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

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Abstract
The influence of initial coronary artery pressure on pressure-flow relationships during long diastoles was studied 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 2 conditions: when pressure was gradually decreasing (40 mm Hg/s); and when pressure and flow were constant beginning 500 ms 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 min 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). Diastolic coronary artery pressure-flow relations obtained when pressure is continuously changing apparently 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. In such a model, 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 75-125 mm Hg.