Acute blood pressure responses and safety considerations in heat therapy interventions: A narrative review

Hypertension is a major global cause of cardiovascular morbidity and mortality, and acute blood pressure (BP) responses to heat therapy are often assumed to be benign. However, the safety of heat therapy in medicated and frail older adults remains unclear. This review synthesizes evidence for acute (≤24-hour) BP responses and safety considerations during heat therapy, with a focus on older adults and beta-blocker users. Electronic databases (MEDLINE, Embase, CENTRAL, Web of Science Core Collection, Ichushi-Web) and relevant clinical trial registries were searched to 13 March 2024 for human studies published in English or Japanese, including randomized controlled trials (RCTs), crossover studies, non-randomized interventions, and observational studies. Two RCTs, one crossover study, seven non-randomized interventional studies, and 11 observational studies including participants aged 18-94 years were included. Across modalities, namely, saunas, mineral hot spring bathing, hot water immersion, and mudpack therapy, systolic BP reductions of approximately 11-27 mmHg within 24 h were reported. However, most studies lacked control groups and showed marked heterogeneity in populations, temperatures, timing of exposure, and outcome assessment. A few small studies suggested greater BP reductions among beta-blocker users. Serious acute adverse events were rare, but few studies specifically targeted adults aged ≥ 65 years or systematically reported comorbidities and background antihypertensive therapy. Heat therapy consistently induced transient reductions in BP. The long-term antihypertensive efficacy of heat therapy is unproven, and safety margins in frail older adults and beta-blocker users remain insufficiently defined, underscoring the need for standardized protocols and high-quality RCTs.