Metabolic acidosis

Clinical Features

• Compensatory respiratory tachypnea

Differential Diagnosis

Anion gap metabolic acidosis

• Lactic acidosis
  • Sepsis, shock, liver disease, CO, CN, metformin, methemoglobin
  • Short bowel syndrome
  • Propylene glycol infusions for lorazepam and phenobarbital
• Renal failure
  • Uremia
• Ketoacidosis
  • DKA
  • Alcoholic Ketoacidosis
  • Starvation ketoacidosis
• Ingestions
  • Acetaminophen toxicity
  • Aspirin toxicity
  • 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

Calculators

Anion Gap

Anion Gap Calculator

Parameter	Value
Sodium (Na⁺) mEq/L
Chloride (Cl⁻) mEq/L
Bicarbonate (HCO₃⁻) mEq/L
Albumin (g/dL) — optional, for correction
Results
Anion Gap	mEq/L
Corrected AG (for albumin)	mEq/L
Delta-Delta Ratio (ΔAG / ΔHCO₃)

Interpretation
AG <12	Normal anion gap — Consider non-AG metabolic acidosis (HARDUPS mnemonic).
AG ≥12	Elevated anion gap — Consider MUDPILES: Methanol, Uremia, DKA, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates.
Delta-Delta Ratio
<1	Concurrent non-AG metabolic acidosis (mixed).
1–2	Pure anion gap metabolic acidosis.
>2	Concurrent metabolic alkalosis (or pre-existing elevated HCO₃).

References
Kraut JA, Madias NE. Serum anion gap: its uses and limitations in clinical medicine. Clin J Am Soc Nephrol. 2007;2:162-174. PMID 17699401. Fenves AZ et al. Increased anion gap metabolic acidosis as a result of 5-oxoproline (pyroglutamic acid). Proc (Bayl Univ Med Cent). 2006;19:364-367.

Winters' Formula

Winters' Formula — Expected pCO₂

Input	Value
Serum Bicarbonate (HCO₃⁻) mEq/L
Results
Expected pCO₂ (low end)	mmHg
Expected pCO₂ (high end)	mmHg

Interpretation
pCO₂ in expected range	Appropriate respiratory compensation — Pure metabolic acidosis with adequate compensation.
pCO₂ > expected	Concurrent respiratory acidosis — Inadequate compensation; concurrent respiratory acidosis present.
pCO₂ < expected	Concurrent respiratory alkalosis — Overcompensation; concurrent respiratory alkalosis present.

References
Winters RW, et al. Studies of Acid Base Disturbances. J Clin Invest. 1956;35:311-318. Formula: Expected pCO₂ = 1.5 × [HCO₃⁻] + 8 (± 2) Albert MS, Dell RB, Winters RW. Quantitative displacement of acid-base equilibrium in metabolic acidosis. Ann Intern Med. 1967;66(2):312-322.

See Also

• Acid-base disorders