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Friday, June 24, 2011

Cold Weather and Air Conditioning Can Raise Your Blood Pressure

High blood pressure, also called hypertension, is defined as a systolic pressure above 140 mm of mercury (mmHg) and/or a diastolic pressure above 90 mmHg. Hypertension increases the risk of several major health problems, including heart disease, stroke, and kidney disease. While it is widely recognized that excess sodium intake increases the risk of hypertension, few people know that the ambient temperature at which people live affects the risk of hypertension. And ambient temperature varies with the seasons of the year. The following studies provide evidence for the seasonal variation in blood pressure and risk of hypertension.

In a study by Woodhouse, Khaw, and Plummer, 96 men and women, aged 65-74 years had their blood pressure taken for a full year. It was found that both systolic (SBP) and diastolic blood pressure (DBP) were greatest during the winter for people with both normal and high blood pressure. There was four times the incidence of blood pressures above 160/90 mmHg in winter than in summer. In a strong seasonal trend, a 1 deg C (1.8 deg F) decrease in living-room temperature was associated with increases of 1.3 mmHg in SBP and 0.6 mmHg in DBP. The authors linked this to the greater incidence of cardiac-related deaths of the elderly in winter.

In a study by Brennan, Greenberg and Miall, blood pressure measurements taken for the Medical Research Council's treatment trial for mild hypertension were analyzed according to the month in which the readings were made. For all age, sex, and treatment groups, both systolic and diastolic pressures were higher in winter than in summer. Blood pressure was also highly and significantly related to maximum and minimum daily air temperature. The seasonal variations in blood pressure were greater in older than in younger people.

In a study by Fujiwara et al., blood pressure was measured on 25 hypertensive outpatients (mean age 57), who spent virtually the entire day indoors in both summer and winter. Both systolic and diastolic blood pressure in the morning and night-time periods were significantly higher in winter than in summer (respective differences of 7.5 and 4.1 mmHg in the morning and 8.2 and 4.5 mmHg at night). Despite the fact that the patients lived essentially indoors at a relatively stable environmental temperature, the seasonal variation in blood pressure was statistically significant.

In a study by Kimura et al. of the Department of Integrated Medicine, Kagawa University, Japan, 15 healthy elderly Japanese (mean age 79) measured their blood pressure at home each morning more than 25 times per month for 3 years. The highest levels of both systolic and diastolic blood pressures (129 and 81 mmHg) occurred in February, the coldest month (avg temp. 5.0 deg C, 41 deg F), while the lowest levels (117 and 73 mmHg) were observed in August, the hottest month (mean temp 29.2 deg C, 84.6 deg F). Thus, both systolic and diastolic blood pressure demonstrated a close inverse relationship to outdoor temperature. A one degree C (1.8 deg F) decrease in the mean outdoor temperature was associated with rises of 0.43 mmHg in systolic blood pressure (SBP) and 0.29 mmHg in diastolic blood pressure (DBP).

In a study by Hozawa et al. at the Tohoku School of Medicine in Japan, 79 male and female volunteers (mean age 72.7 years) measured their blood pressure at least once a month for 3 years beginning in September 2000. The mean number of measurements was 19.0 times per month. Blood pressure levels were lowest in the warmest months. A clear inverse association between temperature and blood pressure values was evident when the outside temperatures was above 10°C, producing a respective decrease in systolic and diastolic blood pressure of 0.40 and 0.28 mmHg for each 1 deg C (1.8 deg F) increment of outside temperature.

A study by Sinha et al. at Maulana Azad Medical College, in India, 275 females 18-40 years of age showed that the prevalence of hypertension based on SBP was 12.7% in summer and 22.2% in winter. The prevalence of hypertension based on DBP was 11.3% in summer vs. 26.6% in winter, a highly statistically significant difference. Overall prevalence of hypertension (SBP = 140 or DBP = 90 mm of Hg) was 1.9 times greater in winter than in summer.

Bottom Line

The temperature at which we live can affect our blood pressure. This is likely related to the fact that, when we are cold, the small arteries in our skin constrict to avoid loss of body heat. That creates resistance to blood flow, thus increasing pressure. When we are hot, the small arteries in the skin widen to allow more heat dissipation, thereby reducing resistance to blood flow. Also, we tend to perspire more when we are hot, thereby losing water and salt, both of which tend to increase blood pressure. The most common medications for reduction of blood pressure are diuretics, which promote loss of water and salt through urination. Sweating can accomplish similar results.

People with hypertension or prehypertension (systolic pressure 120-140) can help control their blood pressure by avoiding being cold. In summer, air conditioning use should be minimized. Most people can adjust to a room temperature of 77 degrees without feeling uncomfortable, and even higher temperatures in locales with low humidity. In winter, the home can be kept warm and, if that is not economically feasible, dressing warmly indoors is a viable alternative. Exercise can be used to warm the body as well.

References

Brennan, P.J., G. Greenberg, W.E. Miall, S.G. Thompson. Seasonal variation in arterial blood pressure. Br Med J (Clin Res Ed) 285 : 919, 2 October 1982.

Fujiwara, T., M. Kawamura, J. Nakajima, Jun, T. Adachi, K. Hiramori. Seasonal differences in diurnal blood pressure of hypertensive patients living in a stable environmental temperature. Journal of Hypertension, vol. 13, no. 12, 1995.

Hozawa A., S. Kuriyama, T. Shimazu, K. Ohmori-Matsuda, I. Tsuji. Seasonal variation in home blood pressure measurements and relation to outside temperature in Japan. Clin Exp Hypertens, vol. 33, no. 3, pp. 153-8, 2011.

Kimura, T., S. Senda, H. Masugata, A. Yamagami, H. Okuyama, T. Kohno, T. Hirao, M. Fukunaga, H. Okada, F. Goda. Seasonal blood pressure variation and its relationship to environmental temperature in healthy elderly Japanese studied by home measurements.Clin Exp Hypertens. 2010 Jan;32(1):8-12.

Sinha P, D.K. Taneja, N.P. Singh, R. Saha. Seasonal variation in prevalence of hypertension: Implications for interpretation.Indian J Public Health, vol. 54, no. 1, pp. 7-10, 2010.

Woodhouse, P.R., K.T. Khaw, M. Plummer. Seasonal variation of blood pressure and its relationship to ambient temperature in an elderly population. Journal of Hypertension, vol. 11, no. 11, 1993.

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