SALT LAKE CITY—Patients treated with hyperbaric oxygen immediately after toxic carbon monoxide exposure are significantly less likely to suffer neurological impairment in memory, concentration, and affect. In a randomized double-blind study, Lindell K. Weaver, MD, and colleagues from LDS Hospital in Salt Lake City found that three treatments administered within 24 hours markedly reduced the risk of persistent cognitive sequelae.[1]

A silent killer, carbon monoxide poisoning accounts for about 40,000 emergency department visits in this country annually. The risk of such poisoning rises each year with the falling temperatures as people fire up their furnaces or other heat sources.

The neurologic consequences of carbon monoxide poisoning may not be apparent in the emergency department, but symptoms usually manifest within 20 days of exposure. In up to half of affected patients, symptoms may be persistent. To determine the optimal treatment for these long-term effects, the researchers compared the rate of cognitive sequelae in 152 patients given three treatments of either normobaric or hyperbaric oxygen. Patients were assessed immediately after the first and third chamber sessions and after two weeks, six weeks, six months, and 12 months.

PATIENTS UNDER PRESSURE

Patients were eligible for enrollment if they met one of the following criteria: if exposure to carbon monoxide could be documented by elevated carboxyhemoglobin levels or ambient carbon monoxide levels, if an obvious incident of exposure was accompanied by related symptoms, or if carboxyhemoglobin levels were below 10% and carbon monoxide poisoning was the only logical diagnosis. Patients were excluded if the exposure had occurred more than 24 hours earlier, if they were younger than 16, or if they were pregnant.

In addition to initial oxygen therapy, patients were randomly assigned to receive three chamber sessions of oxygen. The hyperbaric group was exposed to 3 and then 2 atmospheres absolute of 100% oxygen for the first session and 2 atmospheres absolute for sessions 2 and 3. The normobaric group was exposed to high concentrations of supplemental oxygen for the first session (at 1 atmosphere absolute pressure) and then to room air for sessions 2 and 3.

Researchers gathered data on demographics, coexisting conditions, and details of the exposure. In addition, a physical examination and a battery of neuropsychological tests were performed at baseline and at the aforementioned assessment intervals.

TRIAL STOPPED EARLY

Cognitive impairment at six weeks occurred less frequently in the hyperbaric group (25% of patients) than in the normobaric group (46% of patients). In both groups, the existence of cerebellar dysfunction before treatment was associated with a higher incidence of persistent cognitive sequelae, but hyperbaric therapy was shown to improve outcome even in patients with cerebellar dysfunction. Because hyperbaric oxygen demonstrated a clear efficacy, the trial was stopped after the third interim analysis.

The biggest impediment to therapy appeared to be adverse effects. These included anxiety and aural barotrauma, which occurred in approximately 10% of the patients.

Further research is needed to evaluate the details involved in administering hyperbaric therapy. In an editorial, Stephen Thom, MD, from the University of Pennsylvania pointed out that only 23 investigators in the past 10 years have received NIH funding to study its effects.[2] Now, continuing research is necessary into the optimal number of treatments, the optimal dose of hyperbaric oxygen, the amount of time after exposure when treatment becomes ineffective, and the mechanism by which hyperbaric oxygen prevents damage.

—Lisa Pallatroni

References
1. Weaver LK, Hopkins RO, Chan KJ, et al. Hyperbaric oxygen for acute carbon monoxide poisoning. N Engl J Med. 2002;347:1057-1067.
2. Thom SR. Hyperbaric-oxygen therapy for acute carbon monoxide poisoning. N Engl J Med. 2002;347:1105-1106.

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