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Difficulty: Very hardInterpreting Laboratory Results

A 45-year-old woman is brought to the emergency department by her husband due to progressive confusion, tinnitus, and multiple episodes of vomiting over the past 24 hours. She has a history of major depressive disorder and chronic osteoarthritis, for which she takes several over-the-counter medications. Her temperature is 38.1°C (100.6°F), blood pressure is 112/70 mm Hg, pulse is 118/min, and respiratory rate is 28/min. She appears agitated and diaphoretic.

Laboratory studies show:

TestResult
Sodium142 mEq/L
Potassium3.4 mEq/L
Chloride94 mEq/L
Bicarbonate14 mEq/L
Blood urea nitrogen28 mg/dL
Creatinine1.4 mg/dL
Albumin4.0 g/dL

Arterial blood gas on room air:

TestResult
pH7.43
PaCO221 mm Hg
PaO295 mm Hg

Based on these laboratory results, which of the following is the most accurate interpretation of this patient's acid-base status?

  1. Mixed high anion gap metabolic acidosis, metabolic alkalosis, and respiratory alkalosisAnswer
  2. B
    Mixed high anion gap metabolic acidosis and respiratory alkalosis
  3. C
    Primary high anion gap metabolic acidosis with appropriate respiratory compensation and concurrent metabolic alkalosis
  4. D
    Primary respiratory alkalosis with appropriate metabolic compensation
  5. E
    Mixed normal anion gap metabolic acidosis, metabolic alkalosis, and respiratory alkalosis

Answer

Mixed high anion gap metabolic acidosis, metabolic alkalosis, and respiratory alkalosis
The correct answer accurately identifies three simultaneous primary acid-base disturbances, which is highly characteristic of severe salicylate toxicity with concurrent vomiting. First, the calculated anion gap of 3434 mEq/L (Normal ≈12\approx 12 mEq/L) confirms a high anion gap metabolic acidosis. Second, Winter's formula [Expected PaCO 2_2 =(1.5×HCO3−)+8= (1.5 \times \text{HCO}_3^-) + 8] yields an expected PaCO 2_2 of 29±229 \pm 2 mm Hg. The patient's measured PaCO 2_2 of 2121 mm Hg is significantly lower, indicating a concurrent primary respiratory alkalosis. Finally, calculating the delta gap [Δ\Delta AG == Measured AG (3434) −- Normal AG (1212) =22= 22] and the corrected bicarbonate [Measured HCO 3−_3^- (1414) ++ Δ\Delta AG (2222) =36= 36 mEq/L] reveals a value much greater than 2424 mEq/L, uncovering a concurrent metabolic alkalosis.

Step-by-Step Solution

1
Calculate the Anion Gap (AG) to identify the presence of a metabolic acidosis.
AG =142−(94+14)=34= 142 - (94 + 14) = 34 mEq/L.
An elevated AG (> 12 mEq/L) definitively establishes the presence of a High Anion Gap Metabolic Acidosis (HAGMA), regardless of the overall pH.
2
Apply Winter's formula to determine if respiratory compensation is appropriate.
Expected PaCO 2_2 =(1.5×14)+8=29±2= (1.5 \times 14) + 8 = 29 \pm 2 mm Hg. The actual measured PaCO 2_2 is 2121 mm Hg.
Because the actual PaCO 2_2 (2121 mm Hg) is significantly lower than the expected compensatory range (27−3127-31 mm Hg), a concurrent primary Respiratory Alkalosis is present.
3
Calculate the delta gap and corrected bicarbonate to uncover any hidden concurrent metabolic disorders.
Δ\Delta AG =34−12=22= 34 - 12 = 22. Corrected HCO 3−_3^- =14+22=36= 14 + 22 = 36 mEq/L.
A corrected bicarbonate greater than the normal baseline value (2424 mEq/L) indicates that a concurrent Metabolic Alkalosis is also occurring.

Key Concept

Interpretation of complex triple acid-base disorders using sequential mathematical synthesis: the anion gap, Winter's formula, and the delta gap.
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Topics

Interpreting Laboratory Results

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