Carbon monoxide (CO) poisoning results in approximately 50,000 emergency room visits per year. Further, it is the leading cause of death due to poisoning. When CO binds to hemoglobin it causes toxicity by creating carboxyhemoglobin (COHb). CO binds hemoglobin with 200 times greater affinity than does oxygen. Therefore, CO poisoning reduces the oxygen carrying capacity of the blood.
If untreated, it results in numerous long term complications including intellectual impairment, parkinsonian symptoms, and seizure disorders. Mechanisms through which these symptoms arise include lipid peroxidation, direct binding of CO to intracellular proteins, and induction of apoptosis. These complications can begin to appear as quickly as 3 days after exposure, making prompt treatment essential.
The method of treatment, however, remains controversial. While the standard of treatment includes high flow 100% oxygen via mask or endotracheal tube until symptoms resolve, hyperbaric treatments, which include breathing 100% O2 at 2-3 atmospheres, may reduce long term neurologic sequelae.
Oxygen competitively displaces carbon monoxide from hemoglobin. When breathing room air, this may take up to 300 minutes. The displacement time is reduced to approximately 90 minutes with 100% oxygen. Hyperbaric therapy can further decrease the treatment time to 32 minutes. Furthermore, hyperbaric therapy may reduce lipid peroxidation and decrease long term neurological sequelae.
Critics of hyperbaric therapy point out that there is no standard protocol for hyperbaric dives, and therefore, effectiveness cannot be assessed. Hyperbaric therapy also poses increased risk of having to transport the patient to a hyperbaric center, hyperoxic seizures, and barotrauma. Critics also point out the difficult position providers are placed in should the patient decompensate or code while receiving therapy in a single chamber dive. Pressure must be decreased slowly to allow patients to equilibrate to normal air pressure delaying provider care.
A look at some of the evidence…or lack of evidence
Numerous trials have been conducted to better elucidate the effectiveness of hyperbaric treatments. In 2011, a Cochrane review looked at six randomized controlled trials (RCT) with a total of 1361 patients. Of the 6 trials, only 2 demonstrated a benefit from hyperbaric therapy with reduction of long term neurological symptoms at 1 month. Three trials showed no benefit between hyperbaric therapy and standard 100% oxygen therapy, and 1 trial found worse outcomes for patients who received hyperbaric therapy. The authors of this review concluded that there was insufficient evidence to fully support the use of hyperbaric therapy in carbon monoxide poisoning. They also noted the studies were flawed. The positive trials did not adjust for multiple hypotheses or were stopped early while the negative trials had limited power and poor follow up.
The 2008 ACEP Clinical Policy gives a level C recommendation that hyperbaric therapy is an option for carbon monoxide poisoning, however its use cannot be mandated. It states that there are no clinical variables to identify patients who are more likely to benefit from hyperbaric therapy. Conversely, the Undersea and Hyperbaric Medical Society advocates the use of hyperbaric therapy in the following conditions: loss of consciousness, abnormal neurological signs, cardiovascular dysfunction, severe acidosis, age >36, CO exposure >24 hours, or carboxyhemoglobin level >25%.
Pregnant patients are considered a special population for hyperbaric therapy due to increased concern for fetal distress and death after carbon monoxide poisoning. Steady state levels of COHb in the the fetus are higher than maternal blood. Thus, fetal elimination of carbon monoxide takes longer. Some experts recommend hyperbaric therapy for COHb level >15% or signs of fetal distress. Unfortunately, there are no adequate studies demonstrating its efficacy. Many studies have shown that pregnant women exposed to CO with normal mental status and no loss of consciousness have good outcomes in terms of normal delivery and development of the child.
Carbon monoxide poisoning is commonly seen in the emergency department and requires prompt treatment to prevent serious neurological complications. While hyperbaric therapy may have potential benefits, there is not enough evidence to fully support its use. Factors to consider with hyperbaric therapy include distance to closest hyperbaric chamber and stability of the patient. Given these factors, patients must be considered for hyperbaric therapy on a case by case basis.
Author: Hamil Patel, MD PGY1 @patel_hamil
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