Tschakovsky, Michael Emmerich2006-07-282006-07-2819991999http://hdl.handle.net/10012/411Muscle blood flow at rest and during exercise is determined by the upstream - downstream pressure gradient and the conductance of the vascular bed which is the result of competing vasoconstrictor and vasodilator influences. The general hypotheses of this thesis focused on determinants of forearm blood flow (FBF) as follows: i) changes in venous pressure contribute to the downstream pressure of the vascular bed and can therefore impact FBF and ii) increases in sympathetic nervous activity (SNA, vasoconstrictor influence) impair the adaptation of FBF to exercise. Four studies were conducted in which beat by beat measures of Doppler ultrasound were used to assess the nature of changes in FBF at rest and exercise in response to alterations in venous pressure and vasoconstrictor influences. In the first study, the role of the muscle pump mechanism in elevating FBF at the onset of exercise was examined. It was observed that while mechanical venous emptying did elevate FBF, it could not account for all of the elevation in FBF following a voluntary contraction, indicating that a rapid vasodilation occurred which was detectable within 2 s of contraction. In the second study, elevating and lowering a resting and exercising forearm above and below heart level revealed a transient vasodilation upon arm elevation possibly mediated by the veno-arteriolar reflex, and a hyperemia upon lowering of the arm. The hyperemia appeared to be the result of both vasodilation induced by arm elevation and venous emptying and was reduced the longer the arm remained above heart level. In the third study, lower body negative pressure was used to elevate SNA. The initial rapid increase in FBF at the onset of forearm exercise was attenuated. However, FBF quickly adjusted during the second phase of adaptation to match that during the control condition, but by 5 min of exercise was again significantly less than control. However, O2 extraction compensated for the reduced O2 delivery such that forearm O2 consumption was maintained. In the fourth study SNA was elevated by calf exercise during calf circulatory occlusion. Forearm vasoconstriction was observed at rest. When exercise began this was quickly abolished and blood flow was elevated in the forearm in proprortion to the blood pressure elevations induced by the elevated SNA. It is concluded that venous pressure reductions appear to play a role in determining MBF as hypothesized, by affecting both the downstream pressure and the vascular conductance. However, the hypothesis that elevated sympathetic activity compromises the blood flow adaptation to small muscle mass exercise was not supported by the data. Instead, the evidence from these studies supports the existence of a functional sympatholysis during small muscle mass exercise.application/pdf10942689 bytesapplication/pdfenCopyright: 1999, Tschakovsky, Michael Emmerich. All rights reserved.Harvested from Collections CanadaDoctoral Thesis