Compound heatwaves linked to increased cardiac deaths

Cardiac deaths increase significantly during compound heatwaves-heatwaves where temperatures are elevated both during the day and overnight-according to a new study in JACC, the flagship journal of the American College of Cardiology. The study also found that some types of heart disease are more sensitive to heatwaves than others and that different types of heatwaves may impact heart health in distinct, nonlinear ways.

The research highlights the special risks of compound heatwaves and potential nonlinear relationships between different heatwave types and cardiac mortality. It finds that previous studies using traditional binary definitions-without accounting for different types of heatwaves and their cumulative heat load-may have underestimated this risk by as much as 54%. The findings could help improve clinical guidelines for cardiovascular patients and could also be used to improve public health adaptations to climate change, researchers said.

Compound heatwaves impose a significantly higher risk to cardiac health than either daytime-only or nighttime-only heatwaves. Traditional heatwave definitions fail to capture the full scope of these risks."

Renjie Chen, PhD, coauthor of Fudan University

Previous research has found a link between short-term exposure to heatwaves and an increase in heart disease mortality using conventional binary definitions, but the shape of the risk curve and the role of different heatwave patterns were not well understood.

For the new study, researchers analyzed data for nearly 2.4 million heart disease deaths across mainland China over six years. They compared deaths across daytime-only, nighttime-only, and day-night compound heatwaves and introduced a new metric for measuring heatwave impact called Excess Cumulative Temperatures in Heatwaves, or ECT-HW.

"Unlike traditional binary indicators, ECT-HW captures the full spectrum of heatwave characteristics, including intensity, duration and timing within the season," Chen explained. This provides more precise risk assessment, especially for high-intensity, high-duration and compound heatwaves.

Researchers found that the mortality risk associated with compound heatwaves increased steadily with exposure, without a clear threshold. Daytime-only and nighttime-only heatwaves displayed nonlinear patterns, with effects emerging only beyond specific thresholds and plateauing at moderate intensities.

They also found that risk was not equal across different heart disease subtypes. Sudden cardiac arrest, acute myocardial infarction and heart failure were especially sensitive to compound heatwaves, while pulmonary heart disease only showed elevated risk under high-intensity exposures.

Conventional studies of heatwave deaths that look only at whether a heatwave is present or not significantly underestimate mortality risks, they said.

The findings have several clinical and public health implications, Chen said.

Since different types of heatwaves affect cardiac conditions differently, prevention measures should consider heatwave type and intensity. In addition, given their higher impact, compound heatwaves should be explicitly considered in risk assessment and early warning systems for cardiovascular patients.

The findings also highlight the importance of urban heat mitigation strategies, such as cooling shelters and improved housing infrastructure to protect vulnerable populations.

"As climate change accelerates, the shifting and emerging heatwave exposure pattern demands better metrics and targeted interventions," Chen said. "Given the increasing frequency and intensity of compound heatwaves due to climate change, our findings highlight the need for disease-specific prevention strategies and revised public health guidelines to better protect at-risk populations."

Next, the team is planning follow-up research, including studies to project the future burden of heatwave-related cardiac mortality under different climate change scenarios.