Opposite transmural gradients of coronary resistance and extravascular pressure in the working dog's heart

Abstract

In order to investigate the factors determining flow to the subendocardium independently from functional adjustments of the resistance vessels, pressure-flow curves for the bypassed left circumflex coronary artery were obtained during maximal vasodilatation induced by intra-coronary adenosine infusion in eight anaesthetised mongrel dogs with systolic and diastolic left ventricular pressures of 14.5 ± 2.3 and 0.3 ± 0.2 kPa (109 ± 17 and 2 ± 1.4 mmHg) respectively. For each experiment three or four flow rates were selected in the linear portion of the pressure flow curve for the injection of 15 ± 5 (μm diameter labelled microspheres for the measurement of regional flow in the internal (ENDO) middle (MID) and external (EPI) one third of the perfused left ventricular wall. Linear pressure-flow relationships were obtained for each of the three layers of the left ventricle using the diastolic coronary perfusion pressure and regional flow rates. The slopes (P/F) of the ENDO were in each instance significantly lower than those of the EPI. The extrapolation of the regional pressure-flow curves to zero flow showed the highest value of pressure for the ENDO and the lowest for the EPI, suggesting a transmural gradient of extravascular pressure. The results of this study agree with a waterfall model of the coronary circulation, in which the extravascular pressure decreases from the ENDO to the EPI in contrast to the vascular resistance which decreases from the EPI to the ENDO. This model explains why the subendocardial layer receives an equal or greater blood flow than the subepicardium in normal haemodynamic conditions even in the absence of autoregulation and in spite of a higher extravascular pressure; it also explains why it becomes selectively ischaemic in the presence of a low perfusion pressure such as that produced by a coronary stenosis.