Yet, concern has persisted that excessive DBP lowering may have adverse Rosendorff et al Treatment of Hypertension in Patients With CAD 1383 consequences for the heart. In virtually all instances, lower- ing SBP improves cardiac function and outcomes, probably through a reduction in cardiac work and an improved myocar- dial oxygen balance. On the other hand, it is theoretically pos- sible that lowering of DBP improves cardiovascular outcomes only when coronary perfusion is maintained above the lower limit of coronary autoregulation. Myocardial perfusion occurs almost exclusively during diastole; therefore, DBP is the coronary perfusion pressure. Like most vascular beds, the coronary circulation is capable of autoregulation, so that a decrease in perfusion pressure is accompanied by coronary vasodilation, which maintains a fairly constant coronary blood flow. The problem is that this ability of coronary resistance vessels to dilate in response to a falling perfusion pressure is limited, and at the point of maxi- mal vasodilation, a further decrease in coronary perfusion pressure will result in a decrease in flow. In conscious, instru- mented dogs, contractile function (transmural wall thickening and subendocardial segment shortening) is well maintained at mean coronary filling pressures down to 40 mmHg, which corresponds to a DBP of 30 mmHg.127-129 The lower limit of autoregulation in dogs with LV hypertrophy is shifted upward by 15 to 20 mmHg but can be partially restored byACE inhi- bition, with accompanying regression of LV hypertrophy.129 These studies were in dogs with normal intramural coronary arteries. We do not have good data on equivalent values for the human coronary circulation. In the presence of occlusive CAD, the hemodynamics are much more complicated. Significant CAD will shift the lower autoregulatory limit upward. 