 40-44 mmHg) PO2 50 mmHg (normal value > 80 mmHg) plasma bicarbonate concentration 35 mM (normal value 26 mM) Anion gap 12 mEq/L 4-parameters analysis of this case is as follows: 1) pH is decreased, thus this condition is an acidosis. 2) PCO2 is significantly increased, thus this condition is a respiratory acidosis. 3) Bicarbonate and total CO2 are increased, thus metabolic compensation is present. Since metabolic (renal) compensation requires several days, this condition is a chronic respiratory acidosis. 4) The anion gap is normal; this occurs because the increase in bicarbonate is associated to renal excretion of chloride. Description: chronic hypercapnic acidosis, associated (in this case) to reduced oxygen content and oxygen saturation.       2) Adaptation to high altitude (4.000 m above sea level or higher). A typical hemogas analysis may be as follows: oxygen saturation 75% on room air arterial pH 7.48 PCO2 20 mmHg PO2 60 mmHg plasma bicarbonate concentration 16 mM Anion gap 13 mEq/L 4-parameters analysis of this case is as follows: 1) pH is increased, thus this condition is an alkalosis. 2) PCO2 is significantly decreased, thus this condition is a respiratory alkalosis. 3 and 4) Bicarbonate and total CO2 are decreased, thus metabolic compensation is present, suggesting that the condition is chronic. The normal anion gap is normal because increased urinary excretion of bicarbonate is associated to chloride retention. Diagnosis: chronic respiratory hypocapnic alkalosis due to hyperpnea (this is an attempt to compensate for the reduced atmospheric PO2)       3) Type I diabetes mellitus: plasma glucose 250 mg/dL arterial pH = 7.25 PCO2 20 mmHg PO2 95 mmHg serum bicarbonate 10 mM anion gap 25 mM 4-parameters analysis of this case is as follows: 1) pH is decreased, thus this condition is an acidosis. 2 and 3) PCO2 and bicarbonate are both significantly decreased, thus this condition is a metabolic acidosis with respiratory compensation. 4) The anion gap is increased, i.e. the plasma contains an excess of non-measured anions: HAGMA. Diagnosis: the most important causes of HAGMA are:renal failure, ketoacidosis, lactic acidosis, inherited defects of metabolism (e.g. methylmalonic aciduria), and several types of poisoning. In the present case the increased glycemia suggests diabetic metabolic acidosis due to ketone bodies (acetoacetic acid and 3-hydroxy butanoic acid). The anion gap is increased because of the presence of the non-measured anions acetoacetate and 3-hydroxy butanoate. DIAGNOSTIC CONSIDERATIONS       1) Diseases of the lung and respiratory system usually cause hypoxemia and respiratory acidosis (acute or chronic). Type 1 respiratory failure causes marked hypoxemia associated to mild or absent respiratory acidosis and hypercapnia. It is typically observed in obstructive diseases (e.g. COPD, emphysema). Type 2 respiratory failure causes marked hypercapnia and acidosis and may or may not be coupled to hypoxemia. It is typically observed in the most severe cases of respiratory insufficiency (e.g. interstitial pneumonia, lung fibrosis, poliomyelitis). Hyperventilation causes respiratory alkalosis, and is usually due to hypoxemia (e.g. because of anemia) or to neurological causes (damage of the respiratory centers in the CNS).       2) Diseases of metabolism may produce non volatile acids and cause metabolic acidosis with increased anion gap (HAGMA) (e.g. diabetic ketoacidosis, sepsis, etc.)       3) Diseases of the kidney may cause several metabolic disturbances of blood pH usually in the direction of acidosis: kidney failure with decreased GFR causes a high anion gap metabolic acidosis; proximal tubular insufficiency causes impaired reabsorption of bicarbonate (impaired bicarbonate-chloride exchange; type 2 renal tubular acidosis); distal tubular insufficiency causes type 1 tubular acidosis due to insufficient excretion of acids: the urine pH is relatively high (pH>5.5).       4) Liver failure (e.g. because of cirrosis) may cause alterations of blood pH in both directions. The most commonly observed effect of cirrosis on blood pH are chronic respiratory alkalosis and/or metabolic acidosis. The reason of metabolic acidosis is that the liver metabolizes several acidic substances (e.g. lactic acid, ketone bodies, converting them to CO2, which can be excreted by the lung or to non acidi substances such as glucose or triglicerides). Thus, metabolic acidosis results from impaired metabolism of non volatile acids. The reason of respiratory alkalosis is less clear, but is probably related to a concomitant anemia.       4) Vomiting causes loss of acidic gastric juice and the necessity of replacing the loss produces metabolic alkalosis (see above). Diarrhoea may produce a simila effect.       4) Anemia causes hyperventilation because the reduced O2 capacity of blood, which is met by increased cardiac output and hyperpnea. These responses cause increased loss of CO2.       5) Hyperaldosteronism may be associated to metabolic alkalosis because aldosterone promotes reabsorption of sodium at the expenses of potassium and hydrogen ion. The opposite occurs in Addison's disease.       6) Thiazide diuretics cause loss of sodium and chloride and retention of bicarbonate, and may produce a (mild) metabolic alkalosis.       Clinical example       4) Chronic liver cirrosis. A 56 years old patient presents chronic liver cirrosis, with mixed hyperbilirubinemia. The patient is clearly dyspnoeic with 26 respiratory acts per min. indicating the necessity of a blood gas analysis. The arterial gas analysis yields the following results: pH=7.47 PO2=114 mmHg PCO2=20 mmHg bicarbonate=14.6 mM hemoglobin=13.9 g/dL lactate=4.3 mM anion gap=15 mEq/L total bilirubin=4.6 mg/dL sodium=128 mEq/L potassium=4.8 mEq/L chloride=103 mEq/L Comment this patient present a complex acid-base condition, dominated by chronic respiratory alkalosis (high pH, low PCO2 and bicarbonate). Bicarbonate is low because of the renal compensation. Lactic acid is high (normal value: <2 mM), probably because of impaired liver metabolism; this would suggest a mixed condition in which respiratory alkalosis is coupled to a less relevant degree metabolic acidosis. MORE COMPLEX CASES REQUIRE ADDITIONAL PARAMETERS AND CONCEPTS       In the presence of an acid-base imbalance whose diagnosis is not obvious, more refined measurements are indicated in order to separate the respiratory and metabolic contributions. Special attention is required in the elderly given that metabolic and respiratory conditions of equal or opposite sign may coexist (e.g. pulmonary emphysema, causing chronic respiratory acidosis, may be present together with vomiting, causing acute metabolic alkalosis, or with diabetes, causing chronic metabolic acidosis). In these cases the physician must discriminate the extent of pathological and compensatory changes of the same parameter, be it PCO2 or bicarbonate. Several clinical concepts (and measurements) have been developed to discriminate the metabolic and respiratory components of blood buffers inbalance, as listed below.       Calculated parameters: the "physiological" approach       The physiological approach to the discrimination between compensatory and pathological alterations of PCO2 and bicarbonate is preferred by american physicians. The principle is as follows: given the usual parameters measured in a hemogas analysis of simple cases, in which only one optimally compensated disease is present, one can infer which would be the "expected" parameter in the presence of optimal compensation. If the measured parameter differs from the expected one, a second disease is present. The original formulas were derived by Winter and are often named after him. They are as follows:       expected optimal compensation of metabolic acidosis: PCO2 = 1.5x[HCO3-] + 8       expected optimal compensation of metabolic alkalosis: PCO2 = 0,7×([HCO3-]-24) + 40 Allow an uncertainty margin of at least + or -2 mmHg. If PCO2 >> PCO2, theor then respiratory acidosis is present, in addition to the metabolic disturbance. If PCO2 << PCO2, theor then respiratory alkalosis is present, in addition to the metabolic disturbance.       Expected optimal compensation of chronic respiratory acidosis and alkalosis: [HCO3-] = 24+0,4×(PCO2−40) Allow an uncertainty margin of at least + or -2 mM. If the [HCO3-] of the patient is significantly higher than the calculated value then metabolic alkalosis is present in addition to the compensation of the respiratory disease; if it is significantly lower then the compensation is insufficient or metabolic acidosis is present in addition to the respiratory disease. An utility for calculating the expected parameters using Winter's formulas is provided at this link. Examples: - Compensated metabolic acidosis: pH=7.31; [HCO3-] = 18 mM; PCO2 =36 mmHg Winter's formula: PCO2, theor. in mmHg = 1.5 x 18 + 8 = 35 mmHg Measured PCO2 is within + or - 2 mmHg from calculated one: optimal respiratory compensation is present. - Insufficiently compensated metabolic acidosis: pH=7.22; [HCO3-] = 18 mM; PCO2 = 44 mmHg Winter's formula: PCO2, theor. in mmHg = 1.5 x 18 + 8 = 35 mmHg Measured PCO2 is significantly higher than calculated one: insufficient respiratory compensation. Most probably in this patient we have the contemporaneous presence of a metabolic acidosis (bicarbonate is low) and a respiratory acidosis (PCO2 is higher than expected). Notice that in this example the PCO2 is normal!; but, given the acidosis (pH = 7.22) we do not expect it to be normal, it should be lower if the lung were correctly compensating the disturbance.       Nomograms are graphs in which the measured parameters are reported, together with the indication of statistically determined areas or regions corresponding to single disease conditions. An example is the above reported Arbus' nomogram. The use of nomograms is equivalent and alternative to the use of empirical formulas such as Winter's.       Example:       5) Comatous patient. Hemogas parameters are as follows: pH= 7.0; bicarbonate = 10 mM; PCO2 = 40 mmHg; anion gap = 25 mEq/L.       Our standard four parameters interpretation is metabolic acidosis, becase the pH is low and bicarbonate is low. The anion gap is high, thus this is a case of HAGMA. PCO2 is "normal", but this contradicts our expectation that it should be reduced because of respiratory compensation. Winter's formula for optimal compensation of metabolic acidosis predicts PCO2 = 1.5x10+8 = 23 mmHg, much lower than the actual value; thus we diagnose a concomitant respiratory acidosis. If we locate the data of this patient in the Arbus' nomogram and we find that they fall in a white region, between the metabolic acidosis and the acute respiratory acidosis. Thus the nomogram confirms that the patient has an acute respiratory acidosis superimposed over a (presumably chronic) metabolic acidosis.acidosis. This patient requires immediate resuscitation.       Diagnosis: the patient had diabetic ketoacidosis. Initially his condition was compensated (low PCO2); when the pH fell at 7.0 he entered a comatous state and the respiratory centers in the brain were depressed. Hypoventilation reduced the compensation and added the acute respiratory component. Glycemia was 300 mg/dL. Insulin and potassium therapy were promptly instituted. Note: the patient condition was a medical emergency, and was rapidly progressive, because the increase of PCO2 further depressed the respiratory centers, creating a vicious circle.       Measurement of the "standard" parameters Standard pH, historically the first concept introduced to rationalize complex deviations from the healthy conditions of blood buffers balance was introduced into the clinical practice by Hasselbalch in 1916. It is the pH of the patient's arterial blood measured under standard conditions (P CO2=40 mmHg, hemoglobin fully saturated with O2, T = 37 C). Essentially, the use of standard conditions has the effect of reversing the compensatory effect of respiration and thus to make more evident the eventual presence of a metabolic component in the pH unbalance.       In order to gain an understanding of the concept of standard pH and those that derived from it (standard bicarbonate and base excess) one should consider that CO2 behaves as an acid because of the reaction CO2 + 2 H2O <==> HCO3- + H3O+. Thus, if P CO2 < 40 mmHg, CO2 is absorbed during equilibration, and standard pH < pH. On the contrary, if P CO2 > 40 mmHg, CO2 is released during equilibration, and standard pH > pH.       With the use of three parameters (total CO2, pH, and standard pH), one obtains a better description of the underlying clinical condition, according to the following table:   Typical laboratory data for acidoses and alkaloses   pH     P CO2     total CO2 (or bicarbonate)     standard pH     (Base Excess)   diagnosis   << 7.4 (e.g.: 7.2) >> 40 mmHg (e.g.: 60) > 26 mM (e.g.: 30) 7.4 (BE = 0) uncompensated respiratory acidosis (e.g. acute interstitial pneumonia) < 7.4 (e.g.: 7.34) > 40 mmHg (e.g.: 50) >> 26 mM (e.g.: 50) > 7.4 (BE > 0) respiratory acidosis with metabolic compensation (e.g. chronic obstructive pulmonary disease, COPD) < 7.4 > 40 mmHg   < 7.4, > pH (BE < 0) combined respiratory acidosis and metabolic acidosis (e.g. COPD and diabets mellitus) << 7.4 (e.g.: 7.2) << 40 mmHg (e.g.: 20) << 26 mM (e.g.: 10) < pH (BE < 0) metabolic acidosis with respiratory compensation (e.g. type I diabetes mellitus) > 7.4 << 40 mmHg (e.g.: 20)  < 26 mM (e.g.: 20) 7.4 (BE = 0) uncompensated respiratory alkalosis (e.g. acute adaptation to high altitude) > 7.4   < 40 mmHg << 26 mM (e.g.: 15 mM) < 7.4   (BE < 0) respiratory alkalosis with metabolic compensation (e.g. chronic adaptation to high altitude) > 7.4 > 40 mmHg >> 26 mM (e.g.40 mM) > 7.4, < pH (BE > 0) metabolic alkalosis with respiratory compensation (e.g. severe vomiting) Standard bicarbonate, a concept developed by Astrup and Siggaard Andersen in 1957, is the concentration of bicarbonate one measures when a sample of arterial blood is equilibrated under standard conditions. The rationale of this procedure is that of setting the concentration of one of the components of the major blood buffer (CO2) and determinining that of the other. The number one measures is not the same one would obtain in a standard hemogas analysis, given that part of the bicarbonate originally present in the blood may be lost as CO2 (if the P CO2 was higher than 40 mmHg) or part of the gas may be absorbed and converted to bicarbonate. The standard bicarbonate measures the metabolic component of the acid-base balance of the blood and corrects for respiratory compensation. It can substitute for standard pH. Base excess, again by Astrup and Siggaard Andersen, is the amount of strong acid (or base, in which case the resulting value is negative) required to restore the normal pH of 1 L of blood sample equilibrated under standard conditions. Standard base excess is the same concept except that erythrocytes are partially removed, to a final Hb content of 5 g/dL. BE (or SBE) is another indicator of the metabolic component of the disturbance: acidosis in the presence of positive base excess is an indication of respiratory acidosis. Given the intrinsic limits imposed by the definition of SBE, this parameter can easily be calculated from the sum of bicarbonate in excess with respect to the normal value (assumed 24.4 mM) plus the buffer capacity of all other buffers multiplied by the pH change: SBE = ([HCO3-] - 24.4) + 16.2 x (pH - 7.4) As in the case of Winter's formulas for expected PCO2 or expected [HCO3-], formulas for the expected SBE are available, which allow the physician to diagnose proper compensation or a secondary disorder. An application is provided to implement these formulas. Notice that if acidosis is present (i.e. pH<7.4) and the base excess is positive (i.e. acid is required to restore the pH to 7.4), this implies that the process of equilibrating blood under standard conditions causes the pH to raise above 7.4 (in respiratory acidosis P CO2 > 40 mmHg; thus equilibration with standard P CO2 removes CO2 and bicarbonate). By contrast, acidosis in the presence of a negative base excess (base deficit) indicates an important metabolic component, with respiratory compensation. Base excess provides information analogous to, but more quantitative than, standard pH and standard bicarbonate (see the above Table "Typical laboratory data for acidoses and alkaloses"): BE measures the amount of hydrogen ions lost or absorbed by all blood buffers (proteins and bicarbonate); SBE multiplied by the extracellular volume measures the amount of hydrogen ions lost or absorbed by the whole body. The Base Excess is a crucial element of the nomogram by Siggaard-Andersen, as reported below.       To use the nomogram we report the hemogas analysis parameters of our patient on the nomogram: if the patient falls in a shadowed area we can assume that he/she has a single acid-base defect, with the appropriate compensation (The areas usually include 95% of the "pure" cases); if the patient falls between two areas, then we assume that two concomitant diseases are present.       Advanced, 5-parameters interpretation of the arterial hemogas analysis.       1) The usual 4-parameters evaluation is carried out as above.       2) The standard pH, or standard bicarbonate, or base excess is added to the picture and interpreted as in the Table above.       3) The above parameters are located on specifically designed nomograms to help differential diagnosis.       Examples       6) Chronic obstructive pulmonary disease (COPD)       Severe dispnoea; patients breathes a mixture of 75% air and 25% pure oxygen (O2 partial pressure in inspired air 270 mmHg; O2 content 36%). 60 years old male, heavy smoker.       Hemogas analysis: PO2 = 87 mmHg PCO2 = 79 mmHg HCO3- = 37 mM standard HCO3- = 31.5 mM Base Excess = 10.4 mEq/L pH = 7.28       Comment: chronic respiratory acidosis. Standard pH, calculated from the above parameters is 7.50, indicative of effective renal compensation; this is confirmed by the large Base Excess. A possible metabolic alkalosis component should be inquired (did the patient suffer of vomiting?). The decrease of PO2 is less critical, but this parameter is supported by an O2 enriched gas mixture. 7) Ketoacidosis in diabetes mellitus (see slide below for complete information). 15 years old patient suffering of unstable type I diabetes mellitus. Poor compliance to therapy and dietary prescriptions. Mental confusion. Laboratory data: arterial blood pH = 7.06 PO2 = 71 mmHg PCO2 = 23 mmHg [HCO3-] = 6.5 mMoles/L Anion Gap = 25 mEq/L standard [HCO3-] = 7.4 mMoles/L glucose = 430 mg/dL       Comment: the primary disturbance in this case is a high anion gap metabolic acidosis (HAGMA). The history of the patient and the very high glycemia suggest diabetic ketoacidosis. Very low PCO2 and bicarbonate are mainly due to respiratory (and renal) compensation.       The low PO2 is puzzling: since in this case the pulmonary compensation is achieved via hyperventilation, we expect normal or even slightly increased PO2.       Application of Winter's formula for metabolic acidosis tells us that the expected PCO2 under these conditions is: 1.5x6.5+8 = 18 mmHg, lower that the measured value.       Interpretation: the patient has a severe diabetic ketoacidosis; because of the low pH he is entering a condition of coma (mental confusion preludes to this evolution). The brain overstistimulation of the respiratory centers is reduced, and as a consequence the hyperventilatory response is fading, hence the higher than calculated PCO2 and the low PO2. Coma, if untreated, would add a second reason of acidosis (respiratory) and would prove lethal (the patient was treated with insulin and survived).       8) Primary hyperaldosteronism. Woman, 50 years old, complains weakness and recent onset hypertension, with headaches. Laboratory findings: sodium = 150 mEq/L potassium = 2.2 mEq/L chloride = 103 mEq/L bicarbonate = 32 mMoles/L arterial blood pH = 7.50 PCO2 = 43 mmHg       Comment: hypersodiemia and hypokalemia are characteristic of hyperaldosteronism - prescribe measurement of aldosterone, cortisol and ACTH. The patient also presents metabolic (hypercapnic) alkalosis, a finding that is common in hyperaldosteronism because of excess urinary excretion of hydrogen ions.       Application of Winter's formula for metabolic alkalosis tells us that the expected PCO2 under these conditions is: 0.7x32+20 = 42.4 mmHg, essentially identical to the measured value; thus optimal pulmonary compensation is present, and there is non indication of lung disfunction.       8) Infectious gastroenteritis. 22 year old male presenting with fever, abdominal pain, severe vomiting and diarrhhoea. Laboratory findings: sodium = 140 mEq/L potassium = 3 mEq/L chloride = 86 mEq/L bicarbonate = 38 mMoles/L arterial blood pH = 7.60 PCO2 = 40 mmHg       Comment: the patient has metabolic (hypercapnic) alkalosis, most probably due to vomiting. Electrolyte inbalances are most probably due to excess water loss. Application of Winter's formula for metabolic alkalosis tells us that the expected PCO2 is 0.7x38+20 = 46.6 mmHg, significantly higher than the actual measured value. This suggests that the patient has a superimposed respiratory alkalosis (eliminates more CO2 than expected). Moderate respiratory alkalosis is not uncommon in fever, due to hyperventilation. Further laboratory tests: look for pathogens in the faeces and in the blood; measure glycemia, lactate and osmolarity of blood; if necessary rehydrate the patient orally or parenterally. COMMON CAUSES OF ACIDOSES AND ALKALOSES: A SUMMARY ACIDOSES AND ALKALOSES OF THE ADULT       Acidoses and alkaloses appearing around or after puberty are usually acquired (see below for the congenital acidoses of the newborn and infant), and may be due to several possible causes. Some common examples are listed in the Table below: pH disturbance possible diseases acute respiratory acidosis interstitial pneumonia lung intoxication by fumes or other substances depression of respiratory centers (e.g. eroine abuse, brainstem infarction, coma) chronic respiratory acidosis chronic obstructive pulmonary disease (in heavy smokers!) lung emphisema impaired ventilation lung fibrosis and penumoconioses metabolic acidosis diabetic ketoacidosis lactic acidosis (sepsis!) kidney failure and several kidney diseases (acquired Fanconi's syndrome) respiratory alkalosis hyperventilation due to neurological damage mountain sickness metabolic alkalosis prolonged vomiting diuretic therapy, some defects of tubular function       In the pH disturbances of the adult and the elderly it is important to pay attention to the possible coexistence of multiple pathological conditions: e.g. diabetic ketoacidosis is of the metabolic type, but may cause coma, which in turn depresses the respiratory centers and cause respiratory acidosis: thus two causes of acidosis are present at the same time, one metabolic and one respiratory. In an elderly patient suffering of BPCO and chronic respiratory acidosis, an episode of acute vomiting may superimpose a metabolic alkalosis; thus two opposite alterations of the blood pH may be present at the same time (and the pH may appear almost normal, but with severely altered bicarbonate and total CO2). INHERITED METABOLIC ACIDOSES OF THE NEWBORN       Several inherited metabolic disorders may cause metabolic acidosis because of the accumulation of metabolytes whose elimination is impaired. These must be diagnosed early because appropriate treatment or diet may prevent brain damage and progression of the disease. Acidosis may not be the most prominent symptom, and may actually be mild (in many cases neurological abnormalities may be prominent); however it provides an important diagnostic clue. Acidosis is of the metabolic type, with increased anion gap. Aciduria is usually present (the urine is acidic). Each of these defects is uncommon, but they are many and thus their cumulative incidence is significant, possibly as high as 1 every 100 births.       The nutrients of animals are compounds containing reduced carbon (sugars, aminoacids, fatty acids). Carbon is oxidized using air O2, and since it is a non-metal its oxides are acidic (CO2, carboxylic acids). Blockade of metabolism because of the inherited deficiency of an enzyme may cause carboxylic acid intermediates to accumulate in the blood and in the urine. Each specific defect is discussed in the appropriate lecture or chapter, but it is useful to collect at least the most common defects in a cumulative Table. disease metabolyte and pathway orotic aciduria orotic acid (pyrimidine biosynthesis) methylmalonic aciduria methylmalonic acid (catabolism of propionyl-CoA) maple sirup urine disease branched chain α-ketoacids (catabolism of branched chain aminoacids) phenylketonuria phenylpiruvic acid (catabolism of Phe and Tyr) alkaptonuria homogentisic acid (catabolism of Phe and Tyr) isovaleric aciduria isovaleric acid (catabolism of Leu) argininosuccinic aciduria argininosuccinic acid (urea cycle) mevalonic aciduria mevalonic acid (biosynthesis of cholesterol) lactic aciduria lactic acid (several causes, genetic or acquired) Fanconi's syndrome defect of tubular reabsorption of bicabonate and other solutes (genetic or acquired) Further readings Pasipoularides A. Historical Perspective: Harvey's epoch-making discovery of the Circulation, its historical antecedents, and some initial consequences on medical practice; Journal of Applied Physiology Paul K. Hamilton, Neal A. Morgan, Grainne M. Connolly and Alexander P. Maxwell: Understanding Acid-Base Disorders. Alan W. Grogono Acid-base tutorial Ole Siggaard-Andersen maintains a very informative website on the respiratory and pH properties of blood. HK Walker, WD Hall, and JW Hurst,Clinical Methods Butterworths, Boston, MA, USA. JH Galla Metabolic alkalosis JASN JW Severinghaus The invention and development of the blood gas analysis apparatus Anesthesiology 2002; 97: 253-56. Krbec et al. Non-carbonic buffer power of whole blood is increased in experimental metabolic acidosis: An in-vitro study Front Physiol. 2022; 13: 1009378. CS Breathnach The development of blood gas analysis. Jubran A Pulse oximetry Crit Care. 1999; 3: R11–R17. Arbus GS An in vivo acid-base nomogram for clinical use. Can. Med. Assoc. J. 1973; 109: 291. A collection of clinical cases can be found at this website: https://geekymedics.com/abg-examples-and-case-studies/ Questions and exercises: 1) The normal parameters of arterial O2 are: PO2 > 110 mmHg; O2 content 20 mL/dL; O2 saturation > 95% PO2 > 90 mmHg; O2 content 15 mL/dL; O2 saturation > 95% PO2 > 90 mmHg; O2 content 20 mL/dL; O2 saturation > 95% 2) Normal values for serum CO2 and bicarbonate are: PCO2=40 mmHg (corresponding to 1,3 mM); bicarbonate=26 mM PCO2=26 mmHg (corresponding to 0,8 mM); bicarbonate=26 mM PCO2=40 mmHg (corresponding to 1,3 mM); bicarbonate=40 mM 3) The characteristic features of acute respiratory acidosis are markedly increased PCO2, markedly decreased pH, normal or moderately increased bicarbonate markedly increased PCO2, moderately decreased pH, markedly increased bicarbonate reduced PCO2, markedly decreased pH, reduced bicarbonate 4) The anion gap is increased in acute respiratory acidosis diabetic ketoacidosis adaptation to high altitude. your score: 10 Attendance not registered because of insufficient score - Retry! You can type in a comment or question below (max. length=160 chars.): All comments posted on the different subjects have been edited and moved to this web page (for optimal reading try to have at least 80 characters per line)! Thank you Professor (lecture on bilirubin and jaundice). The fourth recorded part, the one on hyper and hypoglycemias is not working. Bellelli: I checked and in my computer it seems to work. Can you better specify the problem you observe? This Presentation (electrolytes and blood pH) feels longer than previous lectures Bellelli: it is indeed. Some subjects require more information than others. I was thinking of splitting it in two nest year. Bellelli in response to a question raised by email: when we compare the blood pH with the standard pH we do not mean to compare the "normal" blood pH (7.4) with the standard pH. Rather we compare the actual blood pH of the patient, with the pH of the same blood sample equilibrated under standard conditions. Thus, if we say that standard pH is lower than pH we mean that equilibriation with 40 mmHg CO2 has caused absorption of CO2 and has lowered the pH with respect to its value before equilibration. (Lipoproteins) Is the production of leptin an indirect cause of type 2 diabetes since it works as a stimulus to have more adipose tissue that produces hormones? Bellelli: in a sense yes, sustained increase of leptin causes the hypothalamus to adapt and to stop responding. Obesity ensues and this in turn may cause an increase in the production of resistin and other insulin-suppressing protein hormones produced by the adipose tissue. However, this is quite an indirect link, and most probably other factors contribute as well. (Urea cycle) what is the meaning of "dissimilatory pathway"? Bellelli: a dissimilatory pathway is a catabolic pathway whose function is not to produce energy, but to produce some terminal metabolyte that must be excreted. Dissimilatory pathways are necessary for those metabolytes that cannot be excreted as such by the kidney or the liver because they are toxic or poorly soluble. Examples of metabolytes that require transformation before being eliminated are heme-bilirubin, ammonia, sulfur and nitrogen oxides, etc. Talking about IDDM linked neuropathy can be the C peptide absence considered a cause of it?? Bellelli: The C peptide released during the maturation of insulin, besides being an indicator of the severity of diabetes, plays some incompletely understood physiological roles. For example it has been hypothesized that it may play a role in the reparation of the atherosclerotic damage of the small arteries. Thus said, I am not aware that it plays a direct role in preventing diabetic polyneuropathy. Diabetic neuropathy has at least two causes: the microvascular damage of the arteries of the nerve (the vasa nervorum), and a direct effect of hyperglycemia and decreased and irregular insulin supply on the nerve metabolism. Diabetic neuropathy is observed in both IDDM and NIDDM, and requires several years to develop. Since the levels of the C peptide differ in IDDM and NIDDM, this would suggest that the role of the C peptide in diabetic neuropathy is not a major one. If you do have better information please share it on this site! In acute intermitted porphyria and congenital erythropoietic porphyria why do the end product of the affected enzymes accumulate instead of their substrate?? Bellelli: First of all, congratulations! This is an excellent question. Remember that a condition is which the heme is not produced is lethal in the foetus; thus the affected enzyme(s) must maintain some functionality for the patient to be born and to come to medical attention. All known genetic defects of heme biosynthesis derange but do not block this metabolic pathway. Congenital Erythropoietc Porphyria (CEP) is a genetic defect of uroporphyrinogen III cosynthase. This protein associates to uroporphyrinogen synthase (which is present and functional in CEP) and guarantees that the appropriate uroporphyrinogen isomer is produced (i.e. uroporphyrinogen III). In the absence of a functional uroporphyrinogen III cosynthase other possible isomers of uroporphyrinogen are produced together with uroporpyrinogen III, mostly uroporphyrinogen I. The isomers of uroporphyrinogen that are produced differ because of the positions of propionate and acetate side chains, and this in turn is due to the pseudo symmetric structure of porphobilinogen. Only isomer III can be further used to produce protoporphyrin IX. Thus in the case of CEP we observe accumulation of abnormal uroporphyrinogen derivatives, which, as you correctly observed are the products of the enzymatic synthesis operated by uroporphyrinogen synthase. The case of Acute Intermittent Porphyria (AIP) is similar, although there may be variants of this disease. What happens is that either the affected enzyme is a variant that does not properly associate with uroporphyrinogen III cosynthase or presents active site mutations that impair the proper alignement of the phoprphobilinogen substrates. In either case abnormal isomers of uroporphyrinogen are produced, as in CEP. Also remark that in both AIP and CEP we observe accumulation of the porphobilinogen precursor: this is because the overall efficiency of the biosynthesis of uroporphyrinogens is reduced. Thus: (i) less uroporphyrinogen is produced, and (ii) only a fraction of the uroporphyrinogen that is produced is the correct isomer (uroporphyrinogen III). is it possible to take gulonolactone oxidase to synthesize vitamin C instead of vitamin C supplement? Bellelli: no, this approach does not work. The main reason is that the biosynthesis of vitamin C, as almost all other metabolic processes, occurs intracellularly. If you administer the enzyme it will at most reach the extracellular fluid but will not be transported inside the cells to any significant extent. Besides, there are other problems in this type of therapy (e.g. the enzyme if administered orally, may be degraded by digestive proteases; if administered parenterally, may cause the immune system to react against a non-self protein). In theory one could think of a genetic modification of the inactive human gene of gulonolactone oxidase, but the risk and cost of this intervention would not be justified. In addition to these considerations, except for cases of shipwreckage or other catastrophes, a proper diet or administration of tablets of vitamin C is effective, risk-free and unexpensive, thus no alternative therapy is reasonable. However, I express my congratulations for your search on the biosynthesis pathway of ascorbic acid. Resorption and not reabsorption would lead to hypercalcemia ie bone matrix being broken down. Bellelli: I am not sure to interpret your question correctly. Resorption indicates destruction of the bone matrix and release of calcium and phosphate in the blood, thus it causes an increase of calcemia. Reabsorption usually means active transport of calcium from the renal tubuli to the blood, thus it prevents calcium loss. It prevents hypocalcemia, and thus complement bone resorption. To avoid confusion it is better use the terms "bone resorption" and " renal reabsorption of calcium". If you have a defect in renal reabsorption, parthyroid hormone will be released to maintain a normal calcium level by means of bone resorption; the drawback is osteoporosis. In Reed and Frost model: I haven't understood what is the relationship between K and R reproductive index. Thank you Professor! Bellelli: in the Reed and Frost model K is the theoretical upper limit of R0. R the reproductive index is the ratio (new cases)/(old cases) measured after one serial generation time. R0 is the value of R one measures at the beginning of the epidemics, when in principle all the population is susceptible. What is the link between nucleotide metabolism and immunodeficiencies and mental retardation? Bellelli: the links may be quite complex, but the principal ones are as follows: 1) the immune response requires a replication burst of granulocytes and lymphocytes, which in turn requires a sudden increase of nucleotide production, necessary for DNA replication. Defects of nucleotide metabolism impair this phase of the immune defense. Notice that the mechanism is similar to the one responsible of anemia which requires a sustained biosynthesis of nucleotides at a constant rate, rather than in a burst. 2) Mental retardation is mainly due to the accumulation of nulceotide precursors in the brain of the newborn, due to the incompletely competent blood-brain barrier. How can ornithine transaminase defects cause hyperammonemia? Is it due to the accumulation of ornithine that blocks the urea cycle or for other reasons? Bellelli: ornithine transaminase is required for the reversible interconversion of ornithine and proline, and thus participates to both the biosynthesis and degradation of ornithine. The enzyme is synthesized in the cytoplasm and imported in the mitochondrion. Depending on the metabolic conditions the deficiency of this enzyme may cause both excess (when degradation would be necessary) or defect (when biosynthesis would be necessary) of ornithine; in the latter case, the urea cycle slows down. Thus there is the paradoxical condition in which alternation may occur between episodes of hyperammonemia and of hyperornithinemia. When we use the Berthelot's reaction to measure BUN do we also have to measure the concentration of free ammonia before adding urease? Bellelli: yes, in principle you should. Berthelot's reaction detects ammonia, thus one should take two identical volumes of serum, use one to measure free ammonia, the other to add urease and measure free ammonia plus ammonia released by urea. BUN is obtained by difference. However, free ammonia in our blood is so much lower than urea that you may omit the first sample, if you only want to measure BUN. Why do we have abnormal electrolytes in hematological neoplasia e.g. leukemia? Bellelli: I do not have a good explanation for this effect, which may have multiple causes. However, you should consider two factors: (i) acute leukemias cause a massive proliferation of leukocytes (or lymphocytes depending on the cell type affected) with a very shortened lifetime; thus you observe an excess death rate of the neoplastic cells. The dying cells release in the bloodstream their content, which has an electrolyte composition different from that of plasma: the cell cytoplasm is rich in K and poor in Na, thus causing hyperkalemia. (ii) the kidney may be affected by the accumulation of neoplastic white cells or their lytic products. Gaussian curve: If it is bimodal is it more likely to be a "certain diagnosis" than if it is unimodal or does it only show the distinguishment from health? Bellelli an obviously bimodal Gaussian curve indicates that the disease is clearly separated from health: usually it is a matter of how precise and clear-cut is the definition of the disease. For example tuberculosis is the disease caused by M. tuberculosis, thus if the culture of the sputum is positive for this bacterium you have a "certain" diagnosis (caution: the patient may suffer of two diseases, e.g. tuberculosis and COPD diagnosis of the first does not exclude the second). However, in order to have a "certain" diagnosis it is not enough that distribution of the parameter is bimodal, it is also required that the patient's parameter is out of the range of the healthy condition: this is because a distribution can be bimodal even though it is composed by two Gaussians that present a large overlap, and the patient's parameter may fall in the overlapping region. Thus, in order to obtain a "certain" diagnosis you need to consider not only the distribution of the parameter(s) but also the patient's values and the extent of the overlapping region. Prof can you please elaborate a bit more on the interhuman variability and its difference with the interpopulation variability please? Bellelli: every individual is a unique combination of different alleles of the same genes; this is the source of interindividual variability. Every population is a group of individuals who intermarry and share the same gene pool (better: allele pool). Every allele in a population has its own frequency. Two population may differ because of the diffferent frequencies of the same alleles; in some cases one population may completely lack some alleles. The number and frequencies of alleles of each gene determine the variance. If you take two populations and calculate the cumulative interindividual variance of the population the number you obtain is the sum of two contributions: the interindividual variance within each population, plus the interpopulation variance between the means of the allele frequencies. For example, there are human population in which the frequency of blood group B is close to 0% and other populati ons in which it is 30% or more. Prof can you please explain again the graph you have showed us in class about thromboplastin? (Y axis=abs X axis= time) Bellelli: the graph that I crudely sketched in class represented the signal of the instrument (an absorbance spectrophotometer) used to record the turbidity of the sample (turbidimetry). The plasma is more or less transparent, before coagulation starts. When calcium and the tissue factor (or collagen) are added. thrombin is activated and begins digesting fibrinogen to fibrin; then fibrin aggregates. The macroscopic fibrin aggregates cause the sample to become turbid, which means it scatters the incident light. The instrument reads this as a decrease of transmitted light (i.re an increase of the apparent absorbance) and the time profile of the signal presents an initial lag phase, which is called the protrombin or thromboplastin time depending on the component which was added to start coagulation (tissue factor or collagen). Prof can you please explain the concept you have described in class about the simultaneous hypercoagulation and hemorrhagic syndrome? How can this occur? Bellelli: The condition you describe is observed only in the Disseminated Intravascular Coagulation syndrome. Suppose that the patient experiences an episode of acute pancreatitis: tripsin and chymotripsin are reabsorbed in the blood and proteolytically activate coagulation causing an extensive consumption of fibrinogen and other coagulation factors. Tripsin and chymotripsin also damage the vessel walls and may cause internal hemorrages, but at that point the consumption of fibrinogen may have been so massive that not enough is left to form the clot where the vessel has been damaged, causing an internal hemorrage. Pancreatitis is a very severe, potentially lethal condition, and DIC is only one of the reasons of its severity. You said that certain drugs (ethanol, cocaine, cannabis, opiates...) cause a necessity of higher and higher dosage, for two reasons: the enzyme in the liver is inducible and the receptors in the brain are expressed less and less. So, first, I am not sure I got it right, and second I did not understand how expressing less receptors leads to a necessity of higher dosage. Bellelli: You got it correctly, but the detailed mechanism of resistance may vary among different substances, and not all drugs cause adaptation. The reason why reducing the number of receptors may require an increased dosage of the drug is as follows: suppose that a certain cell has 10,000 receptors for a drug. When bound to its agonist/effector, each receptor produces an intracellular second messenger. Suppose that in order for the cell to respond 1,000 receptors must be activated. The concentration of the effector required is thus the concentration that produces 10% saturation. You can easily calculate that this concentration is approximately 1/10 of the equilibrium dissociation constant of the receptor-effector complex (its Kd), the law being Fraction bound = [X] / ([X]+Kd) where [X] is the concentration of the free drug. After repeated administration, the subject becomes adapted to the drug, and his/her cells express less receptors, say 5,000. The cell response will in any case require that 1,000 receptors are bound to the effector and activated, but this now represents 20% of the total receptors, instead of 10%. The drug concentration required is now 1/4 of the Kd. Continuing administration of the drug further reduces the cell receptors, but the absolute number of activated receptors required to start the response is constant; thus the fewer receptors on the cell membrane, the higher the fraction of activated receptors required. Why does hyperosmolarity happen in type 2 diabetes and not in type 1? Bellelli: Hyperosmolarity can occur also in type 1 diabetes, albeit infrequently. The approximate formula for plasma osmolarity is reported in the lecture on electrolytes: osmolarity = 2 x (Na+ + K+) + BUN/2.8 + glucose/18 this is expressed in the usual clinical laboratory units (mEq/L for electrolytes, g/dL for non- electrolytes). The normal values are: osmolarity = 2 x (135 + 5) + 15/2.8 + 100/18 = 280 + 5.4 + 5.6 = 291 mOsmol/L Let's imagine a diabetic patient having normal values for electrolytes and BUN, and glycemia=400 mg/dL: osmolarity = 280 + 5.4 + 22.4 = 307.8 mOsmol/L The hyperosmolarity in diabetes is mainly due to hyperglycemia, even though other factors may contribute (e.g. diabetic nefropathy); however the contribution of glucose to osmolarity is relatively small. As a consequence in order to observe hyperosmolarity the hyperglycemia should be extremely high; this is more often observed in type 2 than in type 1 diabetes, for several reasons, the most relevant of which is that in type 1 diabetes all cells are starved of glucose, and the global reserve of glycogen in the body is impoverished: there is too much glucose in the blood and too few everywhere else, thus reducing, but not abolishing, the risk of extreme hyperglycemia. Usually in type 2 diabetes the glycogen reserve in the organism is not impoverished, thus the risk of extreme hyperglycemia is higher. Hemostasis and Thrombosis lecture: I don't understand why is sodium citrate added to the serum solution to measure the prothrombin time. Bellelli: in order to measure PT or PTT you want to be able to start the coagulation process at an arbitrary time zero, and measure the increase in turbidity of the serum sample. To do so you need (i) to prevent spontaneous coagulation with an anticoagulant; and (ii) to be able to overcome the anticoagulant at your will. Citrate (or oxaloacetate; or EDTA) has the required characteristics: it chelates calcium, and in this way it prevents coagulation; but you can revert its effect at your will by adding CaCl2 in excess to the amount of citrate. You cannot obtain the same effect with other anticoagulants (e.g. heparin) whose action cannot be easily overcome. Dear professor I cannot do the self evaluation test because it says the the time has expired It is not possible because I havent even started them Bellelli: this is due to the fact that the program registers your name and matricola number from previous attempts. I shall fix this bug. Meanwhile try to use a fake matricola number. How is nephrotic syndrome associated hypoalbuminemia as you described in methods of analysis of protein because seems counterintuitive Bellelli: nephrotic syndrome is an autoimmune disease in which the glomerulus is damaged and the filtration barrier is disrupted; diuresis is normal but there is loss of proteins (mostly albumin) in the urine. I m sorry i confused polyurea with hypoalbuminemia but my question still stands during glomerulonephritis you mentioned something of polyurea as compensation i could not follow how this compensation mechanism works and collapse after some time in glomerulonephritis Bellelli: the condition you describe is NOT characteristic of acute glomerulonephritis. In glomerulonephritis there is damage of the glomerulus and severely impaired GFR. Thus the diuresis is severely reduced, and due to impaired filtration proteins appear in the urine. The condition you describe corresponds to the initial stage of chronic kidney failure, usually due to atherosclerosis, diabetes, hypertension or other type of damage of the kidney tissue. In this case GFR is impaired, albeit to a lesser extent than in glomerulonephritis, and the excretion of urea is reduced. This leads to increased BUN. However the increased concentration of urea reduces the ability of the tubuli to reabsorb water, because of osmotic reasons, yielding compensatory polyuria. The patient has reduced GFR but normal or increased diuresis (urine volume in 24 hours). To some extent this effect is beneficial, as it favors the elimination of urea; however it cannot completely solve the problem and in any case the progression of the disease leads to kidney insufficiency. In its essence the point is that a moderately reduced GFR can be partially compensated by reduced tubular reabsorption; a severely reduced GFR cannot. Lecture on Hemogas analysis interpretation of complex cases standard pH Why if PCO2 is less than 40 mmHg it is absorbed during equilibration? Thank you in advance Bellelli: if PCO2 of the patient's blood sample is less than 40 mmHg, when the machine equilibrates with 40 mmHg CO2 the gas is absorbed: i.e. the new PCO2 becomes 40 mmHg and the total CO2 of the sample increases; as CO2 is the acid of the buffer, the standard pH (in this case) decreases, whereas standard bicarbonate will slightly increase. Professor I don't understand how we arrive to this formula: Accuracy = sensitivity x prevalence specificity x (1-prevalence) Bellelli: ok, this relationship is poorly explained in your text, I shall improve its explanation. We use the following definitions: prevalence = sick individuals / total population; accuracy = (true+ + true-) / total population; sensitivity = true+ / sick individuals = true+ / total population x prevalence; specificity = true- / healthy individuals = true- / total population x (1-prevalence); thus we can rewrite: sensitivity x prevalence = true+ / total population; specificity x (1-prevalence) = true- / total population; accuracy = sensitivity x prevalence + specificity x (1-prevalence) Why do we use RNA primer in PCR and not DNA primers? I thought the beginning of the sequence of the gene segment that is going to be formed is made of DNA Bellelli: DNA polimerases require the RNA primers that are synthesized by the enzyme primase. DNA primers do not exist in vivo and would not be recognized by DNA polimerases.       Home of this course Slides for this lecture: