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Difficulty: MediumNephrology

Emergency medical services transport a 41-year-old male to the hospital after his spouse discovered him unresponsive in their garage. Beside him was an open, partially consumed jug of automotive antifreeze. Upon arrival, vital signs demonstrate a temperature of 36.5°C (97.7°F), blood pressure of 105/65 mm Hg, heart rate of 92/min, and a depressed respiratory rate of 10/min. The physical examination is notable for profound lethargy, with the patient only groaning in response to painful stimuli.

Initial laboratory evaluation reveals the following:

TestValue
Sodium140 mEq/L
Potassium4.6 mEq/L
Chloride102 mEq/L
Bicarbonate8 mEq/L
Blood urea nitrogen22 mg/dL
Creatinine1.5 mg/dL
Arterial blood gas parameters on room air:
TestValue
pH7.05
PaCO230 mm Hg
PaO275 mm Hg

Which of the following best characterizes this patient's underlying acid-base disorder?

  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 with appropriate respiratory compensation
  5. E
    Mixed high and normal anion gap metabolic acidosis

Answer

High anion gap metabolic acidosis and concurrent respiratory acidosis
The laboratory results reveal severe acidemia (pH 7.05) driven by a significantly reduced bicarbonate level (8 mEq/L), pointing to a primary metabolic acidosis. The anion gap is calculated as Na+−(Cl−+HCO3−)Na^+ - (Cl^- + HCO_3^-), which equals 140−(102+8)=30140 - (102 + 8) = 30 mEq/L. Since this value exceeds the normal limit of 12 mEq/L, the patient has a high anion gap metabolic acidosis (HAGMA), which is consistent with antifreeze (ethylene glycol) toxicity. To assess whether the respiratory system is appropriately compensating, Winter's formula must be applied: ExpectedPaCO2=(1.5×[HCO3−])+8±2Expected PaCO_2 = (1.5 \times [HCO_3^-]) + 8 \pm 2. For this patient, the expected PaCO2PaCO_2 is (1.5×8)+8=20±2(1.5 \times 8) + 8 = 20 \pm 2 mm Hg. The measured PaCO2PaCO_2 of 30 mm Hg is substantially higher than the 18 to 22 mm Hg range. This relative hypercapnia indicates that the patient is hypoventilating (likely due to toxic CNS depression, reflected by a respiratory rate of 10/min), confirming a concurrent respiratory acidosis.

Step-by-Step Solution

1
Identify the primary disturbance from the arterial blood gas.
The pH of 7.05 indicates acidemia. The low bicarbonate (88 mEq/L) identifies the primary disorder as a metabolic acidosis.
Establishing the primary acid-base status is the foundation for all subsequent calculations.
2
Calculate the serum anion gap.
AnionGap=Na+−(Cl−+HCO3−)=140−(102+8)=140−110=30Anion Gap = Na^+ - (Cl^- + HCO_3^-) = 140 - (102 + 8) = 140 - 110 = 30 mEq/L. Since this is greater than 1212 mEq/L, there is a High Anion Gap Metabolic Acidosis (HAGMA).
Classifying the metabolic acidosis helps identify the etiology (e.g., toxic alcohol ingestion such as ethylene glycol).
3
Calculate the expected respiratory compensation using Winter's formula.
ExpectedPaCO2=(1.5×HCO3−)+8±2=(1.5×8)+8=12+8=20±2Expected PaCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2 = (1.5 \times 8) + 8 = 12 + 8 = 20 \pm 2 mm Hg. The expected range is 1818 to 2222 mm Hg.
Winter's formula determines what the PaCO2 should be if the respiratory system is properly compensating for the metabolic acidosis.
4
Compare the expected PaCO2PaCO_2 to the measured PaCO2PaCO_2 to check for mixed disorders.
The measured PaCO2PaCO_2 (3030 mm Hg) is higher than the expected range (1818-2222 mm Hg). This indicates a superimposed respiratory acidosis.
A PaCO2PaCO_2 higher than the expected compensatory range means the lungs are retaining too much CO2CO_2, characteristic of hypoventilation (consistent with the patient's depressed respiratory rate of 1010/min from intoxication).

Key Concept

Diagnosis of mixed acid-base disorders requires calculating the anion gap and applying Winter's formula to determine if respiratory compensation is adequate. A measured PaCO2 higher than predicted indicates a concurrent respiratory acidosis.
Estimated Time:1m 30s
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Topics

Nephrology

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