Hemodynamic and vascular effects of different durations of lower-leg hot-water immersion

A footbath may be an accessible form of passive heat therapy (PHT) to improve cardiovascular health. As a localized PHT stimulus, it is unclear whether longer durations are superior for promoting adaptive hemodynamic and vascular adjustments. Fourteen older adults (7 females; means ± SD; age: 68 ± 6 yr) performed 30-, 45-, and 60-min lower-leg hot-water (42°C) immersions and a 60-min thermoneutral (36°C) sham immersion on 4 separate days. Superficial femoral artery blood flow and flow-mediated dilation (FMD) were assessed pre- and postimmersion, and the time course of recovery was characterized over the 60 min following immersion. Twenty-four-hour ambulatory blood pressure was monitored over each experimental day. Femoral artery blood flow increased following immersion in all PHT conditions (mean of all PHT conditions: +345 ± 211%; <i>P</i> < 0.001) but was unchanged in sham. The 30- and 45-min PHT conditions sustained a higher leg blood flow than sham for the full hour following immersion (all post hoc comparisons: <i>P</i> < 0.01), whereas the 60-min condition was not different from sham by 30 min into recovery. FMD was greater following 45-min PHT than sham over the hour of recovery (mean of recovery time points: 2.5 ± 0.9% FMD vs. 1.7 ± 1.0%; <i>P</i> = 0.002). Twenty-four hour mean arterial pressure was lower with 45-min and 60-min PHT compared with sham (both -4 ± 4 mmHg; <i>P</i> < 0.05). A 45-min footbath results in comparable hemodynamic adjustments as 60 min of immersion and leads to the largest acute improvement in vascular function. As such, a 45-min footbath may be the preferred dose of PHT to use in future interventions seeking to improve vascular health and blood pressure.<b>NEW & NOTEWORTHY</b> A longer duration footbath may not be better for cardiovascular benefits in older adults. Leg blood flow increases, and ambulatory blood pressure is reduced similarly with 30, 45, and 60 min of lower-leg hot-water immersion. However, only a 45-min footbath improves endothelial function compared with a thermoneutral sham immersion. Forty-five minutes is the most favorable acute dose of lower-leg hot-water immersion and may be the most suitable for chronic interventions seeking to improve cardiovascular health.