[The effect of maximal vasodilation on the distensibility of the coronary vascular bed]

This study aimed at investigating the changes in coronary vascular resistance induced by sudden increases in transmural pressure in the presence of a maximally vasodilated coronary bed. In anaesthetized open-chest dogs under artificial ventilation, aortic blood pressure, left ventricular pressure and the flow in the left circumflex coronary artery were recorded. The flow was derived by means of an electromagnetic flowmeter. Maximal vasodilatation was achieved by intracoronary infusion of dipyridamole (10-40 mg/h), increases in transmural pressure, starting from 70 mmHg, were obtained by constricting for 10 s the descending thoracic aorta with a plastic snare. While in the absence of vasodilatation the increase in pressure was accompanied with an increase in resistance because of an auto-regulatory response, when the coronary bed was maximally dilated the increase in pressure did not alter the coronary vascular resistance. These results seem to be in conflict with the observation that in the coronary circulation the distension of the vascular wall produced by increases in pressure is favoured by the reduction of the vasomotor tone. However, it may be argued that, while a reduction of the vasomotor tone can increase the vascular distensibility, a maximal vasodilatation, as it was seen in the resistance vessels of the skeletal muscles, brings the vascular diameter to such a size that no further distension can be induced when the transmural pressure is increased starting from a value of about 70 mmHg.