CLINICAL RESPIRATORY CARE
CARBON MONOXIDE POISONING by Bill Wojciechowski, MS, RRT
C
arbon monoxide (CO) is a toxic gas that is invisible, odorless, colorless, non-irritating, and tasteless. CO is a by-product of in-
complete combustion of carbon-containing materials. With CO being as old as fire itself, one can only imagine the number of cases of CO poisoning that would have been seen in the emergency de- partment at "Homo Erectus Community Hospital" had it existed 700 to 800 thousand years ago when the cave dwelling Peking Man began using fire for cooking and cave warming! Ironically, CO is manufactured in the body as a result of the
breakdown of hemoglobin. Consequently, normal people may have a COHb concentration of 0.5% in their blood. Smokers often have COHb levels of 10% or higher.
Historical Background Not until coal was used as a domestic fuel did CO poisoning
become a recognized medical problem. Homes and industry re- lied on gas from coal and oil for heating and lighting. Edgar Allen Poe (1809-1849) unwittingly wrote about his symptoms of CO poisoning from gas lighting in many of his short stories. In his fit- tingly titled story, "Loss of Breath," Poe wrote, "The sensations of my illness were much like those of a man upon the gallows: I heard my heart beating with violence - the veins in my hands and wrists were swelling to nearly bursting – my temple throbbed tem- pestuously - and I felt my eyes were starting from my sockets... Confusion crowded upon confusion like a wave upon a wave." Poe related numerous other symptoms in his writings. After Poe’s death on October 7, 1849, the Baltimore health commissioner listed the cause of death as "congestion of the brain." In 1857, in France, a French physiologist named Claude
Bernard carried out an animal experiment and noticed how the animal’s venous blood was as cherry red as blood in arterial cir- culation. Still puzzled as to what CO did to kill the animal, Bernard took a sample of arterial blood from a healthy animal, placed this blood beneath a layer of liquid mercury in a test tube containing CO gas. Maintaining an air-tight seal, Bernard shook the tube to achieve contact between the blood and the CO. He then analyzed the gas in the test tube and found it was oxygen- enriched. Bernard concluded that the CO displaced the O2 in the blood and that the CO "remained fixed in the blood corpuscles," causing the demise of the animal.
Epidemiology CO is the leading cause of morbidity and mortality by poi-
soning in the United States. Because many cases of CO poisoning
go undetected and unreported, the actual incidence of this poi- soning is unknown. Estimates are that approximately one-third of all cases are undiagnosed because CO poisoning mimics a variety of systemic diseases. Common sources of CO are wood and gas burning fire-
places, internal combustion engine exhausts, gas and oil fur- naces, space heaters, coal and wood stoves, electric ovens in the self-cleaning mode, and tobacco smoking.
Pathophysiology CO severely interferes with tissue oxygenation. Both O2
and CO bind reversibly with hemoglobin. These two gases compete for binding with the four hemes in the hemoglobin molecule. However, the magnitude of hemoglobin’s affinity for CO is 210 to 240 times greater than for O2. Additionally, the binding of CO to hemoglobin to form carboxyhemoglobin (HbCO) causes conformational changes to hemoglobin’s tetrameric structure, thereby increasing the affinity of the re- maining heme groups for O2. In other words, when a CO mol- ecule binds with one of the four hemes on a hemoglobin molecule, the other three hemes cling to O2 more avidly. The result is a leftward shift of the oxyhemoglobin dissociation curve, and a compromised tissue O2 delivery. The greater affin- ity of hemoglobin for CO accounts for the fact that only a small amount of inspired CO causes a large COHb level in the blood. For example, inhalation of 0.1% CO produces a COHb level of 10%. Myoglobin is another heme-containing molecule that
binds avidly with CO. Skeletal and cardiac muscle are ad- versely affected by this binding. Carboxymyoglobin poses a se- rious threat to a CO victim because the myocardium can be severely deprived of oxygenation. The associated cardiac is- chemia can cause lethal dysrythmias. The binding of CO to car- diac myoglobin also has a depressant effect on the heart, producing hypotension. Both of these effects exacerbate the de- creased tissue O2 delivery. Interestingly, CO dissociates from myoglobin much more slowly than CO does from hemoglobin. Consequently, as CO leaves myoglobin, CO re-enters the blood where the COHb level rises again. Outside the vasculature CO binds with cytochrome A3, an enzyme involved in the electron transport chain within the process of oxidative phosphorylation. Oxidative phosphorylation involves the production of ATP from ADP and phosphate by harnessing the energy released as elec- trons are transferred during a series of oxi-
continued on page 33 Focus Journal Winter 2012 9
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