High blood pressure (BP) is a clearly established, but modifiable, risk factor for early disability and death . Although most of the adverse outcomes occur in adulthood it has become clear that hypertension (HTN) is a life course problem that can become evident in childhood and can be progressive throughout childhood into adolescence and adulthood. Although few would dispute the importance of taking effective steps to identify and manage this condition in middle-aged and elderly people, relatively little attention has been paid to the problem of high BP in children and adolescents.
The incorporation of BP measurement into routine pediatric healthcare and the publication of norms for BP in children [2,3] has not only enabled detection of significant asymptomatic hypertension secondary to a previously undetected disorder, but it has also confirmed that mild elevations in BP during childhood are more common than previously recognized, particularly in adolescents. It has also become clear that children with elevated BP have an increased risk of left ventricular hypertrophy and increased carotid intima–media thickness as well as an increased risk of hypertension in adulthood [2,3].
These findings support the potential for primordial prevention, that is, the prevention of elevated BP during childhood and adolescence. Prevention and management of hypertension, obesity, and other cardiovascular risk factors, such as dyslipidemia and insulin resistance, are critical for ensuring the health of young people today and the adults of tomorrow.
Epidemiological approaches to studying risk factors for cardiovascular disease have proven to be quite helpful in better understanding disease processes and in generating important hypotheses for future studies. Examining secular trends in childhood BP could help us better understand population-level determinants of HTN. Secular trends are the changes in risk factor level and prevalence that occur in a population over time. These temporal trends can be instructive, because they occur in a time frame where it is unlikely that the genetic background has changed, indicating that they are most likely due to changes in environmental factors .
Trends in childhood BP have been the object of only few studies but are of particular interest in the light of substantial increase in child obesity, the most important environmental factor for high BP in this age group. Because of the association between obesity and BP, it may be expected that levels of SBP and DBP among children would have also changed over the last several decades in parallel to changes in BMI. This parallel association was not found in some of the studies, and in those wherein an association was observed; it did not explain all the BP changes.
In the current issue of the Journal of Hypertension, Peters et al., provide the results of an analysis of secular trends in the level of childhood BP and the impact of obesity in the United Kingdom. On the basis of the data from over 25 000 children, a large increment was found in SBP and BMI values, as well as a triple increment in prevalence of obesity, over the study period. Despite this parallel increment in SBP and BMI, the trends in BMI and height explained only 15% of SBP increases, an association that has weakened over time.
Few studies have assessed the simultaneous secular trends of BMI and BP in children and adolescents . The population-based studies of BP trends, performed in developed and developing countries, have not consistently shown that BP increases in parallel with obesity. Some studies show upward BP trends whereas others show downward BP trends.
In the United States, data from the National Health and Nutrition Examination Surveys (NHANES) show that mean SBP and DBP increased by 1.4/3.3 mmHg from 1988–1994 to 1999–2000 . Increase in SBP was most pronounced for non-Hispanic blacks, Mexican Americans, and those in the 8–12-year-old age group. In contrast, increase in DBP was large and was observed in all subgroups. There was a strong association between BMI and SBP, thus obesity was considered to be a significant determinant of the population increase in BP. Recently secular trends in BMI and BP among children and adolescents in the Bogalusa Heart Study have been published . A total of 24 092 examinations were conducted among 11 478 children and adolescents (aged 5–17 years) from 1974 to 1993. Despite the increase in obesity in Bogalusa between 1974 and 1993, there was no increase in SBP or DBP levels.
In Europe, a Northern Irish study examining BP trends between 1989–1990 and 1999–2001, among children 12–15 years of age reported a decrease of 10 mmHg in SBP and DBP in both sexes . In contrast, in the study performed in the Samos Island, Greece  in adolescents aged 12–17 years, a large increment was observed: 4.3 mmHg for SBP and 10.6 mmHg for DBP in 2007 compared with 2004.
Studies in developing countries also provide potential interesting information due to the rapid changes in socioeconomic conditions. Using data from the China Health and Nutrition Surveys, trends in BP among 8247 Chinese children and adolescents, 6–17 years, were analyzed . Upward trends were observed with significant increase in SBP of 4.7 mmHg and in DBP of 4.0 mmHg from 1991 to 2004. Overweight was strongly related to the presence of hypertension. However, the authors still observed an upward trend in hypertension after adjustment for weight status. A Korean examination of BP trends between 1998 and 2008 in children aged 10–19 years, showed that age-adjusted and height-adjusted mean SBP decreased substantially by 8.7 and 10.0 mmHg among boys and in girls, respectively . These remarkable decreases were found among all age and socioeconomic groups and were not explained by concomitant secular changes in childhood obesity. Likewise, data from annual samples in the Seychelles observed that SBP decreased by 3 mmHg from 1998 to 2006 in both children and adolescents and both sexes .
The huge discrepancies observed among the different studies previously published and the present one in the Journal, bring us to a frequent concern regarding studies of secular trends and international comparisons. It is the issue of the methods of measurement (device used, appropriate cuff sizes, number of BP readings, setting and position of the participants), the inequalities in population samples (age distribution and stage of puberty among study place and ethnicity) and the adjustments performed during the analysis (age, BMI, height, among others). When measurements and analysis are not standardized over time across countries, it can lead to confusion regarding whether the observed differences are real or are artifacts, leading to a false conclusion.
Diagnostic criteria for elevated BP in children is based on the concept that BP in children increases with age and body size, making it impossible to utilize a single BP level to define hypertension, as done in adults. Evaluation of a BP measurement in a child requires determination of the child's height percentile first, and then comparing the child's BP level to the childhood BP tables according to the sex, age, and height percentile of the child. As the normal range of BP shifts with normal growth and development, the BP levels that denote high BP will change accordingly without further increment in the prevalence of hypertension. Despite a lack of increment in the prevalence of hypertension, shifting upward SBP can be translated to high BP in adult life with the consequent increment in hypertension and cardiovascular risk .
In the Peters study, the increment in both BP and BMI is much greater than that observed in previous studies carried out in both developed and developing countries in which controversial results have been published. Confirmation of these trends in the UK is needed. Factors that have resulted in higher BP levels also need to be identified. Although some useful approaches to the prevention and treatment of hypertension are known (prevention of overweight, weight-loss, increased physical activity, and dietary modification) others probably remain to be determined. Additional study of these potentially modifiable factors could lead to further reductions in the prevalence of high BP.
Conflicts of interest
There are no conflicts of interest.
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