The Role of the Venous System in Cardiocirculatory Dynamics during Spinal and Epidural Anesthesia in Man

Effects of spinal and epidural anesthesia uncomplicated by premedication or surgery were studied in 20 normal subjects. Venous blood flow, vascular resistance and venous distensibility of forearm and calf vessels were measured simultaneously before and after induction of spinal or epidural anesthesia. In anesthetized calf vessels, blood flow and vascular distensibility increased and vascular resistance decreased. Changes in cardiac output, whether decreased, increased or unchanged, did not affect the ratio of calf blood flow to cardiac output, which in all instances increased following spinal and epidural anesthesia. Changes in arterial blood pressure did not correlate with changes in cardiac output alone or changes in total peripheral resistance alone. However, changes in arterial blood pressure correlated with the interaction of changes in cardiac output and changes in total peripheral resistance. Evidence presented indicates that arterial hypotension induced by either spinal or epidural anesthesia may be due primarily to increased vascular distensibility of capacitance vessels and secondarily to decreased resistivity in pre- and postcapillary resistance vessels in the anesthetized area.