Hyperemic response of the coronary circulation to brief diastolic occlusion in the conscious dog.

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Abstract
This study was undertaken to determine whether coronary blood flow can be regulated in response to coronary arterial occlusions briefer than a single diastole. The possible involvement of metabolic vs. myogenic mechanisms in such regulation was investigated. Eleven conscious dogs with experimentally produced complete heart block, chronically implanted electromagnetic flow probes, and pneumatic occluders on the left circumflex coronary artery were studied. Diastolic coronary occlusions lasting 100 to 400 msec were performed at paced heart rates of 40, 60, and 120 beats/min. At a heart rate of 60 beats/min, a 200-msec occlusion was sufficiently long to produce a significant reactive hyperemic response; 400-mec occlusions resulted in larger responses, while 100-msec occlusions did not generate a discernible response. The onset of reactive hyperemia was delayed from the end of the occlusion until the first post-occlusion systole. The length of this delay could be altered by changing the heart rate or occlusion duration, but no significant response was detected before the first post-occlusion systole. This characteristic of the data is more consistent with a metabolic than with a myogenic mechanism. If the response is metabolic, the data demonstrate that autoregulation of coronary flow by such a mechanism is very rapid, occurring during the first systole in which a flow deficit is detected by the myocardium.