Temperature exposure and cardiovascular outpatient visits: a distributed lag non-linear analysis in Hangzhou, China

While the association between high temperatures and cardiovascular disease (CVD) hospitalization and mortality is well established, evidence on outpatient morbidity, a key indicator of healthcare burden, remains limited and inconsistent. This study aimed to quantify the association between daily mean temperature (DMT), heatwaves (HW), and CVD outpatient visits during the warm seasons in Hangzhou, China. We analyzed data on 1,906,418 CVD outpatient visits from two Grade A tertiary comprehensive hospitals, alongside daily meteorological and air pollution data from 2018 to 2023. A quasi-Poisson generalized additive model combined with a distributed lag non-linear model (DLNM) was employed to estimate the exposure-lag-response associations. Subgroup analyses by gender and age were conducted, and attributable fractions were calculated. A U-shaped relationship was observed between DMT and CVD outpatient risk, with an optimum temperature of 22.46 °C. Significant delayed effects were identified, with the strongest single-day risks for extreme heat and heatwaves occurring at lag 6 and lag 7 days. The cumulative risk over lag 0–7 days was substantially elevated for both extreme heat (Cumulative Relative Risk, CRR = 1.869, 95%CI: 1.165–3.000) and heatwaves (CRR = 1.462, 95%CI: 1.051–2.035). Subgroup analyses revealed that males and especially the elderly (≥ 60 years) showed stronger responses to cumulative heat stress, with the elderly having the highest risk (CRR = 2.021, 95%CI: 1.143–3.573). High temperatures and heatwaves substantially increase the risk of CVD outpatient visits, with effects pronounced delay and cumulative impact. These findings highlight the necessity for public health strategies to account for delayed risks and to prioritize the protection of vulnerable populations, particularly the elderly.