Age and fitness effects on limb venous compliance, responses and tolerance to maximal lower body negative pressure

摘要

Introduction. It is unknown how aging and endurance training affect tolerance to a maximal orthostatic stressor. Further it is unknown if training an untrained older population will improve venous compliance or affect tolerance. Therefore, the purposes of this project were (1) to determine the effects of age and cardiovascular fitness on venous compliance and tolerance to maximal lower body negative pressure (LBNP) and (2) to develop a model of variables that directly and indirectly affect tolerance to maximal LBNP.;Methods. This project included cross-sectional and longitudinal investigations as well as a compilation of data used to develop a structural equation model of tolerance to the orthostatic stressor of maximal LBNP. All participants underwent graded LBNP to presyncope or 4 minute at -100 mmHg. Cardiovascular variables measured included heart rate, stroke volume, mean arterial pressure and forearm vascular conductance. Venous compliance was determined via venous occlusion plethysmography. Structural equation modeling was used to determine a model of variables associated with tolerance to maximal LBNP.;Results. While some age by fitness differences were found in the responses to LBNP, tolerance to maximal LBNP did not differ between the groups in the cross-sectional design. The more fit groups had greater venous compliance than their unfit peers as did the young groups compared to their older peers. Endurance training improved fitness in the exercising older men and women and tended to improve venous compliance without altering tolerance to maximal LBNP. Structural equation modeling indicated that the manifest variables of gender, % body fat, age, maximal change in heart rate from rest to presyncope and resting total peripheral conductance are the variables most directly and indirectly associated with tolerance.;Conclusions. Neither age nor cardiovascular fitness affect tolerance to maximal LBNP. Aging reduces and chronic exercise increases venous compliance without a significant influence on LBNP tolerance. Six months of endurance training may improve venous compliance without affecting tolerance to maximal LBNP in older men and women. Structural equation modeling, specifically path analysis using easily measured variables can explain an adequate amount of variance in tolerance to maximal LBNP.