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Difficulty: MediumGeneral Internal Medicine

A 42-year-old man is brought to the emergency department by paramedics after his roommate found him unarousable. The roommate reports that the patient has a long-standing history of alcohol use disorder and has been binge drinking for the past two weeks with minimal food intake. Vital signs include a blood pressure of 105/65105/65 mm Hg, heart rate of 112112/min, and respiratory rate of 2424/min. On physical examination, the patient is somnolent but withdraws to painful stimuli. The mucous membranes are parched. There is no asterixis or jaundice. Laboratory studies are obtained and reveal the following:

Laboratory TestResult
Sodium136136 mEq/L
Potassium3.83.8 mEq/L
Chloride9898 mEq/L
Bicarbonate1212 mEq/L
Blood urea nitrogen4545 mg/dL
Creatinine1.51.5 mg/dL
Glucose7575 mg/dL
Arterial blood gas analysis on room air shows:
TestResult
pH7.267.26
PaCO2PaCO_23434 mm Hg
PaO2PaO_29292 mm Hg

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

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

Answer

High anion gap metabolic acidosis and concurrent respiratory acidosis
The patient's presentation and laboratory findings denote a mixed acid-base disorder. The initial step in interpretation is looking at the pH and bicarbonate; a pH of 7.267.26 with a bicarbonate of 1212 mEq/L indicates a primary metabolic acidosis. Next, calculating the anion gap (Na+−[Cl−+HCO3−]Na^+ - [Cl^- + HCO_3^-]) yields 136−(98+12)=26136 - (98 + 12) = 26 mEq/L. An anion gap >12> 12 mEq/L defines a high anion gap metabolic acidosis (likely alcoholic ketoacidosis in this starved, binge-drinking patient). Finally, one must determine if the respiratory compensation is appropriate using Winter's formula (Expected PaCO2=1.5×[HCO3−]+8±2Expected\ PaCO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2). For this patient, the expected PaCO2PaCO_2 is 1.5×12+8=26±21.5 \times 12 + 8 = 26 \pm 2 mm Hg. The patient's measured PaCO2PaCO_2 is 3434 mm Hg. Because the measured PaCO2PaCO_2 is higher than the expected compensatory range (24−2824-28 mm Hg), the patient has inadequate alveolar ventilation, resulting in a concurrent respiratory acidosis. This hypoventilation is likely secondary to central nervous system depression from severe alcohol intoxication.

Step-by-Step Solution

1
Determine the primary acid-base disturbance by evaluating the pH and serum bicarbonate.
The pH is 7.267.26 (acidemia) and the HCO3−HCO_3^- is 1212 mEq/L (low), which indicates a primary metabolic acidosis.
Identifying the primary disturbance is the necessary first step to guide further calculations.
2
Calculate the serum anion gap using the formula: Na+−(Cl−+HCO3−)Na^+ - (Cl^- + HCO_3^-).
The anion gap is 136−(98+12)=26136 - (98 + 12) = 26 mEq/L. Since this is greater than 1212 mEq/L, it is a high anion gap metabolic acidosis.
The anion gap differentiates between causes of metabolic acidosis (e.g., ketoacidosis vs. renal tubular acidosis).
3
Calculate the expected respiratory compensation using Winter's formula: Expected PaCO2=(1.5×HCO3−)+8±2Expected\ PaCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2.
The expected PaCO2=(1.5×12)+8±2=26±2PaCO_2 = (1.5 \times 12) + 8 \pm 2 = 26 \pm 2 mm Hg (range 24−2824-28 mm Hg).
Winter's formula determines if the lungs are adequately compensating for the metabolic acidosis.
4
Compare the measured PaCO2PaCO_2 to the expected PaCO2PaCO_2 to identify any secondary concurrent disorders.
The measured PaCO2PaCO_2 is 3434 mm Hg, which is higher than the expected maximum of 2828 mm Hg. This indicates a concurrent respiratory acidosis.
An inappropriately elevated PaCO2PaCO_2 in the setting of a metabolic acidosis means the patient is hypoventilating relative to their metabolic demand.

Key Concept

Evaluation of a metabolic acidosis requires calculating the anion gap to determine the etiology and applying Winter's formula to identify any concurrent respiratory acid-base disturbances.
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Topics

General Internal Medicine

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