Amy N. Adams

Acute limb heating improves macro- and microvascular dilator function in the leg of aged humans

Local heating of an extremity increases blood flow and vascular shear stress throughout the arterial tree. Local heating acutely improves macrovascular dilator function in the upper limbs of young healthy adults through a shear stress-dependent mechanism but has no such effect in the lower limbs of this age group. The effect of acute limb heating on dilator function within the atherosclerotic prone vasculature of the lower limbs of aged adults is unknown. Therefore, the purpose of this study was to test the hypothesis that acute lower limb heating improves macro- and microvascular dilator function within the leg vasculature of aged adults. Nine young and nine aged adults immersed their lower limbs at a depth of ~33 cm into a heated (~42°C) circulated water bath for 45 min. Before and 30 min after heating, macro (flow-mediated dilation)- and microvascular (reactive hyperemia) dilator functions were assessed in the lower limb, following 5 min of arterial occlusion, via Doppler ultrasound. Compared with preheat, macrovascular dilator function was unchanged following heating in young adults (P = 0.6) but was improved in aged adults (P = 0.04). Similarly, microvascular dilator function, as assessed by peak reactive hyperemia, was unchanged following heating in young adults (P = 0.1) but was improved in aged adults (P < 0.01). Taken together, these data suggest that acute lower limb heating improves both macro- and microvascular dilator function in an age dependent manner. NEW & NOTEWORTHY We demonstrate that lower limb heating acutely improves macro- and microvascular dilator function within the atherosclerotic prone vasculature of the leg in aged adults. These findings provide evidence for a potential therapeutic use of chronic lower limb heating to improve vascular health in primary aging and various disease conditions.

Cognitive and perceptual responses during passive heat stress in younger and older adults.

We tested the hypothesis that attention, memory, and executive function are impaired to a greater extent in passively heat-stressed older adults than in passively heat-stressed younger adults. In a randomized, crossover design, 15 older (age: 69 ± 5 yr) and 14 younger (age: 30 ± 4 yr) healthy subjects underwent passive heat stress and time control trials. Cognitive tests (outcomes: accuracy and reaction time) from the CANTAB battery evaluated attention [rapid visual processing (RVP), choice reaction time (CRT)], memory [spatial span (SSP), pattern recognition memory (PRM)], and executive function [one touch stockings of Cambridge (OTS)]. Testing was undertaken on two occasions during each trial, at baseline and after internal temperature had increased by 1.0 ± 0.2°C or after a time control period. For tests that measured attention, reaction time during RVP and CRT was slower (P ≤ 0.01) in the older group. During heat stress, RVP reaction time improved (P < 0.01) in both groups. Heat stress had no effect (P ≥ 0.09) on RVP or CRT accuracy in either group. For tests that measured memory, accuracy on SSP and PRM was lower (P < 0.01) in the older group, but there was no effect of heat stress (P ≥ 0.14). For tests that measured executive function, overall, accuracy on OTS was lower, and reaction time was slower in the older group (P ≤ 0.05). Reaction time generally improved during heat stress, but there was no effect of heat stress on accuracy in either group. These data indicate that moderate increases in body temperature during passive heat stress do not differentially compromise cognitive function in younger and older adults.

The Effect of Passive Heat Stress and Exercise-Induced Dehydration on the Compensatory Reserve During Simulated Hemorrhage.

ABSTRACT Compensatory reserve represents the proportion of physiological responses engaged to compensate for reductions in central blood volume before the onset of decompensation. We hypothesized that compensatory reserve would be reduced by hyperthermia and exercise-induced dehydration, conditions often encountered on the battlefield. Twenty healthy males volunteered for two separate protocols during which they underwent lower-body negative pressure (LBNP) to hemodynamic decompensation (systolic blood pressure <80 mm Hg). During protocol #1, LBNP was performed following a passive increase in core temperature of ∼1.2°C (HT) or a normothermic time-control period (NT). During protocol #2, LBNP was performed following exercise during which: fluid losses were replaced (hydrated), fluid intake was restricted and exercise ended at the same increase in core temperature as hydrated (isothermic dehydrated), or fluid intake was restricted and exercise duration was the same as hydrated (time-match dehydrated). Compensatory reserve was estimated with the compensatory reserve index (CRI), a machine-learning algorithm that extracts features from continuous photoplethysmograph signals. Prior to LBNP, CRI was reduced by passive heating [NT: 0.87 (SD 0.09) vs. HT: 0.42 (SD 0.19) units, P <0.01] and exercise-induced dehydration [hydrated: 0.67 (SD 0.19) vs. isothermic dehydrated: 0.52 (SD 0.21) vs. time-match dehydrated: 0.47 (SD 0.25) units; P <0.01 vs. hydrated]. During subsequent LBNP, CRI decreased further and its rate of change was similar between conditions. CRI values at decompensation did not differ between conditions. These results suggest that passive heating and exercise-induced dehydration limit the bodys physiological reserve to compensate for further reductions in central blood volume.

Increased postural sway in persons with multiple sclerosis during short-term exposure to warm ambient temperatures

BACKGROUND Multiple sclerosis (MS) is a neurological disease marked by demyelination and axonal loss. Individuals with MS experience increases in clinical signs and symptoms during heat exposure. OBJECTIVE To test the hypothesis that moderate heat exposure adversely affects postural sway in individuals with MS. METHODS Ten individuals with relapsing-remitting MS (50±8y) and nine controls (47±10y) were examined under a Thermal and a Time Control trial. Following a 30min thermoneutral baseline (25°C, 30% relative humidity (RH)), stand tests randomized with eyes open and closed, were performed. For Thermal, subjects were first exposed to 60min of heating (40°C, 30%RH) followed by 60min of cooling (20°C, 30%RH). For Time Control, subjects remained in a thermoneutral environment throughout. Stand tests were repeated at consistent times in both trials. RESULTS No difference in skin and core temperatures between groups were observed for any trial (P>0.05). During heating, postural sway was higher in MS relative to control subjects (eyes open, P=0.03; eyes closed, P=0.011). No differences in postural sway, regardless of eye status, were observed during the Time Control trial for either group (P>0.05). CONCLUSION These data demonstrate that exposure to a moderate heating environment increases postural sway in patients with MS.

Folic acid ingestion improves skeletal muscle blood flow during graded handgrip and plantar flexion exercise in aged humans

Skeletal muscle blood flow is attenuated in aged humans performing dynamic exercise, which is due, in part, to impaired local vasodilatory mechanisms. Recent evidence suggests that folic acid improves cutaneous vasodilation during localized and whole body heating through nitric oxide-dependent mechanisms. However, it is unclear whether folic acid improves vasodilation in other vascular beds during conditions of increased metabolism (i.e., exercise). The purpose of this study was to test the hypothesis that folic acid ingestion improves skeletal muscle blood flow in aged adults performing graded handgrip and plantar flexion exercise via increased vascular conductance. Nine healthy, aged adults (two men and seven women; age: 68 ± 5 yr) performed graded handgrip and plantar flexion exercise before (control), 2 h after (acute, 5 mg), and after 6 wk (chronic, 5 mg/day) folic acid ingestion. Forearm (brachial artery) and leg (superficial femoral artery) blood velocity and diameter were measured via Duplex ultrasonography and used to calculate blood flow. Acute and chronic folic acid ingestion increased serum folate (both P < 0.05 vs. control). During handgrip exercise, acute and chronic folic acid ingestion increased forearm blood flow (both conditions P < 0.05 vs. control) and vascular conductance (both P < 0.05 vs. control). During plantar flexion exercise, acute and chronic folic acid ingestion increased leg blood flow (both P < 0.05 vs. control), but only acute folic acid ingestion increased vascular conductance (P < 0.05 vs. control). Taken together, folic acid ingestion increases blood flow to active skeletal muscle primarily via improved local vasodilation in aged adults.NEW & NOTEWORTHY Our findings demonstrate that folic acid ingestion improves blood flow via enhanced vascular conductance in the exercising skeletal muscle of aged humans. These findings provide evidence for the therapeutic use of folic acid to improve skeletal muscle blood flow, and perhaps exercise and functional capacity, in human primary aging.Listen to this articles corresponding podcast at http://ajpheart.podbean.com/e/folic-acid-and-exercise-hyperemia-in-aging/.