A 62-year-old man with a history of chronic obstructive pulmonary disease (COPD) and coron… — USMLE Step 2 CK

A $62$ -year-old man with a history of chronic obstructive pulmonary disease (COPD) and coronary artery disease is brought to the emergency department during a severe winter snowstorm. His wife found him confused and complaining of a severe, throbbing headache in their unventilated garage, where he had been running a gasoline-powered generator for several hours. On arrival, he is lethargic but arousable. Temperature is $37.1^\circ\text{C}$ ( $98.8^\circ\text{F}$ ), blood pressure is $105/65\text{ mmHg}$ , pulse is $115/\text{min}$ , and respiratory rate is $24/\text{min}$ . Pulse oximetry shows $99\%$ on room air. Physical examination reveals clear lung fields bilaterally and no jugular venous distention. Arterial blood gas analysis on room air shows:

pH: $7.28$
PaCO $_2$ : $32\text{ mm Hg}$
PaO $_2$ : $95\text{ mm Hg}$
HCO $_3^-$ : $14\text{ mEq/L}$

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

A
Initiation of noninvasive positive pressure ventilation
B
Infusion of intravenous sodium bicarbonate
Administration of $100\%$ oxygen via non-rebreather maskAnswer
D
Administration of intravenous furosemide
E
Initiation of intravenous piperacillin-tazobactam

Answer

Administration of

100\%

oxygen via non-rebreather mask

This patient's presentation of confusion, severe headache, and metabolic acidosis after running a gasoline generator in an enclosed space is highly characteristic of acute carbon monoxide (CO) poisoning. CO binds to hemoglobin with an affinity approximately

250

times greater than that of oxygen, forming carboxyhemoglobin. This binding not only significantly reduces the oxygen-carrying capacity of the blood but also shifts the oxygen-hemoglobin dissociation curve to the left, severely impairing oxygen release to tissues and leading to profound lactic acidosis. Importantly, standard pulse oximetry cannot differentiate between oxyhemoglobin and carboxyhemoglobin, resulting in a falsely normal or high reading. Similarly, the arterial PaO

_2

(which measures dissolved oxygen in the plasma) remains normal because lung gas exchange itself is unaffected. The definitive next step in management is the immediate administration of

100\%

oxygen via a non-rebreather mask. This intervention increases the amount of dissolved oxygen in the blood and, more importantly, competitively displaces CO from hemoglobin, reducing the half-life of carboxyhemoglobin from roughly

5

hours on room air to approximately

1

hour.

Step-by-Step Solution

Analyze the patient's history and presenting environment.

The patient was found confused with a severe headache in an enclosed space with a running gasoline generator.

This specific environmental exposure strongly suggests acute carbon monoxide (CO) poisoning.

Interpret the laboratory and objective findings.

The patient has severe metabolic acidosis (pH 7.28, HCO3- 14) indicating tissue hypoxia, despite a falsely reassuring normal pulse oximetry (99%) and normal arterial oxygen tension (PaO2 95 mm Hg).

Standard pulse oximetry cannot differentiate between oxyhemoglobin and carboxyhemoglobin. PaO2 only measures dissolved oxygen in plasma, which remains normal because alveolar gas exchange is unaffected by CO.

Determine the most appropriate immediate intervention.

Administer high-flow

100\%

oxygen.

High-flow oxygen decreases the half-life of carboxyhemoglobin from approximately 4-6 hours (on room air) to 40-90 minutes, rapidly restoring the oxygen-carrying capacity of the blood.

Key Concept

Diagnosis and acute management of carbon monoxide poisoning.

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