Heart rate variability in the general population and its determining factors

Background: As the leading cause of mortality in Switzerland and many other developed
countries, cardiovascular disease (CVD) is of high significance in public health: In 2003, CVD
was the leading cause of death in Switzerland, accounting for 38% of adult deaths. Heart
rate variability (HRV) is a subclinical electrophysiologic marker of cardiac autonomic control
and lower HRV is associated with higher CVD morbidity and mortality. Thus altered HRV
may signify autonomic dysregulation, and study of factors influencing HRV may inform
epidemiologic and clinical evaluation of pathways ultimately leading to cardiac morbidity and
mortality. Literature on HRV in the general population and its influencing factors is relatively
scarce. Literature on HRV in the general population and its influencing factors is relatively
scarce.
Aims: This thesis presents normal values of HRV for the Swiss population and describes its
determinants. As an example of an environmental risk factor, we study the influence of
passive smoking on HRV and possible mechanisms. A further aim of the thesis is to describe
the impact of obesity and a sedentary lifestyle as personal risk factors on HRV and to look at
modification of the effect of obesity by physical exercise.
Methods: Between 2001 and 2003, we measured time and frequency domain parameters of
HRV in a random sample of the SAPALDIA (Swiss cohort study on Air Pollution and Lung
Diseases in Adults) cohort participants aged 50 to 72 using digital 24-hour electrocardiogram
recordings. Subjects with recordings of less than 18 hours, atrial fibrillation, recent
anaesthesia or myocardial infarction or subjects taking digitalis were excluded, and
recordings of insufficient quality discarded, leaving 1742 recordings for analyses. Health
status and personal risk factors were assessed in a detailed interview. Blood pressure,
height and weight as well as markers of cardiovascular health in the blood were measured.
To assess effects of different risk factors on HRV, a structured multivariable linear regression
was performed. To estimate percentiles of gender specific HRV parameters as a function of
age, subjects with known risk factors (smoking, history of cardiovascular disease, high blood
pressure, diabetes, medication) were excluded. In multivariable regression analyses, we
determined the effect of environmental tobacco smoke on HRV, heart rate and blood
pressure. We estimated the effect of physical activity and BMI on HRV in multivariable
regression analyses and investigated the interaction between those two variables.
Results: Taking into account the effects of age, lifestyle factors, variables of cardiovascular
health, medication and cardiovascular risk factors in the blood, women had a lower HRV than
men. The age dependent decline of HRV differed between the sexes. High blood pressure, current smoking, higher levels of uric acid, high-sensitive C-reactive protein and non-HDLcholesterol
were independently associated with lower HRV.
Non-smoking subjects exposed to tobacco smoke pollution at home or at work for more than
2 hours/day had lower HRV values than unexposed subjects. We also found a higher heart
rate in exposed subjects and a tendency for higher diastolic blood pressure.
Obese subjects exercising regularly 2 hours per week or more had higher HRV than their
sedentary peers. Regardless of weight, the improvement of HRV was greatest for those who
exercised regularly. Subjects having gained weight over the past 11 years who were
exercising at least moderately had increased HRV values compared to subjects not
exercising. Exercising regularly significantly modified the relation of obesity with HRV.
Conclusions: HRV shows independent associations with a great variety of known personal
and environmental cardiovascular risk factors. These findings help elucidate cardiac
autonomic pathways through which factors such as passive smoke exposure and inactivity
may adversely influence cardiac morbidity and mortality. As such, the findings strengthen
scientific understanding of the etiology of cardiac disease, and may be useful in the
development of public health strategies to protect and improve cardiovascular health.