Regulation of acid-base equilibrium in chronic hypocapnia. Evidence that the response of the kidney is not geared to the defense of extracellular (H+)

Admission Serum Bicarbonate Predicts Adverse Clinical Outcomes in Hospitalized Cirrhotic Patients

A low serum bicarbonate (SB) level is predictive of adverse outcomes in kidney injury, infection, and aging. Because the liver plays an important role in acid-base homeostasis and lactic acid metabolism, we speculated that such a relationship would exist for patients with cirrhosis. To assess the prognostic value of admission SB on adverse hospital outcomes, clinical characteristics were extracted and analyzed from a large electronic health record system. Patients were categorized based on admission SB (mEq/L) into 7 groups based on the reference range (22-25) into mildly (18-21), moderately (14-17), and severely (<14) decreased groups and mildly (26-29), moderately (30-33), and severely (>30) increased groups, and the relationship of SB category with the frequency of complications (acute kidney injury/hepatorenal syndrome, portosystemic encephalopathy, gastrointestinal bleeding, ascites, and spontaneous bacterial peritonitis) and hospital metrics (length of stay [LOS], admission to an intensive care unit [ICU], and mortality) was assessed. A total of 2,693 patients were analyzed. Mean SB was 22.9 ± 4.5 mEq/L. SB was within the normal range (22-25 mEq/L) in 1,072 (39.8%) patients, and 955 patients (36%) had a low SB. As the SB category decreased, the incidence of complications progressively increased (p < 0.001). Increased MELD-Na score and low serum albumin also correlated with frequency of complications (p < 0.001). As the SB category decreased, LOS, ICU admission, and mortality progressively increased (p < 0.001). On multivariate analysis, the association of decreased SB with higher odds of complications, LOS, ICU admission, and mortality persisted. Conclusion. Low admission SB in patients with cirrhosis is associated with cirrhotic complications, longer LOS, increased ICU admissions, and increased hospital mortality.

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.

Pseudo-Renal Tubular Acidosis: Conditions Mimicking Renal Tubular Acidosis

Hyperchloremic metabolic acidosis, particularly renal tubular acidosis, can pose diagnostic challenges. The laboratory phenotype of a low total carbon dioxide content, normal anion gap, and hyperchloremia may be misconstrued as hypobicarbonatemia from renal tubular acidosis. Several disorders can mimic renal tubular acidosis, and these must be appropriately diagnosed to prevent inadvertent and inappropriate application of alkali therapy. Key physiologic principles and limitations in the assessment of renal acid handling that can pose diagnostic challenges are enumerated.

Acid-base disorders in liver disease

Alongside the kidneys and lungs, the liver has been recognised as an important regulator of acid-base homeostasis. While respiratory alkalosis is the most common acid-base disorder in chronic liver disease, various complex metabolic acid-base disorders may occur with liver dysfunction. While the standard variables of acid-base equilibrium, such as pH and overall base excess, often fail to unmask the underlying cause of acid-base disorders, the physical-chemical acid-base model provides a more in-depth pathophysiological assessment for clinical judgement of acid-base disorders, in patients with liver diseases. Patients with stable chronic liver disease have several offsetting acidifying and alkalinising metabolic acid-base disorders. Hypoalbuminaemic alkalosis is counteracted by hyperchloraemic and dilutional acidosis, resulting in a normal overall base excess. When patients with liver cirrhosis become critically ill (e.g., because of sepsis or bleeding), this fragile equilibrium often tilts towards metabolic acidosis, which is attributed to lactic acidosis and acidosis due to a rise in unmeasured anions. Interestingly, even though patients with acute liver failure show significantly elevated lactate levels, often, no overt acid-base disorder can be found because of the offsetting hypoalbuminaemic alkalosis. In conclusion, patients with liver diseases may have multiple co-existing metabolic acid-base abnormalities. Thus, knowledge of the pathophysiological and diagnostic concepts of acid-base disturbances in patients with liver disease is critical for therapeutic decision making.

Acid-base disturbances in intensive care patients: etiology, pathophysiology and treatment

Acid-base disturbances are very common in critically ill and injured patients as well as contribute significantly to morbidity and mortality. An understanding of the pathophysiology of these disorders is vital to their proper management. This review will discuss the etiology, pathophysiology and treatment of acid-base disturbances in intensive care patients--with particular attention to evidence from recent studies examining the effects of fluid resuscitation on acid-base and its consequences.

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.

The role of hyperventilation: hypocapnia in the pathomechanism of panic disorder

OBJECTIVE: The authors present a profile of panic disorder based on and generalized from the effects of acute and chronic hyperventilation that are characteristic of the respiratory panic disorder subtype. The review presented attempts to integrate three premises: hyperventilation is a physiological response to hypercapnia; hyperventilation can induce panic attacks; chronic hyperventilation is a protective mechanism against panic attacks. METHOD: A selective review of the literature was made using the Medline database. Reports of the interrelationships among panic disorder, hyperventilation, acidosis, and alkalosis, as well as catecholamine release and sensitivity, were selected. The findings were structured into an integrated model. DISCUSSION: The panic attacks experienced by individuals with panic disorder develop on the basis of metabolic acidosis, which is a compensatory response to chronic hyperventilation. The attacks are triggered by a sudden increase in (pCO2) when the latent (metabolic) acidosis manifests as hypercapnic acidosis. The acidotic condition induces catecholamine release. Sympathicotonia cannot arise during the hypercapnic phase, since low pH decreases catecholamine sensitivity. Catecholamines can provoke panic when hyperventilation causes the hypercapnia to switch to hypocapnic alkalosis (overcompensation) and catecholamine sensitivity begins to increase. CONCLUSION: Therapeutic approaches should address long-term regulation of the respiratory pattern and elimination of metabolic acidosis.

Chronic respiratory alkalosis induces renal PTH-resistance, hyperphosphatemia and hypocalcemia in humans

The effects of chronic respiratory alkalosis on divalent ion homeostasis have not been reported in any species. We studied four normal male subjects during a four-day control period (residence at 500 m), during six days of chronic respiratory alkalosis induced by hypobaric hypoxia (residence at 3450 m), followed by a six-day eucapnic recovery period (500 m) under metabolic balance conditions. Chronic respiratory alkalosis (delta PaCO2, -8.4 mm Hg, delta[H+] -3.2 nmol/liter) resulted in a sustained decrement in plasma ionized calcium concentration (delta[IoCa++]p, -0.10 mmol/liter, P less than 0.05) and a sustained increment in plasma phosphate concentration (delta[PO4]p, +0.14 mmol/liter, P less than 0.005) associated with increased fractional excretion of Ca++ (+0.5%, P less than 0.005), decreased phosphate clearance (-6.1 ml/min, P less than 0.025) and decreased excretion of nephrogenous cAMP (-1.5 nmol/100 ml GFR, P less than 0.0025). Urinary phosphate excretion decreased by 15.4 mmol/24 hr on day 1 of chronic respiratory alkalosis (P less than 0.0025), but returned to control values by day 6 despite hyperphosphatemia. Serum intact [PTH] did not change. Sustained hypomagnesuria (-0.8 mmol/24 hr, P less than 0.05) occurred during chronic respiratory alkalosis and was accounted for, at least in part, by decreased fractional excretion of Mg++ (-0.7%, P less than 0.05) in the absence of change in plasma magnesium concentration. Serum 1,25(OH)2D levels were unchanged by chronic respiratory alkalosis. In conclusion, the decrease in nephrogenous cAMP generation despite unchanged serum intact PTH concentration suggests that chronic respiratory alkalosis results in impaired renal responsiveness to PTH as manifested by alterations in PTH-dependent renal calcium and phosphate transport.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic respiratory alkalosis. The effect of sustained hyperventilation on renal regulation of acid-base equilibrium

BACKGROUND

In normal subjects, chronic hyperventilation lowers plasma bicarbonate concentration, primarily by inhibiting the urinary excretion of net acid. The quantitative relation between reduced arterial carbon dioxide tension (PaCO2) and the plasma bicarbonate concentration in the chronic steady state has not been studied in humans, however, and the laboratory criteria for the diagnosis of chronic respiratory alkalosis therefore remain undefined. We wished to provide such reference data for clinical use. Moreover, because chronic hyperventilation paradoxically lowers blood pH still further in dogs with metabolic acidosis, we desired to study the effect of chronic hypocapnia on the plasma bicarbonate concentration (and blood pH) in normal human subjects in whom acidosis had been induced with ammonium chloride.

METHODS

Under metabolic-balance conditions, we used altitude-induced hypobaric hypoxia to produce chronic hypocapnia in nine normal young men, five of whom received ammonium chloride daily to cause metabolic acidosis (the mean [+/- SE] steady-state plasma bicarbonate level in these five was 12.0 +/- 0.5 mmol per liter).

RESULTS

For each decrease of 1 mm Hg (0.13 kPa) in the PaCO2, the plasma bicarbonate concentration decreased by 0.41 mmol per liter in the subjects who started with a normal plasma bicarbonate concentration and by 0.42 mmol per liter in the subjects with acidosis. In contrast to the findings in previous studies of dogs, hypocapnia increased blood pH similarly in both groups; the blood hydrogen ion concentration decreased by about 0.4 nmol per liter for every decrease of 1 mm Hg (0.13 kPa) in PaCO2.

CONCLUSIONS

These results provide reference data for the diagnosis of chronic respiratory alkalosis in humans. Although chronic hypocapnia decreased plasma bicarbonate levels similarly in normal subjects with acidosis and without acidosis, the percent reduction in PaCO2 was always greater than the corresponding percent reduction in the plasma bicarbonate concentration. Therefore, as was not true of the response in dogs, the subjects' blood pH always increased with hyperventilation, regardless of the initial plasma bicarbonate concentration.

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.

The renal response to chronic mineral acid feeding: a re-examination of the role of systemic pH

It has been widely held that systemic acidemia represents the proximate event signaling the kidney to elicit its acidification response to chronic metabolic acidosis. However, a previous study from this laboratory has cast serious doubt on the validity of this conventional viewpoint. When a large acid load (7 mEq/kg/day) was fed chronically to dogs as HCl, H2SO4 or HNO3, net acid excretion increased similarly in all three groups of animals despite wide variability in the prevailing systemic acid-base composition. Marked or moderate hypobicarbonatemia and acidemia were observed in the HCl- or H2SO4-fed animals respectively, but strikingly, plasma [HCO3-] and pH did not change significantly from the control in the HNO3-fed animals. That study concluded that the renal response to chronic mineral acid feeding appears to be triggered, not by acidemia, but by the interplay of sodium delivery to and sodium avidity of the distal nephron as modulated by the reabsorbability of the "acid" anion. We have re-examined the above provocative conclusion in the light of the observation that the only evidence for a dissociation of the renal response from systemic acidemia in that study was derived from preprandial (8:00 a.m.) blood samples obtained some 23 hr after the ingestion of the daily acid load (administered at 9:00 a.m.). We investigated the diurnal variation of plasma acid-base composition in two groups of dogs fed chronically a large acid load (7 mEq/kg/day) as either HCl or HNO3. Both groups exhibited significant diurnal oscillations of plasma acid-base composition.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Secondary hypocapnia fails to protect "whole body" intracellular pH during chronic HCl-acidosis in the dog

Studies have demonstrated that the protective effect of secondary hypocapnia on plasma acidity during chronic HCl-acidosis is undermined by a renal-mediated decrement in plasma bicarbonate concentration induced by the hypocapnia itself. The present study was designed to assess whether the protection of "whole body" intracellular pH (pHi) is similarly undermined by this maladaptive response of the kidney. Whole body pHi was estimated by the 5,5 dimethyl-2,4-oxazolidinedione (DMO) method in seven unanesthetized dogs under each of three conditions: control, chronic HCl-acidosis (10 mEq H+/kg/day) with spontaneous secondary hypocapnia, and chronic HCl-acidosis with a normal level of carbon dioxide tension (maintained by the use of an environmental chamber). pHi was 6.71 +/- 0.02 during control, and 6.57 +/- 0.03 and 6.57 +/- 0.02 during the two acidosis periods, respectively. These results indicate that sustained secondary hypocapnia fails to render the intracellular compartment less acidic because of a maladaptive reduction in intracellular bicarbonate concentration.

Filtered bicarbonate and plasma pH as determinants of renal bicarbonate reabsorption

To examine if bicarbonate reabsorption varies with filtered bicarbonate and plasma pH, we infused anesthetized dogs i.v. with sodium chloride and sodium bicarbonate to alter plasma bicarbonate concentration (PHCO3) without changing hematocrit. Examinations in five dogs over a wide range of glomerular filtration rates (GFR) during ethacrynic acid infusion showed that bicarbonate reabsorption at equal filtered load and equal plasma pH of 7.5 was not significantly changed by increasing PHCO3 from 30.2 +/- 0.4 to 55.2 +/- 0.6 mM and PCO2 from 33.8 +/- 0.7 to 74.1 +/- 2.1 mm Hg. Examinations during respiratory and metabolic alkalosis in five dogs at plasma pH of 7.8 showed that bicarbonate reabsorption at equal filtered load was not significantly different at a PCO2 of 20.2 +/- 0.8 and 36.8 +/- 0.8 mm Hg. Finally, in five dogs that did not receive ethacrynic acid, plasma pH was lowered by inducing respiratory acidosis at a PHCO3 of 30 mM and raised during progressive respiratory and metabolic alkalosis, Bicarbonate reabsorption was linearly related to plasma pH within the range 7.1 to 7.85 (r = 0.92). By altering plasma pH by 0.1 unit, bicarbonate reabsorption was altered by 10 +/- 1%. Thus, filtered bicarbonate rather than GFR and plasma pH rather than PCO2 are important acute regulators of bicarbonate reabsorption. This regulation may be achieved by determining pH and bicarbonate concentration in the luminal fluid along the proximal tubules.

Respiratory acid-base disorders

Both respiratory acidosis and respiratory alkalosis are most likely to occur in combination with some metabolic acid-base disturbance, particularly in the hospitalized patient. After a review of the pulmonary and renal cellular events involved in these complex electrolyte imbalances, principles of diagnosis and treatment are illustrated by means of clinically representative cases.

The nature of the renal response to chronic disorders of acid-base equilibrium

The rate of acid excretion by the kidney appears to be determined by factors regulating the site and the rate of sodium reabsorption, rather than by a homeostatic mechanism that responds to systemic pH. This hypothesis, although unconventional, is supported by much experimental evidence, and it accounts for a wide variety of clinical and physiologic findings that heretofore have been difficult or impossible to explain.

The maladaptive renal response to secondary hypocapnia during chronic HCl acidosis in the dog

It has generally been thought that homeostatic mechanisms of renal origin are responsible for minimizing the alkalemia produced by chronic hypocapnia. Recent observations from this laboratory have demonstrated, however, that the decrement in [HCO(-) (3)], which "protects" extracellular pH in normal dogs, is simply the by-product of a nonspecific effect of Paco(2) on renal hydrogen ion secretion; chronic primary hypocapnia produces virtually the same decrement in plasma [HCO(-) (3)] in dogs with chronic HCl acidosis as in normal dogs (Delta[HCO(-) (3)]/DeltaPaco(2) = 0.5), with the result that plasma [H(+)] in animals with severe acidosis rises rather than falls during superimposed forced hyperventilation. This observation raised the possibility that the secondary hypocapnia which normally accompanies metabolic acidosis, if persistent, might induce an analogous renal response and thereby contribute to the steady-state decrement in plasma [HCO(-) (3)] observed during HCl feeding. We reasoned that if sustained secondary hypocapnia provoked the kidney to depress renal bicarbonate reabsorption, the acute salutary effect of hypocapnia on plasma acidity might be seriously undermined. To isolate the possible effects of secondary hypocapnia from those of the hydrogen ion load, per se, animals were maintained in an atmosphere of 2.6% CO(2) during an initial 8-day period of acid feeding (7 mmol/kg per day); this maneuver allowed Paco(2) to be held constant at the control level of 36 mm Hg despite the hyperventilation induced by the acidemia. Steady-state bicarbonate concentration during the period of eucapnia fell from 20.8 to 16.0 meq/liter, while [H(+)] rose from 42 to 55 neq/liter. During the second phase of the study, acid feeding was continued but CO(2) was removed from the inspired air, permitting Paco(2) to fall by 6 mm Hg. In response to this secondary hypocapnia, bicarbonate concentration fell by an additional 3.0 meq/liter to a new steady-state level of 13.0 meq/liter. This reduction in bicarbonate was of sufficient magnitude to more than offset the acute salutary effect of the hypocapnia on plasma hydrogen ion concentration; in fact, steady-state [H(+)] rose as a function of the adaptive fall in Paco(2), Delta[H(+)]/Delta Paco(2) = -0.44. That the fall in bicarbonate observed in response to chronic secondary hypocapnia was the result of the change in Paco(2) was confirmed by the observation that plasma bicarbonate returned to its eucapnic level in a subgroup of animals re-exposed to 2.6% CO(2). These data indicate that the decrement in plasma [HCO(-) (3)] seen in chronic HCl acidosis is a composite function of (a) the acid load itself and (b) the renal response to the associated hyperventilation. We conclude that this renal response is maladaptive because it clearly diminishes the degree to which plasma acidity is protected by secondary hypocapnia acutely. Moreover, under some circumstances, this maladaptation actually results in more severe acidemia than would occur in the complete absence of secondary hypocapnia.