Metabolic Acidosis

Normal pH of arterial blood = 7.35-7.45

Alkalemia = pH >7.45

Acidemia = pH <7.35

  • High-Anion Gap Metabolic Acidosis (HAGMA): Anion gap > 12
  • Normal-Anion Gap Metabolic Acidosis (NAGMA): Anion gap ≤ 12

Origin of the “Anion Gap”

  • Na+ + unknown+ = HCO3 + Cl + unknown
  • Na+ – (HCO3 + Cl) = unknown – unknown+
  • Na+ – HCO3 – Cl = unknown – unknown+ = ANION GAP = ~10-12

High-Anion Gap Metabolic Acidosis (HAGMA) Differential Diagnosis

  • Mnemonic: KU LOST
    • Ketones (starvation, alcoholism, DKA)
    • Uremia (renal failure)
    • Lactic Acidosis (Type A, B, or D)
    • Osmolar Gap elevation
      • Lab serum osmolality – calculated serum osmolality (2xNa + glucose/18 + BUN/2.8) >10
        • Ethylene Glycol ingestion (metabolized into oxalate, which precipitates with calcium to form kidney stones)
        • Methanol ingestion (metabolized into formic acid, “fixes” optic nerve -> look for visual disturbances)
        • Propylene Glycol (think lorazepam, phenobarbitol, dilantin…medicines that may come mixed with propylene glycol)
        • Treat with fomepizole (competitive inhibitor of alcohol dehydrogenase)
      • IF OSMOLAR GAP IS ELEVATED BUT THERE IS NO ACIDOSIS
        • Think isopropyl alcohol, mannitol, hyperproteinemia, severe hyperlipidemia
    • Salicylate (Aspirin) ingestion: Late; initial presentation will be a respiratory alkalosis as salicylates stimulate the respiratory drive center of the brain
    • Tylenol/Toluene

Normal Anion Gap Metabolic Acidosis (NAGMA) Differential Diagnosis

  • Think renal tubular acidosis or diarrhea (although other more diagnoses do exist, like toluene inhalation)
    • Check urine pH: will generally be elevated in patients with distal RTA
    • Check urine ammonium
      • Urine Anion Gap can be used as a surrogate measure (1)
        • Na+U + K+U – ClU: Elevated if >10. This elevated finding indicates a decreased chloride excretion, which by proxy indicates a low ammonium secretion, as the two are secreted together
      • Type I (Distal) RTA: pH > 5.5, urine anion gap elevated
        • Management: Bicarbonate therapy
      • Type II (Proximal) RTA: pH < 5.5, urine anion gap frequently normal, can be diagnosed by an elevation in the serum and urine osmolality with administration of IV bicarbonate
        • Management: Treat underlying condition if possible (removing offending agents (i.e. topiramate, acetazolamide, multiple myeloma) but if unable to correct underlying condition (i.e. Fanconi Syndrome), bicarbonate therapy is recommended

Respiratory Compensation

  • Winter’s Formula: Expected PaCO2 = (1.5 x HCO3) + 8 +/- 2
    • If patient’s measured PaCO2 > expected PaCO2, there is a secondary respiratory acidosis
    • If patient’s measured PaCO2 < expected PaCO2, there is a secondary respiratory alkalosis

Secondary Metabolic Disturbance

  • Delta Gap = Calculated Anion Gap – Normal Anion Gap (~10)
  • Corrected HCO3 = Normal HCO3 (~24) – Delta Gap
  • If measured HCO3 is significantly higher than the corrected HCO3: secondary metabolic alkalosis
  • If measured HCO3 is significantly lower than the corrected HCO3: secondary metabolic acidosis

References

  1. Alexander, Robert Todd, and Martin Bitzan. “Renal Tubular Acidosis.” Pediatric Clinics of North America, Elsevier, 17 Nov. 2018, http://www.sciencedirect.com/science/article/abs/pii/S0031395518301354?via=ihub.

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