The effect of the coronary vascular wall on the changes in the coronary vascular resistance during a sudden reduction and recovery of the aortic blood pressure.

To demonstrate the effect of elastic and myogenic properties of the vessel wall on the mean coronary vascular resistance during ventricular diastole (m.d.c.r.), this resistance was assessed in twenty dogs anaesthetized with sodium pentobarbitone during a 10 s period of arterial haemorrhage and 20 s of recovery from this haemorrhage. In a first stage of the study ten dogs were investigated following sectioning of the vagal nerves in the neck, beta-blockade, and ligation of the carotid arteries. During haemorrhage and reduction in aortic blood pressure (A.B.P.) in these dogs, there was an initial rise (at 2 s after the start of the haemorrhage) in m.d.c.r. which quickly decreased to a value similar to that obtained during the control period. After the abrupt cessation of haemorrhage, and a rise in A.B.P., there was an initial decrease in m.d.c.r. which eventually increased to the control levels. In the second stage of study identical patterns of changes were obtained in a group of five dogs in which the cervical vagi were not sectioned and beta-blocking agents not given. In another group of five dogs, during infusion of dipyridamole to force maximal coronary vasodilation, the increase in m.d.c.r. was maintained throughout the period of haemorrhage and reduction in A.B.P., and m.d.c.r. did not decrease below the control values following the abrupt cessation of haemorrhage and a rise in A.B.P. The findings in the dogs without dipyridamole infusion suggested that the pattern of changes in m.d.c.r. was due to changes in the perfusion pressure and their effect on the elasticity and myogenic properties of the coronary vascular wall. This suggestion was supported in the second stage of the study, when the reductions in m.d.c.r. were abolished by forcing coronary vasodilation using the infusion of dipyridamole.