A 21-year-old woman is brought to the emergency department by her college roommate due to … — USMLE Step 2 CK

A 21-year-old woman is brought to the emergency department by her college roommate due to confusion and rapid breathing. The roommate reports that the patient has been complaining of severe ear ringing, nausea, and vomiting since last night after taking 'an entire bottle' of an over-the-counter pain reliever for a severe headache. On physical examination, the patient is agitated, diaphoretic, and tachypneic. Temperature is 38.2°C (100.8°F), blood pressure is 112/68 mm Hg, heart rate is 122/min, and respiratory rate is 34/min. Pulse oximetry is 99% on room air. Lungs are clear to auscultation bilaterally. Cardiac examination shows tachycardia with a regular rhythm and no murmurs.

Laboratory results are as follows:

Test	Result
Sodium	141 mEq/L
Potassium	3.6 mEq/L
Chloride	103 mEq/L
Bicarbonate	14 mEq/L
Blood urea nitrogen	16 mg/dL
Creatinine	1.0 mg/dL

Arterial blood gas analysis on room air:

Parameter	Result
pH	7.48
pCO2	18 mm Hg
pO2	97 mm Hg

Which of the following is the most appropriate next step in the management of this patient?

Intravenous sodium bicarbonate administrationAnswer
B
Endotracheal intubation and mechanical ventilation
C
Intravenous furosemide administration
D
Intravenous piperacillin-tazobactam and vancomycin
E
Instruct the patient to breathe into a paper bag

Answer

Intravenous sodium bicarbonate administration

The correct answer involves the administration of intravenous sodium bicarbonate. This patient is presenting with severe salicylate toxicity, characterized by the classic triad of tinnitus, hyperventilation, and a mixed acid-base disorder (primary respiratory alkalosis and primary high anion gap metabolic acidosis). By alkalinizing the serum, sodium bicarbonate shifts the equilibrium of salicylic acid toward its ionized (charged) state. This 'ion trapping' prevents the lipophilic uncharged acid from crossing the blood-brain barrier, thereby protecting the central nervous system from severe neurotoxicity. Furthermore, alkalinization of the urine traps the ionized salicylate in the renal tubules, markedly accelerating its clearance from the body.

Step-by-Step Solution

Identify the primary toxidrome from the clinical history and physical examination.

The patient has a history of an over-the-counter pain reliever overdose, accompanied by tinnitus (ear ringing), nausea, vomiting, fever, and severe tachypnea, which is the classic presentation of salicylate (aspirin) toxicity.

Recognizing the toxidrome is the first step in guiding both diagnostic interpretation and targeted therapy.

Analyze the acid-base disturbance using the laboratory values.

The anion gap is calculated as

Na^+ - (Cl^- + HCO_3^-) = 141 - (103 + 14) = 24

. This indicates a high anion gap metabolic acidosis. Applying Winter's formula, the expected

pCO_2 = 1.5 \times 14 + 8 \pm 2 = 29 \pm 2

mm Hg. Because the actual

pCO_2

is 18 mm Hg, there is a concurrent primary respiratory alkalosis. The alkalemic pH (7.48) confirms the respiratory component is currently dominant.

Accurate acid-base calculation is crucial because the toxic mechanisms of salicylates cause both direct respiratory center stimulation and uncoupling of oxidative phosphorylation.

Determine the most appropriate targeted intervention for the severe toxicity.

Intravenous sodium bicarbonate is indicated.

Sodium bicarbonate alkalinizes the blood, converting salicylic acid to its lipophobic ionized form, preventing it from crossing the blood-brain barrier. It also alkalinizes the urine, accelerating the renal excretion of the toxin.

Key Concept

The management of severe salicylate toxicity requires intravenous sodium bicarbonate to alkalinize the serum and urine, which prevents the toxin from entering the central nervous system and enhances its renal excretion. Intubation should be avoided if possible due to the risk of fatal acidemia during apneic induction.

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