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Carbon Monoxide Poisoning
Dr. Thom is Professor of Emergency Medicine and Chief, Hyperbaric Medicine, University of Pennsylvania School of Medicine, Philadelphia, PA. Within the past 12 months, Dr. Thom reports no commercial conflicts of interest.
It's a deadly, odorless, colorless and tasteless gas we live with every day. You could be dying from it and never know something was wrong. Carbon monoxide is a by-product of the combustion from wood stoves, heaters, automobiles. It kills tens of thousands of people every year and is misdiagnosed and mistreated in thousands of others.
Much more common than most people believe, carbon monoxide poisoning occurs when carbon monoxide (CO) gas is inhaled in unventilated conditions.
Many poisonings could be prevented if people better understood the dangers of CO. Worldwide, CO may be responsible for over half of all fatal poisonings.(1)(2)(3) Often it occurs from cooking or operating combustion engines indoors or leaking heating systems. It is estimated that CO poisoning is the third leading cause of unintentional deaths in the United States and that the incidence of nonfatal poisonings varies from 15,000 to 40,000 cases per year.(4)(5)(6)(7) Because misdiagnosis of CO poisoning is common, however, the real incidence is probably much higher.(8)(9)
How CO Damages the BodyWhen you inhale CO, it enters the bloodstream and takes the place of oxygen by binding with hemoglobin, turning this life-sustaining molecule into a killer: a compound called carboxyhemoglobin (COHb). Hemoglobin's normal role is to bind with oxygen and carry it to all parts of the body. Unfortunately CO binds to hemoglobin several hundred times more strongly than oxygen does; this means that carboxyhemoglobin tends to remain in the body for a long time, where it leads to oxygen starvation. The only way to clear it from the blood is lengthy exposure to fresh air or pure oxygen.
Even worse, because the brain regulates breathing based on the levels of carbon dioxide in the blood rather than on the amount of oxygen in the blood, a CO poisoning victim can easily lose consciousness without realizing that anything is wrong because the brain just doesn't register that oxygen levels in the blood and in the body are falling to dangerous levels.
Five basic processes occur simultaneously within the body that contribute to CO poisoning:
Impaired Delivery and Use of OxygenAs described above, CO binds to hemoglobin. The resulting formation of carboxyhemoglobin (COHb) is a well understood effect of CO exposure.(10) CO binding with hemoglobin interferes with the delivery of oxygen to bodily tissues.(11)
Levels of carbon monoxide in the blood can be determined by a simple blood test to measure carboxyhemoglobin. Carbon monoxide is produced naturally in the body and also functions as a neurotransmitter. Normal carboxyhemoglobin levels in an average person are less than 5%; heavy cigarette smokers may have levels as high as 9%.
Serious health problems tend to be seen when carboxyhemoglobin levels rise above 25%, and a serious risk of death with levels over 70%. Still, the relationship between carboxyhemoglobin levels and its toxic effects vary quite a bit from person to person. For this reason, carboxyhemoglobin levels tell us more about the degree of exposure than they do about how a poisoning victim will do in the short or long term.(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)
Disrupted O2-CO-Nitric Oxide BalanceOur bodies normally use oxygen, carbon monoxide and nitric oxide. The body makes use of all three of these gases in a fixed relationship with each other. When there are elevated levels of CO, this equilibrium is disturbed and the extra nitric oxide that often results can cause further toxic effects.(22)(23)(24)
Damage to Blood VesselsCO poisoning also causes deposits along vascular walls, causing damage to the lining of the blood vessels.(25)(26)(27)(28)(29)(30)(31)(32)
Damage to the Nervous SystemElevations in neurotransmitters initiating nervous system activity occur in the brain during CO poisoning.(33)(34)(35) This could be caused by any (or all) of the three components of CO poisoning listed above.(36) This elevated activity can damage nerve cells and is the same sort of mechanism that is implicated in stroke, traumatic brain injury and diseases of the central nervous system (CNS) such as Lou Gehrig's Disease and amyotrophic lateral sclerosis (ALS).
Immune System Responses During RecoveryOther complications from CO poisoning can happen during treatment and recovery. The damage to blood vessels combined with the other components above damage myelin, an important protein coating neurons that helps transmit messages more quickly within the nervous system and brain. The damage to brain tissues arouses the immune system, which responds by causing brain inflammation, and which, in turn, causes brain damage. This kind of brain damage usually happens during the recovery period, causing memory and learning problems and movement disorders, and may not become apparent until days or weeks after the poisoning incident.
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