Situation: A 40-year-old woman weighing 55 kg is rescued after being trapped for 20 minutes in a closed room of a burning house, where furniture and plastic items were burning. She has partial-thickness burns of the face, neck, anterior trunk, and both arms, estimated at 30% of total body surface area. She does not smoke. On arrival she has a headache, nausea, and confusion. Her results are: Pulse oximetry: 98% on room air Arterial oxygen tension: 92 mmHg Carboxyhemoglobin: 26% (normal in a nonsmoker < 3%) Which interpretation is correct?

Carbon monoxide binds hemoglobin in place of oxygen, so a carboxyhemoglobin of 26% means much of her hemoglobin cannot carry oxygen. Standard pulse oximetry reads carboxyhemoglobin as oxygenated hemoglobin, and arterial oxygen tension reflects dissolved oxygen, so both look normal while tissue oxygen delivery is poor. Her headache, nausea, and confusion fit carbon monoxide poisoning, treated with 100% oxygen.

Why the “normal” numbers are misleading
This patient was trapped in a closed room with burning furniture and plastics, which produces large amounts of carbon monoxide (CO). CO binds to hemoglobin with an affinity roughly 200–250 times greater than that of oxygen ^[1]. The result is carboxyhemoglobin (COHb), a hemoglobin species that cannot carry oxygen effectively and also shifts the oxyhemoglobin dissociation curve, impairing oxygen unloading at the tissue level ^[1][4]. A COHb of 26% in a nonsmoker is markedly elevated, since baseline is normally below 2–3% ^[1].

Pulse oximetry and PaO₂ can look falsely reassuring
Standard two-wavelength pulse oximetry cannot distinguish COHb from oxyhemoglobin. It reads COHb as if it were oxygenated hemoglobin, so the displayed SpO₂ of 98% is misleading ^[2][3]. Arterial oxygen tension, or PaO₂, measures only dissolved oxygen in plasma, not the oxygen bound to hemoglobin. Therefore, a PaO₂ of 92 mmHg can be normal while total oxygen-carrying capacity is severely reduced ^[1][4]. Normal pulse oximetry and normal PaO₂ do not rule out clinically significant carbon monoxide poisoning.

Clinical picture matches tissue hypoxia, not burn pain alone
Headache, nausea, and confusion are classic early neurologic symptoms of CO poisoning ^[1][2][4]. The brain is especially vulnerable because CO also interferes with mitochondrial respiration and promotes oxidative stress and inflammation beyond simple hypoxemia ^[4]. Symptoms correlate poorly with the exact COHb percentage, so the clinical presentation must guide urgency even when the number seems only moderately elevated. In a burn patient, pain can cause agitation, but the combination of a fire in an enclosed space, neurologic symptoms, and a high COHb level points to CO poisoning as the primary driver.

Diagnosis and immediate management
Definitive diagnosis is made by blood co-oximetry, which directly measures COHb and methemoglobin, unlike standard pulse oximetry ^[2][3]. Initial treatment is high-flow 100% oxygen via a nonrebreather mask, which shortens the half-life of COHb and improves oxygen delivery ^[2][3]. Watch out! A normal SpO₂ on a standard pulse oximeter does not exclude CO poisoning; co-oximetry is required when exposure is suspected.

Why the other options are incorrect

Option	Reason it is wrong
1. Pulse oximeter is faulty	The oximeter is functioning normally; it simply cannot differentiate COHb from oxyhemoglobin [2][3].
2. Confusion from burn pain because oxygen values are normal	Oxygen values are falsely normal. COHb of 26% means a large proportion of hemoglobin cannot carry oxygen [1][4].
3. Oxygenation is adequate	PaO₂ reflects dissolved oxygen only, not total oxygen content or tissue delivery. With high COHb, tissue oxygenation is poor despite normal PaO₂ and SpO₂ [1][4].

Key point! In carbon monoxide poisoning, oxygen delivery to tissues is impaired even when pulse oximetry and arterial oxygen tension appear normal. The correct interpretation is that tissue oxygen delivery is poor, and the patient requires immediate high-flow oxygen.