An 18-year-old male is brought to the emergency department by his older sister because he … — USMLE Step 2 CK

An 18-year-old male is brought to the emergency department by his older sister because he has been unable to stand up from the couch for the past two hours. Over the last $3$ weeks, his sister notes he has been increasingly withdrawn, occasionally talking to people who are not present, and failing to show up for his part-time job. His medical history includes major depressive disorder, for which he has been taking fluoxetine for the past year.

On physical examination, the patient is lethargic but arousable. An erythematous, flaky rash is noted around his mouth and nares. Lungs are clear to auscultation. Heart rate is $105/\text{min}$ , blood pressure is $112/70\text{ mm Hg}$ , and respiratory rate is $24/\text{min}$ . Neurologic examination reveals symmetric $2/5$ muscle strength in the lower extremities bilaterally, with absent patellar reflexes.

Laboratory results:
$Na^+$ : $140\text{ mEq/L}$
$K^+$ : $1.8\text{ mEq/L}$
$Cl^-$ : $115\text{ mEq/L}$
$HCO_3^-$ : $14\text{ mEq/L}$
$BUN$ : $10\text{ mg/dL}$
$Cr$ : $0.7\text{ mg/dL}$
$TSH$ : $1.8\text{ }\mu\text{U/mL}$
Urine toxicology screen: Negative for opioids, amphetamines, cocaine, PCP, and cannabis.

Arterial blood gas on room air:
pH: $7.22$
$P_{a}CO_2$ : $36\text{ mm Hg}$
$P_{a}O_2$ : $85\text{ mm Hg}$

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

Administer intravenous potassium chloride and continuously monitor respiratory status.Answer
B
Administer intravenous sodium bicarbonate for an isolated metabolic acidosis.
C
Diagnose schizophreniform disorder and initiate a first-generation antipsychotic.
D
Discontinue fluoxetine and immediately initiate phenelzine for treatment-resistant depression.
E
Notify the patient's employer about his substance use disorder to ensure workplace safety.

Answer

Administer intravenous potassium chloride and continuously monitor respiratory status.

This patient's presentation is classic for inhalant (specifically toluene) toxicity. The erythematous perioral rash ('huffer's rash'), lethargy, and negative standard urine toxicology screen in a young adult point to volatile solvent abuse. Toluene causes a Type 1 (distal) renal tubular acidosis, leading to severe potassium wasting and a non-anion gap metabolic acidosis. The profound hypokalemia (

1.8\text{ mEq/L}

) is responsible for the patient's flaccid paralysis and absent reflexes. Using Winter's formula (

1.5 \times 14 + 8 = 29

), the expected

P_{a}CO_2

27-31\text{ mm Hg}

. The measured

P_{a}CO_2

36\text{ mm Hg}

reveals a superimposed respiratory acidosis, secondary to inhalant-induced CNS depression. Therefore, the most urgent interventions are replenishing potassium to prevent fatal arrhythmias and closely monitoring the airway due to impending respiratory failure.

Step-by-Step Solution

Identify the primary toxidrome from the clinical clues.

The combination of an erythematous perioral rash ('huffer's rash'), central nervous system depression, and severe hypokalemia in an adolescent strongly indicates inhalant abuse, specifically toluene (found in paint thinner or glue).

Recognizing the specific substance of abuse is required to understand the systemic pathophysiology.

Analyze the basic metabolic panel and determine the primary acid-base disturbance.

The anion gap is calculated as

Na^+ - (Cl^- + HCO_3^-) = 140 - (115 + 14) = 11

, which is normal. The patient has a non-anion gap metabolic acidosis.

Toluene metabolism produces hippuric acid, which obligates potassium excretion in the urine, leading to a Type 1 (distal) renal tubular acidosis (RTA) characterized by normal anion gap acidosis and severe hypokalemia.

Evaluate the respiratory compensation using Winter's formula.

Expected

P_{a}CO_2 = 1.5 \times [HCO_3^-] + 8 \pm 2 = 1.5 \times 14 + 8 \pm 2 = 29 \pm 2

. The expected range is

27

31\text{ mm Hg}

To determine if the patient has a simple or mixed acid-base disorder.

Compare expected vs. measured

P_{a}CO_2

The measured

P_{a}CO_2

36\text{ mm Hg}

, which is significantly higher than the expected maximum of

31\text{ mm Hg}

. This means there is a superimposed respiratory acidosis.

Despite being tachypneic (

24/\text{min}

), the patient's central nervous system depression from the inhalant is preventing adequate ventilation to compensate for the metabolic acidosis.

Determine the most critical next step in management.

Intravenous potassium chloride administration and respiratory monitoring.

The patient's flaccid paralysis is due to life-threatening hypokalemia (

1.8\text{ mEq/L}

), which can cause fatal arrhythmias. Concurrently, the uncompensated respiratory acidosis indicates impending respiratory failure requiring close airway monitoring.

Key Concept

Toluene (inhalant) toxicity presents with central nervous system depression, perioral rash, and Type 1 renal tubular acidosis causing a non-anion gap metabolic acidosis with profound hypokalemia. Mixed acid-base disorders often accompany severe intoxication.

Practice More

Review the criteria for distinguishing brief psychotic disorder, schizophreniform disorder, and schizophrenia, as well as the formula for calculating Winter's formula.

Estimated Time:2m 0s

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