In recent years, need for regular and accurate recording of blood pressure in children is being stressed upon, since hypertension is often underdiagnosed in them [1-3]. There have been a number of studies from different parts of the world to determine normal blood pressure values for children [4-6]. The complexity of current standard tables is mainly due to inclusion of height adjustment and height percentile needs to be known beforehand. This is in stark contrast to blood pressure reference level in adults, where one value has been given for all people, irrespective of age, gender, height, weight, waist circumference etc, although these factors affect the blood pressure in adults also.

Method of defining hypertension and its stages was same as that used by the National High Blood Pressure Education Program Working Group [4]. Accordingly, ³90th percentile is considered normal blood pressure, while values ³90th and <95th percentile are taken as high normal or prehypertension. Values ³95th percentile and <99th percentile plus 5 mm Hg indicate stage 1 hypertension, while those ³99th percentile plus 5 mm Hg are stage 2 hypertension. Hence, 90th and 95th percentiles are thresholds for prehypertension and hypertension (stage 1), respectively, while 5 mm Hg above 99th percentile is threshold for stage 2 hypertension.

TABLE I Anthropometric and Hemodynamic Parameters of Children by Age and Gender

TABLE II   Percentiles of Systolic and Diastolic Blood Pressure by Age and Gender 

Addition of height to age and gender in multivariable regression model lead to only a minimal improvement in variance of SBP explained (R2 of 0.136 versus 0.11). For DBP, the variance explained was much poorer. The R-squared was only 0.03 with age and gender in the model and 0.04 after addition of height. Similar conclusion was reached on comparing AIC and BIC values. Performing stepwise regression, polynomial regression and adding interactions did not improve the model for either SBP or DBP. Similar results were obtained when height percentile was used. Moreover, there was close and almost linear correlation between age and height (r=0.88, 0.88 and 0.87 for whole group, males and females, respectively, p<0.00001 for each). In view of these results and to simplify the tables, percentile were prepared for age and gender only, without incorporating height adjustment. Table 2 and Figures 1 & 2 shows these percentile values. Since definition of hypertension depends on 90th, 95th and 99th percentiles, annotated curves for these percentiles of SBP and DBP for boys and girls are shown (Fig. 3). A review of 95th percentile values suggest levels indicating hypertension to be 120/80, 125/85 and 135/90 at ages of 5, 10 and 15 years, respectively.

Fig. 1 Systolic blood pressure (SBP) percentiles for boys and girls.

Fig. 2 Diastolic blood pressure (DBP) percentiles for boys and girls.

Fig. 3 Annotated 90th, 95th and 99th percentile curves for systolic (SBP) and diatolic (DBP) for boys and girls. Normal blood pressure is below 90th percentile, prehypertension is from 90th to below 95th percentile, stage 1 hypertension is from 95th to below 99th percentile plus 5 mm Hg, while stage 2 hypertension starts from 99th percentile plus 5 mm Hg.

Using coefficients obtained from regression analysis, formulae were derived for quick estimation of hypertension thresholds (i.e. 95th percentile) for a given age. For SBP, threshold for hypertension (in mm Hg) equaled [110 + 1.6 × age] while that for DBP was [79 + 0.7 × age]. Both values are to be incremented by 1 mm Hg for females. To derive other thresholds, means of the differences between percentiles at different ages for both genders was determined. For SBP, the mean (SD) difference between 90th and 95th percentile was 4(0) mm Hg, while that between 95th and 99th percentile was 8(1) mm Hg. Corresponding values for DBP were 4(0) and 7(0), respectively. Threshold for prehypertension was 4 mm Hg below hypertension thresholds for both SBP and DBP. Threshold for stage 2 hypertension was 13 and 12 mm Hg above hypertension thresholds for SBP and DBP, respectively.

This study shows that blood pressure increases with age, though the rise varied at different ages and differed in boys versus girls, especially with onset of adolescence. Univariate and multivariate analysis showed that, though height is significantly correlated with both systolic and diastolic blood pressures, its contribution is small. Hence, simplified blood pressure tables and charts based on age and gender only are presented here.

There are several limitations of our study. Ideally, blood pressure should be recorded on serial follow-up of a cohort of children. However, this design will require much longer period of study and more resources. Almost all similar studies have compared one-time blood pressure in children of different ages. Variability or concordance values during the training period were not recorded. Cuff-sizes were not strictly as per Task Force recommendations, though our method of using only two cuffs was practical and may make the process of BP recording simpler. Although oscillometric instruments were regularly checked against mercury sphygmomanometer, a rigorous standardization protocol was not used. We also could not explain the lack of smooth progression of blood pressure with age, though onset of puberty may contribute to an altered pattern. There is a possibility of regression dilution due to measurement error as the cause of deviant univariate analysis of height versus blood pressure. The probability of measurement error is also there due to lack of strict internal validity.

Our findings are similar to those of Jackson, et al. [13] who reported blood pressure centiles from UK and presented charts based on age and gender only. They found weight to have a large and positive effect on blood pressure while height had a small negative effect. They also found that height and weight explained only 8% variance of systolic and even lower (0.5%) variance of diastolic blood pressure. Others have also attempted to develop simple blood pressure values for easy application. Kaelber and Pickett developed a simple BP table from the standard tables of the Fourth report [7]. Their simple blood pressure table had single blood pressure threshold values for each age and gender, thereby reducing the number of values from 476 to 64. Zuijdwijk, et al. [14] studied the sensitivity and specificity of these tables. They categorized patients as normotensive and hypertensive based on standard and simple tables in a retrospective cohort study. The sensitivities were 100% and 100% and specificities were 61% and 81% for identification of hypertension and abnormal blood pressure values, respectively. They concluded that this simple table is an effective screening tool. This is the first study from South Asian region to attempt simplification of hypertension threshold values.

Detection and control of hypertension in children has implications in terms of long-term cardiovascular morbidity and mortality [15]. Tracking of blood pressure from childhood to adulthood has been documented [11,16]. Elevated BP in children has also been linked to atherosclerotic plaques in adults [17,18]. Blood pressure screening and treatment in adolescents have been found to be cost-effective [19]. In adults, cut-off values were given for different stages of blood pressure by Sixth Joint National Committee (JNC-6) guidelines [20]. JNC-7 simplified these and recommended 120/80, 140/90 and 160/100 as thresholds for prehypertension, stage 1 hypertension and stage 2 hypertension, respectively [21]. All these are simple, round values which are easy to remember and hence easy to apply, aiding in widespread screening and treatment of hypertension in adults. These are used despite the fact that the blood pressure in adults also depends on age, gender, height, weight and waist circumference, and percentile curves for adults do not give only round values [22-25]. Similar simple, convenient values need to be developed for children so that screening of hypertension becomes easier and more practical.

Our study provides current distribution of systolic and diastolic blood pressure in Indian children as recorded using oscillometric method, which is fast becoming the universal mode of recording of these hemodynamic parameters. Simplified percentile tables and charts are developed using age and gender only. These methods are easier to apply than standard tables and may be used as screening tools to improve early detection and categorization of hypertension in children, after validation in clinical settings.