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01-10-11 Are we going to discuss CO exposure again?

Question of the Week
January 10, 2011
Are we going to discuss CO exposure again?
Yes we are.  This topic was requested by one of our readers and this topic is important enough to discuss every year.  This exposure causes unrecognized morbidity and mortality and is one of the most litigated exposures. Never underestimate the impact of CO on your patient or you. 
Unfortunately, CO has multiple mechanisms of toxicity and does more than occupy a place on the hemoglobin molecule (carboxyhemoglobin) and create a functional anemia.  CO shifts the oxyhemoglobin dissociation curve to the left, so that hemoglobin holds onto oxygen much tighter, reducing the amount of oxygen released to the tissues.  CO also causes a malfunction of cytochrome oxidase (oxidizing enzyme in mitochondria which transfers electrons from cytochromes to oxygen molecule) which may last for up to three days after a single exposure. The organ systems most impacted by CO exposure are the heart and nervous system.
The symptoms of CO poisoning are nonspecific and frequently described as “flu-like”.  Headache, myalgia, and dizziness may be initial symptoms. Cerebral impairment, confusion, coma and death may subsequently occur. One of the consequences of exposure to CO may be delayed (or persistent) neuropsychological (NP)  sequelae which includes memory and concentration impairment, loss of executive function, decrease in IQ, personality changes, neurologic deficits (such as incontinence) and symptoms of Parkinsons syndrome.  The young and the old are the most vulnerable. The memory and concentration deficits, as well as impairment of executive function are diagnosed by formal NP testing done by a neuropsychologist.  Bedside tests are not sensitive enough to identify these cognitive losses.
The problem is neither the carboxyhemoglobin (COhgb) concentration nor the clinical presentation at the time of evaluation predicts who will develop this sequelae. Not only are we unable to predict who will prevent NP sequelae, but we don’t know if any treatment will prevent or ameliorate the sequelae.
Studies are conflicting regarding the efficacy of hyperbaric oxygen (HBO) and/or normobaric oxygen in preventing or improving this syndrome. There is the possibility that HBO may increase CNS free radicals-although the clinical implications of this are unknown.  There are no conclusive studies.  There is agreement that HBO is a reasonable treatment for pregnant women as fetal hemoglobin has a higher affinity for CO than adult hemoglobin.  Tennessee Poison Center recommends HBO for pregnant women. For other significant exposures, TPC usually recommends 100% oxygen via face mask for at least 12 hours, sometimes longer.   Patients with more severe exposures are admitted for 24 hours of 100% oxygen.  Most patients are referred for NP testing and then followed up by a toxicologist.
Recommendation:  In cases of CO exposure, call the Poison Center, document that you called the PC and their recommendations. These recommendations vary between poison centers.  Know what your PC recommends.  AND tell all your patients to have a CO detector in their house. Prevention of the exposure is easier than treatment of the sequelae.
This question prepared by: Donna Seger, MD   Medical Toxicologist