A 42-year-old woman is evaluated in the emergency department after her roommate found her … — USMLE Step 2 CK

A $42$ -year-old woman is evaluated in the emergency department after her roommate found her confused and breathing rapidly. The roommate reports that the patient has a history of chronic lower back pain and has recently been complaining of a continuous ringing in her ears. Her temperature is $38.2^\circ C$ ( $100.8^\circ F$ ), blood pressure is $112/72$ mm Hg, pulse is $115$ /min, and respirations are $28$ /min. Physical examination shows a diaphoretic and agitated woman. Cardiopulmonary examination is unremarkable except for tachycardia and tachypnea. The lungs are clear to auscultation bilaterally.

Laboratory results are as follows:

Laboratory Test	Result	Reference Range
Sodium	$142$ mEq/L	$136$ - $145$ mEq/L
Potassium	$3.5$ mEq/L	$3.5$ - $5.0$ mEq/L
Chloride	$105$ mEq/L	$98$ - $106$ mEq/L
Bicarbonate	$14$ mEq/L	$22$ - $28$ mEq/L
Blood urea nitrogen	$22$ mg/dL	$7$ - $20$ mg/dL
Creatinine	$1.2$ mg/dL	$0.6$ - $1.2$ mg/dL
Glucose	$98$ mg/dL	$70$ - $100$ mg/dL

Arterial Blood Gas (Room Air)	Result	Reference Range
pH	$7.43$	$7.35$ - $7.45$
$PaCO_2$	$20$ mm Hg	$35$ - $45$ mm Hg
$PaO_2$	$95$ mm Hg	$80$ - $100$ mm Hg

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

Intravenous sodium bicarbonateAnswer
B
Endotracheal intubation and mechanical ventilation
C
Intravenous piperacillin-tazobactam and vancomycin
D
Initiation of noninvasive positive pressure ventilation
E
Intravenous regular insulin and normal saline

Answer

Intravenous sodium bicarbonate

This patient presents with confusion, tachypnea, hyperthermia, and tinnitus (ringing in the ears), alongside a classic mixed acid-base disorder. Her anion gap is calculated as

Na^+ - (Cl^- + HCO_3^-) = 142 - (105 + 14) = 23

mEq/L, confirming an elevated anion gap metabolic acidosis. Applying Winter's formula, her expected

PaCO_2

should be

(1.5 \times 14) + 8 \pm 2 = 27

31

mm Hg. Because her actual

PaCO_2

20

mm Hg, she also has an independent primary respiratory alkalosis. The combination of a primary anion gap metabolic acidosis (due to uncoupling of oxidative phosphorylation and accumulation of organic acids) and a primary respiratory alkalosis (due to direct central stimulation of the medullary respiratory center) strongly suggests salicylate toxicity, typically resulting from aspirin overdose. The most appropriate initial specific treatment is intravenous sodium bicarbonate. This acts to alkalinize both the serum and the urine. Urine alkalinization shifts the salicylic acid into its ionized, lipophobic form (salicylate), which traps it in the renal tubules and rapidly accelerates its excretion.

Step-by-Step Solution

Calculate the anion gap.

Anion Gap =

142 - (105 + 14) = 23

mEq/L.

An elevated anion gap (

> 12

) confirms the presence of a primary metabolic acidosis.

Apply Winter's formula to determine if the respiratory compensation is appropriate.

Expected

PaCO_2 = (1.5 \times 14) + 8 = 29 \pm 2

mm Hg. The actual

PaCO_2

(

20

mm Hg) is significantly lower than the expected range.

The discrepancy reveals an additional primary respiratory alkalosis, independent of the metabolic acidosis.

Synthesize the laboratory data with the clinical presentation.

A mixed primary metabolic acidosis and primary respiratory alkalosis, combined with fever, tinnitus (ringing in the ears), and confusion, points directly to salicylate toxicity.

Salicylates uniquely stimulate the medullary respiratory center (causing respiratory alkalosis) and uncouple oxidative phosphorylation (causing elevated AG metabolic acidosis and hyperthermia).

Determine the most appropriate management step.

Administer intravenous sodium bicarbonate.

Sodium bicarbonate alkalinizes the blood and urine. Alkalinizing the urine keeps salicylate in its ionized (lipophobic) form, preventing renal reabsorption and heavily promoting its excretion.

Key Concept

Diagnosis and management of salicylate toxicity, characterized clinically by tinnitus, hyperthermia, and a classic mixed acid-base disorder (primary elevated anion gap metabolic acidosis and primary respiratory alkalosis).

Estimated Time:1m 30s

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