Abstract
Background
Heart rate (HR) and respiratory rate (RR) are key vital signs, yet few large studies have reported their normal values in toddlers. We thus assessed HR and RR in two-year-old Japanese children. Given that body size may influence vital signs, we first assessed the correlations of height and weight with HR and RR. We also examined associations of HR and RR with physical activity levels and residential region, as these factors have been linked to vital signs in adults.
Methods
This prospective cohort study included children from the Japan Environment and Children's Study. We measured their HR, RR, height, and weight at two years of age, and assessed their outdoor playtime from one to two years of age using a questionnaire. Participants without HR or RR data were excluded from the analysis. Descriptive statistics for HR and RR at two years of age were summarized. In addition, the correlation coefficient between the participants' height or weight and HR or RR were calculated. We investigated the association between the region or outdoor playtime and HR or RR using a one-way analysis of variance and multiple regression analyses. The covariates included medical history, mother's gestational hypertension, parents’ height and weight, parents’ smoking status, and parents’ educational background.
Results
A total of 4,805 participants were included in the HR analysis, and 4,806 in the RR analysis. The mean HR and RR of were 109.2 ± 14.7 beats/min and 29.9 ± 6.1 breaths/min (mean ± standard deviation), respectively. No correlation was found between height or weight and HR or RR. The mean HR of participants who almost never spent time playing outdoors was 111.7 beats/min, whereas that of participants who spent ≥ 3 h/d playing outdoors was 108.2 beats/min (p = 0.038). The mean HR in Eastern Japan was 106.8 /min, whereas that in Western Japan was 112.7 beats/min (p < 0.001).
Conclusion
We assessed the normal HR and RR in Japanese children aged two years, and these normal ranges did not correlate with height and weight. A longer duration of outdoor playtime was associated with a lower HR.
Supplementary Information
The online version contains supplementary material available at 10.1186/s12887-025-05867-3
Keywords: Heart rate, Outdoor playtime, Region, Respiratory rate, Toddlers, Vital signs
Introduction
Heart rate (HR) and respiratory rate (RR) are vital signs for assessing the physiological status of children in several clinical settings [1]. The pediatric advanced life support guidelines (2020) created by the American Heart Association and American Academy of Pediatrics provide normal HR and RR values for children including infants [2]. These values were derived from data cited in two literature sources, one is a systematic review from 69 studies [1] and another is data from a textbook [3]. The studies cited in this systematic review [1] present normal HR and RR values in children. However, the most of these studies are small-scale and, research on the normal ranges of HR and RR specifically targeting children under three years old remains scarce worldwide. Additionally, Liu et al. reported that some vital signs were associated with ethnic differences [4]. However, we could not find any large-scale study in Japan on the normal HR in toddlers and RR in children. For these reasons, we consider that studying HR and RR in Japanese toddlers, which has not yet been examined with large-scale data, is an important topic for future clinical practice and research.
Furthermore, in this study, we not only present normal HR and RR but also examine the factors that influence these vital signs. A study conducted in the UK with participants aged 4–16 years showed no correlation between height or weight and HR or RR [5]. However, whether this relationship also applies to infants and toddlers remains unclear. If height and weight affect vital signs, establishing standard values for each physique becomes necessary. Thus, investigating the impact of height and weight on vital signs is an essential first step. We also explored the relationship between physical activity levels and HR/RR, as previous studies have shown that resting HR tends to decrease with higher physical activity levels in adult athletes [6, 7]. Additionally, we examined regional differences in HR/RR among toddlers, given Japan's extensive geographic span from north to south, which includes a variety of climatic zones from subarctic to subtropical. Japan spans a wide range of latitudes, resulting in diverse climatic conditions across the country. The northern regions experience a cool climate, while the southern areas are characterized by a warm and mild climate. Supplementary Table S1 presents the temperature data reported by the Japan Meteorological Agency for the years 2013 to 2015, corresponding to the period during which the measurements in this study were conducted [8].
Therefore, the objective of this study was to assess the normal HR and RR in Japanese children aged two years and to investigate differences based on body size, physical activity levels, and regional variations using data from the Japan Environment and Children's Study (JECS), a nationwide large-scale birth cohort study.
Methods
This was a prospective cohort study using data from the Japan Environment and Children's Study (JECS), a nationwide birth cohort study in Japan. The JECS protocol, which has been published elsewhere [9], was reviewed and approved by the Institutional Review Board on Epidemiological Studies of the Ministry of Environment and the ethics committees of all participating institutions (Approval number: 100910001; date of approval: September 10, 2010). To ensure the generalizability and applicability of the results of the JECS to the Japanese population, 15 regional centers were selected to cover wide geographical areas with different living environments [9]. These were predetermined as part of the original cohort design. All the procedures and experiments were performed after receiving written informed consent from all the participants. Informed consent to participate was obtained from the parents or legal guardians of any participant under the age of 16.
Participants
The dataset jecs-ta-20190930 contains 104,062 records. A Sub-Cohort Study, which included face-to-face assessment of neuropsychiatric development, body measurement, physical examination by a pediatrician, and blood/urine collection for clinical/chemical analysis, was conducted in randomly selected two-year-olds from among all the participants [10]. The study included 4,988 participants. Children were eligible for inclusion in this analysis if HR and/or RR measurements were available. Participants with missing HR or RR values were excluded from the respective analyses (Figure S1).
Variables and data collection
The clinical team, comprising a pediatrician and nurses, measured the HR, RR, height, and weight of children between the ages of one year eleven months and two years three months. Measurements were conducted at JECS regional centers, hospitals, and clinics related to the regional JECS researchers across Japan. All pediatricians and nurses in charge were trained according to the standardized measurement protocols. We always asked the participants whether the child had fever or the symptoms of upper respiratory infections the day before the examination and the exam was postponed if they showed the symptoms. HR was measured using the palpation method at the radial artery of the wrist, and RR was measured by auscultation. Measurements were taken for one minute duration each. As an indicator of physical activity, we assessed outdoor playtime from the ages of one to two years using a questionnaire completed by the caregivers. The survey was conducted separately for summer (May to September) and winter (November to February). The question asked to parents was:'On average, how many hours did your child play outside during the daytime (9 a.m. to 5 p.m.)?'Respondents were presented with four options to choose from:"Almost never play outdoors,""Less than 1 h per day,""1 h or more but less than 3 h per day,"and"3 h or more per day."We obtained data on medical history, mother's gestational hypertension, parents’ height and weight, parents’ smoking habits, and parents’ educational background from the questionnaire. The height and weight of the father were surveyed using a questionnaire when the child was six months old. The height and weight of the mother, along with the educational background and smoking habits of both parents, were surveyed using a questionnaire during pregnancy. The weight of mother was recorded as her pre-pregnancy weight, and the smoking history of the parents included the period before pregnancy. The exposure characteristics of the mothers participating in the JECS are summarized elsewhere [11].
To comply with STROBE guidelines, we provide clear definitions of all key variables:
Outcomes: HR and RR.
Exposures of interest: height, weight, outdoor playtime, and area of residence.
Covariates [12–16]: medical history, mother's gestational hypertension, parents’ height and weight, parents’ smoking status, and parents’ educational background.
Bias and study size
The random selection of participants for the Sub-Cohort Study helped minimize selection bias. Data collection followed standardized protocols across all centers to reduce information bias. No formal sample size calculation was performed, as this analysis used data from an existing national cohort.
Statistical analyses
We descriptively summarized the ranges of HR and RRs for two-year-olds and calculated mean and standard deviation (SD) as summary statistics for HR and RR.
To investigate the correlation between the participants'height and weight and their HR and RR, scatter plots were created to visually assess the presence of a correlation, and Pearson’s correlation coefficients were calculated. A correlation was considered moderate or stronger if |r|≥ 0.4.
Boxplots were created to visualize the distribution of the differences in HR and RR with outdoor playtime and residential areas, respectively. One-way analysis of variance was performed to determine significant differences (if any) in the mean values of each group. For outdoor playtime and HR/RR, we performed Dunnett's test using"Almost never play outdoors"as the control group. Next, to confirm the regional influence on HR and RR, the participants were divided into two groups based on where they lived. Japan can be divided into two regions based on geography and culture: eastern and western. According to this categorization, Hokkaido to Aichi belongs to Eastern Japan, whereas Kyoto to South Kyushu and Okinawa belong to Western Japan. HR and RR were compared between eastern and western Japan using the Student’s t-test. Multivariable linear regression was also performed using HR and RR as outcomes and height, weight, area of residence, and outdoor playtime as the explanatory variables. The covariates included medical history, mother's gestational hypertension, parents’ height and weight, parents’ smoking status, and parents’ educational background. These covariates were selected based on prior evidence suggesting their relevance to children's physiological regulation. These variables were selected based on previous literature suggesting their potential relevance to children’s physiological development and vital signs [12–16]. We also conducted stratified analyses to evaluate whether the associations between outdoor playtime and HR or RR differed by region. The participants were grouped by residential region, and within each region, mean HR and RR were calculated according to the four categories of outdoor playtime in both summer and winter. To further investigate factors associated with elevated HR and RR, we classified participants into two groups based on whether their HR or RR values were at or above the 95th percentile. We then examined whether there were differences in explanatory variables between the high and normal groups. Student’s t-test was used for continuous variables such as height and weight, while the chi-square test was applied to categorical variables such as residential region and outdoor playtime.
Data with missing values were excluded from the analysis. Statistical analyses were conducted using R (version 4.4.1). A correlation was considered present if the absolute value of the correlation coefficient was 0.4 or higher. The significance level was set at p < 0.05.
Results
The total number of participants in the study was 4,988, which included 2,541 boys and 2,445 girls, and two of unspecified sex. HR and RR data were missing for 183 (3.7%) and 182 participants (3.6%), respectively. These participants were excluded from the analyses. Histograms depicting the HR and RR distribution frequency are shown in Fig. 1. Table 1 indicates mean and SD as summary statistics for HR and RR. The mean HR and RR of the participants were 109.2 ± 14.7 beats/min and 29.9 ± 6.1 breaths/min (mean ± SD).
Fig. 1.
Histograms of HR and RR. The median heart rate was 110/min, with the 25th percentile at 100/min, the 75th percentile at 120/min, and the 95th percentile at 132/min. The median respiratory rate was 30/min, with the 25th percentile at 26/min, the 75th percentile at 32/min, and the 95th percentile at 40/min
Table 1.
Characteristics of participants
| Complete-case for HR (n = 4,805) |
Complete-case for RR (n = 4,806) |
|
|---|---|---|
| Heart rate (beats/min) | 109.2 ± 14.7 | - |
| Respiratory rate (breaths/min) | - | 29.9 ± 6.1 |
| Sex (Male/Female) | 2,441/2,362 | 2,438/2,366 |
| Height (cm) | 83.9 ± 3.0 | 83.9 ± 3.0 |
| Weight (kg) | 11.5 ± 1.2 | 11.5 ± 1.2 |
| Outdoor time in summer (n, %) | ||
| Almost never play outdoors | 168 (3.6) | 169 (3.6) |
| < 1 h per day | 1,607 (34.5) | 1,603 (34.4) |
| 1 to < 3 h per day | 2,653 (56.9) | 2,657 (56.9) |
| ≥ 3 h per day | 234 (5.0) | 237 (5.1) |
| Outdoor time in winter (n, %) | ||
| Almost never play outdoors | 723 (15.5) | 730 (15.7) |
| < 1 h per day | 2,336 (50.2) | 2,335 (50.1) |
| 1 to < 3 h per day | 1,503 (32.3) | 1,501 (32.2) |
| ≥ 3 h per day | 91 (2.0) | 91 (2.0) |
| Region (n, %) | ||
| Hokkaido | 382 (8.0) | 387 (8.1) |
| Miyagi | 406 (8.4) | 410 (8.5) |
| Fukushima | 607 (12.6) | 605 (12.6) |
| Chiba | 291 (6.1) | 288 (6.0) |
| Kanagawa | 296 (6.2) | 296 (6.2) |
| Koshin | 348 (7.2) | 348 (7.2) |
| Toyama | 264 (5.5) | 264 (5.5) |
| Aichi | 280 (5.8) | 280 (5.8) |
| Kyoto | 188 (3.9) | 188 (3.9) |
| Osaka | 382 (8.0) | 378 (7.9) |
| Hyogo | 245 (5.1) | 246 (5.1) |
| Tottori | 136 (2.8) | 137 (2.9) |
| Kochi | 332 (6.9) | 332 (6.9) |
| Fukuoka | 374 (7.8) | 373 (7.8) |
| South-Kyushu and Okinawa | 274 (5.7) | 274 (5.7) |
| Past medical history (n, %) | 209 (4.4) | 209 (4.4) |
| Gestational hypertension (n, %) | 50 (1.0) | 50 (1.0) |
| Mother’s height (cm) | 158.3 ± 5.4 | 158.3 ± 5.4 |
| Mother’s weight (kg) | 53.3 ± 8.9 | 53.3 ± 8.8 |
| Mother’s smoking (n, %) | 139 (2.9) | 140 (2.9) |
| Mother’s low education (n, %) a | 1,468 (30.7) | 1,474 (30.8) |
| Father’s height (cm) | 171.8 ± 5.8 | 171.8 ± 5.9 |
| Father’s weight (kg) | 69.4 ± 11.1 | 69.4 ± 11.1 |
| Father’s smoking (n, %) | 1,947 (41.1) | 1,950 (41.2) |
| Father’s low education (n, %) † | 1,850 (38.8) | 1,858 (39.0) |
Continuous variables: mean ± standard deviation. a Highest level of education was high school or below
Table 1 also shows the participant characteristics. The mean height and weight of the participants were 83.9 ± 3.0 cm and 11.5 ± 1.2 kg (mean ± SD), respectively. In the summer, 3.6% of the participants almost never played outdoors, 34.5% spent < 1 h/d, 56.9% spent between 1–3 h/d, and 5.0% spent ≥ 3 h/d playing outdoors. In the winter, 15.5% of the participants almost never played outdoors, 50.2% spent < 1 h/d playing outdoors, 32.3% spent between 1–3 h, and 2.0% spent ≥ 3 h/d playing outdoors (complete-case for HR). The crude characteristics are presented in Supplementary Table S2.
Histograms depicting the HR and RR distribution frequency are shown in Fig. 1. Figure 2 shows the scatter plots and correlation coefficients between the participants'height and weight and their HR and RR. No correlation was observed in the scatter plot, and the absolute values of the correlation coefficients were below 0.4. HR and RR across the four groups of participants categorized based on outdoor playtime is depicted as a box plot in Fig. 3. The longer the children spent playing outdoors in summer, the lower their HR tended to be, and this relationship was statistically significant. The mean HR of participants who almost never played outdoors was 111.7 beats/min, whereas that of participants who spent ≥ 3 h/d playing outdoors was 108.2 beats/min (p = 0.038). The longer they played outdoors in winter, the higher their HR; however, this relationship was not statistically significant (p = 0.674). Unlike HR, no consistent trend was observed between RR and increased outdoor playtime. Figure 4 depicts a box plot showing HR and RR categorized based on the region of residence of the participants, arranged such that the farther right the category, the more southern the region. Participants from farther south region exhibited significantly higher HR (p < 0.001). The mean HR in Eastern Japan was 106.8 ± 15.3 beats/min (mean ± SD), whereas that in Western Japan was 112.7 ± 12.8 beats/min (p < 0.001). Unlike for the HR, no consistent trend was observed for the RR. The mean RR in Eastern Japan was 30.0 ± 6.5 breaths/min (mean ± SD), whereas that in Western Japan was 29.7 ± 5.3 breaths/min (p = 0.039). Supplementary Table S3 is summary tables for Figs. 3 and 4. Table 2 presents the results of multivariable linear regression, which confirmed a statistically significant relationship between longer outdoor playtime in summer and lower HR. Similarly, a statistically significant relationship was confirmed between HR and belonging to the southern region. To assess multicollinearity among the independent variables, we calculated the variance inflation factors (VIFs). All VIF values were below 3.0, indicating no significant multicollinearity. The stratified results by region and outdoor playtime are summarized in Supplementary Tables S4–S7. These tables present the mean and SD of HR and RR across the four categories of outdoor playtime during both summer and winter in each of the 15 regions.
Fig. 2.
Scatter plots and the correlation coefficients between height/weight and HR and RR. No correlation was observed visually in the scatter plot, and the absolute values of the correlation coefficients were all below 0.05
Fig. 3.
Box plot regarding the relationship between heart/respiratory rate and outdoor playtime. The more time spent outdoor in summer, the lower the heart rate tended to be. Regarding respiratory rate, there was no consistent trend observed with increasing the outdoor playtime
Fig. 4.
Box plot regarding the relationship between heart/respiratory rate and regions. More southern regions are listed more to the right. Hokkaido to Aichi belongs to Eastern Japan, whereas Kyoto to South Kyushu and Okinawa belong to Western Japan. Heart rate tended to be higher in southern regions. Regarding respiratory rate, there was no consistent trend
Table 2.
Multivariable linear regression
| Heart rate | β | Std. error | t | P value |
|---|---|---|---|---|
| Height | −0.125 | 0.109 | −1.155 | 0.248 |
| Weight | 0.090 | 0.259 | 0.348 | 0.728 |
| Outdoor time in summer | −0.177 | 0.393 | −0.451 | 0.652 |
| Outdoor time in winter | −0.444 | 0.365 | −1.218 | 0.223 |
| Hokkaido | −15.900 | 1.184 | −13.430 | < 0.001* |
| Miyagi | −5.503 | 1.146 | −4.804 | < 0.001* |
| Fukushima | 0.320 | 1.071 | 0.299 | 0.765 |
| Chiba | −5.780 | 1.234 | −4.685 | < 0.001* |
| Kanagawa | −1.146 | 1.214 | −0.944 | 0.345 |
| Koshin | −10.950 | 1.184 | −9.252 | < 0.001* |
| Toyama | −5.961 | 1.280 | −4.656 | < 0.001* |
| Aichi | −6.178 | 1.236 | −4.999 | < 0.001* |
| Kyoto | 1.261 | 1.381 | 0.913 | 0.361 |
| Osaka | 3.658 | 1.149 | 3.184 | 0.001* |
| Hyogo | −5.628 | 1.266 | −4.444 | < 0.001* |
| Tottori | −1.595 | 1.565 | −1.019 | 0.308 |
| Kochi | −1.725 | 1.190 | −1.449 | 0.147 |
| Fukuoka | 0.142 | 1.152 | 0.123 | 0.902 |
| Covariates | ||||
| Mother’s height | −0.078 | 0.044 | −1.758 | |
| Mother’s weight | 0.053 | 0.028 | 1.933 | |
| Mother’s low education | 0.782 | 0.490 | 1.595 | |
| Father’s height | 0.017 | 0.041 | 0.401 | |
| Father’s weight | −0.001 | 0.021 | −0.039 | |
| Father’s low education | −0.158 | 0.464 | −0.341 | |
| Parent’s smoking | 0.550 | 0.444 | 1.240 | |
| Past medical history | −0.255 | 1.047 | −0.244 | |
| Gestational hypertension | 0.006 | 2.131 | 0.003 | |
| Respiratory rate | ||||
| Height | −0.080 | 0.046 | −1.750 | 0.080 |
| Weight | 0.221 | 0.110 | 2.014 | 0.044 |
| Outdoor time in summer | −0.266 | 0.167 | −1.596 | 0.111 |
| Outdoor time in winter | 0.011 | 0.154 | 0.068 | 0.946 |
| Hokkaido | 0.673 | 0.501 | 1.343 | 0.179 |
| Miyagi | 0.289 | 0.485 | 0.596 | 0.551 |
| Fukushima | −0.054 | 0.454 | −0.118 | 0.906 |
| Chiba | 0.158 | 0.524 | 0.301 | 0.763 |
| Kanagawa | −0.571 | 0.515 | −1.110 | 0.267 |
| Koshin | −0.306 | 0.502 | −0.609 | 0.543 |
| Toyama | 2.503 | 0.543 | 4.612 | < 0.001* |
| Aichi | −3.574 | 0.524 | −6.821 | < 0.001* |
| Kyoto | −0.867 | 0.585 | −1.482 | 0.138 |
| Osaka | 0.519 | 0.488 | 1.064 | 0.287 |
| Hyogo | 2.186 | 0.536 | 4.076 | < 0.001* |
| Tottori | −3.341 | 0.665 | −5.021 | < 0.001* |
| Kochi | −1.601 | 0.505 | −3.174 | 0.002* |
| Fukuoka | −0.960 | 0.489 | −1.963 | 0.050 |
| Covariates | ||||
| Mother’s height | −0.043 | 0.019 | −2.290 | |
| Mother’s weight | 0.030 | 0.012 | 2.559 | |
| Mother’s low education | 0.444 | 0.207 | 2.144 | |
| Father’s height | −0.015 | 0.017 | −0.841 | |
| Father’s weight | −0.008 | 0.009 | −0.886 | |
| Father’s low education | −0.030 | 0.196 | −0.154 | |
| Parent’s smoking | 0.252 | 0.188 | 1.343 | |
| Past medical history | 0.088 | 0.442 | 0.199 | |
| Gestational hypertension | −1.156 | 0.902 | −1.281 | |
Multivariable linear regression. Regional categories were included as dummy variables in the regression model, with South-Kyushu set as the reference category. All variance inflation factors values were below 3.0, indicating no significant multicollinearity
We compared background characteristics between children in the high HR or RR groups (≥ 95th percentile) and those in the normal groups (Supplementary Tables S8). For HR, children in the high group were more likely to reside in western Japan. No significant differences were observed in outdoor playtime (summer or winter), height, or weight. For RR, children in the high group were less likely to live in western Japan and more likely to have reduced outdoor playtime during winter. No significant differences were observed in summer playtime, height, or weight.
Discussion
In the present study, using data obtained from a nationwide large birth cohort study in Japan, we established the normal HR and RR in two-year-old Japanese children. HR and RR are key vital signs used to assess the physiological status of children in many clinical settings [1]. Therefore, calculation of reference values for HR and RR at each age is important for pediatricians. However, because there are few opportunities to measure HR and RR in healthy children, the number of large-scale studies is limited. Based on literature searches, the most comprehensive cross-sectional study providing normative HR values for healthy children originated from China. The study focused on a cohort of 14,842 Chinese schoolchildren from Hong Kong aged 6–18 years [17]. In the present study, we focused on toddlers, and our study is based on the largest number of participants among prospective cohort studies presenting HR in healthy children. Reports on RR are limited. The most comprehensive cross-sectional study providing normative RR values in healthy children is based on a cohort of 1,109 children aged 4–16 years [5]. Thus, the strength of the present study is that it represents the largest prospective cohort report on HR and RR in healthy 2-year-olds. Consistent with the results of a previous study based on children aged 4–16 years [5], we found no correlation between height or weight and HR or RR in toddlers in the present study. Therefore, we believe that the data on the reference values for HR and RR in children both from our study and previous studies are applicable regardless of their height or weight. This information is crucial for clinicians who use vital signs during physical assessments.
Longer outdoor playtime in summer was associated with a lower HR. As increased outdoor playtime implies increased physical activity in children, we believe that this may have contributed to the lower HR observed in our study. A previous report showed that endurance athletes typically have a higher percentage of slow-twitch muscle fibers than power athletes and non-athletes, and that this increased slow-twitch muscle fiber percentage is associated with a lower resting HR [7]. Additionally, Yoshimoto et al. demonstrated that HR can be voluntarily regulated when individuals receive real-time feedback in a rat model [18]. In the study, rats that received stimulation of the neocortex and medial forebrain as feedback and reward, respectively, reduced their HR within 30 min, achieving an approximate 50% reduction after five days of 3-h feedback sessions. The reduced HR persisted for at least ten days after the training. Other reports have shown that aerobic exercise promotes the brain's reward mechanism [19], and that physical activity and being outdoors are associated with reduced negative affect [20]. Exercise or being outdoors may stimulate the brain's reward system, triggering a mechanism that lowers HR, as observed in our study. To the best of our knowledge, this is the first report on the relationship between physical activity and HR specifically in toddlers who did not undergo rigorous exercise training. An elevated resting HR has been reported to be an independent risk factor for cardiovascular disease and hypertension [13, 21–23]. Our findings emphasize the importance of ensuring outdoor playtime in children to reduce the risk of future cardiovascular diseases and hypertension.
The results of the current study revealed that HR tended to be higher in southern regions, although the underlying mechanism remains unclear. In a previous study conducted in Northern Norway, HR was found to be higher during colder seasons [24]. A study conducted in China reported that extreme cold and hot temperatures are associated with an increase in HR [25]. As it is typically warmer in the southern regions of Japan, our findings contrast with that of the previous reports. We also considered the notion that outdoor playtime, which influences the HR, may vary by region. However, even after controlling for this factor using multiple regression analysis, the relationship between the region and HR remained consistent. Our additional stratified analyses suggest that neither outdoor playtime nor ambient temperature alone fully explain the observed regional differences in HR. Even after adjusting for outdoor playtime in both summer and winter, some southern regions consistently showed higher HR levels. This implies that other unmeasured regional factors, such as diet, stress, air pollution, or cultural differences in daily activity patterns, may also contribute to autonomic regulation in toddlers. These findings suggest that, in addition to temperature and outdoor playtime, other potential factors that vary by region may also affect HR; however, these remain to be clarified. A previous study demonstrated that the reference range of HR and RR established for schoolchildren in the United Kingdom could be applied to children in Cape Town, South Africa [26]. Whether HR varies by region is not yet clear and further investigation is needed to elucidate the regional influences on HR.
The selection of covariates in this study was informed by prior evidence supporting their relevance to cardiovascular and respiratory regulation in children. Maternal weight during pregnancy, particularly obesity, has been associated with higher resting heart rate in neonates [12]. While direct associations between paternal anthropometric traits and toddler vital signs are limited, parental height and weight contribute to child body size, which can influence HR and RR. In addition, parental education was used as a proxy for socioeconomic status, which may affect autonomic regulation in children through pathways involving stress, nutrition, and healthcare access. A previous study in adolescents demonstrated a correlation between parental socioeconomic status and resting HR [13]. Tobacco exposure, both prenatal and postnatal, has also been shown to alter cardiovascular and respiratory control in children [14, 15]. Furthermore, maternal gestational hypertension is known to have long-term effects on offspring cardiovascular outcomes, including elevated blood pressure later in life [16]. These findings collectively support the inclusion of perinatal, socioeconomic, and parental physiological factors as covariates in our analysis to account for their potential influence on HR and RR in early childhood.
This study had several strengths and limitations. A major strength was its large, population-based sample drawn from a nationwide birth cohort, which enhances the generalizability of the normative values for HR and RR among two-year-old children. In addition, we examined the influence of body size, such as height and weight. The use of standardized measurement protocols by trained pediatricians and nurses further contributes to the reliability of the data. Furthermore, we conducted multivariable linear regression analyses to examine the associations between HR or RR and several potential influencing factors, including body size, outdoor playtime, and residential region. However, there were also important limitations. One of the main limitations of the study was the sample size of the low and high groups about outdoor playtime. The majority 80% were in the middle groups. The mean HR of participants who almost never played outdoors was 111.7 beats per minute, whereas that of those who spent ≥ 3 h per day playing outdoors was 108.2 beats per minute (p = 0.009). However, this analysis compared only the two extremes, representing 3.7% and 5% of the total sample, respectively. This limited sample size significantly restricts the generalizability of the findings. Moreover, similar results were not observed during the winter season, when the number of participants in each group was larger, further limiting the strength of the conclusions that can be drawn from this analysis. Second, outdoor playtime was determined based on responses in the questionnaires completed by the caregivers. Consequently, the possibility that the reported playtime may not have been accurately measured remains. Furthermore, as we did not inquire about specific activities during outdoor play, it is conceivable that the reported playtime did not solely reflect physical activity levels. Third, although RR may be influenced by body temperature in children with acute infections [27], we did not assess body temperature in this study. However, we always asked the participants whether the child had fever or the symptoms of upper respiratory infections the day before the examination and the exam was postponed if they showed the symptoms. Therefore, we believe the possibility to include the children with acute infection is relatively low. Finally, as this study used a questionnaire survey targeting the general population, there may have been a selection bias for parents with higher health literacy at the time of participating in the study.
Conclusion
We established the normal HR and RR in Japanese children aged two years, marking the largest prospective cohort study on HR and RR in healthy two-year-olds. Additionally, increased outdoor playtime during the summer months was linked to a lower HR, and HR tended to be higher in southern regions.
Supplementary material
Acknowledgements
The authors thank participants, Research Co-ordinators, doctors and co-operating health care providers (for the medical check-up), the staff and medical editor at Tohoku University.
Members of the Japan Environment and Children’s Study (JECS) Group as of 2024: Members of the JECS Group as of 2024: Michihiro Kamijima (Principal Investigator, Nagoya City University, Nagoya, Japan), Shin Yamazaki (National Institute for Environmental Studies, Tsukuba, Japan), Maki Fukami (National Center for Child Health and Development, Tokyo, Japan), Reiko Kishi (Hokkaido University, Sapporo, Japan), Chiharu Ota (Tohoku University, Sendai, Japan), Koichi Hashimoto (Fukushima Medical University, Fukushima, Japan), Chisato Mori (Chiba University, Chiba, Japan), Shuichi Ito (Yokohama City University, Yokohama, Japan), Ryoji Shinohara (University of Yamanashi, Chuo, Japan), Hidekuni Inadera (University of Toyama, Toyama, Japan), Takeo Nakayama (Kyoto University, Kyoto, Japan), Ryo Kawasaki (Osaka University, Suita, Japan), Yasuhiro Takeshima (Hyogo Medical University, Nishinomiya, Japan), Seiji Kageyama (Tottori University, Yonago, Japan), Narufumi Suganuma (Kochi University, Nankoku, Japan), Shoichi Ohga (Kyushu University, Fukuoka, Japan), and Takahiko Katoh (Kumamoto University, Kumamoto, Japan).
The authors thank participants, Research Co-ordinators, doctors and co-operating health care providers (for the medical check-up), the staff and medical editor at Tohoku University.
Abbreviations
- HR
Heart rate
- JECS
Japan Environment and Children's Study
- RR
Respiratory rate
- SD
Standard deviation
Authors'contributions
Dr. Kanamori drafted the initial manuscript, carried out the initial analyses, and reviewed and revised the manuscript. Dr. Kanamori had full access to all the data in the study and takes responsibility for its integrity and the data analysis. Dr. Suzuki critically reviewed the data analyses and the manuscript. Dr. Ota conceived and designed the study, collected the data, and critically reviewed the manuscript. All the authors approved the final manuscript as submitted and agreed to be accountable for all aspects of the work.
Funding
This study was funded by the Ministry of the Environment, Japan. The findings and conclusions of this article are solely the responsibility of the authors and do not represent the official views of the Japanese Ministry of the Environment.
Data availability
Data are unsuitable for public deposition due to ethical restrictions and legal framework of Japan. It is prohibited by the Act on the Protection of Personal Information (Act No. 57 of 30 May 2003, amendment on 9 September 2015) to publicly deposit the data containing personal information. Ethical Guidelines for Medical and Health Research Involving Human Subjects enforced by the Japan Ministry of Education, Culture, Sports, Science and Technology and the Ministry of Health, Labour and Welfare also restricts the open sharing of the epidemiologic data. All inquiries about access to data should be sent to: jecs-en@nies.go.jp. The person responsible for handling enquiries sent to this e-mail address is Dr Shoji F. Nakayama, JECS Programme Office, National Institute for Environmental Studies.
Declarations
Ethics approval and consent to participate
The JECS protocol was approved by the Institutional Review Board on epidemiologica studies of the Ministry of the Environment, and Ethics Committees of all participating institutions, i.e., NIES, NCCHD, Hokkaido University, Sapporo Medical University, Asahikawa Medical College, Japanese Red Cross Hokkaido College of Nursing, Tohoku University, Fukushima Medical University, Chiba University, Yokohama City Unversity, University of Yamanashi, Shinshu University, Univresity of Toyama, Nagoya City University, Kyoto University, Doshisha University, Osaka University, Osaka Medical Center and Research Institute for Maternal and Child Health, Hyogo College of Medicine, Tottori Unversity, Kochi Univeristy, University of Occupational and Environmenatal University, Kyushu University, Kumamoto University, University of Miyazaki, and University of the Ryukyus. The JECS will be conducted in acoordance to the Helsinki Declaration and to the other nationally valid regulations. All the procedures and experiments were performed after receiving written informed consent from all the participants. Informed consent to participate was obtained from the parents or legal guardians of any participant under the age of 16.
Consent for publication
Not applicable.
Competing interests
The authors declare no competing interests.
Footnotes
Publisher's Note
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Contributor Information
Keita Kanamori, Email: kanamori.keita.q5@dc.tohoku.ac.jp.
The Japan Environment Children’s Study Group:
Michihiro Kamijima, Shin Yamazaki, Maki Fukami, Reiko Kishi, Chiharu Ota, Koichi Hashimoto, Chisato Mori, Shuichi Ito, Ryoji Shinohara, Hidekuni Inadera, Takeo Nakayama, Ryo Kawasaki, Yasuhiro Takeshima, Seiji Kageyama, Narufumi Suganuma, Shoichi Ohga, and Takahiko Katoh
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Data Availability Statement
Data are unsuitable for public deposition due to ethical restrictions and legal framework of Japan. It is prohibited by the Act on the Protection of Personal Information (Act No. 57 of 30 May 2003, amendment on 9 September 2015) to publicly deposit the data containing personal information. Ethical Guidelines for Medical and Health Research Involving Human Subjects enforced by the Japan Ministry of Education, Culture, Sports, Science and Technology and the Ministry of Health, Labour and Welfare also restricts the open sharing of the epidemiologic data. All inquiries about access to data should be sent to: jecs-en@nies.go.jp. The person responsible for handling enquiries sent to this e-mail address is Dr Shoji F. Nakayama, JECS Programme Office, National Institute for Environmental Studies.