Background

Elevated blood pressure (EBP) in children and adolescents increases future risk of cardiovascular disease. Among children and adolescents, increased weight is associated with EBP.

Methods

National cross-sectional data for children and adolescents aged 8–17 years from the National Health and Nutrition Examination Surveys (NHANESs): 1988–1994, 1999–2002, and 2003–2006. The main outcome measures were EBP and pre-EBP estimates.

Results

Overweight boys (odds ratio (OR) 1.54, confidence interval (CI) 1.11–2.13) and both obese boys and girls were significantly more likely to be classified as pre-EBP (boys, OR 2.81, CI 2.13–3.71; girls, OR 2.55, CI 1.75–3.73) and having EBP (boys aged 8–12 years, OR 6.06, CI 2.73–13.44, boys aged 13–17, OR 9.62 CI 4.86–19.06; girls, OR 2.33, CI 1.31–4.13) when compared to the reference weight and controlling for all other covariates.

During 2003–2006, 13.6% (s.e. = 1.2) of boys aged 8–17 years and 5.7% (s.e. = 0.7) of the girls aged 8–17 years were classified as pre-EBP and 2.6% (s.e. = 0.5) of the boys aged 8–17 and 3.4% (s.e. = 0.7) of the girls aged 8–17 were having EBP. After controlling for age, race/ethnicity, and body mass index (BMI), girls only were significantly more likely to have EBP during 2003–2006 than during 1988–1994 (OR 2.17, CI 1.05–4.49). In contrast, adolescent boys aged 13–17 years were significantly less likely to be having EBP during 2003–2006 than during 1988–1994 (OR 0.32, CI 0.13–0.81).

Conclusions

Obesity is strongly, positively, and independently associated with EBP and pre-EBP among youths. However, controlling for all covariates including BMI, EBP has increased among girls but decreased among adolescent boys aged 13–17, during 2003–2006 when compared with 1988–1994.

Methods

Survey description. NHANES of the National Center for Health Statistics, Centers for Disease Control and Prevention, provides cross-sectional, nationally representative health examination data on the US civilian, noninstitutionalized population. A complex, multistage probability sampling design is used to identify the eligible sample.¹⁵ The NHANES 1999–2006 sample included oversampling of youth aged 12–19 years, Mexican Americans, and blacks. The procedures to select the sample and conduct the interview and examination have been previously described.^15,16

Sample. The NHANES III (1988–1994) sample included 6,364 children and youths aged 8–17 years of whom 4,673 (73.4%) were interviewed and examined. The NHANES 1999–2002 included 5,998 children and youths aged 8–17 years of whom 5,110 (85.2%) were interviewed and examined. The NHANES 2003–2006 sample included of 6,630 children and youths aged 8–17 years of whom 4,662 (70.3%) were interviewed and examined.

Measurements. The average of up to three brachial systolic (first Korotkoff phase) and diastolic (fifth Korotkoff phase) BP readings were used as the participants' systolic and diastolic BP values. In NHANES III 94% had three BP measurements; in NHANES 1999–2002 85% had three BP measurements; and in NHANES 2003–2006 72% had three BP measurements. Appropriate BP cuff sizes were used for participants based on measurement of midarm circumference.^16,17 All BP readings were obtained at a single examination visit following a standard protocol.¹⁶

We classified individual participant's BP using the new guidelines detailed in the Fourth Report on the Diagnosis, Evaluation, and Treatment of High Blood Pressure in Children and Adolescents.¹⁸ The new guidelines recommend multiple BP measurements at different times to define persistent hypertension and prehypertension. In this study, all BP readings were obtained at a single examination visit there was a need to differentiate between the guidelines nomenclature for prehypertension and hypertension and our nomenclature for the same outcome of interest. Because our study design only had access to one BP measurement per survey participant, if the single BP measurement was in the prehypertensive BP range it was labeled in this paper as a pre-EBP. Also, if the single BP measurement was in the hypertensive BP range, it was labeled in this paper as an EBP. A somewhat similar approach was taken by Din-Dzietham et al.¹⁹

A child was classified as having pre-EBP if one of the four following criteria was satisfied: (i) the percentile for systolic BP 90% but <95% and the diastolic percentile <90%; (ii) the percentile for diastolic BP 90% but <95% and the systolic BP percentile <90%; (iii) the percentile systolic BP 90% but <95% and diastolic BP percentile 90% but <95%; (iv) observed BP levels 120/80 mm Hg even if systolic and diastolic percentiles are <90%. EBP was defined as the percentile for systolic BP and/or a percentile for diastolic BP that is 95% for gender, age, and height. See the Fourth Report (Appendix B in report—for further details).¹⁸ In classifying an examinee as EBP or pre-EBP, individuals with missing diastolic values but valid systolic values were included. Thus if an examinee' s mean systolic BP was greater than or equal to the sex and age-specific systolic BP threshold, the examinee was classified as pre-EBP or EBP even if the mean diastolic BP was missing. The prevalence of pre-EBP and EBP in youth during NHANES III, NHANES 1999–2002, and NHANES 2003–2006 were based on 4,033 (86.3% (4,033/4,673)), 4,846 (94.8% (4,846/5,110)), and 4,427 (95% (4,427/4,662)) cases, respectively, all nonpregnant.

Height in centimeters and weight in kilograms were measured in the mobile examination center following a standard protocol. BMI was calculated as weight in kilograms divided by the square of height in meters (kg/m²) and was categorized using the age- and sex-specific BMI percentiles based on the year 2000 growth charts of the Centers for Disease Control and Prevention.^20,21 BMI was categorized as (i) underweight defined as BMI less than the 5th sex-specific percentile of BMI for age, (ii) "normal weight" defined as at or above the sex-specific 5th percentile but less than the 85th percentile of BMI for age, (iii) overweight defined as BMI at 85th percentile but less than 95th percentile for age, and (iv) obese defined as BMI >95th percentile for age. (This terminology reflects the labels used by the Institute of Medicine, American Academy of Pediatrics, and other organizations. For more information see Krebs et al.²²) For this analysis, age was categorized into two groups: 8–12 and 13–17 years, similar to previous analyses.¹² Race/ethnicity was based on self-report and categorized as non-Hispanic white, non-Hispanic black, and Mexican American. Estimates for persons classified in the "other race/ethnicity" group, including persons who reported "multiple races" and "other Hispanic" ethnicity were included in the total sample results but are not reported separately.

Statistical methods. Estimates of the prevalence of pre-EBP and EBP among nonpregnant children and adolescents 8–17 years are presented for NHANES III (1988–1994), NHANES 1999–2002, and for NHANES 2003–2006 by the cross classification of race/ethnicity, gender and age, and by gender and BMI category. Estimates are not presented by the cross classification of gender, BMI category and race/ethnicity nor are they presented for underweight because the sample sizes for these subdomains were too small to present statistically reliable results. Sample weights which account for the unequal selection probabilities and adjust for nonresponse and noncoverage are incorporated into the estimation procedure to produce unbiased estimates. Standard errors of these estimates were made using SUDAAN software to account for the complex sample design. To test the association between EBP and time period as well as pre-EBP and time period (1988–1994, 1999–2002, and 2003–2006) for all children and adolescents and within categories defined by race/ethnicity, gender and age group, a modified Rao–Scott test statistic for association (Proc Survey Freq, SAS V9.1) was used. This statistic adjusts the usual weighted Pearson ² by a design effect and follows the F distribution. It is more conservative than ² Statistics.²³

To test for changes in pre-EBP and EBP we used logistic regression to model these prevalences as a function of time period by treating time as a categorical variable and controlling for the possible confounding effects of gender, race ethnicity, age group, and BMI category. Underweight was excluded because of the small sample size (n = 60). Initially the models also included all possible two-way interactions of these main effects. Higher order interactions were not included because the number of variables included in the model would exceed the number of degrees of freedom. To test for significance, the Satterthwaite adjusted F test^24,25 was used. An interaction (P < 0.1) was found between sex and the NHANES survey years for both pre-EBP and EBP. Therefore, separate models were developed for boys and girls; these models included time period, race ethnicity, age group, and BMI category together with all possible two-way and three-way interactions. For pre-EBP and EBP in girls, none of the three-way or two-way interactions was significant. For EBP in boys all of the three-way interactions were significant. Therefore, to help interpret the information because of the significant third-order interaction we stratified the model into the two age categories. Age category was the logical category to stratify by because it is the most inclusive and interpretive category. The separate models by age group (8–12 years and 13–17 years) initially included all the two-way interactions; however none of those were significant. Our final stratified models therefore included main effects only. Odds ratios having a 95% CI not including unity were considered statistically significant for the logistic models.

All statistical analyses were carried out using SAS 9.1 for Windows software (SAS Institute, Cary, NC) and SUDAAN software (RTI, Research Triangle Park, NC).

Estimates for which the relative standard error (defined as the ratio of the standard error of the percent to the percent times 100) exceeded 30% are presented with an asterisk beside them. Estimates whose relative standard errors exceeded 40% are presented with two asterisks beside them. The asterisks and the double asterisks indicate that the estimates are unreliable (Tables 1 and 2).

Table 1 - Percent prevalence and standard error (s.e.) of pre-elevated BP^a among children and adolescents aged 8 through 17 years: United States, NHANES 1988-1994, 1999–2002, and 2003–2006.

Full table (159K)

Table 2 - Percent prevalence and standard error (s.e.) of elevated BP^a Among children and adolescents aged 8 through 17 years: United States, NHANES: 1988–1994, 1999–2002, and 2003–2006.

Full table (155K)

Results

Table 1 presents pre-EBP prevalence estimates from the three survey periods. During 2003–2006, 13.6% of boys aged 8–17 years and 5.7% of the girls aged 8–17 years were classified to have had pre-EBP. The prevalence of pre-EBP did not vary significantly over the three time periods 1988–1994, 1999–2002, and 2003–2006 for those aged 8–17 years old. However, subsample analyses showed that there was a significant association between pre-EBP and time period for Mexican-American girls aged 8–12 years, and for non-Hispanic black adolescent girls aged 13–17 years (Table 1).

Table 2 presents EBP prevalence estimates from the three survey periods. During 2003–2006, 2.6% of the boys aged 8–17 and 3.4% of the girls aged 8–17 were considered to have had EBP. There were no significant differences in observed EBP prevalence estimates for the total sample among the three survey periods. However, subsample analyses showed a significant association between EBP and time period for adolescent girls aged 13–17 years and for non-Hispanic white adolescent girls aged 13–17 years (Table 2). Finally, Tables 1 and 2 describing pre-EBP and EBP estimates included a number of unreliable estimates with large relative standard errors. However, in hypothesis testing, we were concerned with the estimated difference. Therefore, a statistical test was preformed, even if one or both of the proportions were not quite "statistically reliable" because the test statistics was still valid. Obviously, with large relative standard errors the differences must be large to be considered statistically different.

Multivariate analyses

Tables 3 and 4 present the adjusted odds ratios by gender (Table 3 for girls and Table 4 for boys) for the outcome variables pre-EBP and EBP. Non-Hispanic black girls were more likely to be classified as having pre-EBP when compared with non-Hispanic white girls (odds ratio (OR) 1.53, confidence interval (CI) 1.12–2.09). Younger children were less likely to be classified as having pre-EBP than adolescents of the same gender (boys OR 0.22 CI 0.15–0.31; girls OR 0.55; CI 039–0.77). Overweight boys but not girls were significantly more likely to be classified as having pre-EBP (OR 1.54, CI 1.11–2.13) when compared to the reference normal BMI. Obese children and adolescents were significantly more likely to be classified as having pre-EBP (boys, OR 2.81, CI 2.13–3.71; girls, OR 2.55, CI 1.75–3.73) and having had EBP (boys aged 8–12 years, OR 6.06, CI 2.73–13.44, boys aged 13–17, OR 9.62 CI 4.86–19.06; girls, OR 2.33, CI 1.31–4.13) when compared to reference normal BMI. The adjusted models using survey period as a discrete variable, with 1988–1994 as a reference, showed a twofold increase in the odds of being classified as having EBP for girls during 2003–2006 survey period when compared to 1988–1994 (OR 2.17, CI 1.05–4.49). Adolescent boys aged 13–17 years were less likely to be classified as having EBP during the 2003–2006 survey period when compared to 1988–1994 (OR 0.32, CI 0.13–0.81), whereas for boys aged 8–12 years there was no significant changes in the odds of being classified as having EBP during the later time period.

Table 3 - Adjusted odds ratios for pre-elevated BP and elevated BP by Demographic Survey, and body mass index (BMI) categories, children and adolescence girls NHANES 1999–2002 and 2003–2006.

Full table (52K)

Table 4 - Adjusted odds ratios for pre-elevated BP and elevated BP by demographic survey, and body mass index (BMI) categories, children and adolescence boys NHANES 1999–2002 and 2003–2006.

Full table (60K)

Discussion

During 2003–2006, 13.6% of the boys aged 8–17 years and 5.7% of the girls aged 8–17 years were classified as having pre-EBP and 2.6% of the boys aged 8–17 and 3.4% of the girls aged 8–17 were classified as having EBP. Overweight boys and obese boys and girls were significantly more likely to be classified as having pre-EBP and having EBP after controlling for all other covariates. Trend analysis showed that girls only were significantly more likely to be classified as having EBP during 2003–2006 than during 1988–1994. In contrast, adolescent boys aged 13–17 years were significantly less likely to be classified as having EBP during 2003–2006 than during 1988–1994.

The prevalence of EBP in our study is consistent with other recent reports. Hansen et al. reported hypertension prevalence of 3.6% among a cohort of 14,187 children and adolescents with an average age of 9.5 years (range 3–18 years) who had their BP observed at least three times during well-child care visits.²⁶ Our prevalence estimate of EBP for children aged 8–12 years was 3.4% (CI 2.3–4.7%). However, our results can only be obliquely compared to Hansen et al. results because theirs were based on more then one visit. Very recently, Din-Dzietham et al.¹⁹ reported BP trends in children and adolescents using NHANES data from 1963 to 2002. Their overall prevalence estimates for EBP rates were higher for NHANES 1988–1994 (2.7%, s.e. = 0.5) and NHANES 1999–2002 (3.7%, s.e. = 0.4), than our estimates of EBP for the same time period (2.1% (CI 1.3–4.1%) and 2.9% (CI 2–4.8%), respectively).

Although, their prevalence estimates were higher than ours, there were no statistically significant differences in the estimates. We suggest that the discrepancy in the prevalence between our results and Din-Dzietham's results may stem from truncating the age in their calculations (e.g., 156 months/12 would be truncated to 13 years) and excluding from their analyses youth classified as "other" for race/ethnicity category. Still another possibility for the discrepancy, although not as salient, may be the starting point of age calculation; we used date of exam as an index for age and they used date of interview. Up to six weeks may elapse from interview date to exam date. Finally, Gender, age group, race/ethnicity (non-Hispanic blacks), height, and increased weight had been previously associated with increased BP.^{11,12,19,27,28,29}

After controlling for all other covariates including BMI categories, survey period (2003–2006) was associated with increased prevalence of EBP in girls. However, for adolescent boys aged 13–17 years there was a significantly lower prevalence of EBP during this time period. Although the decrease was mainly reflected among white adolescents and among the obese (Table 2); the reason for this discrepant result in one of the 24 (3 2 2 2) time period, gender, age group, EBP, subgroups could be a chance finding or could reflect a BP–obesity interaction among adolescent boys compared to the other subgroups. Furthermore, this suggests that the increase in BMI may not explain all of the factors contributing to the increase in hypertension prevalence, or possibly that BMI does not capture the entire impact of increased body fatness. Environmental factors and lifestyle behaviors such as physical activity are associated with BP levels among adults and may also play a role in the causal pathway for youth.^30,31,32

Race/ethnicity in girls, classification of overweight in boys, and obesity in boys and girls were all independently associated with pre-EBP. It has been suggested that pre-EBP classification in children, as with adults, should alert the clinician of possible subsequent hypertension and a need for a closer medical follow-up—individuals in this category may proceed to develop hypertension and cardiovascular disease in the future.¹⁸

The major strengths of this analysis are that national data were obtained on large samples of US children and adolescents, including two major US population subgroups. The survey uses rigorous quality control monitoring, a standardized BP protocol, calibrated equipment, and well-trained examiners. Moreover, the same data collection protocol and equipment have been used to collect BP since 1988 (ref. 16). Although NHANES 1999–2002 data on EBP was previously reported, this study provides additional 4 years of data and the "other" race/ethnicity category were not excluded from the analyses.

The limitations of this analysis include the fact that BP levels were based on an average of three readings completed at one point in time. According to the new guidelines, elevated BP must be confirmed on repeated visits before characterizing the child or adolescent as having prehypertension or hypertension. Therefore our results may likely to overestimate EBP status for specific individuals. Second, we did not consider other potentially relevant information such as physical activity, dietary intake data, or secondary hypertension caused by other disease etiology.

In conclusion, independent of weight status, age, and race ethnicity the prevalence of EBP has increased among girls aged 8–17 years but decreased among adolescent boys aged 13–17 years. Overweight and obesity were significantly associated with EBP. These observations have clear implications for public health because increased BP in childhood may result in cardiovascular diseases in adulthood.

Disclosure

The authors declared no conflict of interest.

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Acknowledgments

The findings and conclusions of this report are those of the authors and do not necessarily represent the official position of the Centers for Disease Control and Prevention.