Heart Rate Levels in the Populations of the Russian Federation and the United States of America during the Thirty-Year Perspective

Heart rate (HR) acceleration is one of the risk factors of the number of prevalent chronic non-communicable diseases. Aim. To evaluate changes in mean levels of HR in the Russian Federation (RF) and the United States of America (USA) in different age and sex groups from 1975 to 2014. Material and methods. HR levels in men and women of different age groups were evaluated in the populations of the RF and the USA in 1975-1982 and in 2007-2014 by the secondary analysis of cross-sectional trials data. A total number of cases was 48974. Results. In 80th years of XX century Russian men and women of all age groups had lower HR as compared to the USA, the distinctions were statistically significant in all age groups except for men aged 25-34 years (p<0.0001). During the next 3 decades HR significantly decreased in all groups of the USA population, while the RF population demonstrated variety of HR levels trends in different groups, absolute distinctions in mean values were relatively small and did not exceed 2 beats per minute. At the present time HR levels are less in the RF as compared to the USA in women aged 25-55 years and in men aged 25-34 years; in 55-64-year old men HR is somewhat higher in the RF than in the USA, other groups revealed statistically insignificant distinctions. Conclusion. The USA evidently loosed out to the RF in mean levels of HR in all age groups 35 years earlier. During the next 3 decades situation has been worsened as applied to the RF and distinctions with the USA have been largely smoothed over.

Heart rate (HR) of a healthy person is varying from 50 to 75 beats per minute (bpm) depending on age, sex and life style.It is determined by activity of the sinus node cardiac pacemaker under influence of the vegetative nervous system.HR reflects wide range of physiological and pathologic conditions and adapts to support cardiac output and maintain vital organs perfusion [1].
In accordance with experimental and clinical trials data steady HR acceleration can be at the forefront of coronary atherosclerosis pathogenesis and its complications development.Mechanical and metabolic factors resulted from HR increase can promote atherosclerosis progression [2].
High HR is an important factor determining myocardial ischemia as it leads to increase in myocardial oxygen consumption and decreases myocardial perfusion due to reduced diastole.The higher base HR the stronger possibility of an ischemic event.At a base HR of less than 60 bpm, the probability of HR acceleration causing the ischemic event is 8.7% as compared to 18.5% at a HR of more than 90 bpm [3].
There are many evidences that HR acceleration is an important factor influencing unfavorable outcomes in such patients with persistence of the effect after adjustment for other risk factors [4].Prospective trials revealed independent relationship between high HR and increased cardiovascular morbidity and mortality [5,6].In particular, HR of more than 80 bpm was demonstrated to more often cause atherosclerotic plaque disruption which is a basis for acute coronary syndrome development [7].The authors consider a HR of more than 80 bpm to be an important independent risk factor of myocardial infarction morbidity and a predictor of death in such patients [7].Results of examination of a cohort of Russian men and women had also demonstrated progressive reduction in survival and, accordingly, increase in mortality rates at HR of more than 80 bpm [8].
Consequently, existing evidences confirm importance of HR measurement in routine clinical practice to reveal persons at increased risk of unfavorable outcomes.However routine clinical practice often underestimates HR significance.
Our study was aimed at evaluation of changes in HR mean values in various age and sex cohorts of the Russian Federation (RF) and the United States of America (USA) populations from 1975 to 2014.

Material and methods
The work used data of population-based studies conducted in the National Medical Research Center for Preventive Medicine in 1975-1982, protocols of the studies had been published earlier [11,12].These data were combined in a sample with the working title as "RF-1980".Data of the "ESSE-RF" (Epidemiology of cardiovascular diseases and their risk factors in different regions of the RF), cross-sectional observational trial dealt with a random systematic stratified multistage sample formed by the territorial principle and conducted in 2012-2014, have also been used [13].The research was approved by the Ethics Committees of the National Medical Research Center for Preventive Medicine, the National Medical Research Center named after V.A. Almazov and the National Medical Research Center of Cardiology.All participants of the study had signed the informed consent to processing of personal data.
The received data were further compared with data of cross-sectional studies of unorganized civilian population of the USA of the NHANES series: NHANES II (1976II ( -1980) ) and Continuous NHANES (C.NHANES, 2007NHANES, -2012)).The design of these studies and methods of measurements were described in the relevant documentation available at the National Center for Health Statistics (NCHS) site, the USA [14].
Age, sex and HR levels were analyzed.Each population-based study compared HR levels in men and women of the following age groups: 25-34, 35-44, 45-54 and 55-64 years.The number of cases in the groups (n) are listed in the "Results and discussion" section.
Statistical analysis was performed with help of the standard statistical procedures.Nonparametric methods were preferred for the groups comparison, in particular, Mann-Whitney U-test was used.Mean values and standard deviations (M and SD) were also calculated.

Results and discussion
Results of comparison of HR in the populations of the RF and the USA at the end of the tenth years of this century and 30 years earlier are listed in Tables 1  and 2, respectively.
In accordance with ESSE-RF and Continuous NHANES of 2007-2012 data mean HR was less in Russian women under 55 years and men aged 25-34 years as compared to the USA population; in men aged 55-64 years HR was somewhat high in the RF than in the USA; other groups revealed insignificant distinctions (Table 1).
According to the data of 35-year statute of limitations (Table 2) mean HR was less in all age groups of Russian men and women than this in American groups, the distinctions were statistically significant in all groups except in men aged 25-34 years.
So, 35 years earlier the populations revealed distinct unidirectional differences: HR was less in the RF as compared to the USA.At the present time these differences have been largely smoothed over, however the trend to higher HR among Americans remains in majority of age groups (especially in women).
As Table 3 shows HR changes in the RF during 30-year time span were diverse in different groups, absolute distinctions of mean levels were relatively small and did not exceed 2 bpm.
HR has significantly decreased in all the USA groups in the 30-year time span (Table 4).
So, in accordance with our study results American men and women of all age groups have significantly decreased HR during the 30-year time span.In the RF changes of mean HR was relatively small and diverse in different age and sex groups.
Similar data were received at comparison of HR in a sample of American men of the LRC study and Russian men in the Moscow-Leningrad sample which presented two Soviet centers (participants of the LRC) -Russian men revealed lower HR.It could be explained by higher exercise load in Russian men and is in line with our study results.Three decades earlier the RF could not boast of large number of motor-cars which were rather welfare measure than means of transport; while the USA were more urbanized, this led to higher prevalence of low physical activity and higher mean HR in men [15].At the present time the RF is characterized by very large number of motorcars, low physical activity of residents, wide prevalence of stress loads; while the USA, having achieved a higher level of welfare, are looking toward increase in physical activity which results in decreased HRassociation of high level of physical training with low HR is well known [16,17].
Observational studies demonstrate that a lower HR has a protective effect on the cardiovascular system.On the other side, high resting HR is a mortality predictor in wide variety of population groups, that include general population, patients with coronary heart disease, hypertension, heart failure and previous myocardial infarction [19].
So, HR can be potentially useful and simply measured biomarker of general state of health.Lowering of HR in selected subgroups of population may be an effective strategy of interference aimed at diseases risk reduction and improvement of health state prognosis.

Conclusion
Consistent outputs have been made at analysis of four large-scale samples (RF-1980, ESSE-RF, NHANES II and the Continuous NHANES) of the populations of two different countries: the RF and the USA.A total number of analyzed cases was 48974.The USA evidently loosed out to the RF in mean levels of HR in all age groups 35 years earlier.
Acknowledgements.The authors express appreciation to the participants of the ESSE-RF study who had collected the Russian data used in the article.
Besides, our study had become possible due to free access to the data of the trials of the NHANES series provided by the National Center for Health Statistics (NCHS) of the USA.The authors of the article are responsible for analysis results, interpretations and conclusions, the NCHS responsibility is limited by the primary data.
Disclosures.The publication of the article was supported by Servier under the CORAXAN program, but it did not affect the authors' own opinion.