Metabolic acidosis


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Metabolic acidosis

  1. 1. Clinical Review Article Metabolic Acidosis Joseph C. Charles, MD Raymond L. Heilman, MD etabolic acidosis, the most common acid- H2O ↔ H2CO3 ↔ H+ + HCO3–). A metabolic acidosisM base disorder, is associated with many life- threatening conditions. Metabolic acidosis is a state produced by excessive acid produc-tion, reduced acid excretion, or consumption or loss ofbody alkali. Arterial blood gas analysis typically shows can result when either or both of these compensatory responses fails or is overwhelmed. CLINICAL SIGNS AND SYMPTOMS Metabolic acidosis may be asymptomatic. If present,the pH to be less than 7.35 and serum bicarbonate the signs and symptoms of metabolic acidosis are rela-(HCO3–) to be less than 18 mEq/L. The signs and tively nonspecific and may include fatigue, anorexia,symptoms of metabolic acidosis are nonspecific, and its confusion, tachycardia, tachypnea, and dehydration.diagnosis relies on analysis of laboratory data. Delay in Other manifestations depend on the underlying causediagnosis is associated with increased mortality and of the disorder.morbidity.1 Early recognition and prompt initiation of The adverse hemodynamic effects of a deterioratingtreatment are therefore critical. This article discusses metabolic acidosis are profound and, if untreated, canthe evaluation and management of this important be life threatening. An increase in acidity causes pul-acid-base disorder. monary vasoconstriction and an increase in pulmonary vascular pressures. These developments can lead toPATHOPHYSIOLOGY right ventricular failure. At an arterial pH less than 7.2, Cellular metabolism produces carbon dioxide. By a generalized myocardial depression eventually occurs.2reversible intracellular process, CO2 combines with In arteriolar smooth muscle, a decrease in pH leads towater to form carbonic acid (H2CO3–). Carbonic acid is systemic vasodilation, which can cause hypotensionable to dissociate into hydrogen ions and HCO3– ions and circulatory failure. In patients with underlyingin a reversible manner. Acidemia is the state of elevated lung disease, the burden imposed by the compensato-H+ concentration and is measured in units of pH. Cells ry increase in minute ventilation will progress to respi-have a narrow pH range within which they function ratory muscle fatigue and failure. The metabolic conse-optimally. quences include hyperkalemia, hypercalcemia, and There are 2 major mechanisms whereby cells main- hypercalciuria, a catabolic state caused by acceleratedtain a constant H+ concentration. The CO2–HCO3– amino acid oxidation.buffering system is the most important. The primaryresponse to a metabolic acidosis is an increase in venti- BLOOD GAS ANALYSIS AND INTERPRETATIONlation, resulting in increased CO2 excretion by diffu- Metabolic acidosis can be identified by following thesion in the lungs. This results in a drop in the blood 5 steps below, using information from arterial bloodpH. Additionally, an excess of H+ can be excreted by gas analysis and serum electrolyte concentrations. Theconversion to CO2. The formula representing this Figure provides an algorithm for assessing metabolicbuffering system is: H+ + HCO3– ↔ H2CO3– ↔ CO2 + acidosis.H2O. The second mechanism for maintaining pH is a 1. Determine whether the patient is alkalemic or2-tiered response by the kidneys. First, H+ ions are acidemic on the basis of arterial pH (normal,excreted in the proximal tubules, where they combinewith HCO3– to form carbonic acid (H2CO3–). In thebrush borders of the tubular cells, carbonic acid is con- Dr. Charles is division education coordinator and consultant, Divisionverted to CO2 and water, and these are reabsorbed. of Hospital Internal Medicine, Mayo Clinic Hospital, Phoenix, AZ. Dr.Second, bicarbonate can be regenerated by a reverse Heilman is an assistant professor of medicine, Division of Transplant-process of the buffering system in the lungs (CO2 + ation Medicine and Nephrology, Mayo Clinic, Scottsdale, Hospital Physician March 2005 37
  2. 2. Charles & Heilman : Metabolic Acidosis : pp. 37 – 42 pH < 7.38 HCO3– < 18 mEq/L Anion gap < 12 mEq/L Anion gap ≥ 12 mEq/L Check correction for low albumin Urine anion gap Ketones in No ketones blood or urine Glucose Glucose Normal High serum Positive Negative IV fluids? ≥ 200 mg/dL < 200 mg/dL serum osmolality Volume excess? osmolality Diarrhea, Alcoholic Methanol, Paraldehyde, GI loss, Expansion Drugs? DKA ketoacidosis, ethylene salicylates laxative acidosis (eg, lithium) starvation glycol abuse Creatinine Renal loss > 4 mg/dL of HCO3– Low Na+ Elevated K+ Carbonic Renal failure RTA anhydrase inhibitors Hypoaldosteronism Lactate L-Lactate L-Lactate Elevated K+ Low K+ 18–36 mg/dL < 18 mg/dL D-Lactate Type 1 Type 4 Lactic present Type 2 acidosis Abnormal gut floraFigure. Algorithm for assessing metabolic acidosis. DKA = diabetic ketoacidosis; GI = gastrointestinal; IV = intravenous; RTA = renaltubular acidosis. 7.38–7.42). Blood with pH less than 7.38 is aci- (HCO3– + Cl–), and the normal anion gap is demic. 12 ± 2 mEq/L.3 2. Determine whether the primary disorder caus- 4. Determine whether the respiratory system is ing acidemia is metabolic or respiratory. The appropriately compensating by excreting and normal serum level of HCO3– is 18 to 22 mEq/L. lowering CO2 (normal PCO2, 36–44 mm Hg). A serum HCO3– level of less than 18 mEq/L indi- Winter’s formula, (HCO3– × 1.5) + 8 ± 2 = PCO2, cates a primary metabolic acidosis. is an accurate way to calculate the expected 3. Determine whether gap or nongap acidosis is PCO2. The last 2 digits of the pH should roughly present. The anion gap is calculated as Na+ − equal the PCO2 (the “quick look” method).438 Hospital Physician March 2005
  3. 3. Charles & Heilman : Metabolic Acidosis : pp. 37 – 42Table 1. Causes of Increased Anion Gap Metabolic Acidosis Table 2. Causes of Normal Anion Gap Metabolic AcidosisIncreased acid production Hypokalemia-associated causes Alcoholic ketoacidosis Diarrhea Diabetic ketoacidosis Renal tubular acidosis types 1 and 2 Starvation ketoacidosis (mild acidosis only) Carbonic anhydrase inhibitors (eg, acetazolamide)Lactic acidosis Ureteral diversions Type A (with tissue hypoxia) Post-hypocapnic conditions Circulatory and respiratory failure Laxative abuse Sepsis Hyperkalemia-associated causes Myocardial infarction Acid loads and total parenteral nutrition Severe anemia Obstructive uropathy Massive hemorrhage Cholestyramine Carbon monoxide poisoning Addison disease (hypoaldosteronism) Ischemia of large and small bowels Renal tubular acidosis type 4 Ascites and other third-spacing of fluids Sulfur toxicity Type B (without tissue hypoxia) 21-Hydroxylase deficiency Liver failure Potassium-sparing diuretics (eg, triamterene) Enzyme defects of childhood Chlorine gas exposure Leukemia, lymphoma, solid tumors Seizures Poorly controlled diabetes mellitus serum must equal the sum of the negatively charged Severe burns anions. The cations are primarily sodium and potassi- Parenteral nutrition um. The major anions are chlorine and bicarbonate. Epinephrine and norepinephrine infusions Increased anion gap acidosis is generally more severe Idiopathic (in terms of potentially fatal outcome) than normal Bronchodilator anion gap acidosis. The causes of increased and nor- mal anion gap metabolic acidosis are summarized inDrugs and toxins (aspirin, ethylene glycol, methanol, paraldehyde) Tables 1 and 2.Renal failure Gastrointestinal loss can be differentiated fromDilutional (large volume of intravenous fluid replacement) renal pathologies as a cause of normal gap acidosis by using the urinary anion gap ([urine Na+ + urine K+] − urine Cl–), an indirect measure of ammonium secre- 5. Determine whether another metabolic disorder tion.7,8 A negative urinary anion gap (< 0) suggests is present in patients with a high anion gap aci- dosis. Calculate the delta anion gap5 as follows: appropriate renal excretion of ammonium and points delta gap = (anion gap − 10)/(24 − HCO3–). to gastrointestinal loss as the cause of the acidosis. The normal value is between 1 and 1.6. A low Conversely, a zero or positive anion gap suggests delta gap suggests the presence of a concomi- impaired ammonia production and a renal etiology for tant nongap acidosis, whereas a delta gap the acidosis. greater than 1.6 suggests the presence of a con- Calculation of an osmolar gap in a patient with a comitant metabolic alkalosis. metabolic acidosis is essential if the clinical scenario sug- gests ingestion or poisoning. Osmolality is a measure ofDIFFERENTIAL DIAGNOSIS solute particles in a solution. Plasma osmolality reflects Metabolic acidoses can be classified clinically as an both the intracellular and extracellular compartmentsincreased anion gap acidosis or a normal anion gap because they are in osmotic equilibrium. Osmolality canacidosis (hyperchloremic metabolic acidosis). As a be measured or calculated as follows: 2 × Na+ (mEq/L) +concept, the anion gap has shortcomings; nonethe- (blood urea nitrogen [mg/dL]/2.8) + (glucoseless, it is a useful clinical tool in developing a differen- [mg/dL]/18). This formula reflects the major solutes intial diagnosis of metabolic acidosis.6 The concept of extracellular fluid. The calculated plasma osmolalitythe anion gap is derived from the law of electrical neu- should be within 10 mOsm/L water of the measuredtrality. The sum of the positively charged cations in the plasma osmolality. If unmeasured solutes are present Hospital Physician March 2005 39
  4. 4. Charles & Heilman : Metabolic Acidosis : pp. 37 – 42the plasma, the measured osmolality will be much higher mia is present, metabolic acidosis results. Studies sug-than the calculated osmolality; this is called an osmolar gest that this leads to protein malnutrition, depressedgap. An osmolar gap in metabolic acidosis should raise myocardial contractility, increased bone resorption,suspicion for ethylene glycol or methanol poisoning. and decreased thyroid hormone secretion.15 Cor- If the anion gap is being used to assess a metabolic recting the acidosis improves the nutritional status byacidosis, the anion gap must be adjusted if the patient preventing muscle protein breakdown.16 This correc-has hypoalbuminemia. Albumin is a negatively charged tion can be achieved by the oral administration ofprotein, and thus hypoalbuminemia falsely lowers the either sodium bicarbonate or sodium citrate 10% solu-anion gap. The adjustment is made by adding 2.5 to the tion. Sodium citrate has fewer gastrointestinal adversegap for every 1 g/dL that the albumin is below normal.9 effects than sodium bicarbonate; however, caution is required with long-term use of oral citrate in patientsBICARBONATE THERAPY with advanced chronic renal failure because it has Treatment strategies for metabolic acidosis are pri- been shown to increase intestinal absorption of alu-marily directed toward the underlying cause. Bicarbon- minum, which can result in chronic bone toxicity. Inate therapy is a temporary measure used for severe aci- short-term situations in which serum HCO3– is lessdosis (pH < 7.1). The rationale for bicarbonate therapy than 15 mEq/L, it can be slowly corrected with intra-is that at extracellular pH levels lower than 7.1, small venous bicarbonate. Too-rapid correction of acidosisdecreases in the level of HCO3– or increases in PCO2 are can lead to tetany and arrhythmias. In addition, hyper-poorly tolerated. natremia, hypertension, and edema may occur. The use of bicarbonate therapy continues to be con- Hyperkalemia is sometimes present in patients withtroversial.4,10 Risks associated with bicarbonate therapy some degree of renal failure or a disturbance of tubu-include hypernatremia, hyperosmolality, volume over- lar secretion of potassium. Management depends onload, and overshoot alkalosis. Also, bicarbonate para- the degree of hyperkalemia present.doxically shifts the hemoglobin-oxygen dissociationcurve unfavorably, potentially resulting in a worsened Renal Tubular Acidosescerebral metabolic acidosis. Hyperkalemic hyperchloremic metabolic acidosis If bicarbonate is used, it should be given cautiously, (type 4 renal tubular acidosis) results from either aldos-with frequent acid-base monitoring and a goal of terone deficiency or renal tubules not responding toreturning the pH to approximately 7.2. It should be aldosterone. In cases caused by drug-related nephro-given as a slow infusion to lessen the effect of CO2 gen- toxicity, removal of the offending agent is indicated. Aeration during buffering. The goal is to adjust the serum decrease in serum potassium concentrations oftenHCO3– level to 8 to 10 mEq/L. A useful formula for cal- improves the acidosis; the decision to treat is based onculating the bicarbonate requirement is: dose of bicar- the degree of hyperkalemia. A cation exchange resinbonate = (desired HCO3– − serum HCO3–)(mEq/L) × (sodium polystyrene sulfonate) with restriction of diet-weight (kg) × 0.5.11 Intravenous bicarbonate is the main ary potassium is effective. Hypovolemia should be cor-alkalinizing agent. rected. Oral bicarbonate may be beneficial. Cases Although the controversy surrounding bicarbonate caused by mineralocorticoid deficiency may requiretherapy in metabolic acidosis probably overstates its replacement therapy.risks, it has led to a search for alternative agents with Type 1 (distal) renal tubular acidosis is frequentlyfewer adverse effects. Such agents include carbicarb, associated with renal stone formation and hypokal-which consists of equimolar concentrations of sodium emia. The goal of treatment is to eliminate acidosis,bicarbonate and sodium carbonate; tris-hydroxymethyl which will decrease the hypercalciuria. Alkalinizationaminomethane (THAM); and Tribonat, a mixture of with oral sodium bicarbonate or Shohl’s solution (sodi-THAM, acetate, bicarbonate, and phosphate. Only um and potassium citrate) is effective. Potassium sup-THAM, however, is currently available in the United plementation is usually not required.States, and the benefits of these agents have not been Type 2 (proximal) renal tubular acidosis is causedconfirmed.12–14 by defective HCO3– resorption in the proximal renal tubules. Treatment with oral bicarbonate or citrateMANAGEMENT OF SPECIFIC CONDITIONS salts is beneficial. Potassium supplementation is re-Renal Failure quired in the rare severely acidotic cases that require When renal failure progresses to the point that ure- alkali therapy.40 Hospital Physician March 2005
  5. 5. Charles & Heilman : Metabolic Acidosis : pp. 37 – 42Ketoacidosis result of direct stimulation of the respiratory center by Ketoacidosis is caused by increased acid production salicylate. Most commonly, a mixed metabolic acidosisresulting from increased fatty acid metabolism. Alco- and respiratory alkalosis are seen at presentation. Ther-holic ketoacidosis is a syndrome characterized by a apy is directed at reducing drug absorption and promot-high anion gap acidosis and malnutrition in the setting ing renal excretion. The latter is achieved by alkaliniza-of binge drinking and chronic alcoholism. The patho- tion with sodium bicarbonate and promotion of diuresisphysiology is complex.17 The metabolic acidosis is often with adequate intravenous fluid therapy. Hemodialysiscomplicated by acid-base abnormalities caused by may be needed in cases of severe intoxication.coexisting disorders. Treatment with intravenous saline Glue sniffing and inhalant abuse involving tolueneand glucose rapidly corrects the metabolic acidosis by causes toluene toxicity, characterized by a severe meta-facilitating the metabolism of the ketoacids. Volume bolic acidosis that is a mixture of increased anion gapreplacement to correct dehydration is important early and nongap acidosis. Toluene toxicity frequently caus-in the course of treatment. es a type I renal tubular acidosis and can result in renal Fasting and starvation also induce a ketoacidosis. failure. The mainstay of therapy is intravenous fluidsTreatment is directed toward correcting nutritional and potassium replacement.19deficiencies and hypovolemia. The metabolic acidosisis always mild and does not require treatment with Lactic Acidosisbicarbonate. L-Lactic acidosis is caused by the overproduction or Patients with diabetic ketoacidosis present with impaired breakdown of lactate. It is characterized by ahyperglycemia, ketonemia, and acidosis. The metabol- high anion gap, serum lactate level higher thanic acidosis is usually severe owing to coexistent uremic 5 mmol/L, and pH less than 7.3. In the absence ofacidosis and lactic acidosis. Dehydration and hyperos- renal failure, an increased phosphorus level with anmolarity are usually present. Treatment with intra- increased gap acidosis is a strong clue to the presencevenous insulin and fluids rapidly reverses the acidosis of lactic acidosis. Lactic acidosis is classically dividedand ketonemia. With fluids and insulin, the liver rapid- into 2 types: type A is associated with impaired tissuely metabolizes the ketoacids in the liver to bicarbonate, oxygenation; type B is not (Table 1). In practice, thewith prompt improvement in the metabolic acidosis. distinction between the 2 types is often not clear. Se-Although studies have not shown a benefit to treating vere cases of lactic acidosis may be fatal.patients with severe diabetic ketoacidosis with bicarbon- Management of lactic acidosis depends on its causes.ate,18 in practice, it is frequently administered to those Therapy should focus on adequate oxygenation, correc-with a pH of less than 7.1 and HCO3– below 8 mEq/L. tion of extracellular fluid deficits, and treatment specif- ic to the underlying causes. Transient lactic acidosis,Toxin-Related Acidosis such as that resulting from seizure, is frequently of little Ethylene glycol intoxication produces a severe met- consequence.abolic acidosis. It should be suspected in an intoxicat- Judicious use of bicarbonate as a temporary mea-ed patient with an increased anion gap acidosis, oxy- sure in patients whose pH is less than 7.1 and whoselate crystals in the urine sediment, and an osmolar gap serum bicarbonate level is less than 8 mEq/L is gener-greater than 10 mOsm/L. Early correction of the aci- ally recommended.20 Results in experimental studies ofdosis improves the chance of survival, and bicarbonate the use of dichloroacetate in lactic acidosis have beenreplacement is required. encouraging21 but were not replicated in a large con- Methanol, like ethylene glycol, is metabolized to trolled clinical trial.22 D-Lactic acidosis has been report-toxic products by the enzyme alcohol dehydrogenase. ed in patients with short-bowel syndrome. It is usuallyIt likewise produces an osmolar gap. It results in much treated with antibiotics to suppress pathogenic flora.more severe anion gap metabolic acidosis. Visual symp-toms are more prominent in its presentation com- Cardiac Arrestpared with the ataxia and seizures associated with ethy- The exact mechanism by which metabolic acidosis islene glycol poisoning. Intravenous ethanol is effective generated in myocardial cells is unclear.23 Lactic acido-in preventing this metabolic action because the eth- sis and the rapid onset of hypercarbia play a role. Bi-anol competes for the enzyme. If marked acidosis is carbonate is generally used in prolonged attempts atpresent, bicarbonate therapy should be given. resuscitation and when severe acidosis results. However, Patients with aspirin toxicity present initially with the role of bicarbonate during cardiopulmonary resus-respiratory alkalosis caused by hyperventilation as a citation is unclear. Studies provide little evidence Hospital Physician March 2005 41
  6. 6. Charles & Heilman : Metabolic Acidosis : pp. 37 – 42benefit.23,24 Coronary perfusion pressure, not myocar- bolic acidosis. N Engl J Med 1988;318:594–9.dial pH, seems to determine the success of resuscita- 8. Goldstein MB, Bear R, Richardson RM, et al. The urine anion gap: a clinically useful index of ammonium excre-tion. Efforts are best directed at establishing adequate tion. Am J Med Sci 1986;292:198–202.oxygenation and effective circulation. 9. Figge J, Jabor A, Kazda A, Fencl V. Anion gap andDilutional Acidosis hypoalbuminemia. Crit Care Med 1998;26:1807–10. 10. Gabow PA. Sodium bicarbonate: a cure or curse for In patients receiving intravenous solutions of lactate, metabolic acidosis? J Critical Illness 1989;4(5):13–28.acetate, or citrate in large volumes, an increased gap aci- 11. Bersin RM, Arieff AI. Improved hemodynamic functiondosis can develop as a result of incomplete conversion to during hypoxia with Carbicarb, a new agent for thebicarbonate. The addition of bicarbonate to the infu- management of acidosis. Circulation 1988;77:227–33.sion prevents this. By a similar mechanism, the negative- 12. Leung JM, Landow L, Franks M, et al. Safety and efficacyly charged salts of some antibiotics (eg, carbenicillin) of intravenous Carbicarb in patients undergoing surgery:given in large quantities can cause a metabolic acidosis. comparison with sodium bicarbonate in the treatment of mild metabolic acidosis [published erratum appears inCONCLUSION Crit Care Med 1995;23:420]. SPI Research Group. Study of Perioperative Ischemia. Crit Care Med 1994;22:1540–9. Metabolic acidosis may be the result of a transient 13. Brasch H, Thies E, Iven H. Pharmacokinetics of TRISand easily reversible condition such as a seizure. More (hydroxymethyl-)aminomethane in healthy subjects andsevere metabolic acidoses require precise diagnosis and in patients with metabolic acidosis. Eur J Clin Pharmacoltimely treatment of the underlying condition. The 1982;22:257–64.focus of any treatment plan is the underlying disease 14. Bjerneroth G. Alkaline buffers for correction of metabol-process; however, if the pH is lower than 7.2, the effects ic acidosis during cardiopulmonary resuscitation withof acidemia can dominate clinical decision making. focus on Tribonat—a review. Resuscitation 1998;37:The goal of alkali therapy is to reverse severe acidemia 161–71.and protect against the detrimental effects on the car- 15. Mitch WE. Uremia and the control of protein metabo-diovascular system. Intravenous sodium bicarbonate is lism. Nephron 1988;49:89–93.the mainstay of alkali therapy and is given as a continu- 16. Walls J. Effect of correction of acidosis on nutritional sta-ous infusion to prevent the effects of “overshoot” alka- tus in dialysis patients. Miner Electrolyte Metab 1997; 23:234–6.losis. Alternative alkalizing agents such as sodium lac- 17. Wrenn KD, Slovis CM, Minion GE, Rutkowski R. Thetate and citrate are not as reliable because their effects syndrome of alcoholic ketoacidosis. Am J Med 1991;91:depend on oxygenation to bicarbonate. Research 119–28.efforts aimed at finding alternatives to bicarbonate 18. Viallon A, Zeni F, Lafond P, et al. Does bicarbonate ther-therapy continue, as do studies to better identify those apy improve the management of severe diabetic ketoaci-subgroups of metabolic acidosis that benefit from alka- dosis? Crit Care Med 1999;27:2690–3.linization therapy. HP 19. Carlisle EJ, Donnelly SM, Vasuvattakul S, et al. Glue- sniffing and distal renal tubular acidosis: sticking to theREFERENCES facts. J Am Soc Nephrol 1991;1:1019–27.1. Hamblin PS, Topliss DJ, Chosich N, et al. Deaths associ- 20. Adrogue HJ, Madias NE. Management of life-threatening ated with diabetic ketoacidosis and hyperosmolar coma. acid-base disorders. First of two parts [published erratum 1973-1988. Med J Aust 1989;151:439, 441–2, 444. appears in N Engl J Med 1999;340:247]. 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