Does outdoor temperature modify the prognostic value of 24-hour ambulatory systolic blood pressure? the IDACO Study

Background Ambient temperature influences blood pressure (BP), but whether outdoor temperature around the time of ambulatory BP monitoring affects the prognostic interpretation of systolic blood pressure (SBP) for long-term outcomes remains uncertain. Methods We analyzed 14,236 participants from the International Database on Ambulatory Blood Pressure in Relation to Cardiovascular Outcomes (IDACO). Outdoor temperature was summarized as mean temperature over three exposure windows: same-day (1-d), 7-day, and 30-day periods around the ambulatory BP recording. Cohort-specific associations between outdoor temperature and 24-hour SBP were assessed using linear regression. To evaluate whether temperature-related measurement variation affects risk stratification, we derived temperature-standardized 24-hour SBP for each exposure window by standardizing SBP to the cohort-specific median temperature. Cox proportional hazards models estimated hazard ratios (HRs) and 95% confidence intervals (CIs) for total mortality and a composite cardiovascular endpoint. Incremental predictive performance was assessed using Harrell’s C-index, likelihood ratio tests, net reclassification improvement (NRI), and integrated discrimination improvement (IDI). Adjusted cumulative incidence curves were generated by quartiles of observed and temperature-standardized 24-hour SBP (adjusted for cohort, sex, and age). Results Associations between outdoor temperature and 24-hour SBP varied across cohorts and exposure windows (1-d, 7-d, and 30-d), with both positive and negative cohort-specific slopes. In Cox models, the HRs per 20 mm Hg higher observed 24-hour SBP were 1.23 (95% CI 1.17–1.30) for total mortality and 1.47 (1.39–1.55) for cardiovascular endpoints (both P<0.001), and these estimates were essentially unchanged when using temperature-standardized 24-hour SBP. In prediction analyses, adding observed 24-hour SBP increased the C-index from 0.8267 to 0.8293 for total mortality and from 0.8141 to 0.8246 for cardiovascular endpoints, with NRI 4.7% (0.36 to 8.4) and 11.0% (6.7 to 14.2), respectively. Temperature-standardized SBP (1-d, 7-d, or 30-d) yielded similar C-index values and did not consistently improve NRI or IDI compared with observed SBP. Conclusions Outdoor temperature around the time of ambulatory BP monitoring is associated with 24-hour SBP in a cohort-dependent manner, but temperature standardization of SBP provides limited incremental benefit for long-term risk stratification beyond observed ambulatory SBP.