Seasonal variation in blood pressure and its relationship with outdoor temperature in 10 diverse regions of China: the China Kadoorie Biobank

Abstract

Objectives: Mean blood pressure varies moderately with outdoor air temperature in many western populations. Substantial uncertainty exists, however, about the strength of the relationship in other populations and its relevance to age, adiposity, medical treatment, climate and housing conditions.

Methods: To investigate the relationship of blood pressure with season and outdoor temperature, we analysed cross-sectional data from the China Kadoorie Biobank study of 506,673 adults aged 30-79 years recruited from 10 diverse urban and rural regions in China. Analyses related mean blood pressure - overall and in various subgroups - to mean local outdoor temperature.

Results: The mean difference in SBP between summer (June-August) and winter (December-February) was 10 mmHg overall, and was more extreme, on average, in rural than in urban areas (12 vs. 8 mmHg; P for interaction <0.0001). Above 5°C, SBP was strongly inversely associated with outdoor temperature in all 10 areas studied, with 5.7 (SE 0.04) mmHg higher SBP per 10°C lower outdoor temperature. The association was stronger in older people and in those with lower BMI. At lower temperatures, there was no evidence of an association among participants who reported having central heating in their homes.

Conclusion: Blood pressure was strongly inversely associated with outdoor temperature in Chinese adults across a range of climatic conditions, although access to home central heating appeared to remove much of the association during the winter months. Seasonal variation in blood pressure should be considered in the clinical management of hypertension.

Figure 1. Systolic blood pressure by month of measurement among 506 673 men and women

Mean SBP adjusted for age, sex and area. Only includes months with >100 participants. The solid circles joined by a solid line indicate months when all ten areas were actively recruiting and the open circles (joined by a dotted line at the beginning and end of the study period) indicate months when only some were active. There is an open circle for September 2007 when Haikou did not recruit due to re-organisation of the survey team. The horizontal placement of each month indicates, for participants recruited in that month, the mean number of days since the first participant was recruited. J/F = January & February combined (recruitment dropped in January & February due to the Chinese New Year).

Figure 2. Monthly variation in systolic blood pressure and outdoor temperature within each area

For both SBP and outdoor temperature, the mean monthly values are the mean for all participants whose baseline survey happened during that month (regardless of the year). SBP means adjusted for age and sex. Mean outdoor temperature is indicated by a dotted line (note the inverse scale). ^a In Harbin, 100% of participants had some form of heating in their home; 94% had central heating. ^b Areas in which >80% of participants had some form of heating in their home.

Figure 3. Mean systolic blood pressure versus mean monthly outdoor temperature within each area

Mean SBP, but not outdoor temperature, adjusted for age and sex. Regression line is SBP versus outdoor temperature adjusted for age, sex and area, within the temperature range above 5°C. The dotted line represents the hypothetical continuation of the regression line below 5°C. ^a In Harbin, 100% of participants had some form of heating in their home; 94% had central heating. ^b Areas in which >80% of participants had some form of heating in their home.

Figure 4. Monthly variation in systolic blood pressure by various subgroups

(a) By age group (years); (b) By BMI (kg/m²); (c) By alcohol (Male only); (d) By smoking^a (Male only); (e) By blood pressure lowering treatment; (f) By home heating. Body-mass index (BMI) was calculated as weight in kilograms divided by the square of the height in metres. Winter=December, January/February; summer=June, July, August. SBP means adjusted for age, sex and area. a Additionally adjusted for BMI.