Age- and limb-related differences in the vasoconstrictor response to limb dependency are not mediated by a sympathetic mechanism in humans

Abstract

Aims: We tested the hypotheses that vasoconstrictor responses to limb dependency are: (i) greater in the leg than the arm, (ii) impaired with age and (iii) not sympathetically mediated.

Methods: Vascular responses to limb dependency (i.e. lowering the limb from heart level to 30 cm below heart level) were determined in 17 young and 17 older adults. Indices of blood flow were obtained in the brachial and popliteal arteries (Doppler ultrasound) as well as in the cutaneous circulation (forearm and calf using laser-Doppler flowmetry). Vasoconstriction was quantified by calculating the indices of vascular resistance as height corrected mean arterial pressure/limb blood velocity or skin flux. A second group of subjects repeated the limb dependency trials after acute systemic sympathetic blockade.

Results: Limb dependency increased vascular resistance index in the brachial artery (∆59 ± 8%; P<0.05) and popliteal artery (∆99 ± 10%; P<0.05 for change in heart level and brachial vs. popliteal) of young and older adults (∆60 + 9% brachial and ∆61 ± 7% popliteal arteries; P<0.05 for change in heart level and response in popliteal young vs. older adults). In contrast, cutaneous vasoconstrictor responses to limb dependency were similar in the forearm (∆218 ± 29% and ∆200 ± 29% for young and older adults, respectively) and calf (∆257 ± 32% and ∆236 ± 29%; all P<0.05 from heart level) of young and older adults. Vasoconstrictor responses to limb dependency were not affected by sympathetic blockade in young or older adults.

Conclusion: These findings indicate that age-, limb-, and tissue-related differences may exist in the vasoconstrictor response to limb dependency in healthy humans, which are not sympathetically mediated.

Figure 1

Percent change (Δ) in limb and skin vascular resistance index (VR) in the arm (left panels) and leg (right panels) of young and older adults in response to limb dependency. VR index was calculated as mean arterial pressure/mean blood velocity (limb) or flux (skin). Responses are presented as the percent change from the baseline position (limb at heart level) to the dependent position (limb lowered 30 cm below heart level). Mean arterial pressure was corrected to account for the hydrostatic pressure gradient created when the limb was studied in the dependent position (mean arterial pressure in control limb at heart level + 0.766 * 30 cm). Limb VR in the arm was assessed in the brachial artery and in the leg in the popliteal artery. Skin VR in was assessed on the ventral aspect of the forearm (arm) and calf (leg). Values are mean±SE. * P<0.05 from baseline † P<0.05 from response in arm (same age group) ‡ P<0.05 from response in young (same limb)

Figure 2

Percent change (Δ) in vascular resistance index in the arm (left panels) and leg (right panels) of young (n=5) and older adults (n=5) in response to limb dependency. Data obtained before (Pre) and during systemic sympathetic blockade (Post) are presented on both an individual (open symbols) as well as mean basis (closed symbols; mean±SE). Indices of vascular resistance were calculated as mean arterial pressure/mean blood velocity (limb) or flux (skin). Responses are presented as the percent change in vascular resistance index from the baseline (limb at heart level) to the dependent position (limb lowered 30 cm below heart level). Mean arterial pressure was corrected to account for the hydrostatic pressure gradient created when the limb was studied in the dependent position (mean arterial pressure in control limb at heart level + 0.766 * 30 cm). Responses to limb dependency were not altered by sympathetic blockade in young or older adults. Limb VR in the arm was assessed in the brachial artery and in the leg in the popliteal artery. Skin VR in was assessed on the ventral aspect of the forearm (arm) and calf (leg).