Heterogeneous vascular responses to hypoxic forearm exercise in young and older adults

Abstract

We aimed to assess age-related differences in compensatory hypoxic vasodilation during moderate-to-high dynamic exercise at absolute workloads. We hypothesized healthy older adults (n = 12, 61 ± 1 years) would exhibit impaired hypoxic vasodilation at a moderate absolute workload, and this effect would be exaggerated at a higher workload when compared to young adults (n = 17, 27 ± 2 years). Forearm blood flow (FBF) was measured with Doppler ultrasound. Dynamic forearm exercise (20 contractions/min) was completed at two absolute workloads (8 and 12 kg) under normoxic (0.21 FiO2, ~98% SpO2) and isocapnic hypoxic (~0.10 FiO2, 80% SpO2) conditions performed in random order. FBF was normalized as forearm vascular conductance (FBF / mean arterial blood pressure = FVC) to control for differences in blood pressure and to assess vasodilation. FVC increased with exercise and hypoxia (main effects, p < 0.05); vascular responses were not different between young and older adults (interaction effect exercise × group p = 0.37 and hypoxia × group p = 0.96). Results were confirmed when analyzed as either an absolute or relative change in FVC (ΔFVC and %ΔFVC, respectively). Although group responses to hypoxia were not different, individual results were highly variable (i.e., some adults constricted and others dilated to hypoxia). These data suggest (1) compensatory hypoxic vasodilation in older adults is not impaired during forearm exercise at both moderate and higher absolute exercise intensities, and (2) vascular responses to hypoxia are heterogeneous in both young and older adults. Results suggest unique individual differences exist in factors regulating vascular responses to hypoxia.

Fig. 1

Increases in normoxic and hypoxic forearm vascular conductance with exercise. Data are presented as mean ± SE. A change in vascular conductance (ΔFVC) was calculated as FVC_exercise – FVC_{normoxia rest}. Vascular responses to exercise (ΔFVC) increased with increasing exercise intensity (main effect of exercise: p < 0.05 12 vs. 8 kg) and were not different between groups (no effect of group). Both groups exhibited hypoxic-mediated vasodilation that was not different in magnitude between groups (^†main effect of gas condition)

Fig. 2

Compensatory responses to hypoxia during exercise. Data are presented as mean ± SE. Values represent a change in vascular conductance due to hypoxia (compensatory vasodilation) and are expressed as a relative change in ΔFVC with hypoxia $[(\mathrm{\Delta }{\text{FVC}}_{\text{hypoxia}}-\mathrm{\Delta }{\text{FVC}}_{\text{normoxia}})\mathrm{\Delta }{\text{FVC}}_{\text{normoxia}}^{-1}100\%]$. Vascular responses were not different between groups (No effect of group)

Fig. 3

Individual stimulus–response relationships to hypoxia during rest and exercise. Data are presented as individual data points; large open symbols signify group means. Values were calculated as the two-point slope of the relationship between ΔFVC and arterial oxygen saturation (S_pO₂) to assess individual vascular responses to hypoxia. Negative values represent net hypoxia-mediated vasodilation; positive values represent vasoconstriction. Vascular responses to exercise increased at 8 kg exercise intensity (main effect of exercise: ^ap < 0.05 vs. rest) and were not different between groups (no effect of group)