When Is HBOT Required for Carbon Monoxide Poisoning?

Emergency: Carbon monoxide poisoning is a medical emergency. Call 911 immediately. If you suspect CO poisoning, get to fresh air first. Poison Control: 1-800-222-1222.

Carbon monoxide poisoning kills by blocking oxygen delivery to tissues. HBOT is the most effective treatment for serious CO poisoning. But not every CO exposure requires HBOT. Here’s how clinicians decide.

First Steps Before HBOT: 100% Oxygen

Every patient with suspected CO poisoning should receive 100% oxygen by tight-fitting non-rebreather mask immediately. This is not HBOT — it’s room-pressure normobaric oxygen. It works by flooding the body with O2 and accelerating the displacement of CO from hemoglobin.

CO-hemoglobin (COHb) has a half-life of about 5 hours breathing room air. On 100% normobaric oxygen, that drops to about 60-90 minutes. In a hyperbaric chamber at 3 ATA, it drops to about 20 minutes.

Normobaric oxygen begins in the field or emergency department while HBOT candidacy is evaluated.

The UHMS Criteria for HBOT in CO Poisoning

The Undersea and Hyperbaric Medical Society (UHMS) criteria for recommending HBOT in CO poisoning include:

• COHb level at or above 25% on blood draw
• Loss of consciousness at any point, even briefly
• Neurological symptoms: confusion, ataxia, vision changes, seizure, focal deficits
• Cardiac ischemia on ECG
• Age over 36 years in some guidelines (older patients have higher risk of delayed neurological syndrome)

Pregnancy lowers the threshold significantly. In pregnant patients, HBOT is generally recommended when COHb reaches 15% or higher — or even at lower levels if the fetus is symptomatic. Fetal hemoglobin binds CO more tightly than adult hemoglobin, and fetal tissue has less tolerance for hypoxia.

The Timing Problem: COHb May Already Be Normal

Here’s a clinical complication. COHb levels fall quickly once a patient is on oxygen. A patient who was symptomatic at the scene and then received 30-60 minutes of normobaric O2 before blood was drawn may have a normal or near-normal COHb level by the time testing happens.

A normal COHb on blood draw does not rule out significant CO exposure. If the history suggests serious exposure — prolonged exposure to CO source, loss of consciousness, severe symptoms — the clinical picture matters as much as the lab value.

Delayed Neurological Syndrome: The Reason HBOT Matters

10 to 30% of CO-poisoned patients develop delayed neurological syndrome (DNS), a cluster of cognitive and neurological symptoms appearing 2 to 28 days after apparent recovery. This can include memory loss, personality change, parkinsonism, dementia-like symptoms, and motor deficits.

Thom et al. (1995) published an RCT showing that HBOT significantly reduced the incidence of DNS compared to normobaric oxygen. PMID: 7651459. This is the key evidence for HBOT in CO poisoning and the reason it’s the standard of care for serious cases.

Normobaric oxygen clears CO faster than room air. HBOT clears it faster still and appears to address mechanisms beyond CO clearance — including mitochondrial dysfunction and lipid peroxidation that contribute to delayed injury.

Timing: Earlier Is Better

HBOT ideally begins within 6 hours of CO exposure. Research consistently shows better outcomes — and better prevention of delayed neurological syndrome — with early treatment.

HBOT is still used after 6 hours and can have benefit, but the window of maximum effectiveness is the first several hours. This is a reason not to delay seeking care after CO exposure.

What Happens in the Chamber

CO poisoning is typically treated at 3.0 ATA — higher pressure than most wound care or radiation protocols. Sessions are shorter, running approximately 90 minutes. The number of sessions depends on severity. Most uncomplicated cases receive 1-3 sessions. Severe cases with neurological involvement may receive more, guided by clinical response.

FAQ

Q: When is HBOT required for CO poisoning? COHb 25% or higher, any loss of consciousness, neurological symptoms, cardiac ischemia on ECG, or pregnancy with COHb 15% or higher.

Q: What is delayed neurological syndrome? Cognitive and neurological symptoms appearing 2-28 days after CO poisoning. Thom et al. (1995) showed HBOT reduces this risk (PMID: 7651459).

Q: Why is CO more dangerous in pregnancy? Fetal hemoglobin binds CO more tightly than adult hemoglobin. The HBOT threshold is lower in pregnancy.

Q: How quickly should HBOT start? Ideally within 6 hours. Earlier treatment means better outcomes.

Related: Carbon Monoxide Poisoning and HBOT | HBOT and Pregnancy

Medical Disclaimer: This content is for informational purposes only and does not constitute medical advice. Carbon monoxide poisoning is a medical emergency. Call 911 immediately if you suspect exposure. This site does not establish a doctor-patient relationship.