A 24-year-old man is brought to the emergency department by his roommate after being found… — USMLE Step 2 CK

A $24$ -year-old man is brought to the emergency department by his roommate after being found distressed and complaining of a 'loud ringing' in his ears. The patient has been experiencing nausea and vomited once prior to arrival. The roommate reports finding several empty bottles of over-the-counter pain relievers in their shared bathroom. His temperature is $37.9^\circ\text{C}$ ( $100.2^\circ\text{F}$ ), blood pressure is $118/76$ mm Hg, pulse is $112$ /min, and respiratory rate is $28$ /min. Physical examination reveals a diaphoretic and anxious patient with deep, rapid respirations.

Laboratory results:

Test	Result
Serum sodium	$140$ mEq/L
Serum potassium	$4.0$ mEq/L
Serum chloride	$104$ mEq/L
Serum bicarbonate	$14$ mEq/L
Blood urea nitrogen	$18$ mg/dL
Serum creatinine	$1.1$ mg/dL

Arterial blood gas on room air:

Test	Result
pH	$7.47$
PaCO2	$20$ mm Hg
PaO2	$98$ mm Hg

Based on the laboratory findings, which of the following best describes this patient's acid-base status?

Mixed high anion gap metabolic acidosis and respiratory alkalosisAnswer
B
High anion gap metabolic acidosis with appropriate respiratory compensation
C
Respiratory alkalosis with appropriate metabolic compensation
D
High anion gap metabolic acidosis and concomitant respiratory acidosis
E
Mixed high anion gap metabolic acidosis, respiratory alkalosis, and metabolic alkalosis

Answer

Mixed high anion gap metabolic acidosis and respiratory alkalosis

This patient's clinical presentation (tinnitus, nausea, tachypnea, empty pain reliever bottles) strongly suggests salicylate toxicity, which classically causes a mixed acid-base disorder. The calculated anion gap is 22 mEq/L (

140 - [104 + 14]

), confirming a primary high anion gap metabolic acidosis. To determine if the respiratory response is appropriate, Winter's formula is applied: Expected PaCO2 =

(1.5 \times 14) + 8 = 29 \pm 2

mm Hg. The patient's actual PaCO2 is 20 mm Hg, which is significantly lower than the expected compensatory value. This indicates a concurrent primary respiratory alkalosis. Furthermore, the delta gap (Excess AG =

22 - 12 = 10

) equals the delta bicarbonate (

24 - 14 = 10

), yielding a ratio of 1.0, which rules out any superimposed metabolic alkalosis or non-gap acidosis.

Step-by-Step Solution

Calculate the anion gap to determine if a primary metabolic acidosis is present.

Anion Gap = Na - (Cl + HCO3) = 140 - (104 + 14) = 22 mEq/L.

An anion gap > 12 mEq/L confirms the presence of a high anion gap metabolic acidosis.

Apply Winter's formula to calculate the expected PaCO2 for appropriate respiratory compensation.

Expected PaCO2 = (1.5 x 14) + 8 = 29 mm Hg (range: 27-31 mm Hg).

This determines if the respiratory response is merely compensatory or represents a separate primary disorder.

Compare the actual PaCO2 to the expected PaCO2.

The actual PaCO2 of 20 mm Hg is less than the expected range of 27-31 mm Hg.

Because the actual PaCO2 is driven lower than expected, there is a concomitant primary respiratory alkalosis.

Calculate the delta gap and delta ratio to evaluate for a third, hidden metabolic disorder.

Delta AG = 22 - 12 = 10. Delta HCO3 = 24 - 14 = 10. Delta ratio = 10 / 10 = 1.0.

A delta ratio between 1.0 and 2.0 indicates a pure high anion gap metabolic acidosis, ruling out concurrent metabolic alkalosis or non-gap metabolic acidosis.

Key Concept

Salicylate toxicity classically presents 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 and accumulation of organic acids).

Estimated Time:1m 30s

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