Neural-Cardiovascular Responses to Orthostasis Following 8-Weeks of Home-Based Lower Leg Heating in Older Women with Hypertension

Abstract

Background: Hypertension remains a significant health problem for older women (≥60 y). Though effective blood pressure (BP) interventions exist (e.g., exercise, medication), adherence to these options is low. Lower leg heat therapy represents a promising and easily implementable adjuvant treatment that may offer cardioprotective benefits. However, no studies have investigated lower leg heat therapy’s effects on BP and neural-cardiovascular function, let alone in older women with hypertension. Accordingly, we hypothesized that 8-weeks of home-based lower leg heat therapy in older women with hypertension would 1) reduce ambulatory BP and 2) reduce muscle sympathetic nerve activity (MSNA) while augmenting cardiac output and stroke volume during graded head-up tilt (HUT). Methods: Forty older women with hypertension (69 ± 5 y [mean±SD]) were randomized to either heat therapy (INT, 42°C; n = 20) or a thermoneutral control (CON, 35°C; n = 20). Each woman completed 8 wks of at-home sessions (~4/wk; 45 min/session) immersing their legs ~33 cm in a portable spa bath. 24-h ambulatory BP was measured pre- (PRE) and post-intervention (POST). A subset of women (n = 18 for each group) completed supine rest and 5 min of 30° and 60° HUT at each time point, whereby laboratory heart rate, systolic and diastolic BP, MSNA burst frequency and incidence (microneurography), cardiac output (acetylene rebreathing), and stroke volume (cardiac output/heart rate) were measured. Results: Between INT and CON, PRE ambulatory awake systolic BP (140 ± 7 vs. 142 ± 6 mmHg; P = 0.34) and diastolic BP (80 ± 6 vs. 79 ± 6 mmHg; P = 0.51) were not different, nor were PRE supine systolic BP, diastolic BP, or heart rate (all P > 0.05). PRE supine resting MSNA burst frequency (36 ± 13 vs. 33 ± 16 bursts/min), MSNA burst incidence (54 ± 19 vs. 51 ± 22 bursts/100 heartbeats), cardiac output (4.9 ± 0.9 vs. 5.1 ± 1.0 L/min), and stroke volume (72 ± 15 vs. 76 ± 15 mL/beat) were not different between groups (all P > 0.05). From PRE to POST neither ambulatory awake systolic nor diastolic BP differed between groups (all P > 0.05). Despite this, POST laboratory systolic and diastolic BP were greater in INT (both P < 0.05) but not in CON (both P > 0.05) during HUT. POST laboratory heart rate was reduced in INT ( P = 0.025), and trended greater in CON ( P = 0.065) across HUT. Neither MSNA burst frequency nor burst incidence changed during HUT PRE to POST (all P > 0.05). Despite no PRE to POST differences in cardiac output in either group (all P > 0.05), INT had a greater overall POST stroke volume throughout HUT ( P = 0.011), while CON had a reduced stroke volume during 60° HUT ( P = 0.036). Conclusions: Contrary to our hypotheses, 8-weeks of home-based lower leg heat therapy did not improve ambulatory BP or HUT MSNA and cardiac output in older women with hypertension. However, women completing INT appeared to have a greater stroke volume and lower heart rate during HUT. Accordingly, these data suggest that 8-weeks of lower leg heat therapy in older women with hypertension may improve cardiac function during orthostasis without significant changes in sympathetic outflow or ambulatory BP. Supported by the National Institutes of Health (R01AG059314 and F32HL167556). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.