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Difficulty: MediumNephrology

A 56-year-old man is brought to the emergency department by paramedics after being found unresponsive in an alley. The paramedics administered intravenous naloxone en route, which resulted in a transient increase in his respiratory rate, but he remains obtunded upon arrival. The patient is unhoused and has a history of severe alcohol use disorder. His temperature is 36.1°C (97.0°F), blood pressure is 105/65 mm Hg, pulse is 92/min, and respirations are 8/min and shallow. Pulse oximetry shows 88% on room air.

Laboratory studies show the following:
Serum sodium: 142 mEq/L
Serum potassium: 4.8 mEq/L
Serum chloride: 100 mEq/L
Serum bicarbonate: 12 mEq/L
Blood urea nitrogen: 32 mg/dL
Serum creatinine: 1.7 mg/dL

Arterial blood gas analysis on room air:
pH: 7.18
pCO2pCO_2: 50 mm Hg
pO2pO_2: 60 mm Hg

Based on these findings, which of the following best describes this patient's acid-base disturbance?

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

Answer

High anion gap metabolic acidosis and concurrent respiratory acidosis
The patient's laboratory results reveal an elevated anion gap of 30 mEq/L (142 - [100 + 12]), which establishes a primary high anion gap metabolic acidosis (HAGMA), likely due to a toxic alcohol ingestion or lactic acidosis. To determine if there is a secondary acid-base disorder, Winter's formula must be applied to calculate the expected respiratory compensation: expected pCO2=(1.5×[HCO3−])+8±2pCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2. For this patient, the expected pCO2pCO_2 is (1.5×12)+8=26±2(1.5 \times 12) + 8 = 26 \pm 2 mm Hg. The actual pCO2pCO_2 of 50 mm Hg is significantly higher than the expected range (24-28 mm Hg). This indicates inadequate alveolar ventilation resulting in CO2 retention, thereby diagnosing a concurrent primary respiratory acidosis. This is clinically consistent with the patient's shallow respirations and low respiratory rate, likely due to central nervous system depression from an unknown intoxicant.

Step-by-Step Solution

1
Calculate the anion gap to identify the primary metabolic disturbance.
Anion gap = 142−(100+12)=30142 - (100 + 12) = 30 mEq/L.
An anion gap > 12 mEq/L establishes the presence of a primary high anion gap metabolic acidosis (HAGMA).
2
Apply Winter's formula to determine the expected respiratory compensation for the metabolic acidosis.
Expected pCO2=1.5×[HCO3−]+8±2=1.5×12+8±2=26±2pCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2 = 1.5 \times 12 + 8 \pm 2 = 26 \pm 2 mm Hg.
This establishes the target range of pCO2pCO_2 (24 to 28 mm Hg) that would be expected if the patient's respiratory system were appropriately compensating.
3
Compare the actual measured pCO2pCO_2 to the expected pCO2pCO_2 to identify any secondary acid-base disorder.
The actual pCO2pCO_2 is 50 mm Hg, which is significantly higher than the expected maximum of 28 mm Hg.
A pCO2pCO_2 higher than the expected compensated range indicates inadequate alveolar ventilation, signifying a concurrent primary respiratory acidosis. This aligns clinically with the patient's hypoventilation due to suspected intoxication.

Key Concept

Diagnosis of mixed acid-base disorders requires calculating the anion gap to identify a primary metabolic acidosis, followed by applying Winter's formula (pCO2=1.5×[HCO3−]+8±2pCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2) to determine if the respiratory compensation is appropriate, insufficient (indicating concurrent respiratory acidosis), or excessive (indicating concurrent respiratory alkalosis).
Estimated Time:1m 30s
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Nephrology

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