Interaction of Extreme Temperature Events and Nitrogen Dioxide on Out-of-Hospital Cardiac Arrest in a Subtropical...

Extreme temperature events and nitrogen dioxide are established risk factors for cardiovascular disease, often co-occurring during cold spells. However, their additive interactive effects on out-of-hospital cardiac arrest (OHCA) in subtropical climates remain unclear.

Time-series analysis was performed using 16 769 OHCA cases in Shenzhen, China (2015-2019). Extreme temperature events were defined by apparent temperature: cold spells and heatwaves were stratified by percentile thresholds (2.5th-10th for cold, 90th-97.5th for heat) and consecutive durations (2-4 days). Quasi-Poisson regression with distributed lag nonlinear models was applied, and additive interactions were evaluated using relative excess risk due to interaction, attributable proportion, and synergy index.

Cold spells were associated with a significant severity-dependent increase in OHCA risk, with risk ratios (RRs) rising from 1.19 (95% CI, 1.13-1.26) for mildest definition (≤10th percentile for ≥2 days) to 1.50 (95% CI, 1.26-1.78) for most extreme definition (≤2.5th percentile for ≥2 days), whereas heatwaves showed no significant associations. Cold spells and nitrogen dioxide (lag 0-3 days) showed significant synergy. Under the most extreme cold spell, coexposure yielded the highest risk (RR, 1.543 [95% CI, 1.307-1.822]), with 27.7% of OHCA cases attributable to this interaction. This synergy was most pronounced for shockable rhythms (relative excess risk due to interaction up to 1.57 [95% CI, 0.34-2.79]).

Extreme cold and nitrogen dioxide synergistically increase OHCA risk in subtropical megacities, particularly for shockable rhythms-reversible with prompt defibrillation. Integrated early-warning systems targeting compound cold and pollution exposures are needed to reduce mortality.