Coronary pressure-flow relationships. Controversial issues and probable implications.

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Abstract
On the basis of the material discussed, our current assessments of the controversial points mentioned at the beginning of this article may be summarized as follows: Pf = 0, the minimum back pressure to coronary flow associated with a measurable conductance, is indeed greater than coronary outflow pressure (and usually left ventricular diastolic pressure, as well). Pf = 0 needs to be taken into account in attempts to determine coronary driving pressure. In maximally vasodilated beds, Pf = 0 derived from diastolic pressure-flow relationships exceeds coronary outflow pressure by at least a few mm Hg. Pf = 0 varies with coronary outflow and/or diastolic ventricular cavity pressure. When left ventricular preload is elevated, Pf = 0 exceeds outflow pressure by increasing amounts. Pf = 0 appears to be systematically higher and pressure-dependent in beds in which vasomotor tone is operative. An improved understanding of the nature of, and basis for, time-dependent changes in resistance and/or Pf = 0 during long diastoles in nonvasodilated beds is needed. The contour of pressure-flow relationships which are free of reactive effects is curvilinear rather than linear. The degree of curvilinearity is substantial and can change with interventions. Curvilinearity is accentuated at lower pressures and may reflect changes in the number of perfused vascular channels as well as the caliber of individual channels. Capacitive effects need to be dealt with quantitatively in studies of pressure-flow relationships. Values of the capacitance which is involved in these effects vary with both pressure and tone. Capacitive flow also depends upon the instantaneous rate of change of pressure, which has not usually been defined in published studies. Although intramyocardial capacitance is large and plays an important role in systolic-diastolic flow interactions, a controlling role in diastolic coronary arterial pressure-flow relationships has not been established experimentally. In vasodilated beds, in-flow remains remarkably constant for several seconds after the brief transient associated with a step-change in the level of constant pressure perfusion during a long diastole. Calculations of coronary vascular resistance (by whatever method) remain of limited value, particularly when changes in response to an intervention are modest. Because of the curvilinear diastolic pressure-flow relationship, resistance is pressure-dependent and, at any given pressure, is probably best defined by establishing the slope of a diastolic pressure-flow curve which is free of reactive effects.(ABSTRACT TRUNCATED AT 400 WORDS)