Role of venous return in the cardiovascular response following injection of ganglion-blocking agents.

The reduction of arterial pressure in anesthetized dogs following the injection of ganglion-blocking agents is dependent primarily upon a reduction in arterial vasomotor activity and secondarily upon a reduction in venous return associated with an increase in vascular capacity. When venous return is maintained at control levels, the administration of the blocking agents of this study invariably produce a decrease in total peripheral resistance (TPR). Ordinarily in the intact animal such a decrease in TPR is not observed due to a marked reduction in cardiac output which produces a secondary increase in TPR. The results of this stud}' further suggests that the venous and arterial systems may differ in their sensitivity to the effects of ganglion-blocking agents. T HE decrease in blood pressure produced by the injection of ganglion-blocking agents into man or laboratory animals is associated with an increased blood flow 1-3 to and a decreased "vasomotor tone" 4 in certain vas-cular areas. However, total peripheral resistance frequently does not change or may increase. 1 ' 3p 6i 6 Grob and co-workers 5 have reported that peripheral resistance decreased following the injection of hexamethonium in those patients whose cardiac output either did not change or increased and that total peripheral resistance either was unchanged or increased in patients whose cardiac output decreased. Similar findings have been reported by Mover and associates. 7 Recently, we have postulated 4 that the changes in total peripheral resistance following the injection of ganglion-blocking agents depends upon the continued existence of a "critical closing pressure" in various vascular beds, and the degree to which venous return and subsequently cardiac output is reduced. On this basis the effect of ganglion-blocking agents upon total peripheral resistance depends on whether or not the reduced cardiac output and systemic blood pressure following their injection suffice to maintain the patency of peripheral blood vessels. The following study was designed to test this hypothesis. METHODS Twenty-six mongrel dogs of both sexes weighing 14 to 18 Kg., fasted at least 12 hours prior to each experiment, and anesthetized with barbital sodium, 250 mg./Kg., administered intravenously were employed in this study. Experiments were conducted on open chest animals maintained under artificial respiration with the aid of a Palmer respiratory pump. Blood pressure was recorded with Hamilton manometers and blood flow with Shipley-Wilson rotameters. The experimental arrangement is illustrated in figure 1. Total venous return was measured and then returned via the azygos …