A 34-year-old man is brought to the emergency department by emergency medical services aft… — USMLE Step 2 CK

A 34-year-old man is brought to the emergency department by emergency medical services after his partner witnessed him having an episode of rhythmic limb jerking and loss of consciousness lasting approximately 5 minutes. On arrival, he is deeply lethargic and only moans to painful stimuli. His temperature is $37.2^\circ\text{C}$ ( $99.0^\circ\text{F}$ ), blood pressure is $118/76$ mmHg, pulse is $112$ /min, and respirations are $12$ /min. Physical examination reveals a laceration on the lateral aspect of his tongue and generalized muscle tenderness.

Laboratory studies show:
Sodium: $140$ mEq/L
Potassium: $4.6$ mEq/L
Chloride: $104$ mEq/L
Bicarbonate: $12$ mEq/L
Blood urea nitrogen: $16$ mg/dL
Creatinine: $1.1$ mg/dL
Glucose: $118$ mg/dL

Arterial blood gas (room air):
pH: $7.17$
pCO $_2$ : $34$ mmHg
pO $_2$ : $78$ mmHg

Which of the following best describes this patient's primary acid-base disorder(s)?

A
High anion gap metabolic acidosis with appropriate respiratory compensation
B
High anion gap metabolic acidosis and concurrent respiratory alkalosis
High anion gap metabolic acidosis and concurrent respiratory acidosisAnswer
D
High anion gap metabolic acidosis and concurrent normal anion gap metabolic acidosis
E
Normal anion gap metabolic acidosis and concurrent respiratory acidosis

Answer

High anion gap metabolic acidosis and concurrent respiratory acidosis

The correct answer accurately synthesizes the laboratory data. First, the patient has an acidemia (pH 7.17) driven by a low bicarbonate (12 mEq/L), indicating a primary metabolic acidosis. The anion gap is calculated as

140 - (104 + 12) = 24

, confirming a high anion gap metabolic acidosis, which is classically seen following generalized tonic-clonic seizures due to intense muscle exertion producing lactic acid. Next, appropriate respiratory compensation must be evaluated using Winter's formula:

pCO_2 = (1.5 \times 12) + 8 \pm 2 = 26 \pm 2

mmHg. The patient's measured pCO2 is 34 mmHg. Since the measured pCO2 is higher than the expected range of 24-28 mmHg, the patient has a concurrent, superimposed respiratory acidosis. This is clinically consistent with the patient's low respiratory rate of 12/min and deep lethargy, representing a post-ictal depression of the central respiratory drive.

Step-by-Step Solution

Determine the primary acid-base disorder by looking at the pH and bicarbonate.

The pH is 7.17 (acidemia) and the bicarbonate is 12 mEq/L (low), which indicates a primary metabolic acidosis.

Establishing the primary disturbance directs the choice of subsequent diagnostic formulas.

Calculate the serum anion gap using the formula:

AG = Na^+ - (Cl^- + HCO_3^-)

AG = 140 - (104 + 12) = 140 - 116 = 24

. The normal range is typically 10-14, so this is a high anion gap metabolic acidosis.

Determining the anion gap identifies the cause of the metabolic acidosis (e.g., lactic acidosis from the recent seizure).

Calculate the expected respiratory compensation using Winter's formula:

Expected\ pCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2

Expected\ pCO_2 = (1.5 \times 12) + 8 \pm 2 = 18 + 8 \pm 2 = 26 \pm 2

mmHg. The expected pCO2 range is 24 to 28 mmHg.

Winter's formula determines if the respiratory system has appropriately compensated for the primary metabolic acidosis.

Compare the expected pCO2 to the measured pCO2 on the arterial blood gas.

The measured pCO2 is 34 mmHg, which is significantly higher than the expected range of 24-28 mmHg.

Because the measured pCO2 is higher than the expected compensatory range, the patient is retaining carbon dioxide (likely due to a post-ictal depressed respiratory drive), indicating a concurrent respiratory acidosis.

Key Concept

Interpretation of a mixed acid-base disorder requires calculating the anion gap to identify unmeasured anions and applying Winter's formula to determine if respiratory compensation for a metabolic acidosis is appropriate.

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