Metabolic acidosis

Clinical Features

• Compensatory respiratory tachypnea

Differential Diagnosis

Anion gap metabolic acidosis

• Lactic acidosis
  • Sepsis, shock, liver disease, CO, CN, metformin, methemoglobin
• Renal failure
  • Uremia
• Ketoacidosis
  • DKA
  • Alcoholic Ketoacidosis
  • Starvation ketoacidosis
• Ingestions
  • Increased osm gap
    • Methanol
    • Ethylene glycol
  • Normal osm gap
    • Salicylate toxicity
    • Iron
    • INH

Non-gap

• Hyperkalemia
  • Resolving DKA
  • Early uremic acidosis
  • Early obstructive uropathy
  • RTA Type IV
  • Hypoaldosteronism
  • K-sparing diuretics
• Hypokalemia
  • RTA Type I
  • RTA Type II
  • Acetazolamide
  • Acute diarrhea
    • (May be assoc with gap if hypoperfusion -> lactic acidosis)
• CKD
• Intestinal, pancreatic, biliary fistula
• Hyperchloremic IVF infusions
• Hyperalimentation

Evaluation

Osm gap = measured osm - calculated osm (normal 10-15)

Calculated Osm = 2(Na)+(glucose/18)+(BUN/2.8)+(BAL/5)

• Primary acidosis if pH <7.38
• HCO3 <24 = metabolic acidosis
• Always determine if there is another acid/base process occurring
  • Primary respiratory acidosis if pCO2 > pCO2expected
  • Primary respiratory alkalosis if pCO2 < pCO2expected
    • use Winter's formula: PCO2 (expected) = (1.5 x [HCO3–] + 8) ± 2
    • In acute setting PCO2 should fall by 1 mmHg for every 1 mEq fall in HCO3
  • Concurrent metabolic alkalosis if delta-delta > 28
  • Delta-Delta = (AG - 12) + HCO3

Management

• Treat source
• Correct any respiratory acidosis
• Bicarbonate
  • HCO3 dose in mEq = 0.5(wt in kg) x (24 - measured HCO3)
  • Each bicarb 0.5mEq/kg causes 1 meq/L rise in HCO3
  • Consider for:
    • Bicarb <4
    • pH <7.20 AND shock/myocardial irritability
    • Severe hyperchloremic acidemia
    • lower threshold with non-AG acidosis (greater HCO3 loss)
      • Lost bicarbonate would take days to replenish

See Also

• Acid-base disorders

References

Authors

Ross Donaldson