A 32-year-old man is brought to the emergency department by paramedics following a general… — USMLE Step 2 CK

A $32$ -year-old man is brought to the emergency department by paramedics following a generalized tonic-clonic seizure. His roommate witnessed the event, reporting that the seizure lasted for approximately $10$ minutes before stopping on its own. On arrival, the patient is deeply lethargic and only withdraws to painful stimuli. His temperature is $37.2^\circ C$ ( $99.0^\circ F$ ), blood pressure is $118/76$ $mmHg$ , pulse is $110/min$ , and respiratory rate is $8/min$ . Pulse oximetry shows $92\%$ on room air.

Laboratory results are as follows:
Sodium: $140$ $mEq/L$
Potassium: $4.5$ $mEq/L$
Chloride: $100$ $mEq/L$
Bicarbonate: $12$ $mEq/L$
Blood urea nitrogen: $16$ $mg/dL$
Serum creatinine: $1.2$ $mg/dL$

An arterial blood gas drawn on room air reveals:
pH: $7.15$
pCO $_2$ : $40$ $mmHg$
pO $_2$ : $65$ $mmHg$

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

Mixed high anion-gap metabolic acidosis and respiratory acidosisAnswer
B
High anion-gap metabolic acidosis with appropriate respiratory compensation
C
Mixed high anion-gap metabolic acidosis and respiratory alkalosis
D
Mixed normal anion-gap metabolic acidosis and respiratory acidosis
E
Normal anion-gap metabolic acidosis with appropriate respiratory compensation

Answer

The correct answer is a mixed high anion-gap metabolic acidosis and respiratory acidosis.

This patient has an acidemia (

pH = 7.15

) driven by a low serum bicarbonate (

12

mEq/L

), establishing a primary metabolic acidosis. The anion gap is calculated as

Na^+ - (Cl^- + HCO_3^-) = 140 - (100 + 12) = 28

mEq/L

. Because the normal anion gap is approximately

10-14

mEq/L

, an anion gap of

28

indicates a robust high anion-gap metabolic acidosis, likely secondary to profound lactic acidosis from the prolonged muscular exertion and transient hypoxia of the seizure. Next, respiratory compensation must be evaluated using Winter's formula: Expected

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

. For this patient, the expected

pCO_2

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

mmHg

. The patient's actual

pCO_2

on the arterial blood gas is

40

mmHg

. Because the measured

pCO_2

is substantially higher than the expected range of

24-28

mmHg

, there is a concurrent primary respiratory acidosis. Clinically, this is consistent with post-ictal central nervous system depression causing hypoventilation, as evidenced by his low respiratory rate of

8/min

. Therefore, the patient has a mixed high anion-gap metabolic acidosis and respiratory acidosis.

Step-by-Step Solution

Analyze the pH and bicarbonate to determine the primary disorder.

The pH is 7.15 (acidemia) and the bicarbonate is 12 mEq/L (low), indicating a primary metabolic acidosis.

Establishing the initial acid-base disturbance directs the subsequent formulas and calculations needed.

Calculate the serum anion gap.

AG = 140 - (100 + 12) = 28 mEq/L. This indicates a high anion-gap metabolic acidosis (HAGMA).

Metabolic acidosis must always be classified into normal or high anion gap to narrow the differential diagnosis. Here, severe muscle contraction leads to lactic acidosis.

Calculate the expected respiratory compensation using Winter's formula.

Expected pCO2 = (1.5 × 12) + 8 ± 2 = 26 ± 2 mmHg. The expected range is 24 to 28 mmHg.

Metabolic acidosis stimulates central chemoreceptors to increase the respiratory rate, thereby blowing off CO2 to compensate for the acidemia.

Compare actual pCO2 to the expected pCO2.

The actual pCO2 of 40 mmHg is significantly higher than the expected range of 24-28 mmHg.

When the actual pCO2 is greater than the expected pCO2, it indicates a superimposed primary respiratory acidosis. This patient is post-ictal, causing central respiratory depression (respiratory rate of 8/min).

Key Concept

Patients with a primary metabolic acidosis should hyperventilate to reduce their pCO2. If the measured pCO2 is higher than the value predicted by Winter's formula, a concurrent respiratory acidosis is present.

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