Flow in the Major Branches of the Left Coronary Artery during Experimental Coronary Insufficiency in the Unanesthetized Dog

The dynamic changes in the coronary circulation and the response to drugs were studied following experimentally induced coronary insufficiency. Flow measurements were made in the left circumflex and descending branches of the coronary artery, and in the ascending aorta; pressures were measured in the ascending aorta and the left ventricle. As the left circumflex coronary artery branch was gradually constricted, reactive hyperemia following a 10-second occlusion of that vessel decreased. When the Degrees of constriction became such that the control resting flow began to fall, there was no reactive hyperemia, and the contractility index of the heart decreased. Within less than 24 hours, flow in the descending branch of the left coronary artery rose, and the cardiac contractility index returned to control; peak flow rate in the descending branch during reactive hyperemia after a 10-second occlusion also increased. The response to isoproterenol, nitroglycerin, and dipyridamole was similar in direction before and after the partial reduction of flow in the left circumflex coronary artery branch. The response was less in the coronary vessel with partial occlusion; in the unimpeded descending coronary artery branch, the response increased progressively. These results are consistent with the development of collateral vessels.