The relationship between indoor, outdoor and ambient temperatures and morning BP surges from inter-seasonally repeated measurements.

Higher morning blood pressure (BP) surge is a risk factor for cardiovascular disease independent of 24-h mean BP. Although low outdoor temperatures are associated with higher morning BP surges (MBPSs), the influence of indoor temperature and ambient temperatures (temperature while indoors or temperature while outdoors) on MBPS remains unclear. Such information may help prevent excess winter mortality. We simultaneously measured indoor temperatures (living room and bedroom), ambulatory BP and physical activity using wrist actigraphy for 768 person-days during winter and spring/fall in 192 participants (mean age, 69.9 years). Although the indoor and outdoor temperatures showed a strong correlation during periods of moderate temperature (range: 9.8 to 27.7 °C, rp=0.84), the correlation decreased during periods of lower outdoor temperatures (range: -3.37 to 9.73 °C, rp=0.28). In univariate and multivariate analyses, models with ambient temperatures showed the best goodness of fit (lowest Akaike's information criterion (AIC)) followed by models with indoor temperatures and those with outdoor temperatures (AIC: ambient<indoor<outdoor temperature). A multivariate linear mixed-effect regression model showed that a 1 °C decrease in the ambient temperature was significantly associated with a 0.44 mm Hg increase in the sleep-trough MBPS and a 0.52 mm Hg increase in the prewaking BP surge. This was independent of potential confounders, including physical activity.