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Difficulty: MediumNephrology

A 22-year-old woman is brought to the emergency department by her roommate due to agitation, nausea, and ringing in her ears. The roommate reports finding several empty over-the-counter medication bottles in their bathroom. On physical examination, the patient appears distressed and is breathing rapidly.

Temperature is 38.1∘38.1^\circ C (100.6∘100.6^\circ F), blood pressure is 112/68 mm Hg, pulse is 118/min, and respiratory rate is 26/min. Laboratory studies show:

ComponentValue
Serum Na+Na^+142 mEq/L
Serum K+K^+4.1 mEq/L
Serum Cl−Cl^-102 mEq/L
Serum HCO3−HCO_3^-12 mEq/L
BUN18 mg/dL
Serum creatinine0.9 mg/dL

Arterial blood gas on room air:

ComponentValue
pH7.41
PaCO2PaCO_218 mm Hg
PaO2PaO_295 mm Hg

Which of the following best characterizes this patient's underlying acid-base disturbance?

  1. Concomitant primary high anion gap metabolic acidosis and primary respiratory alkalosisAnswer
  2. B
    Primary high anion gap metabolic acidosis with appropriate respiratory compensation
  3. C
    Primary respiratory alkalosis with appropriate metabolic compensation
  4. D
    Concomitant primary non-anion gap metabolic acidosis and primary respiratory alkalosis
  5. E
    Concomitant primary high anion gap metabolic acidosis and primary metabolic alkalosis

Answer

Concomitant primary high anion gap metabolic acidosis and primary respiratory alkalosis
The correct answer identifies a classic mixed acid-base disorder typically seen in salicylate (aspirin) toxicity. Initially, salicylates directly stimulate the medullary respiratory center, causing hyperventilation and a primary respiratory alkalosis. Shortly after, they uncouple oxidative phosphorylation, leading to anaerobic metabolism, accumulation of organic acids (lactic acid, ketoacids), and a primary high anion gap metabolic acidosis. The concurrent primary acidosis and primary alkalosis pull the pH in opposite directions, often resulting in a near-normal or slightly alkalemic pH (e.g., 7.41), while both the HCO3−HCO_3^- and PaCO2PaCO_2 are markedly reduced beyond simple compensatory mechanisms.

Step-by-Step Solution

1
Calculate the serum anion gap.
Anion Gap = Na+−(Cl−+HCO3−)Na^+ - (Cl^- + HCO_3^-) = 142−(102+12)142 - (102 + 12) = 28 mEq/L.
Establishing the anion gap is the critical first step in evaluating any acid-base disturbance. An AG > 12 mEq/L indicates a primary High Anion Gap Metabolic Acidosis (HAGMA).
2
Calculate the expected respiratory compensation using Winter's formula.
Expected PaCO2=(1.5×[HCO3−])+8±2=(1.5×12)+8±2=18+8±2=26±2PaCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2 = (1.5 \times 12) + 8 \pm 2 = 18 + 8 \pm 2 = 26 \pm 2. Range is 24 to 28 mm Hg.
Winter's formula determines if the respiratory response (change in PaCO2PaCO_2) is purely compensatory for the metabolic acidosis, or if an independent respiratory disorder exists.
3
Compare the actual PaCO2PaCO_2 to the expected PaCO2PaCO_2.
Actual PaCO2PaCO_2 (18 mm Hg) < Expected PaCO2PaCO_2 (24-28 mm Hg).
Because the measured PaCO2PaCO_2 is lower than the expected compensatory range, there must be a concurrent primary process actively blowing off CO2CO_2. This establishes the diagnosis of a primary respiratory alkalosis.
4
Calculate the Delta-Delta ratio to check for hidden metabolic disorders.
ΔAG=28−12=16\Delta AG = 28 - 12 = 16. ΔHCO3−=24−12=12\Delta HCO_3^- = 24 - 12 = 12. Ratio = 16/12=1.3316/12 = 1.33.
A ratio between 1 and 2 confirms an uncomplicated HAGMA with no additional hidden metabolic alkalosis or non-anion gap metabolic acidosis (NAGMA).

Key Concept

Mixed acid-base disorders in salicylate toxicity
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Topics

Nephrology

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