Carboxyhemoglobin

Carboxyhemoglobin or carboxyhaemoglobin (symbol COHb or HbCO) is a stable complex of carbon monoxide and hemoglobin (Hb) that forms in red blood cells upon contact with carbon monoxide (CO). Exposure to small concentrations of CO hinder the ability of Hb to deliver oxygen to the body, because carboxyhemoglobin forms more readily than does oxyhemoglobin (HbO₂). CO is produced in normal metabolism and is also a common chemical. Tobacco smoking (through carbon monoxide inhalation) raises the blood levels of COHb by a factor of several times from its normal concentrations.

Hemoglobin contains four heme groups each capable of reversibly binding to one oxygen molecule. Oxygen binding to any of these sites causes a conformational change in the protein, facilitating binding to each of the other sites. Carbon monoxide binds to hemoglobin at the same sites as oxygen, but approximately 200 times more tightly. Normally, oxygen would bind to hemoglobin in the lungs and be released in areas with low oxygen partial pressure (e.g. active muscles). When carbon monoxide binds to hemoglobin, it cannot be released as easily as oxygen. The slow release rate of carbon monoxide causes an accumulation of CO-bound hemoglobin molecules as exposure to carbon monoxide continues. Because of this, fewer hemoglobin particles are available to bind and deliver oxygen, thus causing the gradual suffocation associated with carbon monoxide poisoning.

Since COHb releases carbon monoxide slowly, less hemoglobin will be available to transport oxygen from the lungs to the rest of the body. Conversion of most Hb to COHb results in death - known medically as carboxyhemoglobinemia or carbon monoxide poisoning. Smaller amounts COHb lead to oxygen deprivation of the body causing tiredness, dizziness, and unconsciousness.

COHb has a half-life in the blood of 4 to 6 hours. This time can be reduced to 70 to 35 minutes with administration of pure oxygen (the lower number applying when oxygen is administered with 4 to 5% CO2 to cause hyperventilation). Additionally, treatment in a Hyperbaric Chamber is a more effective manner of reducing the half-life of COHb than administering oxygen alone. This treatment involves pressurizing the chamber with pure oxygen at an absolute pressure close to three atmospheres, allowing the body's fluids, instead of the crippled hemoglobin bonded to CO, to absorb oxygen and to pass free oxygen on to hypoxic tissues. In effect, the need for hemoglobin in the blood is (partially) bypassed.

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