The diastolic hyperemic flow versus pressure relation. A new index of coronary stenosis severity and flow reserve.

The measurement of coronary flow reserve, traditionally calculated as the ratio of maximal hyperemic blood flow divided by basal flow, is difficult to interpret in serial studies because fluctuating hemodynamic parameters may affect either basal or hyperemic flow measurements. To determine the magnitude of this problem and to develop alternative approaches for measuring vascular reserve, 10 anesthetized dogs were instrumented with aortic and inferior vena cava occluders, electromagnetic coronary flow probes, and high-fidelity micromanometers in the left ventricle and aortic root. Coronary flow was measured in the basal state and during maximal hyperemia induced by a steady-state adenosine infusion. Observations were made in the absence of a stenosis and in the presence of two incremental degrees of subcritical stenosis produced by a rigid, external screw occluder. Several parameters of vascular reserve were determined: 1) coronary flow reserve (defined above), 2) mean hyperemic flow divided by mean aortic pressure, 3) mean hyperemic flow divided by the difference between mean aortic pressure and left ventricular end-diastolic pressure, and 4) the slope of the instantaneous relation between diastolic hyperemic flow versus pressure. Each parameter was measured during five steady-state pressure levels achieved by partial occlusion of either the inferior vena cava or the aorta and the levels ranged from 82 +/- 8 mm Hg (mean +/- SD) to 127 +/- 9 mm Hg during hyperemia. All measures of vascular reserve were found to be dependent on hemodynamic parameters such as heart rate and mean aortic pressure. The slope of the instantaneous relation between diastolic hyperemic flow and pressure, however, showed only minimal dependence on heart rate and, in contrast to coronary flow reserve measurements, distinguished between the normal and the two stenotic states. Further, this optimal performance of the hyperemic flow versus pressure slope index was shown in a model in which coronary flow and myocardial work were not independently controlled. This index provides a sensitive and reliable indication of subcritical stenosis severity that may have clinical applications.