Instantaneous hyperemic flow-versus-pressure slope index. Microsphere validation of an alternative to measures of coronary reserve.

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Abstract
BACKGROUND: The instantaneous hyperemic flow-versus-pressure (i-HFVP) slope index is a new method of assessing maximal coronary conductance and can be used as an alternative to conventional measures of coronary reserve. The i-HFVP slope index is determined by measuring the slope of the linear diastolic segment of the relation between instantaneous aortic pressure and hyperemic coronary flow. METHODS AND RESULTS: To validate the i-HFVP slope index as a measure of maximal coronary conductance, we compared this method with a microsphere-derived measurement of maximal coronary conductance (m-HFVP slope index) by determining the slope of the least-squares regression line of the data points for coronary flow during maximal hyperemia and four or five steady-state alterations of aortic pressure in 43 dogs (open-chest, anesthetized preparations) with or without coronary stenoses. The i-HFVP slope index demonstrated no dependence on heart rate, left ventricular end-diastolic pressure, or mean aortic pressure and was highly reproducible within the groups studied (intraclass correlation coefficient, 0.86 for normal arteries, 0.87 for stenotic arteries, and 0.93 for combined groups; for all coefficients, p less than 0.001). The i-HFVP slope index was significantly decreased in the presence of a stenosis (10.3 +/- 3.9 for normal arteries versus 3.6 +/- 1.6 for stenotic arteries, p less than 0.001) as was the transmural m-HFVP slope index (8.9 +/- 4.6 for normal arteries versus 5.3 +/- 3.1, p less than 0.01). Of special importance, the i-HFVP slope index measurement for normal arteries was not significantly different from the transmural and subendocardial m-HFVP slope index measurements (10.3 +/- 3.9 versus 8.9 +/- 4.6 and 9.2 +/- 5.7, respectively). For stenotic arteries, the i-HFVP slope index measurement was also not significantly different from the transmural and subendocardial m-HFVP slope index measurements (3.6 +/- 1.6 versus 5.3 +/- 3.1 and 4.1 +/- 2.3, respectively). The i-HFVP slope index correlated best with subendocardial m-HFVP slope index measurements (correlation coefficient, 0.57; p less than 0.001). When the 95% confidence intervals for the transmural (or subendocardial) m-HFVP slope index in normal arteries were compared with the i-HFVP slope index values, the latter demonstrated a systematic trend to overestimate the m-HFVP slope index. In the presence of a stenosis, this effect was minimized, and the slope values were nearly identical. CONCLUSIONS: The i-HFVP slope index correlates most closely with subendocardial coronary conductance; the index is a hemodynamically independent measure of coronary reserve that is reproducible over a broad range of aortic pressures; and the methodology is applicable to an intact circulation in experimental preparations and may with future developments also prove useful in humans.

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