CHARACTERIZATION OF THE EXTRA-VASCULAR COMPONENT OF CORONARY RESISTANCE BY INSTANTANEOUS PRESSURE-FLOW RELATIONSHIPS IN THE DOG

To investigate the mechanical effects of the myocardium on the blood perfusion of the canine left ventricle, phasic total left ventricular (LV) coronary blood flow, perfusion pressure, LV pressure, aortic flow rate, and LV segment length were recorded continuously in an open-chested dog heart preparation. These variables were analyzed on a digital computer and time synchronized so that coronary pressure-flow curves could be drawn for various instants in the cardiac cycle. During diastole, the pressure-flow relationship is linear, changing to a nonlinear curve with the onset of systole. To estimate phasic patterns of coronary resistance and intramyocardial pressure (IMP), a model based on the vascular waterfall mechanism was developed and fitted to the experimental data. The results of this operation show inferred coronary resistance patterns that increase during ejection and remain constant during diastole and isovolumic contraction. Assuming LV pressure to represent endocardial IMP, the estimated epicardial IMP signal averages 42.1 ± 13.3% of peak LV pressure at this instant of peak pressure. Furthermore, increases in end-diastolic volume reduced the changes in inferred coronary resistance taking place during ejection, but the epicardial IMP signal remained practically unchanged.