Myocardial blood flow and O2-supply following dextran-haemodilution and methaemoglobinaemia in the dog.

The adequacy of oxygen-supply to the left ventricular myocardium was compared in anaesthetised dogs following: a) isovolaemic haemodilution with Dextran 70 (to haematocrit = 20 +/- 2%), b) the acute induction of methaemoglobinaemia (to methaemoglobin-concentration = 53 +/- 7%), bu subjecting them to an exchange-transfusion with methaemoglobin-containing erythrocytes in dextran) and c) a control exchange-transfusion with normal homologous erythrocytes in dextran. Myocardial oxygen-supply was assessed by the measurement of myocardial blood flow (using 15 +/- 5 micron microspheres) and of the PO2 in coronary sinus blood. In dextran-haemodiluted dogs, mean myocardial blood flow was 92% greater than in the control group, allowing the maintenance of normal coronary sinus PO2. In the methaemoglobinaemic group, flow was only 15% greater than in the control group; because oxygen-capacity was halved in this group, myocardial oxygen supply was impaired and coronary sinus PO2 fell. Infusions of isoproterenol (0.1 microgram . kg-1 . min-1) produced similar myocardial flow-increments in the haemodiluted and control groups with similar transmural distribution. In the methaemoglobinaemic group, however, the subendocardial flow-increment was reduced, in association with a further reduction in coronary sinus PO2 and an attenuated contractile response to the drug. These findings suggest that the rheological effects of haemodilution play an important role in the maintenance of myocardial oxygen-supply, but in significant methaemoglobinaemia, the oxygen supply of the myocardium may be severely curtailed.