Research Article
Higher outdoor temperatures are progressively associated with lower blood pressure: a longitudinal study in 100,000 healthy individuals

https://doi.org/10.1016/j.jash.2015.05.003Get rights and content

Highlights

  • Outdoor temperature and blood pressure: stronger associations at higher temperatures.

  • The elderly seem particularly vulnerable to temperature related blood pressure variation.

  • Associations of temperature with blood pressure were independent of humidity.

  • The large study population (n=101,377) allowed for piecewise, stratified analyses.

Abstract

This study investigated the shape of associations between climate parameters (mean daily temperature and humidity) and systolic and diastolic blood pressure in a large longitudinal cohort of healthy individuals. The study population comprised 101,377 Dutch whole blood and plasma donors (50% men), who made 691,107 visits to the blood bank between 2007 and 2009. Climate parameters were acquired from the Royal Netherlands Meteorological Institute. Associations with blood pressure, measured prior to each blood donation, were studied using (piecewise) linear regression analyses within Generalized Estimating Equation models. On average, systolic blood pressure was 0.18 mm Hg, and diastolic blood pressure was 0.11 mm Hg lower per one degree Celsius higher mean daily temperature. Higher daily temperatures were associated with lower blood pressure, independent of humidity and potentially confounding factors. These associations were stronger at older age and higher temperatures. Seasonality should therefore be taken into account when monitoring blood pressure, particularly in older individuals.

Introduction

Both high and low blood pressures are important cardiovascular risk factors and may partly explain seasonal differences in cardiovascular disease. Exposure to temperatures falling below or rising above 20°C–25°C has been related to increased cardiovascular mortality.1, 2, 3, 4, 5 The Eurowinter Group, for instance, showed increased mortality rates with every one degree decrease in temperature below 18°C, particularly in regions with relatively warm winters and cool homes.2 On the other hand, a review published in 2010 reported a 3% increase in death rates per one degree increase in temperature in hot regions (where the temperature exceeds 30°C in the warmest months) and a somewhat milder increase of 2.6% per 10°F (∼4.7°C) in the temperate climate of North America.1

Exposure to cold causes subcutaneous vasoconstriction, which enhances vascular resistance and subsequently increases blood pressure.6, 7 On the other hand, vasodilatation and decreased peripheral resistance at high temperatures may decrease blood pressure. Low blood pressure may hamper perfusion of critical tissues, which also increases cardiovascular risk.8, 9 The influence of outdoor temperature and related seasonality on blood pressure may, therefore, contribute to seasonal variation in cardiovascular mortality, with higher mortality rates in summer and winter compared with spring and fall.

In the past, several studies have been conducted on the effect of outdoor temperature and seasonality on blood pressure. Results pointed in the direction of an inverse relationship between temperature and blood pressure. Thus, high daily temperatures coincided with lower systolic (SBP) and diastolic (DBP) blood pressure.10, 11, 12, 13, 14 Moreover, during a cold winter, blood pressures were significantly higher than in a hot summer.15, 16, 17, 18, 19, 20, 21 In addition to temperature, it has been suggested that other climate parameters like humidity are related to blood pressure.22 In more humid circumstances, there is a decreased ability to evaporate fluids by sweating, which may lead to higher blood pressure because of volume overloads or lower blood pressure if increased body temperature leads to vasodilation.

To date, the precise nature of associations between climate parameters and blood pressure has not been fully elucidated. The association between outdoor temperature and cardiovascular mortality has often been described to be J– or U–shaped.23 There are indications that the association between temperature and blood pressure is non–linear as well.10 A more precise estimate of blood pressure patterns with varying climate parameters may contribute to a better understanding of season–related cardiovascular mortality. Moreover, in health care, it may be helpful to be aware of climate–related blood pressure variation.

We had the opportunity to study associations between temperature and humidity and blood pressure values in a very large cohort of blood donors, for which multiple measurements per donor over time were available. The huge amount of prospectively collected donation data measured before each blood donation, including blood pressure, is unique. By using modern, advanced statistical techniques, it is possible to accurately depict the shape of the association between climate parameters and blood pressure. Therefore, the main objective of this study was to investigate the shape of associations of mean daily temperature and humidity with both SBP and DBP in healthy individuals.

Section snippets

Study Population

Dutch voluntary blood donors comprised the study population. The overall Dutch donor population consists of approximately 400,000 donors, of whom the majority (±97%) is of Caucasian origin. These donors provide around 900,000 donations in the Netherlands annually. To secure both donor and blood safety, several safety measures and deferral criteria are applied according to Dutch guidelines that are based on European guidelines.24 Donors must be between 18 and 70 years of age and, before each

Results

The study data file consisted of 691,107 measurements obtained from 50,641 men aged 49.3 (±12.5) years and 50,736 women aged 42.4 (±13.7) years. The characteristics of the study population at their most recent visit in the period from 2007 to 2009 are presented in Table 1. Mean SBP was 137 mm Hg in men and 126 mm Hg in women, and DBP was 83 and 79 mm Hg, respectively. An overview of mean temperature and humidity per month in the years 2007–2009 is given in Figure 1. In this period, minimum

Discussion

The present study shows that climate parameters were significantly and non–linearly associated with blood pressure. Mean daily temperature was negatively associated with blood pressure, independent of humidity and other potentially confounding factors. This association was stronger at higher temperatures and at older age. Humidity was positively associated with blood pressure, but this was largely attributable to variation in outdoor temperature.

Several potential mechanisms may be proposed for

Acknowledgments

The authors are very grateful to all blood donors for their contribution to the study and for their voluntary donations of blood (components) for patients in need. The authors also express our thanks to all researchers, research assistants, and blood bank personnel for their on–going commitment to DIS and the donor database. The authors thank Pieternel Pasker–de Jong for her help in drafting the manuscript.

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    This study was carried out by the Department of Donor Studies and the Unit Donor Services with the financial support of the Sanquin Blood Supply Foundation.

    Conflict of interest: none.

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