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The purpoae of the present study was to develop a canine model of coronary artery bridging and assess transmural myocardial pcrfusion in this setting. Six adult mongrel dogs were chronically instrumented with aortic and left atrial pressure catheters, a right ventricular bipolar pacing electrode, and a left circumflex coronary artery electromagnetic flow probe and balloon occluder. Seven to 10 days postoperatively, the animals were studied in the awake state. Myocardial bridlng was modeled by totally occluding left circumflex coronary artery inflow for systole (S) only, systole plus one-sixth, one-third, and one-half of diastole (D) by means of an ECG-triggered solenoid valve, at a paced heart rate of 120 beats/min. Regional myocardial flow was determined during each intervention with 3- to 10-ftm radiolabeled microspheres. No change in systemic hemodynamics was noted during the course of any occlusion intervention. However, mean left circumflex coronary flow was significantly reduced during occlusion of S + 1/3 D and S + 1/2 D, and this was associated with decreased flow to the endocardial layers. Peripheral left circumflex coronary artery pressure was assessed in an additional series of five open-chest dogs identically instrumented with the addition of a coronary artery pressure catheter inserted distal to the balloon occluder. During the occlusion of S + 1/2 D, the peripheral coronary pressure markedly decreased to a minimum value of 26 ± 5 mm Hg and a mean value of 53 ± 10 mm Hg. Thus, during the inflow time allowed by this intervention, coronary perfusion pressure was reduced markedly. If this animal model of myocardial bridging is appropriate, these data suggest that, in the rare patient with symptomatic myocardial bridging, occlusion of a coronary artery during systole and a significant portion of diastole results in a decrease in mean flow and a maldistribution of transmural flow.

Transmural myocardial blood flow in a canine model of coronary artery bridging.