Regulation of coronary blood flow during individual diastoles in the dog.

Data derived from instantaneous diastolic coronary artery pressure-flow relationships in the dog have suggested that diastolic coronary vascular resistance may be constant and coronary vasoregulation unimportant during individual diastoles. We tested these hypotheses by using constant pressure perfusion to determine the pattern of coronary blood flow during single diastoles under steady state conditions and when coronary vascular resistance was actively changing. In 20 closed-chest, anesthetized dogs, long diastoles of constant cycle length were obtained by vagal nerve stimulation during simultaneous right ventricular pacing. Cholinergic vasodilation was prevented with intracoronary atropine. There were no differences between early and late diastolic flows (P > 0.8) for cycle lengths of 0.5 to 5 seconds and flows ranging from 40 to 125 ml/min per 100 g. When coronary resistance was actively altered with adenosine, brief coronary inflow occlusions, or abrupt changes in heart rate, diastolic flow varied continuously during each beat (13.0 ± 1.1 (SD) to 33.4 ± 5.4 ml/min per 100 g per sec). In 10 additional animals, instantaneous diastolic coronary artery pressure-flow relations obtained with initial diastolic resistance constant were compared with those obtained when resistance was actively changing. Despite a changing diastolic resistance, the resulting pressure-flow curves remained linear. Values for slope and zero flow pressure were increased during vasoconstriction (P < 0.001) and decreased during vasodilation (P < 0.001). We conclude that, during constant pressure perfusion, coronary vascular resistance is constant during diastole, and when coronary resistance is actively changing, regulation of coronary blood flow occurs continuously during individual diastoles. Furthermore, instantaneous diastolic coronary artery pressure-flow relations may be linear despite changes in coronary vasomotor tone, and thus cannot be used to estimate resistance or back pressure in such cases.