To determine the distribution of resistance in the coronary vasculature, measurements of microvascular pressure and diameter were obtained with vasomotor tone intact and during coronary dilation produced by papaverine. We studied anesthetized, open-chest cats and used jet ventilation synchronized to the cardiac cycle to eliminate respiratory-induced cardiac motion. The system for measuring microvascular pressure compensated for cardiac motion with stroboscopic illumination of the microvessels and a computer-controlled electromechanical micromanipulator that moved a micropipette in synchrony with the heart. Pressures were measured with the servonull technique, and diameters were measured via a video system. Resistance was estimated from the pressure gradient from the aorta to a particular class (size) of coronary microvessel. During control conditions, with coronary vasomotor tone intact, myocardial perfusion was 139 +/- 9 ml X min-1 X 100 g-1 and was increased to 339 +/- 52 during papaverine infusion (P less than 0.05). During control conditions (mean arterial pressure 70-80 mmHg), approximately 25% of total coronary resistance was proximal to 200-microns diameter arterioles and approximately 20% was produced by arterioles between 100 and 200 microns diameter; there was approximately 20-mmHg pressure gradient from the aorta to 200-microns arterioles and a 30- to 35-mmHg gradient from the aorta to 100-microns arterioles. Present results also indicate that 55% of total coronary resistance is distal to the 100-microns arterioles under control conditions. Coronary vasodilation with papaverine (mean arterial pressure maintained at 70-75 mmHg with an aortic snare) produced a redistribution of resistance; only 10% of total resistance was proximal to the 200-microns arterioles, and the pressure gradient from the aorta to these vessels was only 5-10 mmHg. Thus, under these experimental conditions with vasomotor tone intact, a substantial portion (45%) of coronary resistance resides in relatively large (greater than 100-microns) coronary arterioles, and the distribution of resistance and microvascular pressures can be shifted with coronary vasodilation.
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