Hyperbaric Oxygen Therapy for Acute Carbon Monoxide Exposure

Abstract

Source: Weaver LK, Hopkins RO, Chan KJ, et al. Hyperbaric oxygen for acute carbon monoxide poisoning. N Engl J Med. 2002;347:1057–1067.The authors from Salt Lake City, Utah, randomized 152 patients (>16 years of age) who had documented exposure to carbon monoxide (CO) and any of the following symptoms: loss of consciousness, confusion, headache, malaise, fatigue, dizziness, forgetfulness, visual disturbances, vomiting, heart ischemia, or a metabolic acidosis to either hyperbaric-oxygen (HBO) or normobaric-oxygen. HBO was administered 3 times at 6–12 hour intervals in a monoplace chamber. The first exposure to HBO occurred within 24 hours of exposure to CO and used 100% inspired oxygen (FiO2) at 3 atm for 1 hour followed by 2 atm for an additional 2 hours. The 2 remaining HBO sessions used 100% FiO2 at 2 atm for the entire 3 hours. Patients who received normobaric oxygen received 100% FiO2 at 1 atm for 3 hours each session.A battery of neuropsychological tests were administered immediately after the first and third oxygen session and then after 2 weeks, 6 weeks, 6 months, and 12 months. The patients in each group had similar ages and symptoms of headache, vomiting, and confusion. Thirteen percent were admitted to the hospital and 8 percent were mechanically ventilated. The carboxyhemoglobin level (COHb) at time of the first treatment session was 4.3% in the HBO group and 4.6% in the normobaric group.The first HBO session was stopped in 9 patients (7 because of anxiety, 1 due to tympanic membrane rupture, and 1 due to cough). The second or third session was omitted in 4 patients due to middle ear pressure. Data were analyzed using the intent to treat principle. Testing at 6 weeks showed that 25% of the HBO patients had cognitive sequelae compared to 46% of the normobaric group (P=.007). Patients in the HBO group continued to have lower rates of impairment at 6 months (21% versus 38%, P=.02) and 12 months (18% versus 33%, P=.04). The frequency of cognitive sequelae did not differ between patients who completed 3 sessions (24%) from those who did not (28%). The authors concluded that 3 sessions of HBO within the first 24 hours after carbon monoxide poisoning reduced cognitive deficits at both 6 weeks and 12 months after injury.In this study, patients were transported over a 3-state region and several hours had typically elapsed prior to treatment. Treatment with HBO occurred after COHb levels were reduced by treatment with conventional oxygen administration. The half-life of COHb is approximately 5 hours and 20 minutes at room air. This is reduced to 90 minutes with FiO2 of 1.0 at 1 atmosphere, and to 23 minutes at 3 atm.1 Toxic symptoms typically occur when COHb levels are 15–20%.In addition to causing tissue hypoxia, carbon monoxide reacts with reduced metallic ions and forms weak associations with proteins. HBO therapy has been shown to decrease lipid peroxidation, neutrophil adherence, and CO binding to cytochromes resulting in improved brain metabolism.2 Hypoxia is not the sole mechanism by which carbon monoxide causes brain injury and HbCO levels correlate poorly with neurologic injury. Carbon monoxide toxicity has 2 patterns. Patients can present with acute brain and heart injuries and/or with a delayed neurological syndrome that manifests itself in approximately 15–50% of severely poisoned patients between 2 to 28 days after exposure. Age and loss of consciousness have been identified as risk factors for delayed injury.3Complications of HBO therapy include bronchospasm, seizures, progression of untreated pneumothorax, pain and/ or rupture of tympanic membranes, anxiety, and need for patient transport to centers that can offer HBO.4 Delivery of HBO to critically ill patients is difficult. Chambers are monoplace or multi-place. Care providers are pressurized with a patient in a multi-place chamber but the monitoring devices are located outside the chamber with cables that penetrate the chamber hull. Providers are not immediately available to patients in a monoplace chamber. Critically ill patients with COHb and shock from burns may require frequent interventions and be too unstable for HBO.Although children were not included in the above study, children exposed to carbon monoxide may also benefit from HBO treatment; infants may require placement of myringotomy tubes.4 Primary care providers need to be aware of the signs and symptoms of CO poisoning and of the nearest HBO facilities in their region.5It is interesting to note that although patients had near normal carboxyhemoglobin concentrations prior to getting HBO therapy, cognitive impairment occurred nonetheless —suggesting a mechanism other than hypoxia for the cause of this impairment.