The relation of leg venous function and structure to orthostatic hypotension in humans

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BACKGROUND AND PURPOSE: Despite extensive investigation, the effects of either cardiovascular fitness or age on orthostatic remain uncertain. Calf venous compliance may be a critical factor in determining the cardiovascular stress associated with change in orthostasis. Preventing fluid shifts into the legs markedly reduces reflex responses to an orthostatic stressor. The extent to which exercise training in a previously sedentary older and young population will affect venous compliance and tolerance to the orthostatic stress is unknown. An evaluation of the compliance of the whole venous system (including venule to large vein) in human calf was developed with a venous congested plethysmography (VCPG) (Halliwill, et al., J.Appl.Physiol. 1999) which has already been used in several application studies. Utilizing the B-mode ultrasonography (US), we have proposed a similar technique for estimating the compliance in medium venous vessels (i.e., Popliteal vein) using US images, which is deri … More ved from the CSA-venous pressure (Pv) relationship.Therefore, we investigate:1. The relation of leg venous compliance to orthostatic intolerance in humans.2. Effect of 10days successive endurance training on leg venous compliance.Secondly, to clearly define the venous vascular function concerning venous pooling, we examined:3. The effect of ischemic handgrip exercise on leg venous compliance in humans4. The relationship between venous compliance and capillary filtration in the lower limb.RESULT: Venous compliance was not different between tolerance and intolerance group. This result suggested that the leg venous compliance is not dominant determinant of the orthostatic intolerance in humans. Venous pooling decreases cardiac filling pressure and blood pressure (BP); therefore, arterial baroreceptors (carotid and aortic) and baroreceptors in the cardiopulmonary region must adapt to maintain BP over a fairly wide range of hemodynamic conditions. And no change in calf venous compliance was found after 10 days successive of endurance training (60 min, 40% peak Vo2). Under the sympathetic stimulation (sympatho-excitatory stimulation) induced by the ischemic handgrip exercise to fatigue followed post-exercise ischemia. Compared to the control, the calf volume was consistently smaller under the sympathetic stimulation. The venous compliance at lower Pv was significantly lower. These data shows to existence of sympathetic venous vascular regulation in humans. In addition, there is negative correlation was observed between venous compliance and capillary filtration in the lower limb. Less