THE RESPONSE OF EXTRACELLULAR HYDROGEN ION CONCENTRATION TO GRADED DEGREES OF CHRONIC HYPERCAPNIA: THE PHYSIOLOGIC LIMITS OF THE DEFENSE OF PH

Acute sodium bicarbonate administration improves ventilatory efficiency in experimental respiratory acidosis: clinical implications

Administering sodium bicarbonate (NaHCO3) to patients with respiratory acidosis breathing spontaneously is contraindicated because it increases carbon dioxide load and depresses pulmonary ventilation. Nonetheless, several studies have reported salutary effects of NaHCO3 in patients with respiratory acidosis but the underlying mechanism remains uncertain. Considering that such reports have been ignored, we examined the ventilatory response of unanesthetized dogs with respiratory acidosis to hypertonic NaHCO3 infusion (1 N, 5 mmol/kg) and compared it with that of animals with normal acid-base status or one of the remaining acid-base disorders. Ventilatory response to NaHCO3 infusion was evaluated by examining the ensuing change in PaCO2 and the linear regression of the PaCO2 vs. pH relationship. Strikingly, PaCO2 failed to increase and the ΔPaCO2 vs. ΔpH slope was negative in respiratory acidosis, whereas PaCO2 increased consistently and the ΔPaCO2 vs. ΔpH slope was positive in the remaining study groups. These results cannot be explained by differences in buffering-induced decomposition of infused bicarbonate or baseline levels of blood pH, PaCO2, and pulmonary ventilation. We propose that NaHCO3 infusion improved the ventilatory efficiency of animals with respiratory acidosis, i.e., it decreased their ratio of total pulmonary ventilation to carbon dioxide excretion (VE/VCO2). Such exclusive effect of NaHCO3 infusion in animals with respiratory acidosis might emanate from baseline increased VD/VT (dead space/tidal volume) caused by bronchoconstriction and likely reduced pulmonary blood flow, defects that are reversed by alkali infusion. Our observations might explain the beneficial effects of NaHCO3 reported in patients with acute respiratory acidosis.

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

Clinical Approach to Assessing Acid-Base Status: Physiological vs Stewart

Evaluation of acid-base status depends on accurate measurement of acid-base variables and their appropriate assessment. Currently, 3 approaches are utilized for assessing acid-base variables. The physiological or traditional approach, pioneered by Henderson and Van Slyke in the early 1900s, considers acids as H⁺ donors and bases as H⁺ acceptors. The acid-base status is conceived as resulting from the interaction of net H⁺ balance with body buffers and relies on the H₂CO₃/HCO₃^- buffer pair for its assessment. A second approach, developed by Astrup and Siggaard-Andersen in the late 1950s, is known as the base excess approach. Base excess was introduced as a measure of the metabolic component replacing plasma [HCO₃^-]. In the late 1970s, Stewart proposed a third approach that bears his name and is also referred to as the physicochemical approach. It postulates that the [H⁺] of body fluids reflects changes in the dissociation of water induced by the interplay of 3 independent variables-strong ion difference, total concentration of weak acids, and PCO₂. Here we focus on the physiological approach and Stewart's approach examining their conceptual framework, practical application, as well as attributes and drawbacks. We conclude with our view about the optimal approach to assessing acid-base status.

Determinants of hypokalemia following hypertonic sodium bicarbonate infusion

The hypokalemic response to alkali infusion has been attributed to the resulting extracellular fluid (ECF) expansion, urinary potassium excretion, and internal potassium shifts, but the dominant mechanism remains uncertain. Hypertonic NaHCO₃ infusion (1 N, 5 mmol/kg) to unanesthetized dogs with normal acid-base status or one of the four chronic acid-base disorders decreased plasma potassium concentration ([K⁺]_p) at 30 min in all study groups (Δ[K⁺]_p, - 0.16 to - 0.73 mmol/L), which remained essentially unaltered up to 90-min postinfusion. ECF expansion accounted for only a small fraction of the decrease in ECF potassium content, (K⁺)_e. Urinary potassium losses were large in normals and chronic respiratory acid-base disorders, limited in chronic metabolic alkalosis, and minimal in chronic metabolic acidosis, yet, ongoing kaliuresis did not impact the stability of [K⁺]_p. All five groups experienced a reduction in (K⁺)_e at 30-min postinfusion, Δ(K⁺)_e remaining unchanged thereafter. Intracellular fluid (ICF) potassium content, (K⁺)_i, decreased progressively postinfusion in all groups excluding chronic metabolic acidosis, in which a reduction in (K⁺)_e was accompanied by an increase in (K⁺)_i. We demonstrate that hypokalemia following hypertonic NaHCO₃ infusion in intact animals with acidemia, alkalemia, or normal acid-base status and intact or depleted potassium stores is critically dependent on mechanisms of internal potassium balance and not ECF volume expansion or kaliuresis. We envision that the acute NaHCO₃ infusion elicits immediate ionic shifts between ECF and ICF leading to hypokalemia. Thereafter, maintenance of a relatively stable, although depressed, [K⁺]_e requires that cells release potassium to counterbalance ongoing urinary potassium losses.

Extracorporeal CO2 removal (ECCO2R) in patients with stable COPD with chronic hypercapnia: a proof-of-concept study

Domiciliary non-invasive ventilation (NIV) effectively reduces arterial carbon dioxide pressure (PaCO2) in patients with stable hypercapnic chronic obstructive pulmonary disease, but a consistent percentage of them may remain hypercapnic. We hypothesised that extracorporeal CO2 removal (ECCO2R) may lower their PaCO2. Ten patients hypercapnic despite ≥6 months of NIV underwent a 24-hour trial of ECCO2R. Six patients completed the ECCO2R-trial with a PaCO2 drop ranging between 23% and 47%. Time to return to baseline after interruption ranged 48–96 hours. In four patients, mechanical events led to ECCO2R premature interruption, despite a decreased in PaCO2. This time window ‘free’ from hypercapnia might allow to propose the concept of ‘CO2 dialysis’.

Changes in the SIDActual and SID Effective Values in the Course of Respiratory Acidosis in Horses With Symptomatic Severe Equine Asthma-An Experimental Study

Equine asthma syndrome is an allergic, inflammatory airway disease that usually affects older horses. Respiratory acidosis is an acid-base imbalance caused by alveolar hypoventilation. The acid-base balance may be assessed using the Henderson-Hasselbalch equation as well as the Stewart model. The authors hypothesized that systemic respiratory acidosis changes the ionic concentrations affecting water dissociation. The study group included 16 Warmblood, mixed breed horses of both sexes with a history of severe equine asthma, and 10 healthy horses were used as controls. Arterial and venous blood were collected from all the horses. The pH, pO₂, and pCO₂ and HCO_3- were assessed in the arterial blood. Na, K, Cl, albumin, and P_inorganic (P_i) were assessed in the venous blood. The obtained results were used to calculate the anion gap (AG), modified AG, actual strong ion difference (SID_a), weak non-volatile acids, and effective strong ion difference (SID_e) values for all the horses. A systemic, compensatory respiratory acidosis was diagnosed in the study group. The concentration of Na in the blood serum in the study group was significantly higher, whereas the concentration of Cl was significantly lower than the values in the control group. The SID_a and SID_e values calculated in the horses from the study group were significantly higher than those in the control group. Significantly higher SID_a and SID_e values confirm the presence of ionic changes that affect water dissociation in the course of respiratory acidosis in horses. The SID_a and SID_e values may be useful in the diagnosis and treatment of respiratory acidosis in horses, which warrant further investigation.

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.

Compensation for Acid-Base Disorders

Hydrogen concentration is a critical determinant of many physiologic functions and is tightly regulated. Any alteration in acid-base equilibrium sets into motion a compensatory response by either the lungs or the kidneys. The compensatory response attempts to return the ratio between Pco₂ and [HCO₃^-] to normal and thereby minimize the pH change. A primary increase or decrease in one component is associated with a predictable compensatory change in the same direction in the other component, and the expected compensation can be estimated clinically in dogs and cats.

Assessing Acid-Base Status: Physiologic Versus Physicochemical Approach

The physiologic approach has long been used in assessing acid-base status. This approach considers acids as hydrogen ion donors and bases as hydrogen ion acceptors and the acid-base status of the organism as reflecting the interaction of net hydrogen ion balance with body buffers. In the physiologic approach, the carbonic acid/bicarbonate buffer pair is used for assessing acid-base status and blood pH is determined by carbonic acid (ie, Paco₂) and serum bicarbonate levels. More recently, the physicochemical approach was introduced, which has gained popularity, particularly among intensivists and anesthesiologists. This approach posits that the acid-base status of body fluids is determined by changes in the dissociation of water that are driven by the interplay of 3 independent variables: the sum of strong (fully dissociated) cation concentrations minus the sum of strong anion concentrations (strong ion difference); the total concentration of weak acids; and Paco₂. These 3 independent variables mechanistically determine both hydrogen ion concentration and bicarbonate concentration of body fluids, which are considered as dependent variables. Our experience indicates that the average practitioner is familiar with only one of these approaches and knows very little, if any, about the other approach. In the present Acid-Base and Electrolyte Teaching Case, we attempt to bridge this knowledge gap by contrasting the physiologic and physicochemical approaches to assessing acid-base status. We first outline the essential features, advantages, and limitations of each of the 2 approaches and then apply each approach to the same patient presentation. We conclude with our view about the optimal approach.

Oxygen therapy in acute exacerbations of chronic obstructive pulmonary disease

Acute exacerbations of chronic obstructive pulmonary disease (COPD) are important events in the history of this debilitating lung condition. Associated health care utilization and morbidity are high, and many patients require supplemental oxygen or ventilatory support. The last 2 decades have seen a substantial increase in our understanding of the best way to manage the respiratory failure suffered by many patients during this high-risk period. This review article examines the evidence underlying supplemental oxygen therapy during exacerbations of COPD. We first discuss the epidemiology and pathophysiology of respiratory failure in COPD during exacerbations. The rationale and evidence underlying oxygen therapy, including the risks when administered inappropriately, are then discussed, along with further strategies for ventilatory support. We also review current recommendations for best practice, including methods for improving oxygen provision in the future.

Using corrected serum chloride and predicted bicarbonate concentrations to interpret acid-base status in dogs

BACKGROUND

Changes in water balance and the presence of unmeasured anions perturb the inverse relationship between serum chloride (Cl) and bicarbonate (HCO(3) ) concentrations in people, affecting accurate interpretation of acid-base status.

OBJECTIVES

The aim of this study was to demonstrate that corrected serum Cl and predicted HCO(3) concentrations, based on serum sodium (Na) concentration and anion gap (AG), would be inversely correlated and could be used to better characterize causes of acid-base disorders in dogs.

METHODS

In this retrospective study, electrolyte data from dogs with at least one abnormality in serum Na, Cl, or HCO(3) concentrations were analyzed. Profiles were classified before and after calculations using 2 methods, a modified Feldman and an institutional method, to correct Cl concentration and predict HCO(3) concentrations based on Na concentration and AG. Dogs were classified as low (L), normal (N), or high (H) based on Cl (first letter) and HCO(3) (second letter) concentrations, as follows: LL, LN, LH, NL, NN, NH, HL, HN, or HH.

RESULTS

For profiles from 261 dogs, reclassifying corrected Cl and predicted HCO(3) concentrations resulted in a shift from the initial classification into a different one in 73% of dogs; in most cases, the shift was to LH, NN, or HL categories. Albumin concentration was a significant factor in acid-base balance.

CONCLUSIONS

When interpreting acid-base status based on results of a standard biochemical panel, erroneous conclusions can be drawn if concentrations of Na, unmeasured anions, and albumin are not taken into account. The inverse relationship between serum Cl and HCO(3) concentrations may be used to identify frequent acid-base disorders as well as to unmask abnormalities obscured by irregularities in water balance or AG.

Secondary responses to altered acid-base status: the rules of engagement

Each of the four canonical acid-base disorders expresses as a primary change in carbon dioxide tension or plasma bicarbonate concentration followed by a secondary response in the countervailing variable. Quantified empirically, these secondary responses are directional and proportional to the primary changes, run a variable time course, and tend to minimize the impact on body acidity engendered by the primary changes. Absence of an appropriate secondary response denotes the coexistence of an additional acid-base disorder. Here we address the expected magnitude of the secondary response to each cardinal acid-base disorder in humans and offer caveats for judging the appropriateness of each secondary response.

Renal compensation to chronic hypoxic hypercapnia: downregulation of pendrin and adaptation of the proximal tubule

The molecular basis for the renal compensation to respiratory acidosis and specifically the role of pendrin in this condition are unclear. Therefore, we studied the adaptation of the proximal tubule and the collecting duct to respiratory acidosis. Male Wistar-Hannover rats were exposed to either hypercapnia and hypoxia [8% CO(2) and 13% O(2) (hypercapnic, n = 6) or normal air (controls, n = 6)] in an environmental chamber for 10 days and were killed under the same atmosphere. In hypercapnic rats, arterial pH was lower than controls (7.31 +/- 0.01 vs. 7.39 +/- 0.01, P = 0.03), blood HCO(3)(-) concentration was increased (42 +/- 0.9 vs. 32 +/- 0.24 mM, P < 0.001), arterial Pco(2) was increased (10.76 +/- 0.4 vs. 7.20 +/- 0.4 kPa, P < 0.001), and plasma chloride concentration was decreased (92.2 +/- 0.7 vs. 97.2 +/- 0.5 mM, P < 0.001). Plasma aldosterone levels were unchanged. In the proximal tubule, immunoblotting showed an increased expression of sodium/bicarbonate exchanger protein (188 +/- 22 vs. 100 +/- 11%, P = 0.005), confirmed by immunohistochemistry. Total Na/H exchanger protein expression in the cortex was unchanged by immunoblotting (119 +/- 10 vs. 100 +/- 11%, P = 0.27) and immunohistochemistry. In the cortex, the abundance of pendrin was decreased (51 +/- 9 vs. 100 +/- 7%, P = 0.003) by immunoblotting. Immunohistochemistry revealed that this decrease was clear in both cortical collecting ducts (CCDs) and connecting tubules (CNTs). This demonstrates that pendrin expression can be regulated in acidotic animals with no changes in aldosterone levels and no external chloride load. This reduction of pendrin expression may help in redirecting the CNT and CCD toward chloride excretion and bicarbonate reabsorption, contributing to the increased plasma bicarbonate and decreased plasma chloride of chronic respiratory acidosis.

Respiratory effects in humans of a 5-day elevation of end-tidal PCO2 by 8 Torr

The aims of this study were to determine 1) whether ventilatory adaptation occurred over a 5-day exposure to a constant elevation in end-tidal Pco2 and 2) whether such an exposure altered the sensitivity of the chemoreflexes to acute hypoxia and hypercapnia. Ten healthy human subjects were studied over a period of 13 days. Their ventilation, chemoreflex sensitivities, and acid-base status were measured daily before, during, and after 5 days of elevated end-tidal Pco2 at 8 Torr above normal. There was no major adaptation of ventilation during the 5 days of hypercapnic exposure. There was an increase in ventilatory chemosensitivity to acute hypoxia (from 1.35 ± 0.08 to 1.70 ± 0.07 l/min/%; P < 0.01) but no change in ventilatory chemosensitivity to acute hypercapnia. There was a degree of compensatory metabolic alkalosis. The results do not support the hypothesis that the ventilatory adaptation to chronic hypercapnia would be much greater with constant elevation of alveolar Pco2 than with constant elevation of inspired Pco2, as has been used in previous studies and in which the feedback loop between ventilation and alveolar Pco2 is left intact.

Size and composition changes in diaphragmatic fibers in rats exposed to chronic hypercapnia

OBJECTIVE

To test the hypothesis that chronic hypercapnia changes the composition of the respiratory muscle by continuous augmentation of ventilation.

MATERIALS AND METHODS

Eighteen male Wistar rats were housed in 10% CO(2) in air for 19 weeks, and their minute ventilation V(E) was measured every 6 weeks. The diaphragm, excited at 19 weeks of exposure, was classified as fiber type I, IIa, or IIb. Cross-sectional areas of individual fibers were measured. Fibers with a target-like appearance on reduced nicotinamide adenine dinucleotide-tetrazolium reductase (NADH-TR) stain also were counted. The data were compared with those of rats kept in room air.

RESULTS

The mean (+/- SD) PaCO(2) after 19 weeks of sustained hypercapnia was 71.0 +/- 4.7 mm Hg. The V(E) remained at a high level until 12 weeks of exposure, and then it significantly decreased at week 18. In a comparison with the control rats, a larger number of type I fibers and a smaller number of type IIb fibers were found in the diaphragm of the chronically hypercapnic rats. In addition, the latter group's cross-sectional area revealed fibers of a significantly smaller diameter. Target-like fibers were observed in 5% of the NADH-TR-stained fibers in the chronically hypercapnic rats but were not seen in the control rats.

CONCLUSION

By increasing the ratio of fatigue-resistant fibers, the diaphragm was able to adapt to a sustained load induced by hypercapnia. However, this adaptive process was accompanied by a degenerative change in the tissue.

Ventilatory responses to hypercapnia and hypoxia following chronic hypercapnia in the rat

This study investigated the effects of an 18 week exposure to 10% CO(2) in air on minute ventilation (V(E)), breathing pattern and the chemoresponiveness of rats to hypoxic and hyperoxic stimuli. We found that V(E) remained elevated over the 18 weeks. Nonetheless, the breathing pattern changed significantly. Tidal volume increased and the durations of inspiration and the total cycle decreased. After the sustained hypercapnia the mean Pa(CO(2)) was 72.0+/-5.1 (S. D.) mmHg. Every 6 weeks the chemoresponiveness of the CO(2)-exposed rats was tested by an acute exposure sequentially to room air, then a 6% O(2), 10% CO(2) and 84% N(2) gas mixture, and finally a 90% O(2) in 10% CO(2) mixture. On either room air or the hyperoxic-hypercapnic mixture V(E) fell to its pre-hypercapnic level. On the hypoxic-hypercapnic mixture V(E) increased significantly. These results demonstrate that the initial stimulating effect of 10% CO(2) on V(E) persisted for the entire 18 weeks without altering hypoxic or hyperoxic ventilatory responses.

Flow diagrams for the diagnosis of acid-base disorders

This article discusses flow diagrams and tables intended to provide a systematic approach to the rapid laboratory differential diagnosis of acid-base disorders in the emergency department.

Exercise-associated hyponatremia in Alaskan sled dogs: urinary and hormonal responses

Exercise-associated hyponatremia occurs in horses and humans, both species that sweat, and in sled dogs, which do not sweat. To investigate the mechanism of exercise-associated hyponatremia in sled dogs, we measured water turnover, serum electrolyte concentrations and osmolality, plasma renal hormone concentrations, and urine composition of 12 fit Alaskan sled dogs before, during, and after a 490-km sled dog race (Ex group). Water turnover and serum electrolyte concentrations were measured in six similarly fit dogs that did not run (Sed group). Water turnover was significantly larger (P < 0.001) in Ex [190 +/- 19 (SD) ml . kg-1 . day-1] than in Sed dogs (51 +/- 13 ml . kg-1 . day-1). There were significant (P < 0.001) decreases in serum sodium concentration (from 148.6 +/- 2.8 to 139.7 +/- 1.9 mmol/l) and osmolality (from 306 +/- 9 to 296 +/- 5 mosmol/kgH2O) of Ex, but not Sed, dogs during the race. Plasma concentrations of arginine vasopressin decreased, whereas aldosterone and plasma renin activity increased significantly (P < 0. 01) during the race. Urine osmolality was unchanged, whereas urine sodium, potassium, and chloride concentrations decreased significantly (P < 0.05) and urine urea concentration increased (P = 0.06). These results demonstrate increased water turnover associated with hyponatremia and renal sodium conservation with maintained high urine osmolality in exercising Alaskan sled dogs.

Prolonged asystolic hyperkalemic cardiac arrest with no neurologic sequelae

We report the case of a 70-year-old man who developed cardiac arrest secondary to hyperkalemia that complicated severe chronic renal failure due to obstructive uropathy. The patient experienced electromechanical dissociation and approximately 26 minutes of asystole after which the resuscitation was suspended. However, 8 to 10 minutes after declaration of death, the patient was noted to have developed spontaneous return of circulation as the emergency department personnel were preparing to transport him to the morgue. The patient survived and was discharged without apparent neurologic sequelae. This case demonstrates the challenges facing physicians to predict the outcome of hyperkalemic cardiac arrest based on usual parameters. It also highlights the relative paucity of resuscitation guidelines to assist in the management of this medical emergency.

Application of multivariate autoregressive modelling for analysing chloride/potassium/bicarbonate relationship in the body

The authors repeatedly analysed course data of acid-base disturbances accompanying hypochloraemia and/or hypokalaemia by means of multivariate autoregressive modelling. It was found that the regulatory relationship between chloride and bicarbonate is inverse between the following two hypochloraemic hyperbicarbonataemic states: the one induced by chloride depletion and the other induced by CO2 retention. Also, the study revealed an independent role of potassium deficiency in the development of metabolic alkalosis, especially in cases with mineralocorticoid-induced alkalosis. The present approach enabled the authors to solve a long-standing problem, i.e. to differentiate between the roles of chloride and potassium in the development of metabolic alkalosis.

Potassium and anaesthesia

Potassium is the principle intracellular ion, and its concentration and gradients greatly influence the electrical activity of excitable membranes. Because anaesthesia is so intimately involved with electrically active cells, potassium concentrations in surgical patients have received considerable attention in diagnostic and therapeutic applications. With the ongoing evolution in the indications for potassium, it is important to review the role of potassium in cellular activity, in storage and regulation, in diseases that alter potassium homeostasis, and in the therapeutic implications of perioperative alterations of potassium concentration. A rational approach to abnormal potassium values and the use of potassium in the operating room is sought, based on a physiological understanding of risks and benefits.

Understanding serum electrolytes. How to avoid mistakes

Serum electrolyte levels by themselves may be difficult to interpret. For example, an abnormal serum potassium concentration cannot be understood without taking into account the patient's history, other serum electrolyte values, and possibly results of other laboratory tests. Knowledge of the principles of serum electrolyte concentrations, therefore, is an important adjuvant to understanding their implications.

Delivery dependence of early proximal bicarbonate reabsorption in the rat in respiratory acidosis and alkalosis

In the intact rat kidney, bicarbonate reabsorption in the early proximal tubule (EP) is strongly dependent on delivery. Independent of delivery, metabolic acidosis stimulates EP bicarbonate reabsorption. In this study, we investigated whether systemic pH changes induced by acute or chronic respiratory acid-base disorders also affect EP HCO3- reabsorption, independent of delivery (FLHCO3, filtered load of bicarbonate). Hypercapnia was induced in rats acutely (1-3 h) and chronically (4-5 d) by increasing inspired PCO2. Hypocapnia was induced acutely (1-3 h) by mechanical hyperventilation, and chronically (4-5 d) using hypoxemia to stimulate ventilation. When compared with normocapneic rats with similar FLHCO3, no stimulation of EP or overall proximal HCO3 reabsorption was found with either acute hypercapnia (PaCO2 = 74 mmHg, pH = 7.23) or chronic hypercapnia (PaCO2 = 84 mmHg, pH = 7.31). Acute hypocapnia (PaCO2 = 29 mmHg, pH = 7.56) did not suppress EP or overall HCO3 reabsorption. Chronic hypocapnia (PaCO2 = 26 mmHg, pH = 7.54) reduced proximal HCO3 reabsorption, but this effect was reversed when FLHCO3 was increased to levels comparable to euvolemic normocapneic rats. Thus, when delivery is accounted for, we could find no additional stimulation of proximal bicarbonate reabsorption in respiratory acidosis and, except at low delivery rates, no reduction in bicarbonate reabsorption in respiratory alkalosis.

Effect of chronic respiratory acidosis on urinary calcium excretion in the dog

It is currently believed that the two chronic acidemic disorders exert disparate effects on urinary calcium excretion: chronic metabolic acidosis induces consistent hypercalciuria, but no appreciable change or even a decrease in calcium excretion is reported to attend chronic respiratory acidosis. Whereas the effect of metabolic acidosis is well documented, little work has been carried out in chronic hypercapnia. In fact, most of the studies on chronic respiratory acidosis were short in duration, had employed only mild hypercapnia, or had failed to control carefully the prevailing metabolic conditions. We have carried out balance observations in nine dogs exposed to a 10% CO2 atmosphere in an environmental chamber for a period of two weeks. Chronic respiratory acidosis led to a significant increase in urinary calcium excretion from a mean control value of 0.4 +/- 0.1 mmol/day to 0.6 +/- 0.1 mmol/day during both week 1 and 2 of hypercapnia (P less than 0.05). Hypercalciuria occurred even though filtered load of calcium fell. Mean fractional excretion of calcium increased significantly during each week of hypercapnia averaging 0.60 +/- 0.12% during control, 1.05 +/- 0.13% during week 1, and 1.26 +/- 0.17% during week 2 of hypercapnic exposure (P less than 0.05). There were no changes in plasma levels of immunoreactive parathyroid hormone or 1,25-dihydroxyvitamin D3. These findings suggest that chronic respiratory acidosis, just like chronic metabolic acidosis, augments urinary calcium excretion by a direct depressive effect on the tubular reabsorption of calcium.

Postoperative metabolic alkalosis following general surgery: its incidence and possible etiology

A prospective clinical study was performed on 293 patients, in order to elucidate the abnormalities in acid-base balance following general surgery. Six arterial blood gas and pH determinations were taken from each patient before surgery and on postoperative days zero, one, three, five and seven. A total of 1699 determinations were obtained. Although the majority of patients (87.5 per cent) had a normal acid-base balance before surgery, a postoperative metabolic alkalosis was seen in 50.5 per cent of the patients. However, there was an extremely low incidence of other postoperative acid-base abnormalities, apart from a transient increase in metabolic acidosis on the operative day. A significantly high mortality rate (32.3 per cent) was observed in 31 patients who had continuous metabolic alkalosis during the postoperative period. An excessive bicarbonate load resulting from the administration of fresh frozen plasma following surgery was strongly suggested as one of the major causes of postoperative metabolic alkalosis. Further investigation is required to elucidate the mechanism of the generation of metabolic alkalosis induced by the postoperative bicarbonate load in surgical patients.

Mixed acid-base disorders

Mixed acid-base disturbances are combinations of two or more primary acid-base disturbances. Mixed acid-base disturbances may be suspected on the basis of findings obtained from the medical history, physical examination, serum electrolytes and chemistries, and anion gap. The history, physical examination, and serum biochemical profile may reveal disease processes commonly associated with acid-base disturbances. Changes in serum total CO2, serum potassium and chloride concentrations, or increased anion gap may provide clues to the existence of acid-base disorders. Blood gas analysis is usually required to confirm mixed acid-base disorders. To identify mixed acid-base disorders, blood gas analysis is used to identify primary acid-base disturbance and determine if an appropriate compensatory response has developed. Inappropriate compensatory responses (inadequate or excessive) are evidence of a mixed respiratory and metabolic disorder. The anion gap is also of value in detecting mixed acid-base disturbances. In high anion gap metabolic acidosis, the change in the anion gap should approximate the change in serum bicarbonate. Absence of this relationship should prompt consideration of a mixed metabolic acid-base disorder. Finding an elevated anion gap, regardless of serum bicarbonate concentration, suggests metabolic acidosis. In some instances, elevated anion gap is the only evidence of metabolic acidosis. In patients with hyperchloremic metabolic acidosis, increases in the serum chloride concentration should approximate the reduction in the serum bicarbonate concentration. Significant alterations from this relationship also indicate that a mixed metabolic disorder may be present. In treatment of mixed acid-base disorders, careful consideration should be given to the potential impact of therapeutically altering one acid-base disorder without correcting others.

Mechanisms of adaptation to chronic respiratory acidosis in the rabbit proximal tubule

The hyperbicarbonatemia of chronic respiratory acidosis is maintained by enhanced bicarbonate reabsorption in the proximal tubule. To investigate the cellular mechanisms involved in this adaptation, cell and luminal pH were measured microfluorometrically using (2",7')-bis(carboxyethyl)-(5,6)-carboxyfluorescein in isolated, microperfused S2 proximal convoluted tubules from control and acidotic rabbits. Chronic respiratory acidosis was induced by exposure to 10% CO2 for 52-56 h. Tubules from acidotic rabbits had a significantly lower luminal pH after 1-mm perfused length (7.03 +/- 0.09 vs. 7.26 +/- 0.06 in controls, perfusion rate = 10 nl/min). Chronic respiratory acidosis increased the initial rate of cell acidification (dpHi/dt) in response to luminal sodium removal by 63% and in response to lowering luminal pH (7.4-6.8) by 69%. Chronic respiratory acidosis also increased dpHi/dt in response to peritubular sodium removal by 63% and in response to lowering peritubular pH by 73%. In conclusion, chronic respiratory acidosis induces a parallel increase in the rates of the luminal Na/H antiporter and the basolateral Na/(HCO3)3 cotransporter. Therefore, the enhanced proximal tubule reabsorption of bicarbonate in chronic respiratory acidosis may be, at least in part, mediated by a parallel adaptation of these transporters.

Acute respiratory failure and chronic obstructive lung disease

Patients with COPD who develop acute respiratory failure require special attention in their management. Patients with severe COPD often have cor pulmonale, complex acid/base compensations, and altered respiratory control mechanisms. These need to be considered when approaching the patient with an acute decompensation. Because of the improving prognosis in this group of patients, aggressive management should be undertaken using combinations of bronchodilator medications, oxygen, bronchial hygiene, and antibiotics.

Renal acidification during chronic hypercapnia in the conscious dog

Enhanced renal acidification during chronic hypercapnia (CH) results in transient augmentation in net acid excretion (NAE) (adaptation phase) and persistent acceleration in renal bicarbonate reclamation (adaptation and steady-state phases). The mechanisms responsible for the return of NAE to control values despite persistent acidemia during the steady state phase of CH remain undefined. In addition, it remains unsettled whether the enhancement of renal ammoniagenesis known to occur during the adaptation phase of CH persists during the steady-state phase. Furthermore it is uncertain if the alteration in whole-kidney acidification observed in CH originates from augmentation in the acidification of both proximal and distal nephronal segments. To shed further light on these issues, observations on the profile of the urine acid-base moieties during the adaptive and steady-state phases of CH were carried out in dogs chronically exposed to hypercapnia (10% FiCO2) in an environmental chamber (13 days). Additionally, collecting duct hydrogen ion secretion (CDH+S) was evaluated by employing the U-B PCO2 in alkaline urine in intact unanesthetized dogs with either CH (10% FiCO2) or eucapnia. The balance studies demonstrated that NAE increased in early hypercapnia (4.84 meq/kg body weight, control 3.27 meq/kg body weight, p less than 0.05) and returned to baseline thereafter; by contrast, urine NH+4 which was augmented during the adaptation phase (3.71 meq/kg body weight, control 1.97 meq/kg body weight, p less than 0.05) remained elevated throughout (3.25 meq/kg body weight).(ABSTRACT TRUNCATED AT 250 WORDS)

[Practical approach to complex acid-base disorders using a slide rule]

Diagnosis of mixed acid-base disturbances is often difficult. Nowadays it depends on biochemical and statistical interpretation, coupled with clinical data. The acid-base slide-rule is a useful tool to carry out this five step procedure, which it simplifies, giving rapidly at the patient's bed-side an objective support for the diagnosis of acid-base disturbances.

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.

Regulation of acid-base equilibrium in chronic hypercapnia

Previous studies from this laboratory have demonstrated that the decreased renal bicarbonate reabsorption prevailing during chronic hypocapnia is not mediated by the alkalemia that normally accompanies this acid-base disturbance but by some direct consequence of the change in PaCO2 itself. Based on the reasonable expectation that the mechanisms underlying the kidney's response to primary respiratory disturbances would be similar over the entire spectrum of physiologic carbon dioxide tensions, the present study was designed to assess whether an acidic change in systemic pH is a critical factor in the renal response to chronic hypercapnia. For this purpose, the plasma and renal responses to chronic respiratory acidosis in normal dogs were compared to those in dogs chronically fed a large hydrochloric acid (HCl) load (7 mmoles/kg/day). Exposure to 6% carbon dioxide for 7 days in a large environmental chamber induced a stable increment in PaCO2 which averaged 17 +/- 0.5 and 22 +/- 1.3 mm Hg in normal and HCl-fed animals, respectively. Steady-state plasma bicarbonate concentration rose from 22.0 +/- 0.4 to 27.1 +/- 0.5 mEq/liter in normals and from 14.7 +/- 0.7 to 24.2 +/- 0.8 mEq/liter in the HCl-fed group. As a result of these changes in PaCO2 and plasma bicarbonate, steady-state plasma hydrogen ion concentration rose in normals from 41 +/- 0.8 to 49 +/- 0.9 nEq/liter (pH 7.39 +/- 0.01 vs. 7.31 +/- 0.01) but did not change significantly in the HCl-fed group (55 +/- 1.4 vs. 56 +/- 1.4 nEq/liter; pH 7.26 +/- 0.01 vs. 7.25 +/- 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Acid-base and ventilatory adaptation in conscious dogs during chronic hypercapnia

Ventilation and cisternal cerebrospinal fluid (CSF) and arterial acid-base balance were measured in awake dogs during air control and from 1 h to 26 days of breathing 5% CO2 in air. Ventilation increased 4-fold during acute hypercapnia and then declined to a minimum at 5-10 days. Between 1-3 days and 16-26 days of hypercapnia ventilation was relatively stable at 2.5 times control. [HCO3-]CSF increased rapidly by 12 h of hypercapnia and in the steady-state [HCO3-]CSF was correlated with PCSFCO2. Between 1 h and 1.5 days of hypercapnia, increase in [HCO3-]CSF was also correlated with increase in [NH3]CSF. Despite increase in [HCO3-]CSF, there was no compensation of [H+]CSF throughout 26 days of hypercapnia. Hydrogen ion may have contributed to the control of ventilation during chronic hypercapnia since ventilation was correlated with [HCO3-]a and [HCO3-]CSF. However, a relationship between ventilation and [H+] of arterial blood and CSF during chronic hypercapnia was relatively poor or absent. Ventilatory adaptation to chronic hypercapnia could not be related to metabolism or to [NH3]CSF. The mechanism(s) by which the increase in PCO2 during chronic respiratory acidosis results in sustained elevation of ventilation remains to be resolved.