Pressure dependence of the canine aortic characteristic impedance and the effects of alterations in smooth muscle activity.

Abstract

Experiments were performed on anaesthetised dogs to separate the effects of changes in aortic smooth muscle activity on the characteristic impedance Zc from its passive pressure dependence. Zc was measured from the ascending aortic pressure/flow relationship in a wide pressure range caused by forced slow oscillations in pressure induced with an external pump. Changes in smooth muscle tone were elicited by haemorrhage two levels of volume expansion and alpha-adrenergic blockade with phenoxybenzamine. Curves relating Zc to pressure usually showed a minimum. When analysed at a given pressure haemorrhage increased Zc whereas both volume expansions and alpha blockade reduced Zc over a wide pressure range. In the case of haemorrhage, the total increase in Zc was shown to be the sum of a passive pressure-dependent component and an active (pressure-independent) component. This study has clarified the mechanisms responsible for the change in Zc observed after acute alterations in blood volume and after alpha-blockade.