Cardiovascular Responses to Lower Body Negative Pressure in the Elderly: Role of Reduced Leg Compliance

Abstract

Background: It is warranted to test the hypothesis that the orthostatic tolerance does not diminish in the aging process per se in healthy individuals. Objective: The purpose of the present study was to examine the effects of aging on cardiovascular response and baroreflex sensitivity during lower body negative pressure (LBNP) with a special reference to leg compliance. Methods: Fifteen healthy old male subjects [mean age 68.2 ± (SE) 0.8 years] and 22 young male subjects [mean age 21.4 ± (SE) 0.3 years] underwent a 21-min bout of ramped LBNP (from 0 to –60 mm Hg, 10 mm Hg each for 3 min). Heart rate (HR), blood pressure, stroke volume (SV), forearm blood flow, and leg volume were measured throughout the experimental period. The arterial baroreflex sensitivity was calculated from spontaneous changes in beat-to-beat arterial pressure and HR during LBNP. Results: The leg compliance was lower, and the orthostatic tolerance index was higher in old than in young participants. The LBNP-associated increases in leg volume and HR and the decreases in SV were lower in old subjects, suggesting that the reduction of venous return was less in magnitude in old subjects during LBNP. The baseline value of baroreflex sensitivity evaluated by the sequence analysis was smaller, and no LBNP-related change was observed in old subjects, whereas a gradual LBNP-related reduction was observed in young subjects. The slope of regression between ΔSV and change in forearm vascular resistance during LBNP was identical in both age groups. Conclusions: We conclude that: (1) aging per se does not increase the intolerance to orthostatic stress induced by LBNP; (2) a low magnitude of venous return reduction during LBNP contributes to a higher tolerance in the old because of lower leg compliance, and (3) the sensitivity of baroreflex control of the HR is attenuated in the old; however, there is no deterioration of the sensitivity of the peripheral vasoconstriction during LBNP.

Sex-related effects on venous compliance and capillary filtration in the lower limb

“…This, in turn, elicits an increased sympathetic response with higher peripheral resistance and increased heart rate (4,5,10,44). In studies on men, a greater calf venous compliance has been linked to an increased venous capacitance response, which seems to have an impact on cardiovascular responses to orthostatic stress and orthostatic tolerance (43,56). Women are more susceptible to orthostatic stress than men (4,10,13,40,51,58), and in accordance with some findings in the arterial tree, it may be hypothesized that women have greater venous compliance in the lower limbs predisposing to orthostatic intolerance (52).…”

“…During upright posture, however, the pooling of blood in the veins of the lower part of the body decreases central blood volume and venous return (4,5,16). The venous compartment in the legs, rather than the pelvic or abdominal region, seems to have a hemodynamic impact during lower body negative pressure (LBNP) (16), and in studies on men, a greater calf venous compliance has been linked to an increased venous capacitance response with a concomitant reduction in central blood volume (44,56). This, in turn, elicits an increased sympathetic response with higher peripheral resistance and increased heart rate (4,5,10,16,44,58).…”

Effects of a 6-mo endurance-training program on venous compliance and maximal lower body negative pressure in older men and women

“…Preventing fluid shifts into the legs markedly reduces reflex responses to an orthostatic stressor (15). Calf venous compliance may be lower in older adults (17,19,23), which some have suggested may help protect older participants during orthostatic stress (19,38). Calf venous compliance is higher in endurance-trained young and older individuals (5,10,17,22,23).…”

“…Calf venous compliance is higher in endurance-trained young and older individuals (5,10,17,22,23). Tsutsui and colleagues (38) suggested that lower leg compliance was associated with higher orthostatic tolerance in old compared with the young participants. However, the generalizability of this observation is limited: since compliance was assessed during the LBNP protocol rather than during rest, 65% of the participants tolerated the orthostatic challenge, and fitness was not considered (38).…”

“…Tsutsui and colleagues (38) suggested that lower leg compliance was associated with higher orthostatic tolerance in old compared with the young participants. However, the generalizability of this observation is limited: since compliance was assessed during the LBNP protocol rather than during rest, 65% of the participants tolerated the orthostatic challenge, and fitness was not considered (38). Recent crosssectional data suggest that endurance training may improve venous compliance without affecting tolerance to maximal LBNP (17).…”