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Difficulty: MediumNephrology

A 4545-year-old man is brought to the emergency department by emergency medical services after a witnessed generalized tonic-clonic seizure on the street. Bystanders report the seizure lasted approximately 33 minutes. The patient has a known history of severe alcohol use disorder and was found unresponsive outside a liquor store. On arrival, he is lethargic and arousable only to painful stimuli. His temperature is 36.8∘C36.8^\circ C (98.2∘F98.2^\circ F), blood pressure is 112/72112/72 mm Hg, heart rate is 108108/min, and respiratory rate is 1010/min. Pulse oximetry shows 92%92\% on room air.

Laboratory studies show:

TestResult
Serum sodium140140 mEq/L
Serum potassium4.24.2 mEq/L
Serum chloride9898 mEq/L
Serum bicarbonate1414 mEq/L
Blood urea nitrogen1818 mg/dL
Serum creatinine1.21.2 mg/dL

Arterial blood gas on room air:

TestResult
pH7.177.17
pCO2pCO_24040 mm Hg
pO2pO_26868 mm Hg

Which of the following best explains this patient's acid-base findings?

  1. Concurrent alveolar hypoventilationAnswer
  2. B
    Appropriate respiratory compensation for the primary disorder
  3. C
    Accumulation of unmeasured serum cations
  4. D
    Excessive loss of bicarbonate from the gastrointestinal tract
  5. E
    Primary stimulation of the medullary respiratory center

Answer

The correct answer is concurrent alveolar hypoventilation, which explains the superimposed respiratory acidosis seen alongside the high anion gap metabolic acidosis.
The correct answer identifies that the patient has a mixed acid-base disorder: a high anion gap metabolic acidosis (HAGMA) and a concurrent respiratory acidosis. The patient's anion gap is calculated as Na+−(Cl−+HCO3−)=140−(98+14)=28Na^+ - (Cl^- + HCO_3^-) = 140 - (98 + 14) = 28 mEq/L, which confirms HAGMA, likely secondary to lactic acidosis from his recent generalized tonic-clonic seizure. To determine if there is a mixed disorder, Winter's formula is utilized to calculate the expected pCO2pCO_2: Expected pCO2=1.5×HCO3−+8±2pCO_2 = 1.5 \times HCO_3^- + 8 \pm 2. For this patient, expected pCO2=1.5×14+8=29±2pCO_2 = 1.5 \times 14 + 8 = 29 \pm 2 mm Hg. The patient's actual pCO2pCO_2 is 4040 mm Hg. Because the actual pCO2pCO_2 is significantly higher than the expected range of 27−3127-31 mm Hg, there is a superimposed primary respiratory acidosis. This is clinically consistent with concurrent alveolar hypoventilation, supported by his bradypnea (respiratory rate of 1010/min) and lethargy, likely caused by his postictal state and underlying alcohol intoxication.

Step-by-Step Solution

1
Identify the primary acid-base disorder using the pH and serum bicarbonate.
The patient has a pH of 7.177.17 (acidemia) and a low serum HCO3−HCO_3^- of 1414 mEq/L, indicating a primary metabolic acidosis.
A primary decrease in bicarbonate characterizes metabolic acidosis, which lowers the blood pH.
2
Calculate the anion gap to further classify the metabolic acidosis.
Anion gap = Na+−(Cl−+HCO3−)=140−(98+14)=28Na^+ - (Cl^- + HCO_3^-) = 140 - (98 + 14) = 28 mEq/L. Since the normal range is 8−128-12 mEq/L, this is a high anion gap metabolic acidosis (HAGMA).
Categorizing the acidosis into normal or high anion gap narrows the differential diagnosis. A HAGMA in a postictal patient is typically due to a transient lactic acidosis from muscle contraction.
3
Calculate the expected respiratory compensation using Winter's formula.
Expected pCO2=1.5×HCO3−+8±2=1.5×14+8±2=29±2pCO_2 = 1.5 \times HCO_3^- + 8 \pm 2 = 1.5 \times 14 + 8 \pm 2 = 29 \pm 2 mm Hg. The expected range is 2727 to 3131 mm Hg.
To determine if there is a secondary (mixed) acid-base disorder, the actual pCO2pCO_2 must be compared to the expected physiological respiratory compensation.
4
Compare the actual pCO2pCO_2 to the expected pCO2pCO_2 to identify any concurrent disorders.
The actual pCO2pCO_2 is 4040 mm Hg, which is significantly higher than the expected range of 27−3127-31 mm Hg. This indicates a concurrent respiratory acidosis.
An inappropriately elevated pCO2pCO_2 in the setting of metabolic acidosis means the patient is hypoventilating. In this scenario, the alveolar hypoventilation is secondary to CNS depression from the postictal state and underlying alcohol intoxication.

Key Concept

In patients with metabolic acidosis, Winter's formula must be used to verify if respiratory compensation is adequate. If the actual pCO2pCO_2 is higher than the expected calculated value, a concurrent respiratory acidosis is present.
Estimated Time:1m 30s
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Topics

Nephrology

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