Mechanisms of Coronary Flow Reserve Impairment in Human Hypertension

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Abstract
The purpose of this study was to investigate the different mechanisms responsible for an impairment of coronary vasodilator capacity in hypertensive subjects by an integrated echocardiographic approach, including transesophageal Doppler echocardiography, which allows noninvasive monitoring of coronary flow velocity in the left anterior descending artery during pharmacological vasodilation. The study population consisted of 17 healthy control subjects and 33 hypertensive subjects: 10 without hypertrophy, 16 with mild to moderate hypertrophy, and 7 with severe left ventricular hypertrophy. Mean systolic and diastolic flow velocities were monitored basally (together with indexes of myocardial oxygen demand, such as heart rate, myocardial inotropism, and left ventricular wall stress) and during infusion of low-dose (0.56 mg/kg per 4 minutes) and high-dose (0.84 mg/kg per 9 minutes) dipyridamole. Coronary reserve was assessed as the ratio of mean diastolic velocity after high-dose dipyridamole and basal diastolic velocity, and minimum coronary resistance as the ratio of diastolic blood pressure and diastolic velocity after high-dose dipyridamole. Compared with the control group, in all hypertensive groups, coronary reserve was similarly decreased (3.54±0.84 versus 2.59±0.42, 2.29±0.46, and 2.43±0.71; P <.01) and minimum resistance increased (0.56±0.15 versus 0.75±0.31, 0.75±0.19, and 0.78±0.21 mm Hg·s −1 ·cm −1 ; P =NS). These results confirm that coronary reserve in hypertensive individuals is reduced even before the occurrence of left ventricular hypertrophy. The reduction in coronary reserve depends on both an increase in resting coronary flow and an impairment in maximal vasodilator capacity. An increase in resting flow is dependent on higher heart rate and wall stress in hypertensive subjects without ventricular hypertrophy and on increased myocardial mass in hypertensive subjects with hypertrophy. Hypertensive subjects with ventricular hypertrophy also demonstrated a significantly blunted response to low-dose dipyridamole.