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Difficulty: MediumNephrology

A 45-year-old man is brought to the emergency department by paramedics after being found unresponsive in a park. His medical history is significant for severe alcohol use disorder. On physical examination, he is unarousable and his breath has a fruity odor. Temperature is 36.1°C (97.0°F), blood pressure is 105/65 mmHg, pulse is 102/min, and respiratory rate is 8/min and shallow. Pulse oximetry shows 92% on room air.

Laboratory results are as follows:
Serum sodium: 142 mEq/L
Serum potassium: 4.2 mEq/L
Serum chloride: 98 mEq/L
Serum bicarbonate: 14 mEq/L
Blood urea nitrogen: 28 mg/dL
Serum creatinine: 1.5 mg/dL

Arterial blood gas analysis on room air:
pH: 7.12
PaCO2PaCO_2: 45 mmHg
PaO2PaO_2: 65 mmHg

Which of the following describes the most accurate interpretation of this patient's acid-base status?

  1. High anion gap metabolic acidosis and concurrent primary respiratory acidosisAnswer
  2. B
    High anion gap metabolic acidosis with adequate respiratory compensation
  3. C
    High anion gap metabolic acidosis and concurrent primary respiratory alkalosis
  4. D
    Normal anion gap metabolic acidosis and concurrent primary respiratory acidosis
  5. E
    Normal anion gap metabolic acidosis with adequate respiratory compensation

Answer

High anion gap metabolic acidosis and concurrent primary respiratory acidosis
The patient presents with acidemia (pH 7.12) and a low serum bicarbonate (14 mEq/L), indicating a metabolic acidosis. To correctly interpret the acid-base status, two main steps are required. First, the anion gap (AG) is calculated: AG=Na+−(Cl−+HCO3−)=142−(98+14)=30AG = Na^+ - (Cl^- + HCO_3^-) = 142 - (98 + 14) = 30 mEq/L. A normal AG is typically 8-12 mEq/L; therefore, the patient has a high anion gap metabolic acidosis, likely secondary to alcoholic ketoacidosis given his history. Next, Winter's formula must be applied to determine if the respiratory compensation is adequate: Expected PaCO2=(1.5×HCO3−)+8±2PaCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2. Using the patient's bicarbonate of 14 mEq/L, the expected PaCO2PaCO_2 is 29±229 \pm 2 mmHg (range 27-31 mmHg). The patient's actual PaCO2PaCO_2 is 45 mmHg. Because the actual PaCO2PaCO_2 is significantly higher than expected, there is inadequate respiratory compensation, which constitutes a concurrent primary respiratory acidosis. This is clinically consistent with his central nervous system depression and hypoventilation (respiratory rate of 8/min) due to severe alcohol intoxication. Therefore, the correct interpretation is a high anion gap metabolic acidosis with a concurrent primary respiratory acidosis.

Step-by-Step Solution

1
Calculate the serum anion gap.
AG=Na+−(Cl−+HCO3−)=142−(98+14)=30AG = Na^+ - (Cl^- + HCO_3^-) = 142 - (98 + 14) = 30 mEq/L.
To determine the primary etiology of the metabolic acidosis. A normal AG is typically 8-12 mEq/L. Since 30 > 12, there is a primary high anion gap metabolic acidosis (HAGMA).
2
Calculate the expected PaCO2PaCO_2 using Winter's formula.
Expected PaCO2=(1.5×HCO3−)+8±2PaCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2. Using a bicarbonate of 14 mEq/L, the expected PaCO2=(1.5×14)+8±2=29±2PaCO_2 = (1.5 \times 14) + 8 \pm 2 = 29 \pm 2 mmHg (range 27-31 mmHg).
To evaluate whether the respiratory compensation for the metabolic acidosis is adequate or if there is a secondary respiratory acid-base disorder.
3
Compare the expected PaCO2PaCO_2 with the measured PaCO2PaCO_2.
The measured PaCO2PaCO_2 of 45 mmHg is greater than the expected compensatory range of 27-31 mmHg.
A measured PaCO2PaCO_2 higher than expected indicates inadequate respiratory compensation, defining a concurrent primary respiratory acidosis (hypoventilation).

Key Concept

Diagnosis of mixed acid-base disorders using the anion gap and Winter's formula
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Topics

Nephrology

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