Antihypertensive Medications and Risk of Heat-Related Illness During Heatwaves: Self-Controlled Case Series.

PURPOSE: To investigate whether antihypertensive medications modify the risk of heat-related illness during heatwaves among adults with hypertension in Japan. METHODS: We identified adults with hypertension in Tsukuba City administrative claims data between April 2014 and March 2019. Average daily temperature was derived from Japan Meteorological Agency data. We used a self-controlled case series (SCCS) design to estimate incidence rate ratios (IRRs) for heat-related illness during 5-day pre-heatwave, heatwave and 5-day post-heatwave periods compared to baseline within individuals. The main analysis was stratified by person-time prescribed antihypertensives (angiotensin-converting enzyme [ACE] inhibitors/angiotensin II receptor blockers [ARBs], calcium channel blockers [CCBs] or non-loop diuretics) and person-time not prescribed antihypertensives, with drug-temperature interaction assessed. Subgroup analyses were conducted for each antihypertensive class. All models were adjusted for age and season. RESULTS: We observed a greater increase in the rate of heat-related illness during heatwaves versus baseline among person-time prescribed antihypertensives (IRR: 1.64, 95% CI: 1.53-1.75) than among person-time not prescribed antihypertensives (IRR: 1.42, 95% CI: 1.27-1.58), with evidence of drug-temperature interaction (p < 0.01). When examining individual drug classes, ACE inhibitors/ARBs and CCBs were associated with an increased risk of heat-related illness during heatwaves, whereas there was no evidence of an effect with diuretics. CONCLUSIONS: Our findings indicate that people taking antihypertensives are more vulnerable to heat-related illnesses during heatwaves. More research is required to explore whether this is a causal effect of the medication or due to differences between those who are treated and untreated with antihypertensives that are related to the outcome.