Influence of autoregulation and capacitance on diastolic coronary artery pressure-flow relationships in the dog.

We studied the influence of initial coronary artery pressure on pressure-flow relationships during long diastoles in 22 closed-chest, anesthetized dogs under basal conditions and after maximum coronary vasodilation.The circumflex artery was perfused from a pressurized arterial reservoir through a cannula inserted in the carotid artery.Diastolic pressure-flow relations were obtained under two conditions: (1) when pressure was gradually decreasing (40 mm Hg/sec); and(2) when pressure and flow were constant beginning 500 msec after a rapid step decrease in pressure.Pressure-flow curves obtained with pressure gradually decreasing were linear in the basal state and during maximum coronary vasodilation r = 0.98 ± 0.01 (SE).In autoregulating animals, 25 mm Hg increments in coronary artery pressure from 75 to 125 mm Hg resulted in progressive decreases in the slope (2.2 ± 0.1 to 1.2 ± 0.1 ml/min per 100 g per mm Hg, P < 0.01) and increases in the zero flow pressure intercept (37.3 ± 1.4 to 51.2 ± 2.1 mm Hg, P < 0.01) of the pressure-flow curves.These changes were not observed when autoregulation was abolished with intracoronary adenosine.Diastolic pressure-flow curves obtained under constant pressure conditions were also linear in the basal state and during maximum coronary vasodilation (r = 0.98 ± 0.01).Increasing initial coronary artery pressure from 75 to 125 mm Hg resulted in a decrease in the slope (1.6 ± 0.1 to 0.9 ± 0.0 ml/ min per 100 g per mm Hg, P < 0.01) and an increase in the zero flow pressure intercept (21.1 ± 2.1 to 37.2 ± 2.3 mm Hg, P < 0.01) of the pressure-flow curves.With maximum coronary vasodilation, the slope increased to 4.7 ± 0.1 ml/min per 100 g per mm Hg (P < 0.001) and the zero flow pressure intercept decreased to 15.2 ± 0.7 mm Hg (P < 0.001).When pressure-flow curves obtained with pressure gradually decreasing were compared to ones obtained during constant pressure perfusion, the constant pressure values for slope averaged 10-25% lower (P < 0.01), whereas values for the zero flow pressure intercept were 8-12 mm Hg lower (P < 0.01).These data indicate that diastolic coronary artery pressure-flow relations obtained when pressure is continuously changing are influenced by coronary vascular capacitance.When capacitance effects were minimized, the slope and zero flow pressure intercept of the diastolic pressure-flow curves were dependent on coronary vasomotor tone, with the zero flow pressure intercept ranging from 15 to 37 mm Hg.These data are consistent with a vascular waterfall model of diastolic flow regulation.Based on such a model, we estimate that changes in coronary vascular resistance and zero flow pressure intercept can account for 70-80% and 20-30%, respectively, of the coronary autoregulation observed over the pressure range of 75 to 125 mm Hg.