Pediatric Nephrology Unit, Department of Pediatrics, General Hospital, University of Valencia, Spain.
Correspondence and requests for reprints to Empar Lurbe, Pediatric Nephrology Unit, Department of Pediatrics, General Hospital, University of Valencia, Avda Tres Cruces s/n, 46014 Valencia, Spain. Tel: +34 96 3862900; fax: +34 96 3862647; e-mail: email@example.com
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Assessment of blood pressure in children and adolescents is firmly recommended in the course of routine health care , even though the answers to many key questions remain unclear. Blood pressure values have long been identified as a risk factor for the development of cardiovascular disease morbidity and mortality in the adult population . However, this relationship is more difficult to evaluate in children and adolescents because the endpoints of concern are often decades into the future. This circumstance has led to a distributional approach for the evaluation of blood pressure in the pediatric population, with a systolic or a diastolic blood pressure greater than the 95th percentile being designated as elevated .
All that is known about the levels and distribution of casual blood pressure in childhood and adolescents is that blood pressure increases during growth and maturation, and adolescence is a fast growth period during which body mass and blood pressure change rapidly. These are the main reasons why reference blood pressure values over the last few decades have been referred to as specific for sex, age, and/or height in children and adolescents up to 18 years of age.
Based on the above, normal blood pressure is defined by the Task Force as systolic and diastolic blood pressure less than the 90th percentile for age, sex and height. Borderline, or high-normal blood pressure, is defined as an average systolic and/or average diastolic blood pressure between the 90th and 95th percentiles for age, sex and height. Hypertension is defined as an average systolic and/or average diastolic blood pressure greater than the 95th percentile for age, sex and height, measured on at least three separate occasions .
Statistically, 5% of children have a blood pressure measurement higher than the 95th percentile during a single office visit. However, blood pressure tends to normalize on subsequent measurements due to the accommodation of the child to the measurement procedure and to the statistical phenomenon of regression towards the mean. Consequently, the prevalence of hypertension decreases to 1% after the second examination .
Considering that the prevalence of essential hypertension is low and that there is little immediate risk to most children, can routine blood pressure measurements as part of pediatric care be justified? Blood pressure tracks from childhood to adulthood; therefore, some have recommended screening to identify those children at risk for the development of essential hypertension as adults. Others have argued against this strategy because they believe that correlations between childhood and adult blood pressure levels are too weak. These viewpoints call into question the usefulness of routine blood pressure measurements to identify children at high risk for the development of essential hypertension.
Few large-scale epidemiologic studies have revealed long-term persistent elevated blood pressure from childhood into adulthood and, more importantly, how the persistent elevation in blood pressure ultimately has developed into adult hypertension [5,6]. Unfortunately, some aspects of design and methodology differ among the studies, making it difficult to pool data.
The degree of blood pressure tracking has been shown to vary among studies demonstrating positive correlations, from low to high magnitude [5–9]. In general, a lower correlation was found for diastolic blood pressure compared to systolic blood pressure . The differences among the studies with respect to the strength of the juvenile–adult blood pressure correlations may be due to the differences in ages of children at entry, in adult outcome ages, in blood pressure recording and in the time of follow-up. The same happens with studies which analyse the risk to develop hypertension. Although studies of blood pressure tracking have shown lower correlation coefficients with increasing length of time between examinations, it is important to know if blood pressure remains predictive as the cohort enters the age when hypertension begins to develop. As the prevalence of hypertension increases, predictive values will also increase and may become clinically more important.
In the present issue of the journal, Vos et al.  focus on whether routine blood pressure measurement in young adolescence is a predictor of hypertension and total cardiovascular risk in young adulthood. The authors analysed two Dutch cohorts screened during adolescence, and re-examined these cohorts between 15 and 20 years later, to determine whether blood pressure measured at the beginning accurately predicts hypertension and the 10-year cardiovascular risk calculated at the final examination. As a concurrent objective, the authors investigated whether such blood pressure measurements add to the value of indicators of later hypertension and cardiovascular risk routinely collected at present. The authors conclude that adolescent blood pressure screening would detect a large proportion of all young adult hypertensive women. In addition, the authors tried to analyse the predictive value of adolescent blood pressure on the 10-year cardiovascular risk as calculated with the Framingham risk score. As expected, this calculation was made only for men because young women have a low cardiovascular risk. Given the large and continuous impact of cardiovascular disease in individuals and on public health, the authors believe that youth health care should consider blood pressure screening in adolescents. The article contributes interesting information and leads to a discussion of important issues.
The first issue is the period selected for the baseline blood pressure measurement. Besides the biological variability of blood pressure values, the timing of blood pressure measurements introduces a potential bias. This is especially relevant during adolescence, due to the impact of pubertal development on blood pressure when considering that age does not accurately predict the pubertal stage of the subject . Only before the beginning of pubertal changes, or when the pubertal growth spurt is over, and the impact of hormonal status is stable, are blood pressure values the most reliable in both boys and girls. Whether or not the different results in boys and girls for predicting hypertension observed in the study by Vos et al.  could be explained by different levels of sexual maturity at the time needs to be taken into account. In this study, pubertal stage was evaluated in only one of the cohorts. From the data released, it can be deduced that pubertal development was practically complete in girls, but not in boys.
Besides the question of when to measure blood pressure, no less important is the question of how many measurements are necessary to better predict hypertension. When only a cross-sectional survey is used, it is convenient to minimize blood pressure variability by increasing the number of measurements from which the average is calculated. However, predicting from various surveys over time may offer a better estimation. In the Bogalusa Heart Study , the prevalence of hypertension in subjects with four or five previous elevations was much higher than for those with only one elevation, thus indicating that the higher the subjects were at risk, the more likely they were to be identified. In addition to blood pressure measurements, it is important to remember that the use of semi-automatic oscillometric devices in a young population tends to overestimate blood pressure values . This may explain the difference of systolic blood pressure values between the two cohorts included in the study by Vos et al. , which have their blood pressure measured with different devices , even though age and body mass indices were similar between the two cohorts.
Second, what is the predictive value of baseline blood pressure for developing hypertension later in life. Using the data compiled from single blood pressure measurements in adolescents, Vos et al.  found that for every 1 mmHg increase in systolic blood pressure, the risk of hypertension increased 8% in girls and 3% in boys. When adolescent systolic blood pressure was categorized into three levels, a low adolescent blood pressure (≤ 99 mmHg in girls) excluded young adult hypertension, and among those with systolic blood pressure ≥ 120 mmHg, 16.4% became hypertensive. These cut-off points were less suitable for boys in predicting future hypertension.
Predictive value had previously been analysed in two different studies [5,6] that had similarities in the time of follow-up and the age at the last examination. In the Bogalusa Heart Study , blood pressure was monitored as part of the cardiovascular risk factors surveys in a biracial population (64% White and 36% Black). The study showed that childhood blood pressure elevation serves as a good predictor of elevated blood pressure and hypertension in adulthood, especially when multiple observations are available. At the follow-up examination, subjects who were in the highest quintile, given by age, race, and sex-specific quintiles of blood pressure level, were much more likely to develop hypertension (3.6-fold for systolic blood pressure and 2.6-fold) for diastolic blood pressure. All other subjects in every other quintile had a similar low prevalence, indicating that the hypertensive subjects were primarily from the top quintile. Concordant results were published from a cohort analysed 20 years apart in Lithuania .
Both studies [5,6] introduced an important factor that needs to be considered. They showed that changes in weight and body mass index had a strong influence on future blood pressure. Weight gain from childhood to adulthood was independently associated with future elevation of blood pressure, reflecting strong influences exerted by behaviour and by the environment. Therefore, inclusion of nutritional information and activity levels in the surveys may improve not only the predictive value of early blood pressure measurements, but also offer information about the positive effect of intervention with life style changes .
The crux of this discussion is that early life risk prediction only makes sense if there are effective interventions to reduce risk based on the information obtained during childhood. Considering the available information, those children who remain in the higher distribution of blood pressure values need special guidance to reduce weight if they are overweight or obesity is present or to avoid excessive weight gain over time with nutritional counselling and increased physical activity.
In the face of the great impact that adult cardiovascular disease has in Westernized societies, it seems crucial to examine further the relationships among cardiovascular risk factors at the childhood–adolescence–adulthood transition, the putative earliest point in the development of cardiovascular risk. It is important to stress that, despite the variability and uncertainties of blood pressure values, they are the main measurable marker of cardiovascular risk later in life. Unfortunately, a better test than blood pressure measurement has yet to be described. This justifies looking for the applicability of blood pressure measurements during childhood and adolescence for prognostic value.
Understanding the persistence of blood pressure elevation over time and its progression into clinical hypertension would aid in the early identification and prevention of hypertension. This would be improved by the inclusion of information concerning genetic background  and fetal growth , both clearly related to blood pressure values and whose impact increases over time.
In summary, the goal of blood pressure measurement in children and adolescents is to provide strategies for promoting cardiovascular health, and these should be integrated into a comprehensive pediatric health care programme.
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