Effects of heatwaves on emergency medical service activity in Vienna: a 4-year analysis

Heatwaves increase the risk of morbidity and strain emergency medical services. However, prehospital data from Central Europe remain limited. Overall, 936,461 emergency dispatch records were linked to spatially matched meteorological data collected from a 506-point grid across Vienna. Heatwaves were defined based on the daily minimum, mean, and maximum temperatures using the duration-and-threshold approach. During the 2018-2021 study period, group comparison analyses and generalized linear models with a negative binomial distribution were used to estimate incidence rate ratios (IRRs), adjusting for calendar effects. Subgroup analyses assessed heterogeneity by age, sex, diagnostic category, and timing during and after heat events. Daily minimum temperature of ≥ 20.5 °C for 2 consecutive days yielded the strongest association with increased dispatch activity (IRR = 1.104, 95% CI 1.077-1.131, p < 0.001). The effects intensified with increasing heatwave severity. Subsequent heatwave days exhibited decreased but statistically significant impacts. Female patients (IRR = 1.094) and those aged 0-18 and 76-85 years presented with a disproportionately greater increase in dispatches. Dispatches for heat-related illness, COPD, unconsciousness, and trauma were significantly higher. The first heatwave each year had stronger effects (IRR = 1.118) than the subsequent events. Minimum temperature-based definitions had the highest predictive value. Our results support the need to adapt local heat-health warning systems to account for cumulative exposure, early season risks, and diagnosis-specific vulnerabilities.