Surgical Critical Care: Metabolic Acidosis

Anion Gap

• Anion Gap = Na – (HCO3 + Cl)
• Anion Gap Metabolic Acidosis (AGMA):
  • Anion Gap > 12
  • Caused by Adding Organic Acids
• Non-Anion Gap Metabolic Acidosis (Non-AGMA):
  • Anion Gap ≤ 12
  • Caused by Adding HCl or Losing HCO3/NaCl – Think GI or Renal Causes

Causes

• Anion Gap Metabolic Acidosis (AGMA): Mn
  • Methanol
  • Uremia
  • DKA
  • Propylene Glycol
  • Iron/INH
  • Lactic Acidosis
  • EtOH
  • Rhabdomyolysis or Renal Failure
  • Salicylates (Late)
• Non-Anion Gap Metabolic Acidosis (Non-AGMA): Mn
  • Hyperalimentation
  • Addison Disease
  • RTA
  • Diarrhea or Dialysis
  • Acetazolamide
  • Spironolactone
  • Saline: Resuscitation with Normal Saline
    • Hyperchloremic Metabolic Acidosis
    • Cause: Decreased SID (Strong Ion Difference) Causes Increased H+
      • SID = (Na + K) – (Cl + Lactate)
      • Primarily from Excess Cl

Physiologic Changes

• *See Surgical Critical Care: Acid-Base Disorders – Physiologic Changes

Determination if Respiratory Compensation is Appropriate

• Winters Formula
  • PCO2 = (1.5 x HCO3) + 8 +/- 2
• Other Approximations of pCO2
  • pCO2 = HCO3 + 15
  • pCO2 Approximates the Decimal Digits of Arterial pH (pH 7.27 with pCO2 27 mmHg)
• Interpretation:
  • PCO2 Within Expected Values: Adequate Respiratory Compensation
  • PCO2 Above Expected Values: Secondary Respiratory Acidosis or Mixed Acid Base Disorder
  • PCO2 Under Expected Values: Secondary Respiratory Alkalosis or Mixed Acid Base Disorder

Delta Gap (ΔΔ)

• Formula Used in Evaluating AGMA for Additional Disturbances
• Tests:
  • Delta Gap (ΔΔ) = Change in Anion Gap – Change in Bicarb
    • Change in Anion Gap = AG – 12 (Normal Value)
    • Change in Bicarb = 24 (Normal Value) – HCO3-
  • Delta Ratio = Change in Anion Gap / Change in Bicarb
• Gap Interpretation:
  • Significantly Positive (>6): Mixed AGMA & Metabolic Alkalosis
    • Decrease in Bicarbonate is Less than Expected
  • Near-Equal (-6 to +6): AGMA Alone
  • Significantly Negative (<-6): Mixed AGMA & Non-AGMA
    • Rise in Anion Gap is Not as Significant as the Observed Decrease in Bicarbonate
• Ratio Interpretation:
  • < 0.4: Non-AGMA Alone
  • 0.4-0.8: Mixed AGMA & Non-AGMA
  • 0.8-2.0: AGMA Alone
  • > 2.0: Mixed AGMA & Metabolic Alkalosis

Treatment

• Treatment of Underlying Etiologies
• Augment Respiratory Compensation if Mechanically Ventilated
• Sodium Bicarbonate
  • Use Debated & Generally No Survival Improvement
  • Indications:
    • pH < 7.1
    • pH < 7.2 with Severe AKI
    • Renal Tubular Acidosis
  • Side Effects:
    • Myocardial Depression
    • Left-Shift Oxygen Dissociation Curve (Impairs Oxygen Delivery)
    • Increased Lactate Production
    • Intracellular Acidification
    • Hypokalemia