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Difficulty: EasyNephrology

A 3838-year-old man is brought to the emergency department by emergency medical services after his roommate found him unresponsive. The roommate reports that the patient was severely depressed about a recent job loss and had been drinking a large amount of homemade, illicitly distilled liquor (moonshine). On arrival, the patient is difficult to arouse. His temperature is 36.5∘C36.5^\circ\text{C} (97.7∘F97.7^\circ\text{F}), blood pressure is 105/65105/65 mm Hg, pulse is 9292/min, and respiratory rate is 88/min.

Laboratory results are as follows:
Sodium: 142142 mEq/L
Potassium: 4.84.8 mEq/L
Chloride: 102102 mEq/L
Bicarbonate: 1212 mEq/L

Arterial blood gas analysis on room air reveals:
pH: 7.127.12
PaO2\text{PaO}_2: 6565 mm Hg
PaCO2\text{PaCO}_2: 3838 mm Hg

Which of the following best describes this patient's current acid-base status?

  1. A
    Normal anion gap metabolic acidosis and concurrent respiratory acidosis
  2. Elevated anion gap metabolic acidosis and concurrent respiratory acidosisAnswer
  3. C
    Elevated anion gap metabolic acidosis with appropriate respiratory compensation
  4. D
    Primary respiratory acidosis with compensatory metabolic alkalosis
  5. E
    Normal anion gap metabolic acidosis with appropriate respiratory compensation

Answer

Elevated anion gap metabolic acidosis and concurrent respiratory acidosis
The correct answer accurately synthesizes the data. The acidemia (pH 7.127.12) and low bicarbonate (1212 mEq/L) indicate a metabolic acidosis. The anion gap is elevated at 2828 mEq/L (Normal: 8−12\text{Normal: } 8-12 mEq/L). Based on Winter's formula, the expected PaCO2\text{PaCO}_2 is 26±226 \pm 2 mm Hg. The patient's actual PaCO2\text{PaCO}_2 is 3838 mm Hg. Since 3838 mm Hg is greater than the expected maximum of 2828 mm Hg, the patient has a superimposed respiratory acidosis, likely secondary to CNS depression and hypoventilation from severe intoxication.

Step-by-Step Solution

1
Identify the primary acid-base disorder.
Primary metabolic acidosis.
The patient is acidemic (pH 7.127.12) with a severely low bicarbonate level (1212 mEq/L), establishing a primary metabolic acidosis.
2
Calculate the anion gap to classify the metabolic acidosis.
The anion gap is 2828 mEq/L, which is elevated.
Anion Gap = Na−(Cl+HCO3−)=142−(102+12)=142−114=28\text{Na} - (\text{Cl} + \text{HCO}_3^-) = 142 - (102 + 12) = 142 - 114 = 28 mEq/L. A normal gap is 8−128-12 mEq/L. The elevated gap suggests ingestion of unmeasured toxic anions, such as methanol or ethylene glycol from the illicit liquor.
3
Apply Winter's formula to determine expected respiratory compensation.
Expected PaCO2\text{PaCO}_2 is 26±226 \pm 2 mm Hg (range: 24−2824-28 mm Hg).
Expected PaCO2=1.5×[HCO3−]+8=1.5×12+8=18+8=26\text{PaCO}_2 = 1.5 \times [\text{HCO}_3^-] + 8 = 1.5 \times 12 + 8 = 18 + 8 = 26. This formula predicts how much the patient should hyperventilate to compensate for the metabolic acidosis.
4
Compare the measured PaCO2\text{PaCO}_2 to the expected PaCO2\text{PaCO}_2.
The measured PaCO2\text{PaCO}_2 (3838 mm Hg) is significantly higher than the expected range (24−2824-28 mm Hg).
Because the measured PaCO2\text{PaCO}_2 is higher than the calculated compensatory response, the patient is inadequately exhaling CO2\text{CO}_2 (hypoventilation, consistent with a respiratory rate of 88/min). This confirms a concurrent primary respiratory acidosis.

Key Concept

Identifying a mixed acid-base disorder requires determining the primary disturbance, calculating the anion gap, and applying Winter's formula to evaluate the adequacy of the respiratory response.
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Topics

Nephrology

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