Coronary microvascular resistance in hypertensive cats

Chronic systemic hypertension has been shown to alter the distribution of vascular resistance in many microvascular beds. The purposes of this study were to assess the effects of chronic systemic hypertension on the pressure distribution in the coronary microcirculation and to determine the microvascular site where coronary vascular resistance is increased. Cats were made hypertensive using a one-kidney, one-wrap model (Page model). A servonulling system was used to directly measure pressures in the epimyocardial microvessels of the beating left ventricle in normotensive and hypertensive cats. In chronically hypertensive cats, mean arterial pressure was 153 ± 5 mm Hg compared with 98 ± 3 mm Hg in normotensive cats (p<0.05). Left ventricular mass was increased approximatelv 34% in hypertensive cats (9.4 ± 0.3 versus 7.0 ± 0.3 g, p<0.05). Myocardial perfusion measured using radiolabeled microspheres was not different between hypertensive and normal cats. Coronary vascular resistance of the left ventricle was increased in hypertensive cats (0.90 ± 0.08 versus 0.66 ± 0.05 mm Hg x min x 100 g/ml, p<0.05). Microvascular pressures were measured in three groups of microvessels: small, <200 μm; medium, 200-300 μm; and large, ≥300 μm. Mean microvascular pressures of large, medium, and small arterial microvessels in hypertensive cats were 144 ± 8, 127 ± 6, and 115 ± 7 mm Hg, respectively. Mean microvascular pressures of large, medium, and small arterial microvessels in normotensive cats were 95 ± 4, 79 ± 3, and 71 ± 4 mm Hg, respectively. Venous microvascular pressures in small and medium vessels were the same level in normotensive and hypertensive cats (12-16 mm Hg). Coronary microvascular segmental resistances were calculated from the distribution of microvascular pressures and the total coronary vascular resistance. Arterial resistance in vessels >150 μm was similar in hypertensive and normotensive cats, whereas microvascular resistance in vessels <150 μm was twofold higher in hypertensive cats when compared with normotensive controls. Therefore, the distribution of microvascular resistance was shifted downstream in cats with chronic hypertension.