Hemodynamic effects of vasodilator agents in dogs with experimental ventricular septal defects

Abstract

The ratio of pulmonary to systemic vascular resistance (Rp/Rs) largely determines the amount of left to right shunting and pulmonary to systemic flow ratio (Qp/Qs) in the presence of a large isolated ventricular septal defect. The possibility that pharmacologic reduction of systemic vascular resistance with α adrenergic receptor blockade or β adrenergic receptor stimulation would increase the ration Rp/Rs, and therefore reduce the ratio Qp/Qs, was studied in dogs in which ventricular septal defects had been surgically created. Administration of phentolamine and phenoxybenzamine caused a 42% reduction in Rs and no reduction in Rp. Qs was unchanged and Qp declined by 24% and the ratio Qp/Qs fell by 32%. Infusion of the β adrenergic receptor stimulant isoproterenol also reduced Qp/Qs. However, this was accomplished as a result of an increase in Qs and at the expense of an increase in heart rate. As a decline in the ratio Qp/Qs has been shown to be beneficial to patients with large left to right shunts, pharmacologic reduction of systemic vascular resistance may prove to be helpful in treating congestive heart failure in those patients with large left to right shunts at the ventricular level who are refractory to the usual decongestive measures.