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Difficulty: MediumGeneral Internal Medicine

A 2828-year-old man is brought to the emergency department by paramedics after being found obtunded in his home. His roommate reports finding empty bottles of unknown medications nearby. His temperature is 36.8∘C36.8^\circ\text{C} (98.2∘F98.2^\circ\text{F}), blood pressure is 105/65105/65 mm Hg, pulse is 110110/min, and respiratory rate is 66/min and shallow. Pulse oximetry shows 88%88\% on room air. Laboratory results are as follows:

Laboratory TestResult
Serum sodium142142 mEq/L
Serum potassium4.54.5 mEq/L
Serum chloride104104 mEq/L
Serum bicarbonate1414 mEq/L
Blood urea nitrogen1818 mg/dL
Serum creatinine1.11.1 mg/dL

Arterial blood gas analysis on room air reveals a pHpH of 7.107.10, pCO2pCO_2 of 4646 mm Hg, and pO2pO_2 of 5858 mm Hg.

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

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

Answer

Mixed high anion gap metabolic acidosis and respiratory acidosis
The patient's laboratory values reveal a mixed acid-base disorder consisting of a high anion gap metabolic acidosis and a concurrent respiratory acidosis. The first step in evaluating this disorder is identifying the primary disturbance. The severe acidemia (pHpH of 7.107.10) combined with a low serum bicarbonate (1414 mEq/L) strongly indicates a primary metabolic acidosis. Next, the anion gap is calculated as Na+−(Cl−+HCO3−)=142−(104+14)=24Na^+ - (Cl^- + HCO_3^-) = 142 - (104 + 14) = 24 mEq/L. Since a normal anion gap is 8−128-12 mEq/L, a high anion gap metabolic acidosis is present. Finally, expected respiratory compensation must be evaluated using Winter's formula: expected pCO2=(1.5×HCO3−)+8±2pCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2. For this patient, the expected pCO2pCO_2 is (1.5×14)+8=29±2(1.5 \times 14) + 8 = 29 \pm 2 mm Hg. The patient's actual pCO2pCO_2 is 4646 mm Hg, which is significantly higher than the expected range (27−3127-31 mm Hg). This indicates an inadequate respiratory response and a superimposed primary respiratory acidosis, which aligns with the patient's obtunded state and hypoventilation (respiratory rate of 66/min) secondary to a suspected toxic ingestion.

Step-by-Step Solution

1
Identify the primary acid-base disorder based on pHpH and bicarbonate.
The patient is acidemic (pH=7.10pH = 7.10) with a low bicarbonate (1414 mEq/L), indicating a primary metabolic acidosis.
Establishing the primary driver of the acidemia is required before calculating expected compensatory responses.
2
Calculate the serum anion gap.
Na+−(Cl−+HCO3−)=142−(104+14)=24Na^+ - (Cl^- + HCO_3^-) = 142 - (104 + 14) = 24 mEq/L. This indicates a high anion gap metabolic acidosis.
Differentiating between normal and high anion gap metabolic acidosis narrows the differential diagnosis (e.g., toxic ingestion, lactic acidosis).
3
Calculate the expected pCO2pCO_2 using Winter's formula to determine if respiratory compensation is adequate.
Expected pCO2=(1.5×14)+8±2=29±2pCO_2 = (1.5 \times 14) + 8 \pm 2 = 29 \pm 2 mm Hg (Range: 27−3127-31 mm Hg).
This establishes the baseline target for appropriate respiratory compensation in a primary metabolic acidosis.
4
Compare the actual pCO2pCO_2 to the expected pCO2pCO_2.
The actual pCO2pCO_2 is 4646 mm Hg, which is greater than the expected range of 27−3127-31 mm Hg. Therefore, there is a concomitant primary respiratory acidosis.
A pCO2pCO_2 higher than the expected calculated value implies inadequate ventilation, diagnosing a superimposed respiratory acidosis.

Key Concept

In patients with a primary metabolic acidosis, actual pCO2pCO_2 must be compared to expected pCO2pCO_2 (calculated via Winter's formula) to identify concomitant respiratory acid-base disorders.
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Topics

General Internal Medicine

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