Contribution of local blood flow regulation mechanisms to the maintenance of arterial pressure in upright position during epidural blockade.

The contribution of local blood flow regulation mechanisms to the maintenance of arterial pressure in upright position was studied in 5 normal subjects. Central sympathetic blockade was induced by epidural anesthesia. Blood flow in anterior tibial muscle in both legs and in brachioradial muscle in one arm was measured by the local 133Xe washout technique. Arterial blood pressure was recorded directly from the radial artery. Slow head-up tilt (about 30 degrees) caused a decrease in blood flow of about 36% in the dependent legs and in arm remaining at heart level. Arterial pressure decreased by about 10%. Blockade of the local sympathetic veno-arteriolar "axon reflex" in one leg by injection of phentolamine into the common femoral artery caused a vasodilatation in the ipsilateral muscle, while muscle blood flow did not change in the other leg or arm. Within 20 s after the injection of phentolamine arterial pressure decreased by about 7%. This cannot be explained by a "systemic" effect because injection of phentolamine into the femoral vein did not effect arterial pressure within the first 40 s. Vasoconstriction due to blood-borne factors is ruled out since preventing the increase in vascular transmural pressure in the leg by inducing counterpressure locally, abolished the vasoconstriction. Thus, the results suggest that the local veno-arteriolar "axon reflex" together with myogenic mechanisms contribute to the maintenance of arterial pressure in the upright position.