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Instantaneous and steady-state pressure-flow relations of the coronary system in the canine beating heart
Abstract:
Steady-state and instantaneous pressure-flow relations were both obtained from the pump-perfused left coronary bed of the beating heart in seven mongrel dogs. The steady-state pressure-flow relation was obtained by changing flow, and measuring pressure after it reached a steady level; it showed a sigmoid shape, with flow-regulation around 70 ml . min-1 . 100 g-1, and it had an average zero-flow pressure intercept of 1.9 kPa (14 mmHg). This curve was represented by an equation, using four parameters. The quality of regulation of the coronary bed could be quantified with this equation by determining the pressure range, when flow was changed from 25% below to 25% above control level. We found this pressure range to be 8.7 +/- 2.4 kPa (65 +/- 18 mmHg) on the average. The tangent at each point of steady-state pressure-flow relation was called differential resistance. Instantaneous pressure-flow relations were obtained by superimposing stepwise changes of flow of different amplitude, at several steady-state levels of flow. Pressure followed these steps with a time-constant of 0.3 +/- 0.1 s, due to capacitive effects, then remained constant during 3 to 4 s, and thereafter changed due to regulation. Pressure was measured during the plateau, assuming it to be a regulation-free period. The instantaneous pressure-flow relations were found to be linear, and the slope was called instantaneous resistance. In the physiological range of flows, instantaneous resistance increased with flow. The ratio between instantaneous and differential resistance, the regulatory index, is suggested to quantify regulation at each point of the steady-state curve. This index was between one and zero up to the upper limit of the regulatory range; at higher flows it was negative. In the maximally vasodilated bed the instantaneous pressure-flow relations fell along the steady-state relation, and the regulatory index was thus equal to zero at all flow-levels.