Abstract Body (Do not enter title and authors here): Introduction: The hyperemic longitudinal myocardial blood flow (MBF) gradient (i.e., hyperemic MBF from the base to the apex of the left ventricle) has been proposed as an emerging non-invasive index to identify hemodynamically significant epicardial coronary artery disease (CAD). However, whether the utility of this parameter is influenced by the magnitude of the hyperemic stimulus has not been studied. Accordingly, we measured the longitudinal MBF gradient at rest and during submaximal and maximal adenosine (ADO)-induced hyperemia in swine with a fixed epicardial coronary stenosis.

Methods: Swine (n=11) were chronically instrumented with a fixed LAD stenosis (89±3%). Regional MBF was assessed with fluorescent microspheres at rest and during submaximal (150 or 300 µg/kg/min) and maximal (900 µg/kg/min) ADO vasodilation, with concomitant phenylephrine (8±3 µg/kg/min) during maximal ADO to avoid hypotension. Coronary flow reserve (CFR) was calculated as the quotient of MBF during ADO divided by MBF at rest and the longitudinal MBF gradient was calculated as the difference in absolute MBF between the base and apex of full-thickness myocardium in LAD (ischemic) and PDA (non-ischemic) perfusion territories.

Results: ADO elicited a dose-dependent increase in MBF that was blunted in the LAD perfusion territory vs. the PDA perfusion territory, such that a dose of at least 300 µg/kg/min was necessary to identify a significant regional difference in CFR (Figure A). The longitudinal MBF gradient was negligible in the normally perfused region of the left ventricle at rest and across all ADO doses. In contrast, an abnormal decrease in the longitudinal MBF gradient was evident In the LAD perfusion territory, but only when ADO was given at a dose of at least 300 µg/kg/min (Figure B).

Conclusion: These results demonstrate that a proximal LAD stenosis leads to an abnormal decrease in the base-to-apex perfusion gradient during ADO hyperemia in swine. However, the reduction in the longitudinal MBF gradient is not evident at relatively low doses of ADO, suggesting that the diagnostic utility of this parameter is dependent on the magnitude of the hyperemic stimulus.