Intraoperative assessment of regional myocardial perfusion using quantitative myocardial contrast echocardiography: An experimental evaluation

To test the hypothesis that myocardial contrast echocardiography can be used to quantitate regional myocardial flow in the arrested heart at the time of delivery of cardioplegic solution, data were acquired in 13 dogs on cardiopulmonary bypass. Different degrees of stenosis were placed in random order on the left anterior descending coronary artery. For each stenosis, myocardial contrast echocardiography was performed by injecting sonicated albumin microbubbles into the cross-clamped aortic root at the time of delivery of cardioplegic solution. The resultant echocardiographic images were analyzed on an off-line computer. Background-subtracted time-intensity plots were generated, and an exponential function, f(t) = Ce^-αt + De^-βt, was applied to each plot. Variables that reflected the total number of microbubbles entering the coronary artery bed, such as the area under the curve and the peak height of the curve, correlated best with radiolabeled microsphere-measured myocardial flow (r = 0.92 and r = 0.91, respectively). Variables that reflected the appearance of contrast microbubbles in the myocardium, such as the initial slope and the slope at 1 s, also had a good correlation with myocardial flow (r = 0.84 and r = 0.89, respectively). Variables that reflected the washout of contrast medium from the myocardium, such as the slope of the descending portion of the curve, had only a fair correlation with myocardial flow (r = 0.65). In six dogs, the technique of injecting contrast medium into the cross-clamped aortic root was also examined. Although continuous infusion of contrast medium produced smaller perturbations in mean aortic and distal left anterior descending artery pressures compared with a bolus injection (p < 0.01), the correlation between the variables of the time-intensity curves and flow was equally close with both techniques. It is concluded that it is possible to quantitate myocardial flow by using myocardial contrast echocardiography at the time of delivery of cardioplegic solution in dogs on cardiopulmonary bypass. The implementation of this technique in humans might be useful in guiding the sequence of graft placement and thereby improving myocardial preservation during coronary artery bypass operations.