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Difficulty: MediumNephrology

A previously healthy female collegiate athlete is admitted to the medical intensive care unit shortly after collapsing at the finish line of a summer half-marathon. Emergency medical personnel found her lethargic and disoriented. On arrival, her temperature is 39.4 °C (102.9 °F), blood pressure is 88/54 mm Hg, heart rate is 132/min, and respiratory rate is 26/min. Physical examination reveals dry mucous membranes and diffuse muscle tenderness, but lungs are clear to auscultation.

Initial laboratory studies demonstrate the following:
Laboratory TestResult
Sodium138 mEq/L
Potassium6.2 mEq/L
Chloride102 mEq/L
Bicarbonate12 mEq/L
Blood urea nitrogen58 mg/dL
Creatinine3.2 mg/dL
Arterial blood gas (on room air):
ParameterResult
pH7.12
pCO2pCO_238 mm Hg
pO2pO_285 mm Hg

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

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

Answer

High anion gap metabolic acidosis and respiratory acidosis
This patient presents with a severe acute kidney injury, likely due to exertional heat stroke and rhabdomyolysis, resulting in an acid-base disturbance. Stepwise analysis is required. First, the anion gap is 138−(102+12)=24138 - (102 + 12) = 24 mEq/L, confirming a primary high anion gap metabolic acidosis (HAGMA). Next, respiratory compensation is evaluated using Winter's formula: Expected pCO2=(1.5×HCO3−)+8±2Expected\ pCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2. For a bicarbonate of 12 mEq/L, the expected pCO2pCO_2 is 24-28 mm Hg. The patient's actual pCO2pCO_2 is 38 mm Hg. Because the actual pCO2pCO_2 is significantly higher than the expected range, she has a concomitant respiratory acidosis. This often occurs when patients develop respiratory muscle fatigue from the work of breathing associated with severe metabolic acidosis (Kussmaul respirations) or from altered mental status.

Step-by-Step Solution

1
Calculate the serum anion gap.
AG=Na+−(Cl−+HCO3−)=138−(102+12)=24AG = Na^+ - (Cl^- + HCO_3^-) = 138 - (102 + 12) = 24 mEq/L.
Determines the presence of a primary high anion gap metabolic acidosis (HAGMA).
2
Calculate the expected respiratory compensation using Winter's formula.
Expected pCO2=(1.5×12)+8=26±2Expected\ pCO_2 = (1.5 \times 12) + 8 = 26 \pm 2 mm Hg (range 24-28 mm Hg).
Evaluates whether the lungs are adequately compensating for the metabolic acidosis.
3
Compare the actual pCO2pCO_2 to the expected pCO2pCO_2.
Actual pCO2pCO_2 (38 mm Hg) > Expected pCO2pCO_2 (24-28 mm Hg).
A pCO2pCO_2 higher than expected defines an uncompensated or mixed state with primary respiratory acidosis.
4
Calculate the delta-delta ratio to rule out a third mixed disorder.
ΔAG/ΔHCO3−=(24−12)/(24−12)=1.0\Delta AG / \Delta HCO_3^- = (24 - 12) / (24 - 12) = 1.0.
A ratio between 1 and 2 confirms a pure high anion gap metabolic acidosis without an additional normal anion gap metabolic acidosis.

Key Concept

In patients with a primary metabolic acidosis, adequate respiratory compensation must be confirmed via Winter's formula; an actual pCO2 higher than the expected value indicates a concomitant respiratory acidosis.
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Topics

Nephrology

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