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The local dilatory reserve of the canine coronary vasculature was studied with the particle distribution technique. Normal ventricles and hearts with slowly progressive narrowing of both the left circumflex coronary artery and the right coronary artery were studied. In spite of chronic occlusion of 2 coronary arteries myocardial infarction did not occur in the majority of animals because of collateral development. Coronary reserve was determined by producing graded to maximal coronary vasodilation. In normal hearts flow increased homogeneously over the entire left ventricle. In hearts with chronic coronary occlusion coronary vasodilation produced non-homogeneous increases in flow: collateral dependent myocardium received less blood flow than myocardium supplied by normal coronary arteries. Early after coronary occlusion the total coronary reserve was less than normal and the dilatory reserve of collateral dependent vessels was markedly diminished. Late (6 months) after coronary occlusion the total coronary reserve was still below normal but the dilatory reserve of collateral dependent vessels had improved. A new quantitative index of collateral function is defined as the level of coronary flow (delivered through normal coronary arteries) at which collateral flow deviates from homogeneous perfusion. Collateral function, when so defined, increases by a factor of almost 6 times between 4 weeks (early after coronary occlusion) and 6 months (late after occlusion) after the implantation of occluding devices.

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