THE NATURE OF THE CARBON DIOXIDE TITRATION CURVE IN THE NORMAL DOG

Combining the Physical-Chemical Approach with Standard Base Excess to Understand the Compensation of Respiratory Acid-Base Derangements: An Individual Participant Meta-analysis Approach to Data from Multiple Canine and Human Experiments

BACKGROUND

Several studies explored the interdependence between Paco2 and bicarbonate during respiratory acid-base derangements. The authors aimed to reframe the bicarbonate adaptation to respiratory disorders according to the physical-chemical approach, hypothesizing that (1) bicarbonate concentration during respiratory derangements is associated with strong ion difference; and (2) during acute respiratory disorders, strong ion difference changes are not associated with standard base excess.

METHODS

This is an individual participant data meta-analysis from multiple canine and human experiments published up to April 29, 2021. Studies testing the effect of acute or chronic respiratory derangements and reporting the variations of Paco2, bicarbonate, and electrolytes were analyzed. Strong ion difference and standard base excess were calculated.

RESULTS

Eleven studies were included. Paco2 ranged between 21 and 142 mmHg, while bicarbonate and strong ion difference ranged between 12.3 and 43.8 mM, and 32.6 and 60.0 mEq/l, respectively. Bicarbonate changes were linearly associated with the strong ion difference variation in acute and chronic respiratory derangement (β-coefficient, 1.2; 95% CI, 1.2 to 1.3; P < 0.001). In the acute setting, sodium variations justified approximately 80% of strong ion difference change, while a similar percentage of chloride variation was responsible for chronic adaptations. In the acute setting, strong ion difference variation was not associated with standard base excess changes (β-coefficient, -0.02; 95% CI, -0.11 to 0.07; P = 0.719), while a positive linear association was present in chronic studies (β-coefficient, 1.04; 95% CI, 0.84 to 1.24; P < 0.001).

CONCLUSIONS

The bicarbonate adaptation that follows primary respiratory alterations is associated with variations of strong ion difference. In the acute phase, the variation in strong ion difference is mainly due to sodium variations and is not paralleled by modifications of standard base excess. In the chronic setting, strong ion difference changes are due to chloride variations and are mirrored by standard base excess.

EDITOR’S PERSPECTIVE

The role of the clinical laboratory in diagnosing acid-base disorders

Acid-base homeostasis is fundamental for life. The body is exceptionally sensitive to changes in pH, and as a result, potent mechanisms exist to regulate the body's acid-base balance to maintain it in a very narrow range. Accurate and timely interpretation of an acid-base disorder can be lifesaving but establishing a correct diagnosis may be challenging. The underlying cause of the acid-base disorder is generally responsible for a patient's signs and symptoms, but laboratory results and their integration into the clinical picture is crucial. Important acid-base parameters are often available within minutes in the acute hospital care setting, and with basic knowledge it should be easy to establish the diagnosis with a stepwise approach. Unfortunately, many caveats exist, beginning in the pre-analytical phase. In the post-analytical phase, studies on the arterial reference pH are scarce and therefore many different reference values are used in the literature without any solid evidence. The prediction models that are currently used to assess the acid-base status are approximations that are mostly based on older studies with several limitations. The two most commonly used methods are the physiological method and the base excess method, both easy to use. The secondary response equations in the base excess method are the most convenient. Evaluation of acid-base disorders should always include the assessment of electrolytes and the anion gap. A major limitation of the current acid-base laboratory tests available is the lack of rapid point-of-care laboratory tests to diagnose intoxications with toxic alcohols. These intoxications can be fatal if not recognized and treated within minutes to hours. The surrogate use of the osmolal gap is often an inadequate substitute in this respect. This article reviews the role of the clinical laboratory to evaluate acid-base disorders.

Secondary Response to Chronic Respiratory Acidosis in Humans: A Prospective Study

Introduction

The magnitude of the secondary response to chronic respiratory acidosis, that is, change in plasma bicarbonate concentration ([HCO₃⁻]) per mm Hg change in arterial carbon dioxide tension (PaCO₂), remains uncertain. Retrospective observations yielded Δ[HCO₃⁻]/ΔPaCO₂ slopes of 0.35 to 0.51 mEq/l per mm Hg, but all studies have methodologic flaws.

Methods

We studied prospectively 28 stable outpatients with steady-state chronic hypercapnia. Patients did not have other disorders and were not taking medications that could affect acid−base status. We obtained 2 measurements of arterial blood gases and plasma chemistries within a 10-day period.

Results

Steady-state PaCO₂ ranged from 44.2 to 68.8 mm Hg. For the entire cohort, mean (± SD) steady-state plasma acid−base values were as follows: PaCO₂, 52.8 ± 6.0 mm Hg; [HCO₃⁻], 29.9 ± 3.0 mEq/l, and pH, 7.37 ± 0.02. Least-squares regression for steady-state [HCO₃⁻] versus PaCO₂ had a slope of 0.476 mEq/l per mm Hg (95% CI = 0.414–0.538, P < 0.01; r = 0.95) and that for steady-state pH versus PaCO₂ had a slope of −0.0012 units per mm Hg (95% CI = −0.0021 to −0.0003, P = 0.01; r = −0.47). These data allowed estimation of the 95% prediction intervals for plasma [HCO₃⁻] and pH at different levels of PaCO₂ applicable to patients with steady-state chronic hypercapnia.

Conclusion

In steady-state chronic hypercapnia up to 70 mm Hg, the Δ[HCO₃⁻]/ΔPaCO₂ slope equaled 0.48 mEq/l per mm Hg, sufficient to maintain systemic acidity between the mid-normal range and mild acidemia. The estimated 95% prediction intervals enable differentiation between simple chronic respiratory acidosis and hypercapnia coexisting with additional acid−base disorders.

Experimental critical care in ventilated rats: effect of hypercapnia on arterial oxygen-carrying capacity

PURPOSE

We have previously demonstrated an increased arterial O2-carrying capacity in normal ventilated dogs subjected to both acute and prolonged exogenous hypercapnia. In the present study, we tested if arterial hypercapnia, during controlled ventilation, can increase O2-carrying capacity also in rats.

MATERIALS AND METHODS

Twenty young male Sprague Dawley rats were anesthetized (60 mg/kg pentobarbital), tracheostomized, intubated, and one femoral vein and artery were cannulated. Anesthesia and paralysis were maintained using 15 mg/kg/h pentobarbital intravenously, and 2 mg/kg/h vecuronium bromide. The fluid balance (5 mL/kg/h saline), normothermia, and minute volume were maintained. The mean arterial blood pressure and heart rate were continuously monitored. Experiments included the following: (1) a control group, ventilated with normoxic air for 150 minutes (n = 5); (2) mild hypercapnia, a group of eight rats ventilated with normoxic air for 30 minutes and then ventilated with a mixture of normoxic air at 60 mm Hg CO2 (8 kPa) for 1 hour; and (3) severe hypercapnia, a group of seven rats were treated exactly as in group II, except a 90 mm Hg (12 kPa) CO2 during hypercapnia. Gas-exchange profile, arterial hemoglobin (Hb) concentration, arterial Hb-oxygen saturation (Hb-O2), and arterial O2 content were periodically determined during normocapnia and 1 hour of hypercapnia.

RESULTS

Exposures to mild and severe hypercapnia, in rats with maintained ventilation, significantly reduced the arterial O2 content by 20% and 33%, respectively, without significant changes in the arterial Hb concentration (-2%). Severe hypercapnia generated a significant reduction of -14% in the PaO2, but not in PaO2/ FiO2 ratio.

CONCLUSION

Rats subjected to controlled ventilation and permissive hypercapnia, unlike dogs and perhaps humans, show no augmentation of Hb concentration. Hypercapnia in rats also provokes much stronger Bohr effect than in dogs. Hypercapnia-induced Bohr effect in rats is accompanied with extreme desaturations of Hb-O2, and substantial reduction in the O2-carrying capacity. We speculate that the strong hypercapnia-induced Bohr effect in rats may prevent hypoxia at the tissue level. However, to maintain a stable oxygen-carrying capacity in rats used for pulmonary critical care studies with hypercapnia, we suggest to use hyperoxia, with or without a mild hypothermia.

Log-jam in acid-base education and investigation: why make it so difficult?

Medical students frequently have difficulty in interpreting acid-base data particularly when pH values are used. The difficulty persists when students qualify and has implications for the safe management of patients who require investigation of acid-base status. Simplification of tuition is required together with a change of practice in the reporting of acid-base data by the laboratories. To improve understanding, we recommend that the teaching and reporting of acid-base status should be changed to use [H+] instead of pH, and a greater emphasis placed on the logical interpretation of primary measurements--that is [H+] and PCO2--with less reliance on derived variables.

History of blood gas analysis. II. pH and acid-base balance measurements

Electrometric measurement of the hydrogen ion concentration was discovered by Wilhelm Ostwald in Leipzig about 1890 and described thermodynamically by his student Walther Nernst, using the van't Hoff concept of osmotic pressure as a kind of gas pressure, and the Arrhenius concept of ionization of acids, both of which had been formalized in 1887. Hasselbalch, after adapting the pH nomenclature of Sørensen to the carbonic-acid mass equation of Henderson, made the first actual blood pH measurements (with a hydrogen electrode) and proposed that metabolic acid-base imbalance be quantified as the "reduced" pH of blood after equilibration to a carbon dioxide tension (PCO2) of 40 mm Hg. This good idea, coming 40 years before simple blood pH measurements at 37 degrees C became widely available, was never adopted. Instead, Van Slyke developed a concept of acid-base chemistry that depended on measuring plasma CO2 content with his manometric apparatus, a standard method until the 1960s, when it was displaced by the three-electrode method of blood gas analysis. The 1952 polio epidemic in Copenhagen stimulated Astrup to develop a glass electrode in which pH could be measured in blood at 37 degrees C before and after equilibration with known PCO2. He introduced the interpolative measurement of PCO2 and bicarbonate level (later base excess) using only pH measurements and, with Siggaard-Andersen, developed clinical acid-base chemistry. Controversy arose when blood base excess was noted to be altered by acute changes in PCO2 and when abnormalities of base excess were called metabolic acidosis or alkalosis, even when they represented compensation for respiratory abnormalities in PCO2. In the 1970s it became clear that "in-vivo" or "extracellular fluid" base excess (measured at an average extracellular fluid hemoglobin concentration of 5 g) eliminated the error caused by acute changes in PCO2. Base excess is now almost universally used as the index of nonrespiratory acid-base imbalance.

Blood acid-base status, whole blood and whole body buffer values in sand rats (Psammomys obesus) exposed to hypercapnia

Arterial blood acid-base status of unanesthetized sand rats (Psammomys obesus) were studied under normocapnic and hypercapnic conditions, and compared to those obtained for the albino rat (Rattus norvegicus). The average control blood pH: 7.396 +/- 0.034; PaCO2: 30.5 +/- 2.9 mmHg; HCO-3: 18.8 +/- 2.5 mM/l; and HCO-3 std: 20.9 +/- 2.1 (N = 15) obtained here for the sand rat are in the lower range of values found in other mammals and indicate a status of partially compensated metabolic acidosis. The blood buffer values of the sand rat, delta log PCO2/delta pH = -2.32 +/- 0.35 (N = 25) are significantly higher than those found here for the rat, delta log PCO2/delta pH = -1.51 +/- 0.10 (N = 39), and those reported for other mammals. This high blood buffer value may be related to the natural high mineral diet of the sand rat. The in vivo (whole body) buffer value delta log PaCO2/delta pH = -1.41 and -1.65 for the sand rat and the rat found here are higher than those reported for the man and dog and may represent a physiological adaptation to the hypercapnic conditions prevailing in underground burrows.

The effects of the inhibition of the renal carbonic anhydrase on the blood acid-base status in hypercapnic rats

Arterial blood acid-base status was measured in unanesthetized rats treated with benzolamide (a selective renal carbonic anhydrase inhibitor). These measurements were carried out in rats exposed to different levels of CO2 in air (0-10% CO2) for periods of up to 6 hr. In untreated rats the whole body buffer value showed a continuous increase and after 6 hr of exposure to hypercapnia its value was twice that measured initially. On the other hand, the whole body buffer value of benzolamide treated rats did not change during the 6 hr of exposure to hypercapnia. The whole body buffer value of normal rats, measured after 6 hr of hypercapnia is similar to that reported for chronic (3-5 days) hypercapnia in the normal dog. The whole body buffer value in benzolamide treated rats was similar to that reported for the normal dog and man, during acute CO2 exposures. It is suggested that mechanisms involving the renal carbonic anhydrase are responsible for the significant, rapid changes in the whole body buffer value that take place during the initial phase of acute exposure to CO2 in the rat. This may represent a mechanism of adaptation to burrow hypercapnic conditions.

In vivo and in vitro CO2 titration curves in the rabbit: adaptation to hypercapnic conditions

Arterial blood acid-base status of unanesthetized, unrestrained rabbits was studied during 6-12 hr of exposure to 7, 10 and 14.5% CO2. Most of the changes in blood acid-base status occurred during the first 20-60 min of exposure to hypercapnia and only minor changes occurred during the remaining exposure period (up to 12 hr). Blood buffer values obtained were not different from those reported for other terrestrial mammals. The whole body buffer values obtained here for the rabbit (0.58 nM H+/mmHg PCO2) is higher than that reported previously for man and dog. This relatively high whole body buffer value complies well with the high tolerance to CO2 reported for the rabbit.

Regulation of blood acid-base status in guinea pigs exposed to hypercapnia

1. Arterial blood acid-base status of unanesthetized, unrestrained guinea pigs was studied during 6 hr of exposure to 4, 8, 10 and 14.5% CO2. 2. During exposure to 4% CO2, blood pH was kept within the range of control values, despite significant increase of 5-8 mmHg in PaCO2. 3. Most of the changes in blood acid-base status occurred during the first 30-60 min of exposure to CO2, and only minor changes were observed during the remaining exposure period (up to 6 hr). 4. In-vivo CO2 titration curves were not linear over the CO2 range studied here. The slope of the in-vivo H+/PaCO2 line became much more steep at PaCO2 values higher than 65-75 mmHg. 5. The apparent whole body buffer value (beta = -delta HCO-3/delta pH), being 42.6 slykes after 1 hr for the 4-10% CO2 range, changed to -18.1 slykes when calculated for 1 hr at the 10-14.5% CO2 range. 6. It is concluded that guinea pigs can regulate their blood pH better than rats, rabbits, dogs and men when exposed for short periods to CO2 levels up to 10%. 7. When exposed to higher levels (14.5%) of CO2, they show a very limited capability for regulating their blood pH--much less than rats, rabbits and dogs.

Adaptations to hypercapnic conditions in the nutria (Myocastor coypus)--in vivo and in vitro CO2 titration curves

Arterial blood acid-base status of unanesthetized, unrestrained nutria was studied during exposure to 5, 10 and 14.5% CO2 for 6 hr. Control values, pH = 7.426 +/- 0.037, PaCo2 = 36.5 +/- 3.1 mmHg and [HCO-3] = 24.3 +/- 2.5 mM/1 (n = 24), are within the normal range reported for other mammals. Values after 6 hr of exposure to 10% CO2 were: pH = 7.355 +/- 0.043, PaCO2 = 71.0 +/- 3.6 mmHg and [HCO-3] = 38.0 +/- 4.1 mM/l (n = 5). Arterial blood buffer slopes, obtained from the in vitro titration curve, did not show any pattern of adaptation to hypercapnia. Whole body buffer slopes, calculated from the in vivo CO2 titration curve, showed significantly higher values for the nutria than for the rat, dog and man, under comparable conditions [beta(delta HCO-3/delta pH)] = 57.0 slykes for nutria, 32.6 for rat and 11.8 for man. delta H+/delta PaCO2 = 0.38. mM/l/mmHg for nutria, 0.55 for rat and 0.76 for man. The results suggest that the nutria possesses an efficient metabolic mechanism for regulation of pH level during exposure to hypercapnic conditions.

Arterial blood gases and acid-base status in awake rats

Arterial blood gases and acid-base balance were measured in adult rats using a cannula implanted in the aortic arch. These measurements were performed both in awake, unrestrained animals and in animals submitted to various circumstances i.e. (a) different diet: high and low sodium chloride intake, (b) anesthesia by pentobarbital or inactine and, (c) repeated blood sampling with concomitant replacement with the same volume of blood. For each group investigated the [HCO3 -]a vs. PaCO2, [H+] vs. PaCO2, PaCO2 vs. PaO2 relationships were determined. The values obtained (m +/- SD) from awake, unrestrained adult rats were respectively 7.47 +/- 0.02 for arterial pH, 34.5 +/- 3.0 Torr for PaCO2 and 90 +/- 5.5 Torr for PaO2; the calculated [HCO3 -]a concentration was 25.5 +/- 1.5 mmol . 1-1. The present results indicate that plasma bicarbonate concentration, within normal range, highly depends on the prevailing resting level of PaCO2 (n = 202; r = 0.82; P less than 10(-3)). In addition, the PaCO2 versus PaO2 relationship was highly statistically significant (n = 202; r = -0.43; P less than 10(-3). In the other experimental groups of rats, these relationships were virtually the same as above although mean values (+/- SD) for PaCO2, PaO2, pHa and [HCO3 -]a might vary with the group investigated. The mean value for whole pHi, obtained by the DMO method, reached 6.81 for pHa = 7.47 and was not correlated to PaCO2 level in normal conditions. The present data argue for the existence of a respiratory component mediating individual acid-base variations in a normal population of rats. Arterial carbon dioxide partial pressure, by determining bicarbonate ions reabsorption rate, would ensure pH regulation under normal circumstances.

A comparison of carbon dioxide titration curves of arterial mixed venous and coronary sinus blood

Two types of titration curve (log PCO2 vs. pH and whole blood [HCO3-] vs. pH) were obtained in anaesthetized dogs by altering FICO2. Both plots reveal a significant progressive decrease in slope from arterial(a) to mixed venous (v) to coronary sinus (cs) blood. Lines drawn between corresponding points on the arterial and mixed venous or coronary sinus curves have slopes determined by the Respiratory Exchange Ratio (R) and are called R lines. Differences in slope between arterial and venous (v or cs) curves may be due to changes in blood flow or of delivery of CO2 to the blood. The former is indicated by constant R values at different CO2 tensions whilst the latter is a reflection of changing R values due to fluctuations in CO2 stores. The myocardium is not an important store for CO2 but CO2 has a profound effect on coronary blood flow and it is this that causes the difference in a-cs slopes. The reverse is true for skeletal muscle where the charging and discharging of CO2 stores brings about the difference in a-v slopes.

Balance of net base in the rat: adaptation to and recovery from sustained hypercapnia

Net base and mineral balances were evaluated in a group of male 350 g Wistar rats exposed to 10% carbon dioxide in air for 10 days with a view to identifying the source of net base subject to retention during renal compensation of sustained respiratory acidosis. In response to hypercapnia, the rate of renal net acid excretion rose but insignificantly. However, a rise in whole body net base concentration from about 215 mmol/kg to about 250 mmol/kg came about by ongoing gastrointestinal absorption in the weight-losing animal, absorbed net base being distributed to extracellular and non-extracellular compartments of the body, presumably including bone. During an 8-day recovery period, a small decrement in whole body net base concentration was observed.

Effects of acute hypercapnia and hypocapnia on plasma and red cell potassium, blood lactate and base excess in man during anesthesia

In order to test the relationship between changes in plasma potassium concentration and pH changes of respiratory origin, we produced hypercapnia (mean PaCO2 71 mmHg = 9.5 kPa) in a group of 17 patients and hypocapnia (mean PaCO2 21 mmHg = 2.8 kPa) in another 20 patients during neurolept analgesia and intraabdominal operations. A control group of 19 patients was studied under normocapnia but otherwise identical conditions. During hypercapnia, serum potassium rose, deltaK/deltapH amounting to -0.82, -1.05 and -1.34 after 30, 60 and 90 min, respectively. During hypocapnia, serum potassium decreased, deltaK/deltapH being a little more negative than during hypercapnia (mean values -1.62, -2.44 and -1.60). Red cell potassium concentration decreased in all three groups to a similar extent. Blood lactate levels during hypercapnia decreased to 75% of control and during hypocapnia rose to a maximum of 186% of control. In order to obtain reasonable values for base excess in primarily respiratory acid-base disorders, it is necessary to use nomograms based on in vivo ECF-CO2-titration curves. With this premise, hypercapnia or hypocapnia in our patients was not associated with significant changes in base excess.

Correction of hyperkalemia by bicarbonate despite constant blood pH

Patients having hyperkalemia often are given bicarbonate to raise blood pH and shift extracellular potassium into cells. Blood pH in many hyperkalemic patients, however, is compensated. To determine whether bicarbonate, independent of its pH action, affects plasma potassium, 14 hyperkalemic patients were treated with bicarbonate in 5% dextrose. In five patients (changed pH group), blood pH rose at least 0.08, while in nine (constant pH group), it changed less than 0.04. In the first group, pH rose 0.12, bicarbonate rose 5.9 mEq/liter, and plasma potassium fell 1.6 mEq/liter, and plasma potassium fell 1.4 mEq/liter. The correlation between changes in plasma potassium and bicarbonate was identical in the two groups and independent of urinary potassium excretion. Four additional patients, who were treated with 5% dextrose alone, did not significantly lower their plasma potassium, although subsequent treatment with bicarbonate in 5% dextrose lowered their plasma potassium. Thus, bicarbonate lowers plasma potassium, independent of its effect on blood pH, and despite a risk of volume overload, should be used to treat hyperkalemia in compensated acid-base disorders, even in the presence of renal failure, provided the plasma bicarbonate concentration is decreased.

Relationship between phosphaluria and acute hypercapnia in the rat

Standard clearance studies were performed in mechanically ventilated intact and acutely thyroparathyroidectomized (TPTX) rats to document and characterize the effect of hypercapnia (HC) on urinary phosphorus excretion (U(P)V). HC as compared to normocapnia (NC) was associated with an increase in U(P)V in intact (62.5 vs. 7.93 mug/min) and TPTX (30.5 vs. 0.59 mug/min) rats, an increase in filtered load of phosphorus in intact (218 vs. 191 mug/min) and TPTX (243 vs. 146 mug/min) rats, an increase in blood bicarbonate concentration in intact (27.8 vs. 26.0 meq/liter) and TPTX (24.5 vs. 22.3 meq/liter) animals, and a decrease in blood pH in intact (7.15 vs. 7.42) and TPTX (7.07 vs. 7.39) rats. Additional TPTX rats with NC and HC were studied during phosphorus infusion at a comparable filtered load of phosphorus (NC = 307 mug/min and HC = 328 mug/min). U(P)V was 18.5 mug/min in NC and 85.2 mug/min in HC animals. Intact NC animals infused with NaHCO(3) achieved a blood bicarbonate of 45.9 meq/liter compared to 26.0 meq/liter in intact NC NaCl-infused rats. U(P)V was 10.0 mug/min in the NaHCO(3) and 7.93 mug/min in NaCl-infused animals. In intact HC animals infused with NaHCO(3), blood pH was 7.36 compared to 7.42 in NC intact NaCl-infused animals. U(P)V was 83.2 mug/min in the HC bicarbonate-infused and 7.93 mug/min in the NC NaCl-infused rats. These experiments demonstrate that elevated blood carbon dioxide tension per se increases U(P)V. Increases in filtered load of phosphorus and blood bicarbonate which are associated with HC contribute to the phosphaturia as does parathyroid hormone. The phosphaturia is not dependent upon reduction of extracellular pH.

Intracellular pH of isolated rat diaphragm muscle with metabolic and respiratory changes of extracellular pH

Relationships between intracellular and extracellular pH isolated rat diaphragms were determined both during respiratory and metabolic changes of extracellular pH. Metabolic changes of extracellular pH were produced by varying bicarbonate concentration of the suspending Krebs-Ringer solution and respiratory changes were produced by varying PCO2 of the suspending medium. At any defined extracellular pH, the bicarbonate concentration ratios between intracellular and extracellular space were the same during both metabolic and respiratory changes of extracellular pH. However, when extracellular pH varied within 7.15 and 7.4 intracellular pH remained essentially constant. In order to maintain the intracellular pH constant during extracellular pH changes, a bicarbonate efflux during metabolic changes from the intracellular compartment, and a bicarbonate influx during respiratory changes to the intracellular compartment must occur. The maintenance of identical intracellular/extracellular bicarbonate concentration ratios regardless of the mechanisms of extracellular pH changes (metabolic or respiratory) suggests an active mechanism for the transport of bicarbonate or H-+ ions.

Serial observations of arterial and mixed-venous blood gases after step change in ventilation

In 22 dogs, subjected to a step change in ventilation, serial changes in blood gas composition and lactate and pyruvate concentrations of arterial as well as mixed venous blood were studied. The change of PaCO2 was approximately 20 mm Hg both in hypo- and hyperventilation. During hypoventilation the difference in various forms of CO2 between arterial and mixed venous blood showed first a downward shift and then gradually increased, whereas during hyperventilation they progressively increased and reached a constant level within 10-20 min. This difference was assumed to be mainly due to more efficient CO2 elimination through lung ventilation in hyperventilation as compared with CO2 accumulation from tissue metabolism in hypoventilation. In vivo buffer slopes for CO2 during hypoventilation were about half those in vitro, whereas during hyperventilation both slopes were approximately the same. In vivo arterial buffer slope was higher during hypoventilation and lower during hyperventilation as compared to that of mixed venous blood in the respective state of ventilation.

The influence of graded degrees of chronic hypercapnia on the acute carbon dioxide titration curve

Studies were carried out to determine the influence of the chronic level of arterial carbon dioxide tension upon the buffering response to acute changes in arterial carbon dioxide tension. After chronic adaptation to six levels of arterial CO(2) tension, ranging between 35 and 110 mm Hg, unanesthetized dogs underwent acute whole body CO(2) titrations. In each instance a linear relationship was observed between the plasma hydrogen ion concentration and the arterial carbon dioxide tension. Because of this linear relationship, it has been convenient to compare the acute buffering responses among dogs in terms of the slope, dH(+)/dPaco(2). With increasing chronic hypercapnia there was a decrease in this slope, i.e. an improvement in buffer capacity, which is expressed by the equation dH(+)/dPaco(2)=-0.005 (Paco(2))(chronic) + 0.95. In effect, the ability to defend pH during acute titration virtually doubled as chronic Paco(2) increased from 35 to 110 mm Hg. The change in slope, dH(+)/dPaco(2), was the consequence of the following two factors: the rise in plasma bicarbonate concentration which occurs with chronic hypercapnia of increasing severity, and the greater change in bicarbonate concentration which occurred during the acute CO(2) titration in the animals with more severe chronic hypercapnia. These findings demonstrate the importance of the acid-base status before acute titration in determining the character of the carbon dioxide titration curve. They also suggest that a quantitative definition of the interplay between acute and chronic hypercapnia in man should assist in the rational analysis of acid-base disorders in chronic pulmonary insufficiency.