Basic Science and Pathophysiology

The mechanism of CO poisoning is not completely understood. CO binds to haemoglobin (Hb) with an affinity of approximately 220 times that of oxygen. CO shifts the oxygen dissociation curve to the left. It also binds to myoglobin and mitochondrial cytochrome oxidase, impairing adenosine triphosphate production.

CO poisoning causes platelet and neutrophil activation, free radical formation and lipid peroxidation in brain and other tissues through an immunological mechanism. The importance of these non-hemoglobin-mediated effects has been best documented in the heart, where mitochondrial dysfunction due to CO can produce myocardial stunning despite adequate oxygen delivery [7].

CO binds to fetal haemoglobin and shifts the already left-shifted fetal oxyhaemoglobin dissociation curve further to the left. The half-life of CO in the fetus is longer than that in the mother and is of particular concern when considering treatment options.

CO is produced continuously in the body as a by-product of haem breakdown. This leads to a normal baseline COHb concentration of about 0.5%. In pregnancy and especially in haemolytic anaemias this can rise towards 5%. Cigarette smoking leads to COHb concentrations of up to about 12% in heavy smokers [2].

Administration of oxygen speeds the elimination of CO from the body. Without therapy, the elimination half life of CO is 4-6 hours. Administration of high flow oxygen by a tightfitting mask at normal atmospheric pressure reduces half life to approximately 76 mins. In hyperbaric oxygen chamber at 2.5 atmosphere absolute pressure, the elimination half life is further decreased to 20 mins.