A 21-year-old female university student is evaluated in the urgent care clinic after her r… — USMLE Step 2 CK

A 21-year-old female university student is evaluated in the urgent care clinic after her roommates noticed her acting increasingly confused and breathing rapidly over the past $4\text{ hours}$ . She has a history of major depressive disorder and recently failed an important exam. She complains of a constant "buzzing" sound in her ears. Her temperature is $37.9^\circ\text{C}$ ( $100.2^\circ\text{F}$ ), pulse is $118\text{/min}$ , respirations are $30\text{/min}$ , and blood pressure is $112/68\text{ mm Hg}$ . Physical examination reveals an agitated, tachypneic patient with clear lung fields and no focal neurologic deficits.

Laboratory studies reveal the following:

Laboratory Test	Result
Sodium	$140\text{ mEq/L}$
Potassium	$3.5\text{ mEq/L}$
Chloride	$104\text{ mEq/L}$
Bicarbonate	$13\text{ mEq/L}$
Blood urea nitrogen	$18\text{ mg/dL}$
Creatinine	$0.9\text{ mg/dL}$

Arterial Blood Gas	Result
pH	$7.48$
PaCO $_2$	$18\text{ mm Hg}$
PaO $_2$	$98\text{ mm Hg}$

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

A
High anion gap metabolic acidosis and concurrent respiratory acidosis
B
High anion gap metabolic acidosis with appropriate respiratory compensation
C
Normal anion gap metabolic acidosis and concurrent respiratory alkalosis
High anion gap metabolic acidosis and concurrent respiratory alkalosisAnswer
E
Primary respiratory alkalosis with appropriate metabolic compensation

Answer

High anion gap metabolic acidosis and concurrent respiratory alkalosis

The correct answer identifies that this patient has a mixed acid-base disorder characterized by both a high anion gap metabolic acidosis and a primary respiratory alkalosis, which is the classic presentation of acute salicylate (aspirin) toxicity. The clinical context of a suicide attempt, tinnitus ('buzzing' in ears), fever, and tachypnea strongly suggest this diagnosis. The first step in evaluating this patient's acid-base status is calculating the serum anion gap:

\text{Na}^+ - (\text{Cl}^- + \text{HCO}_3^-) = 140 - (104 + 13) = 23\text{ mEq/L}

. Since the normal range is

10-14\text{ mEq/L}

, a high anion gap metabolic acidosis (HAGMA) is present. Salicylates cause this by uncoupling oxidative phosphorylation and interfering with the Krebs cycle, leading to the accumulation of organic acids. The second step is to use Winter's formula to determine if the respiratory response is an appropriate physiological compensation or if a concurrent respiratory disorder exists. Expected

\text{PaCO}_2 = 1.5 \times [\text{HCO}_3^-] + 8 \pm 2

. For this patient,

1.5 \times 13 + 8 = 27.5 \pm 2\text{ mm Hg}

(range

25.5 - 29.5\text{ mm Hg}

). The patient's measured

\text{PaCO}_2

18\text{ mm Hg}

, which is substantially lower than the expected range. This excessive hyperventilation indicates a concurrent primary respiratory alkalosis, caused by salicylates directly stimulating the medullary respiratory center. Therefore, the patient has a mixed HAGMA and respiratory alkalosis.

Step-by-Step Solution

Calculate the serum anion gap to evaluate for a primary metabolic acidosis.

Anion Gap = Na - (Cl + HCO3) = 140 - (104 + 13) = 23 mEq/L.

An anion gap > 14 mEq/L establishes the presence of a High Anion Gap Metabolic Acidosis (HAGMA), a key component of salicylate toxicity.

Use Winter's formula to determine the expected respiratory compensation (expected PaCO2) for the calculated metabolic acidosis.

Expected PaCO2 = (1.5 * HCO3) + 8 +/- 2 = (1.5 * 13) + 8 +/- 2 = 27.5 +/- 2 mm Hg (Range: 25.5 to 29.5 mm Hg).

Winter's formula determines if the patient's breathing pattern is an appropriate physiologic response to the acidosis or if a second primary acid-base disorder is present.

Compare the patient's measured PaCO2 to the expected PaCO2 range.

The measured PaCO2 is 18 mm Hg, which is significantly lower than the expected minimum of 25.5 mm Hg.

Because the measured PaCO2 is lower than what would be expected from compensatory hyperventilation alone, a concurrent primary respiratory alkalosis must also be present.

Key Concept

Patients with salicylate (aspirin) toxicity classically present with a mixed acid-base disorder: a primary respiratory alkalosis (due to direct medullary respiratory center stimulation) and a primary high anion gap metabolic acidosis (due to uncoupling of oxidative phosphorylation).

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