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Full-Text PDF: 401-409.pdf
Prevalence and Correlates of Elevated Blood Pressure in Chinese Children Aged 6-13 Years: a Nationwide School-Based Survey
 

Original Article

Prevalence and Correlates of Elevated Blood Pressure in Chinese Children Aged 6-13 Years: a Nationwide School-Based Survey*

ZHAI Yi1, LI Wei Rong2, SHEN Chong3, Frank QIAN4, and SHI Xiao Ming5,#

1. Division of Non-communicable Disease Control and Community Health, Chinese Center for Disease Control and Prevention, Beijing 102206, China; 2. Office of Medical Statistics, Beijing Friendship Hospital, Beijing 100050, China; 3. Department of Division of Epidemiology and Biostatistics, School of Public Health, Nanjing Medical University, Nanjing 210029, Jiangsu, China; 4. Center for Global Health, University of Chicago, Chicago, IL 60637, USA; 5. Institute of Environmental Health and Related Product Safety, Chinese Center for Disease Control and Prevention, Beijing 100021, China

*This work was supported by the Young Scholar Scientific Research Foundation of the Chinese Center for Disease Control and Prevention (grant # China CDC 2010A205).

#Correspondence should be addressed to Dr. SHI Xiao Ming, E-mail: sxmcdc@163.com, Tel: 86-10-87715709.

Biographical note of the first author: ZHAI Yi, female, born in 1978, master, majoring in non-communicable diseases control and prevention.

 

Abstract

Objective  To estimate the prevalence of elevated blood pressure (EBP) in Chinese children and identify individual and family factors associated with EBP.

Methods  A nationwide cross-sectional study was conducted in 2010 using stratified cluster sampling. Participants’ blood pressure was measured, and their parents completed a questionnaire on personal and family characteristics. Prevalence and correlates of EBP were assessed.

Results  Among a total of 24,333 participants, 20.2% of boys and 16.3% of girls had EBP. The prevalence of EBP increased with the ascending trend of waist circumference, Waist-to-height ratio, and body mass index. The adjusted prevalence ratios (aPRs) for obese boys and girls were 2.50 and 2.97, respectively. Fewer urban boys (16.2%) had EBP than rural boys (21.7%). Boys with a family history of hypertension were 12% more likely to have EBP. Children whose mothers received a college education tended to have lower likelihood of EBP; with an aPR was 0.85 among boys and 0.78 among girls.

Conclusion  EBP is common among obese students and those who have a family history of hypertension. A negative association between mothers’ education levels and EBP risk in children was found.

Key words: Elevated blood pressure; Waist circumference; Waist-to-height ratio; Body mass index; Children; Cross-sectional study; China

Biomed Environ Sci, 2015; 28(6): 401-409        doi: 10.3967/bes2015.057         ISSN: 0895-3988

www.besjournal.com (full text)                CN: 11-2816/Q      Copyright ?2015 by China CDC

 

 

INTRODUCTION

E

levated blood pressure (EBP) is a leading cause of premature death in China[1]. Globally, hypertension, along with obesity, is a major risk factor for cardiovascular diseases in adults, which are major causes of premature death in both high-income as well as low-and middle-income countries[2-3]. The prevalence of these two intertwined conditions is also increasing among the pediatric population[4]. EBP in childhood predicts cardiovascular risk later in life[5]. Identifying childhood hypertension and its risk factors could contribute to better prevention of hypertension and cardiovascular disorders in adults[6].

Childhood obesity, an unintended consequence of rapid economic growth in the past several decades, has become widespread in China. The overall percentage of children and adolescents aged 7-17 years who were overweight and obese increased by 15.8% in one decade, from 5.7% in 1992 to 6.6% in 2002[7]. It further increased to 19.0% of boys and 10.9% of girls in 2010[8]. Obesity is closely linked to hypertension[8-10]. Earlier national data showed that the prevalence of hypertension among children aged 6-17 years increased from  7.1% in 1991 to 14.6% in 2004[11]. A study conducted in 2010 in Shandong Province in eastern China showed higher prevalence of hypertension: 26.2%  of boys and 19.8% of girls[12]. The literature on   EBP among Chinese children is sparse, and the factors associated with children’s EBP are not   well understood. We analyzed a national sample from a 2010 survey in order to, (1) estimate the prevalence of EBP in Chinese school-age children and (2) identify individual and family factors associated with EBP.

METHODS

Study Design and Subjects

This nationwide cross-sectional study was conducted in 2010. Subjects were recruited using a multi-stage cluster random sampling method in school-age children. In the first stage of sampling, eight provinces in China were selected including: Inner Mongolia, Jiangsu, Anhui, Shandong, Hunan, Guangxi, Chongqing, and Gansu. The counties in each province were sorted by average per-capita gross domestic product, and were stratified into three strata by economic level (low, middle, and high). The second stage involved randomly selecting three counties or county-level cities in each stratum. In the third stage, in each selected county, one urban and one rural elementary school were randomly selected; and in each school, two or three classes were randomly selected from each grade. Finally, all students in these selected classes (grades 1-6 and aged 6-13 years) were eligible for participation. The students who were absent on the day of the physical examination or did not complete the questionnaires were excluded. The overall participation rate was 90.0%.

The survey included two components: a physical examination of the subjects and a questionnaire interview with a parent or guardian of the participating students. The parent or guardian filled out a self-administered questionnaire under the guidance of staff from local branches of the Center for Disease Control and Prevention (CDC). The questionnaire elicited information on the subject’s age and sex, mother’s education level, and family history of hypertension. Mother’s education level was used as a proxy for socioeconomic status, which was divided into three categories: (1) ≤6 years of schooling (elementary school or lower); (2) 7-12 years of schooling (high school); and (3) ≥13 years of schooling (junior college or higher). The family history of hypertension was defined as a history of diagnosed hypertension in any biological parent or grandparent. Ethics approval was obtained from the Ethics Committee of China CDC (IRB code: 201020). Informed consent forms were signed by the subjects’ parents or guardians.

Health Measurements

The physical examinations were performed by health professionals from local county CDCs. Blood pressure (BP) was measured in the right arm using a mercury sphygmomanometer with appropriate cuff size (7, 9, or 12 cm) after the subject had been seated quietly for five min, making sure that the cuff covered at least two-thirds of the upper arm. Korotkoff phases 1 and 5 were used for defining systolic BP (SBP) and diastolic BP (DBP), respectively. Two readings were obtained within a 2-min interval. If the pressure readings differed by ≥6 mmHg, a third measurement was performed. The mean of the last two measurements was used in the analysis. The definition of EBP in children is based on the normative distribution of BP in healthy children. EBP was defined as SBP or DBP or both equal to or above the 95th percentile for children of the same age, gender, and height according to The fourth report on the diagnosis, evaluation, and treatment of high blood pressure in children and adolescents[13].

Body height and weight of the participants were measured using standard protocols, without shoes and outerwear. Height was measured to the nearest 0.1 cm on a column stadiometer and weight to the nearest 0.1 kg on a digital scale. Waist circumference (WC) was measured to the nearest 0.1 cm by a nonelastic flexible tape. With the participant in a standing position, the tape was applied horizontally midway between the lowest rib margin and the iliac crest. Waist-to-height ratio (WtHR) was calculated as waist circumference (cm)/height (cm). Both WC and WtHR were categorized into quartiles by sex. Body mass index (BMI) was calculated as weight (kg)/height squared (m2). BMI that is less than the 5th percentile for children of the same age and gender is considered underweight, above the 95th percentile as obese, and between the 85th and 95th percentiles as overweight[14].

Statistical Analysis

Participants’ personal and family characteristics were compared between boys and girls using student’s t-test or Wilcoxon rank-sum test for continuous variables and χ2 test for categorical variables. All P-values reported are two-sided. Mean and standard error (SE) of SBP and DBP were calculated by age, WC, WtHR, BMI, residence, family history of hypertension, and mother’s education.

The sex-specific prevalence and 95% confidence interval (CI) of EBP were calculated by age group. The relationship between EBP and WC, WtHR, and BMI were assessed using generalized linear models adjusting for age, and the relationship between EBP and residence, family history of hypertension, and mother’s education was assessed adjusting for age and BMI. The model distribution type with the ‘binomial distribution’, and the ‘logit’ link function in the model is chosen. All analyses were performed using SAS software, version 9.3 (SAS Institute Inc., Carey, NC, USA).

RESULTS

Personal and Family Characteristics

A total of 24,333 children (13,006 boys and 11,327 girls) were included in the analysis (mean age was 9.3 years (range 6-13); mean height, 133.0 cm; mean weight, 30.3 kg; mean WC, 57.8 cm; mean WtHR, 0.43; and mean BMI, 16.8 kg/m2). About 40% of children had at least one parent or grandparent with hypertension. The majority of the children’s mothers received a high school education. On average, boys were taller and heavier than girls, and had larger WC, WtHR, and BMI than girls (all P<0.05). There were no statistical differences between boys and girls with respect to family history of hypert- ension and mothers’ educational level (Table 1).

Systolic and Diastolic Blood Pressure

Overall, SBP and DBP increased with age, WC, WtHR, and BMI status in both boys and girls (all P<0.05) (Table 2). Children residing in rural areas had higher age- and BMI-adjusted SBP and DBP than those residing in urban areas (P<0.05). A family history of hypertension was more likely to contribute to higher age- and BMI-adjusted DBP in boys and age- and BMI-adjusted SBP and DBP in girls (P<0.05). SBP and DBP in boys and SBP in girls decreased as of mother’s education level decreased after adjusting for age and BMI (P<0.05).

Elevated Blood Pressure

The overall prevalence of EBP among children in this study was 18.4%; 20.2% for boys and 16.3% for girls with children aged 10-11 years having the highest prevalence (Table 3). The prevalence of EBP increased with the ascending trend of WC, WtHR, and BMI in both boys and girls. The prevalence of EBP increased from 17.2% in boys with normal weight to 26.1% for overweight boys and 49.3% for obese boys; the adjusted prevalence ratios (aPRs) for overweight and obese boys were 1.66 (95% CI, 1.48-1.85) and 2.50 (95% CI, 2.26-2.76), respectively. Among girls, the prevalence of EBP increased from 14.6% for normal weight to 29.6% for overweight and 45.9% for obese, and the aPRs were 1.94 (95% CI, 1.70-2.20) and 2.97 (95% CI, 2.53-3.45), respectively. Compared with the participants at the 1st quartile of WtHR, the prevalence of EBP increased significantly from 13.7% to 33.0% for boys and from 10.2% to 25.7% for girls, and the aPRs were 1.76 (95% CI, 1.58-1.96) and 1.64 (95% CI, 1.45-1.85), respectively.

 

Table 1. Characteristics of Chinese School-age Children, 2010

Characteristic

Total (n=24,333)

Boys (n=13,006)

Girls (n=11,327)

P-value

Age (mean±SD), y

9.3±1.8

9.3±1.8

9.2±1.8

0.16

Height (mean±SD), cm

133.0±11.4

133.2±11.1

132.8±11.8

<0.01

Weight (mean±SD), kg

30.3±8.6

30.9±8.9

29.5±8.3

<0.001

Waist circumference (mean±SD), cm

57.8±7.8

59.0±8.3

56.4±7.0

<0.001

Waist to Height Ratio (mean±SD)

0.43±0.05

0.44±0.05

0.42±0.04

<0.001

BMI (mean±SD), kg/m2

16.8±2.6

17.1±2.8

16.5±2.4

<0.001

Family history of hypertension

9636 (39.6)

5135 (39.5)

4501 (39.7)

0.69

Mother’s education level

 

 

 

0.43

Elementary school or lower

5908 (24.3)

3179 (24.4)

2729 (24.1)

 

High school

15034 (61.8)

8031 (61.8)

7003 (61.8)

 

Junior college and higher

3391 (13.9)

1796 (13.8)

1595 (14.1)

 

Note. SD: standard deviation.

 

 

 

There was a significantly lower prevalence of EBP among boys living in urban (16.2%) compared with rural (21.7%) areas, and the aPR was 0.89  (95% CI, 0.83-0.97). Boys with a family history of hypertension were 12% more likely to have EBP (aPR, 1.12; 95% CI, 1.04-1.20) than those without it. Children whose mothers received college education tended to have a lower likelihood of EBP, and aPR was 0.85 (95% CI, 0.75-0.97) among boys and 0.78 (95% CI, 0.78-0.79) among girls.

DISCUSSION

Our study estimated the prevalence of EBP among Chinese children aged 6-13 years to be 20.2% in boys and 16.3% in girls. The prevalence of EBP estimated for China was higher in our study compared to studies conducted for other countries. The prevalence of hypertension among urban school children aged 5-14 years in southern India was 7.2 % (6.6% in boys and 7.9% in girls)[15]. A cross- sectional study involving 794 Brazilian children aged 6-13 years from public elementary schools found 7% prevalence of high blood pressure (hypertension or prehypertension)[16]. In Japan, the prevalence of high blood pressure was 15.9% in 4th-grade boys and 15.8% in 4th-grade girls, and 11.1% in 7th-grade boys and 10.8% in 7th-grade girls[17]. In the United States, 13.6% of boys and 5.7% of girls aged 8-17 years were classified as having pre-EBP, and 2.6% of boys and 3.4% of girls as having EBP during 2003-2006[18]. However, caution should be taken when making cross-country comparisons. First, our study represented one of the most recent studies. Increasing trends of childhood EBP have been observed around the world[11,19]. Second, the method of measuring blood pressure may also contribute to the variety of study outcomes across studies; for example, repeated measurements could result in lower estimates of EBP. The estimated prevalence of EBP on the first, second, and third screens among 6193 students in Hong Kong, China were 9.54%, 2.77%, and 1.44%, respectively[20]. However, the high prevalence of EBP among Chinese children suggested an emerging health issue among children in the countries where people are becoming wealthy and adopting new and often sedentary lifestyles.

Our study also showed personal and family factors associated with EBP. Boys and girls who were overweight, obese, or had high WC, WtHR or lower mother’s education levels were more likely to have EBP. Our data were in agreement with other studies that suggest high BMI and large WC are associated with EBP[21-24]. Obesity and central adiposity may impair microvascular function, and therefore result in the development of hypertension[25-26]. Among boys, EBP was higher for those having a family history of hypertension or residing in rural areas. Family history of hypertension is an important factor associated with children’s EBP; our study finding is consistent with previous studies[27-28]. Place of residence is a key indicator of socioeconomic position. C Lazarou et al.[29] found that 70% of children with EBP lived in rural areas, and 58.8% had low socioeconomic status. Socioeconomic inequalities have been linked to childhood obesity in developed countries[30-31], and children with lower socioeconomic status were at a higher risk for hypertension in adulthood[32-33]. Systematic reviews also found that lower SES during childhood was related to greater risk of cardiovascular diseases during adulthood[34-35]. In this study, we also used mothers’ education levels as a proxy for children’s socioeconomic status, and found that mother’s high education level lowered the risk of EBP among children.

This study has several strengths. First, the study participants consisted of a nationwide sample of children in grades 1-6 from both urban and rural areas. Participants were selected using a multistage sampling strategy, and the sampling weights, including design weight, post-stratification weight, and non-response weight, were used in the data analysis. The study findings can be extrapolated to all children aged 6-13 years because over 98% of Chinese children in this age group are enrolled in elementary schools due to the nine-year compulsory education policy implemented in 1986. Second, we calculated prevalence ratio instead of odds ratios. Prevalence ratio offers conservative, consistent, and interpretable relationships of EBP among different categories in this cross-sectional study. Third, we evaluated both personal and family factors associated with EBP.

The study also has limitations. Adult residents in northern China have a higher prevalence of hypertension than those in southern China[36]. However, we did not consider the northern and southern areas as a stratum in the sampling stage of this study, and hence the understanding of the difference of EBP among children in the two stata within China is not possible.

Blood pressure was only measured on one visit in this study. Other studies have suggested that initial readings may inflate the estimate of EBP, and multiple readings measured on different occasions are needed[37-38].

CONCLUSIONS

The prevalence of EBP is high among Chinese school-age children, particularly among those who are overweight or obese. Screening of blood pressure among obese children is useful for early identification of at-risk children and provides a good opportunity for health education. Children with a family history of hypertension should also be screened. The negative association between mothers’ education level and EBP risk in children may be due to mothers’ roles as primary caregivers who greatly influence children’s dietary and lifestyle behaviors; providing mothers with health information may be an effective strategy in preventing childhood EBP. Prevention intervention programs should also target rural schools, as children in rural areas also have high prevalence of EBP.

Conflict of interest

The authors declare no conflict of interest.

Acknowledgments

The authors appreciate the children and their family members for participation in this study. The authors also thank the staff of local CDCs for conducting the study activities. Special thanks to Drs. Hong Yu Ling, Yang Quan He, Michael Engelgau, and Wang Gui Jing who have put considerable time and valuable effort in statistical calculations, discussion, and comments on the manuscript.

Received: March 8, 2015;

Accepted: June 24, 2015

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