Effects of Heat Waves on Hospitalizations, Emergency Department Visits, and Outpatient Care in Frail Older Adults: A Systematic Review and Meta-Analysis

Background/Objectives: Heat waves are increasingly frequent and intense climate events with significant implications for public health, particularly among frail older adults. While most evidence has focused on mortality and morbidity, healthcare service utilization represents an additional and potentially more sensitive indicator of heat-related health burden. Methods: A systematic review and meta-analysis was conducted following the PRISMA guidelines and prospectively registered in PROSPERO (CRD420251107598). PubMed/MEDLINE, Scopus, and Web of Science were searched up to August 2025. This study aimed to systematically review and quantitatively synthesize the evidence on the association between heat wave exposure and healthcare utilization—including hospitalizations, emergency department (ED) visits, and outpatient care—among frail older adults. Pooled effect estimates (RRs, IRRs, and ORs) were calculated using random-effects models. Heterogeneity was assessed using the I2 statistic, and sensitivity analyses were performed by outcome type, effect measure, and risk of bias. Results: Fifty-five studies met the inclusion criteria. Heat wave exposure was consistently associated with increased healthcare utilization. Both hospitalizations and ED visits showed significant increases during heat wave periods, with results remaining robust across sensitivity analyses. Evidence on outpatient care was limited but suggested a similar pattern. Substantial heterogeneity was observed across studies, reflecting variability in exposure definitions, populations, and study designs. Overall, the methodological quality of the included studies was acceptable, with most presenting a low-to-moderate risk of bias. Conclusions: Heat waves are associated with increased healthcare utilization among frail older adults, indicating a relevant burden on healthcare systems. Healthcare utilization may represent a sensitive indicator of heat wave impact, complementing traditional clinical outcomes.