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Difficulty: MediumNephrology

A 32-year-old man is brought to the emergency department by paramedics after being found unresponsive in his apartment. Bystanders report that he may have used heroin and had been lying in the same position for an unknown duration. On arrival, his temperature is 36.8°C (98.2°F), blood pressure is 105/65 mm Hg, pulse is 88/min, and respiratory rate is 10/min. Physical examination reveals pinpoint pupils and marked swelling and induration of the right lower extremity.

Laboratory studies show the following:

TestResult
Serum sodium142 mEq/L
Serum potassium5.6 mEq/L
Serum chloride102 mEq/L
Serum bicarbonate14 mEq/L
Blood urea nitrogen48 mg/dL
Serum creatinine3.2 mg/dL
Creatine kinase42,000 U/L

Arterial blood gas analysis on room air reveals:

ParameterResult
pH7.21
PaCO2PaCO_236 mm Hg
PaO2PaO_268 mm Hg

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

  1. High anion gap metabolic acidosis and respiratory acidosisAnswer
  2. B
    High anion gap metabolic acidosis with adequate 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 anion gap and normal anion gap metabolic acidosis

Answer

High anion gap metabolic acidosis and respiratory acidosis
This patient has a mixed acid-base disorder. First, calculate the anion gap (AG): AG=[Na+]−([Cl−]+[HCO3−])AG = [Na^+] - ([Cl^-] + [HCO_3^-]) AG=142−(102+14)=26 mEq/LAG = 142 - (102 + 14) = 26\ mEq/L A normal AG is 10-14 mEq/L, so the patient has a high anion gap metabolic acidosis (HAGMA), likely due to uremia from acute kidney injury (creatinine 3.2 mg/dL) and lactic acidosis secondary to prolonged immobilization and rhabdomyolysis (creatine kinase 42,000 U/L).

Next, assess the adequacy of respiratory compensation using Winter's formula: Expected PaCO2=(1.5×[HCO3−])+8±2Expected\ PaCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2 Expected PaCO2=(1.5×14)+8±2=29±2 mm HgExpected\ PaCO_2 = (1.5 \times 14) + 8 \pm 2 = 29 \pm 2\ mm\ Hg The expected range for PaCO2PaCO_2 is 27 to 31 mm Hg. The patient's measured PaCO2PaCO_2 is 36 mm Hg. Since the actual PaCO2PaCO_2 is greater than the expected range, there is a concurrent, primary respiratory acidosis. This is consistent with his clinical presentation of an opioid overdose (pinpoint pupils, hypoventilation with a respiratory rate of 10/min).

Step-by-Step Solution

1
Analyze the primary acid-base status using pH and serum bicarbonate.
The pH is 7.21 (acidemia) and serum bicarbonate is low (14 mEq/L), indicating a primary metabolic acidosis.
Establishing the primary disorder determines the direction of the subsequent compensatory calculations.
2
Calculate the serum anion gap.
AG = 142 - (102 + 14) = 26 mEq/L. This indicates a high anion gap metabolic acidosis (HAGMA).
Differentiating between normal and high anion gap acidosis identifies underlying etiologies (in this case, likely acute kidney injury and rhabdomyolysis leading to uremia and lactic acidosis).
3
Calculate expected respiratory compensation using Winter's formula.
Expected PaCO2 = (1.5 × 14) + 8 ± 2 = 29 ± 2 mm Hg (range: 27-31 mm Hg).
This determines if the lungs are adequately compensating for the metabolic acidosis by blowing off CO2.
4
Compare expected PaCO2 to the measured PaCO2 to detect additional acid-base disorders.
Measured PaCO2 (36 mm Hg) is greater than the expected range (27-31 mm Hg).
Because the actual PaCO2 is higher than expected, the patient has a concurrent primary respiratory acidosis (hypoventilation due to opioid overdose).

Key Concept

Patients with a metabolic acidosis must be evaluated for appropriate respiratory compensation using Winter's formula; an actual PaCO2 higher than the calculated expected range signifies a concomitant primary respiratory acidosis.
Estimated Time:1m 30s
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Topics

Nephrology

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