September 2, 2013
Cardiovascular Risk Factors Predominant During Cold Winter Months
Lawrence LeBlond for redOrbit.com - Your Universe Online
A new multinational study has linked cardiovascular risk factors to cold weather. Based on cross-sectional data from 10 studies over seven countries, researchers found such risk factors occur more in the winter than in the summer.
"Deaths from cardiovascular disease are higher in winter and lower in summer. We decided to conduct a large scale study to see whether cardiovascular risk factors have a seasonal pattern which could explain the seasonality in deaths," said Dr. Pedro Marques-Vidal of Institute of Social and Preventive Medicine, University of Lausanne (IUMSP).
The researchers obtained their information from 107,090 subjects between the ages of 35 and 80. The countries and number of patients involved are as follows: 21,128 subjects in Belgium, 15,664 in Denmark, 1,626 in France, 18,370 in Italy, 25,532 in Norway, 9,359 in Russia and 15,411 in Switzerland.
The researchers compared factors such as blood pressure, lipids, glucose, BMI and waist circumference for each season. Data were adjusted for age, gender and smoking. Some data, such as blood pressure, lipids and glucose were also adjusted for BMI and whether or not the patient was on medication.
The team of researchers discovered several of the cardiovascular risk factors were higher in winter (January and February) and lower in summer (June to August) compared to the annual average.
Systolic blood pressure levels were about 3.5 mmHg lower in summer than in winter on average.
"Although this difference is almost irrelevant for an individual, it is considerable for a whole population because the whole blood pressure distribution is shifted to higher values, increasing cardiovascular risk,” said Dr. Marques-Vidal. “Indeed, the impact of season on blood pressure levels might have as great an impact on cardiovascular risk as genetic markers for blood pressure. This is because the joint effect of genetic markers on blood pressure is modest, between 2 and 3 mmHg."
They also found waist circumference was on average 1 cm (0.4 in) smaller in summer than in winter, while total cholesterol was 0.24 mmol/L lower in summer than in winter.
"We observed a seasonal variation in waist circumference but BMI did not change throughout the year. We have no clear explanation for this finding,” noted Dr. Marques-Vidal. “Total cholesterol may increase during the winter because of changes in eating habits. There was no seasonal variation in glucose, probably because several cohorts did not collect blood samples in the fasting state. We have begun a study on seasonality of food intake which may help explain these findings.
"Our large scale study shows that some cardiovascular risk factors take holidays over the summer. This may explain why deaths from cardiovascular disease are higher in winter than summer. People need to make an extra effort to exercise and eat healthily in the winter to protect their health," he added.
"Our team is currently conducting another study to find out if the seasonal pattern in cardiovascular risk factors reverses in the southern hemisphere, where seasons are inverted relative to the northern hemisphere. Based on preliminary data, it does seem to be the case. The overall study is expected to collect information on almost 200,000 subjects from over 12 countries."
The team is also conducting a study across 18 countries, involving 50 million deaths, to discern if seasonal changes of risk factors “affect risk of dying from myocardial infarction or stroke.”
In a second, separate study, one researcher looked to see if there was an association between heart attacks and air pollution.
Prof. Marc Claeys of Belgium, noted in his study, which involved nearly 16,000 patients, no apparent associations were found between the two.
"Air pollution and temperature changes are the most frequently reported environmental triggers for acute myocardial infarction (AMI). Epidemiologic studies have focused mainly on one environmental condition, but most environmental triggers are related to each other and may attenuate or reinforce the triggering effect of a single environmental factor,” said Prof. Claeys.
"Better knowledge of the impact of environment on AMI will help medical care providers and policy makers to optimize prevention strategies for a target risk population," he added.
The study evaluated independent environmental triggers of AMI in a multifactorial nationwide environmental model. The researchers extracted data from 32 Belgian percutaneous coronary intervention (PCI) centers from 2006 to 2009 looking at weekly counts of AMI patients who underwent primary PCIs.
The AMI counts were correlated with average weekly meteorological data obtained from daily measurements in 73 meteorological sites across Belgium. The team investigated measurements from air pollution, black smoke, temperature and relative humidity.
During the study period, 15,964 AMI patients (average 63 years old, 24.8 percent female) were admitted with a weekly average admission rate of 77 +/- 11 patients. The analysis showed for all measurements, only temperature was significantly linked with AMI, which increased seven percent for each 18°F decrease in minimal temperature.
"Additional analysis showed that the triggering effect of low temperature was also present outside the winter period. Apparently, smaller differences in temperature between indoor and outdoor can also precipitate AMI. In addition, below a minimal temperature of [50°F] there is no additional effect of temperature decrease on the occurrence of AMIs.
"A potential mechanism to explain the increased risk of coronary events associated with decreasing temperature is the stimulation of cold receptors in the skin and therefore the sympathetic nervous system, leading to a rise in catecholamine levels. Moreover, increased platelet aggregation and blood viscosity during cold exposure promotes thrombosis and clot formation," he added.
"In a global environmental model, low temperature is by far the most important environmental trigger for AMI, whereas air pollution has a negligible effect. People at risk of AMI (for example elderly patients with diabetes and hypertension) can minimize their risk by avoiding big changes in temperature. This means wearing suitable clothes when going from the warm indoors to the colder outdoors, even beyond winter time," concluded Prof. Claeys.
The studies of Dr. Marques-Vidal and Prof. Claeys were presented at the European Society of Cardiology Congress 2013 in Amsterdam this week.