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

Abstract

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%, by 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 μm 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 μg·kg⁻¹·min¹) 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.