Myocardial and systemic vascular responses to low concentration of carboxyhemoglobin.

Carboxyhemoglobin alters the oxyhemoglobin dissociation curve in such a manner that oxygen is released to the tissues with great difficulty and at a lower oxygen tension. The known effects on heart and brain of breathing low concentrations of carbon monoxide are primarily related to this leftward shift and perhaps also to its combination with myoglobin and certain iron-contain­ ing enzymes. Hemoglobin-oxygen equilibria in the presence of carboxyhemo­ globin resemble the equilibria of more primitive forms of hemoglobin and give rise to the suggestion that this decrease in the access to oxygen is a form of counter-evolution. The human toxicology of carbon monox­ ide was first studied in detail by Haldane in 1895.14 A series of courageous experi­ ments performed on himself led to the con­ clusion that the symptoms of carbon mon­ oxide intoxication closely resembled the effects produced by exposure to decreased concentrations of oxygen. Although he did not observe serious symptoms at rest until his hemoglobin was at least one third sat­ urated with carbon monoxide, exertion pro­ duced mild dyspnea and palpitations when as little as 14 percent carboxyhemoglobin (COHb) was present. Recently presented data from our laboratory suggest that COHb concentrations as low as 5 percent may interfere with oxygen delivery to the diseased myocardium. 5 More recent studies have confirmed this and demonstrated that low concentrations of carbon monoxide may provoke electrocardiographic abnor­ malities in patients with coronary artery disease. 1 T heoretical Considerations The respiratory characteristics of hemo­ globin may be defined by ( 1 ) the oxygen capacity and ( 2 ) the shape of the dissocia­ tion curve. Since one gram of hemoglobin will reversibly bind 1.34 ml of oxygen, the oxygen capacity is closely related to hemo­ globin concentration. Invertebrates have oxygen capacities ranging from one to two ml per 1 0 0 ml of blood; fish, amphibia and reptiles carry between 5 and 15 ml per 100 ml of blood while mammals carry between 15 and 20 ml per 100 ml of blood. The capacity may be as high as 40 ml per 100 ml of blood in certain diving vertebrates. Equal in importance to the oxygen ca­ pacity of hemoglobin is the affinity of he­ moglobin for oxygen. Many attempts to