CHANGES IN MYOCARDIAL BLOOD-VOLUME WITH GRADED CORONARY STENOSIS

Vasodilation of microvessels distal to a stenosis results in an increa se in myocardial blood volume (MBV). The purpose of this study was to examine the changes in MBV induced by graded coronary artery stenoses by using myocardial. contrast echocardiography (MCE). Accordingly, 21 dogs underwent progressive stenosis of a coronary artery in a random o rder, the severity of which was judged by the pressure distal to it. T otal myocardial blood flow (MBF) to the bed distal to the artery (both anterograde and collateral) was measured by injection of radiolabeled microspheres into the left atrium. In seven dogs, anterograde and tot al MBF were measured at each stenosis stage by injection of different microspheres into the left atrium and directly into the coronary arter y, respectively. MBV was calculated by dividing MBF by the mean transi t rate of microbubbles injected directly into the coronary artery duri ng MCE. The perfusion bed size of the artery was also measured by MCE. Our major findings are as follows: 1) there is a nonlinear increase i n MBV with increasing degrees of coronary stenosis until the coronary stenosis becomes critical; 2) at moderate levels of coronary stenosis, MBV remains constant despite ongoing autoregulation because of reduct ion in the size of the perfusion bed supplied by the stenotic vessel; and 3) after exhaustion of autoregulation, a decrease in MBV is noted with increasing levels of stenosis. We conclude that assessment of MBV provides insights into myocardial perfusion distal to a coronary sten osis above and beyond that provided by the measurement of MBF alone.