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Abstract
Objective Resting heart rate (RHR) is increasingly recognised as a prognostic marker for long term cardiovascular outcomes in adults. This study assessed associations of RHR with blood pressure (BP), anthropometry and exercise in a large representative sample of Hong Kong children.
Study design, setting and subjects A territory-wide growth survey carried out in 2005–2006 included students sampled from each of Hong Kong's 18 districts. RHR and BP were measured by validated oscillometric BP devices and anthropometric data and exercise frequency were recorded. Multiple linear regressions were used to test associations among RHR and BP, anthropometry and exercise frequency.
Results Data on 14 842 children aged 6–18 years were available. Multiple linear regression analyses showed that RHR was positively associated with BP, and negatively associated with age and exercise frequency (p<0.001). RHR was more positively linked to waist circumference among the anthropometric measurements, and positive independent association was only identified in boys (p<0.001).
Conclusions Elevated RHR is independently associated with elevated BP in children, whereas increased structured exercise is related to lower RHR.
- Cardiology
- Comm Child Health
- Epidemiology
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What is already known on this topic
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Resting heart rate has been shown to be a cardiovascular prognostic marker in adults.
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High resting heart rate has been demonstrated to be associated with elevated blood pressure and physical inactivity in some small-scale studies in adolescents.
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Associations between resting heart rate and obesity in children have been inconsistent.
What this study adds
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High resting heart rate is associated with younger age, female sex, elevated blood pressure and physical inactivity in children.
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Resting heart rate is more closely linked to fatness than body size.
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High resting heart rate is associated with increased waist circumference in boys but not girls.
Background
Resting heart rate (RHR) is a readily measurable and important clinical parameter. Apart from serving as an aid in triage screening and an early warning marker of clinical deterioration,1 associations between RHR and all-cause and cardiovascular death rate in adults have been noted.2 ,3 In adults, there is a strong association between RHR and cardiovascular events that is independent of systolic blood pressure (SBP), level of physical activity, increase in body mass index (BMI) and waist circumference (WC).4 Elevated RHR in young adults is predictive of later development of hypertension.5 However, to our knowledge there are no reports on the relation between RHR in children and the later development of cardiovascular diseases. A few studies have indirectly investigated the association between RHR in children with other potential cardiovascular prognostic factors, like blood pressure (BP) and obesity.6–8 However, these studies included only adolescents or had small sample sizes, perhaps explaining an inconsistent association between RHR and obesity. We analyse a large dataset to determine the associations between RHR and potential cardiovascular prognostic factors, namely BP, anthropometry and exercise level.
Methodology
The study used data collected in a 2005/2006 Hong Kong Growth Survey. Details of this study of a large representative sample of primary and secondary Hong Kong school students have been previously reported.9 The study was approved by the Joint Chinese University of Hong Kong and New Territories East Cluster Clinical Research Ethics Committee and the Ethics Committee of the Department of Health of the Hong Kong Government.9
Sampling method
In the first stage, from each of the 18 districts in Hong Kong, one primary and one secondary school were randomly selected. In the second stage, two classes in each grade were selected based on operational needs of the school. All students of the selected classes were invited to join the study. The study included a self-administered questionnaire relating to exercise for students of grade 4 or above. Students enter grade 4 at the age of 8 or 9 years.
Measurement of RHR and BP
RHR and BP were measured by previously validated oscillometric devices (Datascope Accutorr Plus).10 ,11 Children were asked to sit in a chair and rest for at least 5 min before the measurements were taken with the right arm supported at heart level. Cuffs of appropriate size were used. Two sets of BP and RHR measurements at a 1-min interval were taken from which average values were used for analysis. Prehypertension was defined as SBP or diastolic BP within 90th–95th centile, whereas hypertension was defined as BP above or equal to 95th centile, adjusted for age and gender.
Measurement of anthropometric parameters
A Harpenden stadiometer was used to measure the standing height without shoes to the nearest 0.1 cm. Body weight and percentage body fat (% body fat) were measured using a portable Tanita body fat monitor/scale (Model BF-522). The body weight was measured to the nearest 0.1 kg in light clothing. An inelastic measuring tape was used to measure WC midway between the lowest rib and the superior border of the iliac crest at the end of normal expiration to the nearest 0.1 cm. High WC and excessive % body fat were defined as that greater than or equal to the 85th percentile of the specific parameters.12 ,13 BMI was calculated as weight divided by height squared (kg/m2). The International Obesity Task Force cut-offs for overweight and obesity were used.14
Exercise frequency
Two questions were included in the self-administered questionnaire to assess the frequency of activity in students: (1) Do you currently participate in any regular exercise class other than school physical education class? (2) If yes, please tick the frequency of training: 1, 2 or 3 and above per week. A detailed list of typical exercise training classes commonly offered in Hong Kong together with an explanation of these two questions was provided to all participants.
Statistical analysis
Data were summarised and presented separately by sex using appropriate descriptive statistics. Before further analysis, height, weight and BMI were converted into z-scores using local reference data in 1993,15 whereas BP parameters, WC and % fat were converted into z-scores with reference derived from the current study population. The participants’ characteristics (z-scores or percentage distribution) were compared using student t test or Pearson χ² test, depending on the underlying variable.
Univariate analyses were performed to assess the association of RHR with the studied variables stratified by sex and age. The age of 12 years was used to divide the children into young children and adolescents. Since there were collinearities between SBP and diastolic BP, and among height, weight, BMI, WC and % body fat, only the factor with the strongest correlation with RHR in each of the collinearity groups was chosen for further multivariable analyses. Multiple linear regressions were used to examine the association of the studied variables with RHR among children.
Statistical analyses were performed using SPSS 17.0 (SPSS Inc., Chicago, Illinois, USA). All statistical tests were two-sided and a p value <0.05 was considered statistically significant.
Results
A total of 14 842 Chinese school children (7472 boys, 7370 girls) aged 6–18 years were recruited. Overall 93% and 90% of the invited primary and secondary school students respectively participated. The main reasons for non-participation were absence from school (primary n=312, 4%; secondary n=530, 6%) or refusal to take part (primary n=208, 3%; secondary n=350, 4%). Among all the recruited students, 10 514 were grade 4 (aged 8–9 years) or above and provided information regarding their exercise frequency. A total of 14 605 students had their body fat composition assessed.
The characteristics of study sample are shown in table 1. There was a trend towards higher RHR in overweight and obese children, as well as in children with higher WC or % body fat (see online supplementary table 1). Hypertensive children also had higher RHR, whereas RHR decreased with increasing exercise frequency in a dose–response fashion.
Characteristics of the study sample (n=14 842)
Univariate and multivariable analyses showed that RHR was negatively associated with age, and positively associated with BP, even after adjustment (p<0.001) (table 2). WC had the strongest correlation with RHR among the anthropometric measures in all groups. Height was weakly associated with RHR in young boys (p=0.023), but not associated with RHR in older boys or girls (p>0.05). RHR was positively associated with all body fatness measures in univariate analyses (p<0.05), except BMI in adolescent girls (p=0.07). After adjustment for age and BP, only boys showed an independent positive correlation with the body fatness measures (p<0.001), but the association was lost in young girls (p=0.86), and even became negative in adolescent girls after adjustment (β=−0.04, p=0.02). For the analyses of students of grade 4 (aged 8–9 years) or above, RHR had a negative association with exercise frequency, in which the association and effect were stronger with increasing exercise level (p<0.001) (table 3). Further multivariable analysis using the combined dataset identified an independent association between RHR and sex, where girls had higher RHR.
Association among resting heart rate and age, BP and anthropometry indices in Hong Kong Chinese children and adolescents (n=14 842) stratified by age and sex
Association among resting heart rate and age, blood pressure, anthropometry indices and exercise frequency in Hong Kong Chinese children and adolescents (n=10 514)
Discussion
Using a large representative dataset we show that age and exercise were negatively associated with higher RHR, whereas elevated BP was positively associated. A weak association between RHR and obesity was noted. The negative association between higher RHR and age is consistent with the findings from previous studies.6 ,7 We also show that the gender difference of RHR in children was independent of age, anthropometry and exercise frequency. This suggests that the regulation of RHR is different in the two sexes, and that the gender difference in RHR is not related to boys doing more exercise than girls. Our positive association between higher RHR and BP has been noted in other studies. A cross-sectional study of 356 Brazilian male children showed that HR was positively correlated with BP, independent of BMI.8 A similar finding was reported in 2230 Italian adolescents.6 These consistent findings suggest a possible direct physiological link between BP and RHR, which could be independent of other factors acting on the autonomic nervous system.8 Common mechanisms related to sympathetic tone may exist that regulate both BP and RHR.16 ,17 Although hypertension is a well established cardiovascular risk factor in adults, it is yet to be shown that hypertension in children, particularly mild hypertension, has a long term impact on cardiovascular health. Thus, longitudinal studies will be needed on determine whether elevated BP and RHR in children can predict future adverse health outcomes.
For BP, weight has a higher and opposite impact than height.18 For RHR, we show that increased weight and BMI were associated with a higher RHR, whereas height was not. This is consistent with previous findings.6 Similarly WC and % body fat showed a similar positive association with RHR, suggesting that the association is more likely related to fatness, rather than body size. In adults, obese subjects tend to have a higher RHR, possibly related to angiotensinogen secretion by adipose tissue acting on the sympathetic system.16 A positive correlation between BMI and RHR has been noted in Brazilian boys,8 and between weight and RHR in Italian adolescents.6 However, in another study from Brazil of 11-year-old adolescents, no and even a negative association was found between BMI and RHR in both sexes after adjusting for confounders.7 In our study, WC was found to have the highest correlation in univariate analyses in all groups of children and adolescents. BMI assesses both muscle and fat which predicts subcutaneous adiposity, whereas WC is more related to visceral adiposity.19 This suggests RHR could be more closely linked to visceral adiposity in children. Further multivariable analyses showed the positive association only held true for boys after adjustment, but not for girls.
Physical training increases parasympathetic and decreases sympathetic activity at rest,20 and may explain our finding of a positive association between exercise frequency and lower RHR. Together with reduced catecholamine release, exercise training plays a role in RHR reduction.21 Although the positive association between physical training and lower RHR may be beneficial for future cardiovascular health, the evidence for such an effect is limited in children. This is in contrast to stronger evidence in adults for RHR to be a marker of long term disease. It has been suggested that elevated RHR may be a potential measure to screen children with dyslipidaemia who are at risk of future development of atherosclerosis.22 Further study of the potential role of RHR in predicting disease seems warranted, given that it is a simple measurement to undertake and has associations with cardiometabolic risk factors such as elevated BP, physical inactivity, dyslipidaemia and possibly central obesity.
The main strength of our study is that it uses data from a large representative sample (n=14 842) of Hong Kong Children. However, there were also a number of important limitations. First, the method of measuring RHR might have influenced our results. Although children were asked to rest for 5 min prior to measuring RHR, it is possible that obese children may have slower recovery of HR.23 Our RHR values for children older than 8 years are higher than the RHR reference values derived from a systemic review by Fleming et al (see online supplementary figure 1).24 Our RHR values are also higher than those reported in another smaller Hong Kong community-based survey using similar methodology to ours.25 The reasons why our RHRs are higher than these other studies are unclear. One possibility could be related to our sampling environment. Our study was done in a school setting where a large group of students assembled in a common area for assessment. Although students were kept seated and rested for 5 min prior to measurements, it is possible that the students may have got excited easily through the surrounding activity and interactions with their classmates. It should be noted that the lack of a standard method for measuring RHR has also been an important limitation in previous studies.26 ,27 Despite this limitation, we believe that with our study aiming to explore the association of RHR with important physiological parameters, it is less likely that the absolute values of RHR would impact on the direction and strength of their relationships. Second, it is possible that the oscillometric devices used to measure RHR might have affected RHR measurements. These devices are widely used in local practice and have been employed in large epidemiological surveys, based on the recommendations from experts.28 RHR obtained from oscillometric devices have been shown to approximate the accuracy and reliability of measures obtained from the standard method.29 Third, we did not document pubertal stage. Sexual maturation has been a demonstrated independent determinant of RHR.6 Fourth, physical activity in this study was only assessed with two simple questions. Physical activity is a broad term, and its heterogeneous nature makes it extremely difficult to characterise and quantify. Although there are some validated physical activity questionnaires, these are lengthy. Since our study was not primarily aimed to assess the physical activity of the participants, we instead used two direct and simple questions to capture the participants’ exercise frequency from a public health prospective. The questions used in our study were similar to those used by other investigators.30 Finally, the cross-sectional study design limits our ability to determine whether any of the associations identified are causal.
Conclusions
With adjustment for potential confounders, high RHR was independently associated with younger age, female sex, elevated BP and physical inactivity in children. A weak association with WC and RHR was noted in boys but not girls.
Acknowledgments
The authors thank the school principals, teachers, parents and students who participated in the study.
References
Supplementary materials
Supplementary Data
This web only file has been produced by the BMJ Publishing Group from an electronic file supplied by the author(s) and has not been edited for content.
Files in this Data Supplement:
- Data supplement 1 - Online supplement
Footnotes
Contributors SYK had full access to the study data and prepared the initial draft of the article. HKS coordinated the study and assisted in the supervision of data collection. HKS and KCC took active part in the statistical work. EASN, RYTS, AML and AFCL critically revised the manuscript for important intellectual content. RYTS, EASN and AML prepared the proposal and supervised the study. All authors contributed to the paper's submitted form and have read and approved the final paper.
Funding The study was supported by internal funding from the Department of Paediatrics, The Chinese University of Hong Kong and the Hong Kong Paediatric Society.
Competing interests None.
Ethics approval Ethics Committee, Chinese University of Hong Kong.
Patient consent Obtained.
Provenance and peer review Not commissioned; externally peer reviewed.