Seasonal variation in blood pressure recorded in routine primary care

Objective Seasonal variations in blood pressure (BP) exist. There is limited information about important clinical factors associated with increased BP and the strength and amplitude of seasonal variation in primary care. Methods This was a repeated cross-sectional observational study of routinely measured BPs in primary care using data from electronic medical records in the greater Toronto region, from January 2009 to June 2019. We used time-series models and mean monthly systolic BPs (SBPs) and diastolic BPs (DBPs) to estimate the strength and amplitude of seasonal oscillations, as well as their associations with patient characteristics. Results 314,518 patients were included. Mean SBPs and DBPs were higher in winter than summer. There was strong or perfect seasonality for all characteristics studied, except for BMI less than 18.5 (underweight). Overall, the mean maximal amplitude of the oscillation was 1.51mmHg for SPB (95% CI 1.30mmHg to 1.72mmHg) and 0.59mmHg for DBP (95% CI 0.44mmHg to 0.74mmHg). Patients aged 81 years or older had larger SBP oscillations than younger patients aged 18 to 30 years; the difference was 1.20mmHg (95% CI 1.15mmHg to 1.66mmHg). Hypertension was also associated with greater oscillations, difference 0.53mmHg (95% CI 0.18mmHg to 0.88mmHg). There were no significant differences in SBP oscillations by other patient characteristics, and none for DBP. Conclusion Strong seasonality was detected for almost all patient subgroups studied and was greatest for older patients and for those with hypertension. The variation in BP between summer and winter should be considered by clinicians when making BP treatment decisions.