亞裔與白人幼童其血壓心跳及BMI並無不同,但亞裔之LF/HF較白人高;不論族裔,女孩子的LF/HF較男孩子高

Differences in heart rate variability between Asian and Caucasian children living in the same

Canadian community

Katharine E. Reed, Darren E.R. Warburton, Crystal L. Whitney, and

Heather A. McKay

Abstract: Heart rate variability (HRV) is an umbrella term for a variety of measures that assess autonomic influence on the heart. Reduced beat-to-beat variability is found in individuals with a variety of cardiac abnormalities. A reduced HRV positively correlates with obesity, poor aerobic fitness, and increasing age. Racial (black–white) differences are apparent in adults and adolescents. We aimed to evaluate (i) Asian–Caucasian differences in HRV and (ii) differences in HRV between girls and boys. Sixty-two children (30 male (15 Caucasian, 15 Asian) and 32 female (15 Caucasian,

17 Asians)) with a mean age of 10.3 ± 0.6 y underwent 5 min resting HRV recording, fitness testing (Leger’s 20 m

shuttle), and self-assessed maturity. Outcome HRV measures were a ratio of low to high frequency power (LF:HF), standard deviation of R–R intervals (SDRR) and root mean square of successive R–R intervals (RMSSD). Data were compared between groups using analysis of covariance (ANCOVA). There were no race or sex differences for time domain variables, mean R–R, body mass index, or blood pressure. Compared with Caucasian children, Asian children displayed a higher adjusted (fitness, R–R interval) LF:HF ratio (72.9 ± 59.4 vs. 120.6 ± 85.3, p < 0.05). Girls demonstrated

a higher adjusted LF:HF power than boys (117.2 ± 85.1 vs. 76.6 ± 62.4, p = < 0.05). In conclusion, Asian and

Caucasian children display different frequency domain components of heart rate variability.



Key words: autonomic nervous system, sympathetic, vagal, race, aerobic fitness, sex.

Résumé : La variabilité de la fréquence cardiaque (HRV) est un terme général qui englobe une série de mesures afin

d’évaluer l’influence autonome du coeur. Chez les individus présentant une anomalie cardiaque, on observe une moins

grande variation de battement à battement. Une HRV moindre est corrélée positivement avec l’obésité, une piètre

condition physique et le vieillissement. On observe des différences raciales (Noirs–Blancs) chez des adultes et des adolescents.

Nous voulons analyser (i) les différences de la HRV entre les Caucasiens et les Asiatiques et (ii) les différences

de la HRV entre les garçons et les filles. Soixante-deux enfants dont 30 garçons (15 Caucasiens et 15 Asiatiques)

et 32 filles (15 Caucasiennes et 17 Asiatiques), âge : 10,3 ± 0,6 ans, participent à une évaluation de la HRV durant

5 min au repos, au test de Léger (course-navette sur 20 m) et font une autoévaluation de leur maturité. Les mesures

retenues sont le ratio de la puissance de basse fréquence sur la puissance de haute fréquence (LF:HF), l’écart type des

intervalles R–R (SDNN) et la valeur quadratique moyenne d’intervalles R–R successifs (RMSSD). On analyse les différences

au moyen d’une analyse de covariance. On n’observe aucune différence entre les races et les genres dans les résultats

suivants : variables à réponse temporelle, R–R moyen, IMC et pression sanguine. Comparativement aux enfants

caucasiens, les Asiatiques présentent un meilleur ratio LF:HF ajusté (condition physique, intervalle R–R) : 72,9 ± 59,4

vs. 120,6 ± 85,3, p < 0,05. Les filles présentent une plus grande puissance LF:HF ajustée : 117,2 ± 85,1 vs. 76,6 ±

62,4, p = < 0,05. En conclusion, les composantes de la variabilité de la fréquence cardiaque des enfants caucasiens et

asiatiques n’ont pas la même réponse temporelle.

Mots clés : système nerveux autonome, sympathique, vagal, race, puissance aérobie, sexe.

[Traduit par la Rédaction] Reed 6

Appl. Physiol. Nutr. Metab. 31: 1–6 (2006) doi:10.1139/H05-015 © 2006 NRC Canada

1

Pagination not final/Pagination non finale

Received 8 January 2005. Accepted 18 July 2005. Published on the NRC Research Press Web site at http://apnm.nrc.ca on 23 March

2006.

K.E. Reed, D.E.R. Warburton, and C.L. Whitney. School of Human Kinetics, University of British Columbia, Vancouver, BC,

Canada.

H.A. McKay.1 Department of Orthopaedics / Family Practice, University of British Columbia, 5th Floor, Research Pavilion, 828

West 10th Avenue, Vancouver, BC V5Z 1L8, Canada.

1Corresponding author (e-mail: mckayh@interchange.ubc.ca).

Introduction

Heart rate variability (HRV) measured by power spectral

analysis provides a quantitative marker of autonomic nervous

system influence on heart rate and has been shown to

reflect cardiovascular health. In adults, impaired variability

has been reported following myocardial infarction

(Sosnowski et al. 2002), in chronic heart failure, and in left

ventricular dysfunction (Nolan et al. 1992). In young children,

reduced HRV values are associated with atrial septal

defects (Finley et al. 1989) and increased likelihood of sudden

infant death syndrome (Edner et al. 2002). An unfavourable

autonomic profile balance (manifesting as reduced beatto-

beat variability) reflects a predominately sympathetic influence

on control of heart rate and is positively correlated

with general obesity (Martini et al. 2001; Nagai et al. 2004;

Gutin et al. 2000), high visceral fat deposition (Gao et al.

1996), lower aerobic fitness (Gregoire et al. 1996; Aubert et

al. 2001), male gender (Sinnreich et al. 1998), and increasing

age (Umetani et al. 1998); (Reardon and Malik 1996).

Racial (black–white) differences in HRV have been previously

studied in adults, with blacks having a lower sympathetic

drive than age-matched whites (Guzzetti et al. 2000)

(Liao et al. 1995). There are only limited data in young children

and none that compare Asians and Caucasians. Previous

data concerning race differences in youth have been

equivocal. Whilst one investigation found that black adolescents

display less favourable HRV measures (i.e., a greater

sympathetic contribution to total power) than age-matched

whites (Faulkner et al. 2003), another found reduced sympathetic

activity in blacks (Urbina et al. 1998). Heart rate variability

has been explored exclusively in Asian populations in

adults and children, but has not been compared with other

races. Results from independent studies of Asian (Kikuchi et

al. 2003; Kazuma et al. 2002; Nagai et al. 2004) or Caucasian

children (Faulkner et al. 2003; Mandigout et al. 2002)

suggest there may be racial differences in time and frequency

domains of HRV, with Asian children living in Asia

displaying a lower HRV than Caucasian children living in

western societies. Canada is a multiracial society with more

than 2 million Canadians reporting an Asian origin (Statistics

Canada 2004). There have, however, been no comparisons

between Asian and Caucasian children living in the

same North American community.

The effect of male or female gender on HRV is well documented

in adults, with the majority of researchers reporting

that women, at least until late middle age, demonstrated a

higher vagal influence on heart rate control than men

(Gregoire et al. 1996; Liao et al. 1995; Antelmi et al. 2004).

However, the influence of sex appears to be modulated by

age (Umetani et al. 1998) and studies that have examined

sex differences in children have found that girls have lower

variability than boys (Umetani et al. 1998; Faulkner et al.

2003). These studies, which involved 24 h HRV monitoring,

showed lower time domain values in girls aged 14–16 y and

1–20 y, respectively. To our knowledge, no studies have used

5 min recording to examine sex differences.

Thus, our primary objective was to determine whether

differences in HRV existed between Asian–Canadian (AC)

and Caucasian–Canadian (CC) children living in the same

community. Our second objective was to explore differences

in HRV between prepubertal girls and boys using a 5 min

recording.

We hypothesize that compared with CC children, AC children

will have lower levels of the time domain variables

(standard deviation of R–R intervals (SDRR) and root mean

square of successive R–R intervals, (RMSSD)) accompanied

by greater sympathetic predominance, evidenced by a higher

LF:HF in the frequency domain variables. We also hypothesize

that girls will demonstrate altered HRV profiles, specifically

lower variability, compared with age-matched boys using

5 min recordings similar to those derived from 24 h recordings.

Materials and methods

Participants

Sixty-six Asian and Caucasian children from grades 4 and

5 were randomly selected from a larger cohort of participants

in a school-based exercise intervention (Action

Schools!, B.C., N = 514). Parents of the children completed

a health history questionnaire on the child’s behalf. Children

with cardiovascular disease were deemed ineligible to participate

in the intervention; thus, no further children were excluded

from the present study. Children were classified as

Caucasian, Asian, East Indian, or Other based on the birthplace

of both parents. Children were classified as “Asian” if

both parents, or all 4 grandparents, were born in Hong Kong,

China, Japan, Taiwan, or Korea; “Caucasian” providing both

parents or all 4 grandparents were born in Europe or North

America; and “Other” if the child had parents of other origins

(i.e., Africa or India) or had parents of 2 distinct races.

The University of British Columbia Clinical Research Ethics

Board approved the investigation and all participants and

their legal guardians provided written consent.

Measurements

Short-term, 5 min resting HRV was taken using Polar

S810 Heart Rate Monitors (Polar Electro, Oy, Finland). As

measurement error attenuates the correlation observed between

variables, we attempted to control potentially confounding

variables by (i) instructing children not to consume

a caffeinated beverage for at least 2 h before HRV measurement,

(ii) taking all measurements before lunch break activity,

and (iii) making all recordings on school premises.

Children lay supine on a padded mat in a quiet, softly lit

room. Recording began immediately and lasted for 6 min.

Digitally coded R–R interval length was input into Polar

software (Polar Electro) using an infrared transmitter to display

a tachogram on screen. After the first minute of data

was discarded, R–R intervals were automatically filtered

using median- and moving-average-based filtered methods.

Acquisition and filtering of R–R data using Polar software

has been previously described (Jurca et al. 2004). Data (R–R

intervals) were then exported as text to HRV Analysis Software

(Biomedical Signal Analysis, Kuopio, Finland). The

R–R series were transformed to the frequency domain via

fast Fourier transformation. Spectral power was determined,

in accordance with the Task Force of the European Society

of Cardiology and the North American Society of Pacing

Electrophysiology (Task Force 1996), as very low frequency

(VLF; 0.01–0.04 Hz), low frequency (LF; 0.04–0.15 Hz), or

high frequency (HF; 0.15–0.5 Hz). LF:HF was chosen as the

© 2006 NRC Canada

2 Appl. Physiol. Nutr. Metab. Vol. 31, 2006

primary measure of interest, as it provides information regarding

relative vagal or sympathetic predominance (Pagani

et al. 1986). Values for HF and LF are given as normalized

units (nu). For HF, normalized units are calculated as

HF / (total power – VLF) × 100

where HF is measured in ms2. For LF, normalized units are

calculated as

LF / (total power – VLF) × 100

where LF is measured in ms2. Secondary variables of interest

were the global time domain measure, the standard deviation

of normalized R–R intervals (measured in milliseconds

(ms); SDRR), and the root mean squared of successive R–R

intervals (also measured in ms; RMSSD), which is thought

to represent vagal activity.

Height in bare feet was measured to the nearest 1 mm.

Weight, in light indoor clothing, was measured using an

electronic scale (SECA, Hamburg, Germany) to the nearest

0.1 kg. Duplicate measures were taken unless measures differed

by ± 0.4 cm or ± 0.2 kg when a third measure was

made. The average of 2 values or the median of 3 values was

used for analysis. Body mass index (BMI) was calculated as

kg/m2. Aerobic fitness was determined via Leger’s 20 m progressive

shuttle run (Leger et al. 1988). Children begin running

at 8.5 km/h and increase in speed by 0.5 km/h each

minute. Children continued running until they were unable

to maintain the required pace at a given level. This maximal

test was developed for children and estimates aerobic capacity

from running speed and duration. Blood pressure was

measured in duplicate on the left arm after 5–10 min quiet

rest using an automated sphygmomanometer (VSM

MedTech, Canada) using an appropriately sized cuff. If values

were within 5 mmHg for systolic blood pressure, the

lowest value was recorded. If the difference exceeded 5 mmHg

then a third measurement was taken. Children self-assessed

their physical maturity using line drawings and descriptions

of pubic hair (boys and girls) and breast stage (girls) based

on Tanner staging (Tanner 1955). Stage 1 represents prepuberty,

stage 2 represents early puberty, stage 3 represents

middle puberty, stage 4 late represents puberty, and stage 5

is considered post pubertal.

Statistical procedures

HRV measures that were skewed (Kolmogorov–Smirnov

Z, p < 0.05) were transformed (natural logarithm (ln)) to

normalize the distribution. In these cases, statistical comparisons

were based on the ln scales. Analysis of covariance

(ANCOVA) was used to evaluate race and sex differences in

HRV. Owing to their established relationship with HRV, we

controlled for aerobic fitness (Leger’s 20 m shuttle) and

mean resting heart rate by including them as covariates in

the analysis. Statistical significance was set at alpha level p <

0.05. All statistical analyses were performed using SPSS

version 12.0 for Windows (SPSS Inc., Chicago, Ill.).

Results

Sixty-two children (32 AC children comprising 15 boys

and 17 girls; 30 CC children comprising 15 boys and 15

girls) aged 9–11 years were included in the analysis. Data

from 4 children were excluded owing to excessive movement

during recording. Descriptive characteristics (means ±

SD) of the participants by sex and race are provided

(Table 1). All children reported to be in Tanner stage 1 or 2.

There were no racial differences for age, BMI, heart rate,

or blood pressure. CC children ran a significantly greater

number of 20 m laps compared with AC children (26.5 ±

11.3 and 19.9 ± 5.7, respectively, p = 0.006).

There were no sex differences for age, BMI, aerobic fitness,

heart rate, or blood pressure.

Heart rate variability measurements

Group means (non-adjusted) for raw and log-transformed

data by race and sex are provided (Table 2). There were no

race or sex differences for total power, SDRR intervals, or

RMSSD.

There was a difference in LF:HF between AC and CC

children (F = 5.8, p = 0.01), with AC children having a

higher LF:HF than CC children (Table 3). Analysis by sex

showed a higher LF:HF ratio (F = 4.3, p = 0.04) in girls than

in boys (Table 3). Within-sex differences showed that AC

children had higher (NS) LF:HF than CC children (93.9 ±

16.6 and 59.2 ± 16.5, F = 2.1, p = 0.17 for AC and CC boys,

respectively; 144.5 ± 20.7 and 86.2 ± 22.1, F = 3.5, p = 0.71

for AC and CC girls, respectively).

Discussion

Comparison between Asian and Caucasian children

This was the first investigation to examine differences in

HRV between Caucasian and Asian children living in the

same Canadian community. AC children had higher LF:HF

when compared with CC children of the same age. A lower

© 2006 NRC Canada

Reed 3

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Asians Caucasians Boys Girls

n 32 30 30 32

Age (y) 10.3 (0.6) 10.5 (0.6) 10.2 (0.6) 10.5 (0.6)

BMI (kg/m2) 18.6 (3.5) 18.6 (2.6) 18.8 (2.9) 18.4 (3.2)

No of. 20 m laps run 19.9 (5.7)* 26.5 (11.3)* 21.8 (8.5) 24.4 (10.2)

SBP (mmHg) 102.2 (9.1) 105.7 (10.5) 103.9 (8.1) 103.6 (11.2)

DBP (mmHg) 65.1 (7.5) 65.5 (8.2) 66.6 (6.8) 64.1 (8.5)

Heart rate (b/min) 81.1 (12.1) 76.9 (10.1) 77.9 (12.1) 80.0 (10.1)

Note: Values are means (± SD). BMI, body mass index; DBP, diastolic blood pressure; SBP, systolic blood

pressure; b/min, beats per minute).

*Race difference at a significance level of p < 0.05.

Table 1. Descriptive characteristics of participants by sex and race.

LF:HF is indicative, although not definitive, of a higher

sympathetic and (or) lower vagal influence on heart rate.

However, this finding of a differential influence on heart

rate according to race is further supported by the higher

(NS) values for RMSSD seen in CC children. This time domain

measure is strongly influenced by vagal modulation of

the SA node. Additionally, when measured in the supine position,

as in the present study, global measure of HRV such

as SDNN are modulated primarily by the activity of the

vagus. Although the differences in SDNN and RMSSD between

AC and CC children failed to reach statistical significance

here, both measures are clearly higher in the latter

group.

Our findings support previous work that highlights a racial

difference in adults and adolescents in HRV (Liao et al.

1995; Faulkner et al. 2003); (Urbina et al. 1998). Investigations

that compared African–American with Caucasian–

American adolescents have been equivocal. While one study

revealed that 15-year-old African–American males generally

had less favourable HRV outcome measures (greater sympathetic

modulation of heart rate) (Faulkner et al. 2003), a similar

study (Urbina et al. 1998), reported that 13- to 17-yearold

African–American male youth had less sympathetic tone

and greater short-term variability. Racial differences in HRV

were evident at rest and during cardiovascular stress tests

such as the Valsalva manoeuvre (Urbina et al. 1998). Similar

comparisons have not been previously made in younger age

groups or, until now, between Asian and Caucasians.

Comparison between girls and boys

In this cohort, boys had a greater contribution of highfrequency

power to total power than girls. Boys and girls

scored similarly on aerobic fitness tests, but girls had an elevated

LF:HF power ratio compared with boys. These results

support previously reported sex differences (Faulkner et al.

2003) (Silvetti et al. 2001) wherein girls, aged 15 y and 1–

20 y, respectively, demonstrated a lower HRV than boys.

Both of these investigations found that time domain variables

(namely SDRR and the standard deviation of normal R–R

intervals for 5 min segments, SDANN) were higher in boys.

In the same investigation, however, Silvetti and colleagues

(Silvetti et al. 2001) reported no sex differences for

RMSSD. Conversely, the present study found that differences

in HRV measures were significant when measured in

the frequency domain (LF:HF). Differences were also evident

in the time domain variable measures (SDRR and

RMSSD), but these failed to reach statistical significance.

Previously, it has not been shown that sex differences in

HRV were observed with short-term (5 min) recordings. The

discrepancies between the findings of this study, and those

of other studies i.e., no difference in time domain, only in

frequency domain variables, is likely because of the duration

of the recording. Frequency domain methods should be preferred

to the time domain methods when short-term recordings

are investigated (Task Force 1996).

The relationships between physical activity and indices of

spectral power parallel those reported in adults (i.e., more

active individuals typically demonstrate greater vagal predominance)

(Gregoire et al. 1996). Aerobic training is believed

to improve the electrical stability of the myocardium,

with regular exercise improving the cardiac autonomic profile

(Melanson and Freedson 2001). Nagai and colleagues

(Nagai et al. 2004) conducted a cross-sectional investigation

and separated 96 girls and boys into 4 groups; lean physically

active, lean sedentary, obese physically active, and

obese sedentary. Lean active children had significantly better

HRV parameters compared with the other group. However,

physical activity also appeared to contribute to enhanced autonomic

nervous system activity in both lean and obese children.

In the present investigation, differences in heart rate

variability between girls and boys, and between AC and CC

children persisted after adjusting for physical fitness.

Although a substantial proportion of the variance in HRV

can be accounted for by factors such as age, sex, BMI, physical

activity, and fitness, the Framingham Heart Study and

the Kibbutzim Family study found that up to 34% of the

variance was accounted for by genetic factors (Singh et al.

1999; Sinnreich et al. 1999). Although sex differences in

© 2006 NRC Canada

4 Appl. Physiol. Nutr. Metab. Vol. 31, 2006

Asian Caucasian Boys Girls

LF 48.78 (16.62) 37.57 (14.81) 38.71 (14.81) 47.62 (17.41)

Ln 3.82 (0.38)* 3.54 (0.40)* 3.57 (0.41)† 3.79 (0.39)†

HF 51.22 (16.62) 62.53 (14.83) 61.25 (14.81) 52.33 (17.39)

Ln 3.87 (0.35)* 4.10 (0.28)* 4.07 (0.28)† 3.89 (0.36)†

LF:HF 120.57 (85.31) 72.92 (59.42) 76.57 (62.4) 117.19 (85.12)

Ln 4.54 (0.73)* 4.05 (0.67)* 4.11 (0.68)† 4.51 (0.74)†

RMSSD 55.11 (34.45) 75.50 (54.77) 65.18 (38.21) 64.12 (53.17)

Ln 3.83 (0.59) 4.06 (0.72) 4.03 (0.56) 3.87 (0.76)

SDRR 56.41 (27.37) 64.92 (38.02) 60.17 (30.01) 60.81 (35.91)

Ln 3.89 (0.59) 4.01 (0.62) 3.94 (0.63) 3.94 (0.59)

TP 559 (660) 626 (824) 624 (855) 558 (735)

Ln 5.66 (1.27) 5.58 (1.41) 5.64 (1.41) 5.61 (1.27)

Note: Statistical significance refers to log transformed and raw units data in each case. HF, high frequency

power; LF, low frequency power; RMSSD, root mean squared of successive R–R intervals; SDRR, standard

deviation of R–R intervals; TP, total power; Ln, log-transformed value).

*Race difference at p < 0.05.

†Sex difference at p < 0.05.

Table 2. Group (race and sex) mean (± SD) of raw and log-transformed heart rate variability

data (non-adjusted).

adult HRV measures are well documented, race differences

and sex differences in children have not been well investigated.

We acknowledge some limitations in the study. Although

we controlled for aerobic fitness, we were unable to match

children on physical activity. Second, we performed a crosssectional

comparison of sex and race. Analysis of heart rate

variability in children is a complex issue, and the evolving

nature of the autonomic nervous system as maturation occurs

adds further difficulty. Longitudinal investigations of

HRV are necessary to determine whether time and frequency

domain measures show similar age-, race-, and sex-related

patterns over time.

Conclusions

There is a paucity of literature describing HRV in young

children and none that compared Asian to Caucasian children.

We demonstrated that AC children aged 9–11 y had

significantly elevated LF:HF power compared with CC children

living in the same community. These findings support

previous investigations suggesting black–white differences

in heart rate variability in children, but introduce new findings

regarding altered variability profiles between Asian and

Caucasian children. Although this difference is unlikely to

result in adverse health implications during childhood, racial

norms for HRV measures should be determined and considered

during clinical examinations and experimental investigation.

Further studies to establish racial norms may be

warranted.

Acknowledgements

We acknowledge support from the Ministry of Health

Planning, B.C., the Canada Foundation for Innovation,

Ottawa, Ont., and the Michael Smith Foundation for Health

Research, Vancouver, B.C.

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Group Adjusted log LF:HF

Asians 4.52 (0.12)*

Caucasians 4.07 (0.13)*

Males 4.11 (0.13)†

Females 4.48 (0.12)†

*Race difference at p < 0.05.

†Sex difference at p < 0.05.

Table 3. Adjusted (aerobic fitness and heart

rate) means of log-transformed low : high frequency

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Summary

Chronobiology International

1997, Vol. 14, No. 6, Pages 597-606