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Difficulty: MediumNephrology

A 42-year-old woman is brought to the emergency department with sudden-onset shortness of breath and right-sided pleuritic chest pain. Her medical history includes ulcerative colitis, and she reports experiencing a severe disease flare with 6 to 8 watery bowel movements daily for the past week. Vital signs show a temperature of 37.6°C (99.7°F), heart rate of 115/min, blood pressure of 112/68 mm Hg, and respiratory rate of 28/min. Pulse oximetry is 88% on room air. Physical examination reveals clear lungs bilaterally but right calf swelling and tenderness. A computed tomography pulmonary angiogram confirms a segmental pulmonary embolism. Laboratory studies are as follows:

LabValue
Sodium138 mEq/L138 \text{ mEq/L}
Potassium3.4 mEq/L3.4 \text{ mEq/L}
Chloride118 mEq/L118 \text{ mEq/L}
Bicarbonate10 mEq/L10 \text{ mEq/L}

Arterial blood gas (room air):
pH: 7.39
PaCO2\text{PaCO}_2: 17 mm Hg17 \text{ mm Hg}
PaO2\text{PaO}_2: 55 mm Hg55 \text{ mm Hg}

Based on these results, which of the following best characterizes this patient's acid-base status?

  1. Non-anion gap metabolic acidosis and an additional respiratory alkalosisAnswer
  2. B
    Non-anion gap metabolic acidosis with adequate respiratory compensation
  3. C
    Non-anion gap metabolic acidosis and an additional respiratory acidosis
  4. D
    Elevated anion gap metabolic acidosis and an additional respiratory alkalosis
  5. E
    Elevated anion gap metabolic acidosis with adequate respiratory compensation

Answer

Non-anion gap metabolic acidosis and an additional respiratory alkalosis
The correct answer identifies a non-anion gap metabolic acidosis and a concurrent primary respiratory alkalosis. This patient presents with an acidemic pH (7.39) and a low bicarbonate (10 mEq/L10 \text{ mEq/L}), which establishes a primary metabolic acidosis driven by gastrointestinal bicarbonate losses from her ulcerative colitis flare. The anion gap is calculated as Na+−(Cl−+HCO3−)=138−(118+10)=10 mEq/L\text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) = 138 - (118 + 10) = 10 \text{ mEq/L}. Because this falls within the normal range (8−12 mEq/L8-12 \text{ mEq/L}), the disorder is a non-anion gap metabolic acidosis. To determine if there is a mixed disorder, Winter's formula must be applied: Expected PaCO2=1.5×[HCO3−]+8±2\text{Expected PaCO}_2 = 1.5 \times [\text{HCO}_3^-] + 8 \pm 2. For this patient, the expected PaCO2\text{PaCO}_2 is (1.5×10)+8=23±2 mm Hg(1.5 \times 10) + 8 = 23 \pm 2 \text{ mm Hg} (range 21−25 mm Hg21-25 \text{ mm Hg}). Her measured PaCO2\text{PaCO}_2 is 17 mm Hg17 \text{ mm Hg}, which is significantly lower than the expected compensatory range. A PaCO2\text{PaCO}_2 lower than expected indicates an additional primary respiratory alkalosis. In this clinical scenario, the superimposed respiratory alkalosis is caused by hypoxia and pain-driven hyperventilation secondary to her acute pulmonary embolism.

Step-by-Step Solution

1
Identify the primary acid-base disorder from the pH and bicarbonate.
The pH is acidemic (7.39) and the bicarbonate is low (10 mEq/L10 \text{ mEq/L}), establishing a primary metabolic acidosis.
pH < 7.40 defines acidemia, and a low bicarbonate indicates a metabolic origin.
2
Calculate the serum anion gap.
Anion gap = 138−(118+10)=10 mEq/L138 - (118 + 10) = 10 \text{ mEq/L}. This is normal, confirming a non-anion gap metabolic acidosis.
Differentiating between normal and elevated anion gap metabolic acidosis helps identify the underlying etiology (gastrointestinal losses in this case).
3
Apply Winter's formula to determine the expected respiratory compensation.
Expected PaCO2=(1.5×10)+8±2=23±2 mm Hg\text{Expected PaCO}_2 = (1.5 \times 10) + 8 \pm 2 = 23 \pm 2 \text{ mm Hg} (range: 21−25 mm Hg21-25 \text{ mm Hg}).
Winter's formula evaluates if the respiratory response to metabolic acidosis is appropriate.
4
Compare the measured PaCO2\text{PaCO}_2 to the expected range.
The measured PaCO2\text{PaCO}_2 (17 mm Hg17 \text{ mm Hg}) is lower than the expected range (21−25 mm Hg21-25 \text{ mm Hg}).
A PaCO2\text{PaCO}_2 lower than expected indicates excessive hyperventilation, revealing a concurrent primary respiratory alkalosis (due to the pulmonary embolism).

Key Concept

Identifying mixed acid-base disorders requires calculating the anion gap to classify metabolic acidosis and applying Winter's formula to determine if respiratory compensation is appropriate or if a secondary respiratory disorder exists.
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Topics

Nephrology

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