The ability to maintain myocardial perfusion at constant levels in the face of changing driving presence is termed autoregulation. In normal cases, autoregulation is maintained at mean arterial pressures as low as 60 mms of Hg and as high as 130 mm of Hg. Autoregulation plays an important role. In arteries, reduction in perfusion pressure distal to the stenosis are compensated by autoregulatory dilation of resistance vessels. Coronary collaterals do not exhibit autoregulation.