Indian Pediatrics 2006; 43:491-501
Blood Pressure Reference Tables for Children and Adolescents of Karnataka
Pushpa Krishna, *PrasannaKumar K.M., §Nagaraj Desai, and ¶Thennarasu K.
From the Department of Physiology, *Department of
Endocrinology & Metabolism, §Department of Cardiology,
Correspondence to: Dr. Pushpa Krishna, Lecturer, Department of Physiology, M.S. Ramaiah Medical College, Bangalore, India. E-mail:email@example.com
Manuscript received: February 21, 2005, Initial review
completed: April 8, 2005,
α + β (Z height) + X.σ where σ2 was estimated from the residual mean square from regression model and X = 1.280, 1.645 or 2.326 for the 90th, 95th and 99th percentiles, respectively(8,10).
After excluding undernourished (n = 283) and obese (n = 282), data of 5208 children were considered for analysis. The study sample characteristics are shown in Table I. The maximum numbers of children were in the age group of 14 years (n = 834) with minimum number of children in the age group of 4 and 5 years (n = 70). Age and height were principal determinants of blood pressure (Table II). The height percentile values of the 5th through 95th percentiles for each one-year age group were calculated and reported in Table III. The height was ranging from 81 to 179 centimeters. In the age groups 3 to18 years the values of systolic and diastolic blood pressure ranged from 60-180 mm Hg and 30-110 mm Hg respectively. The mean values of body mass index (BMI) show steady increase with age from 9 year onwards and were ranging from 13.6 to 20.4.
Characteristics of Study Sample
Figures in parentheses are standard deviations; SBP–Systolic blood pressure; DBP–Diastolic blood pressure; BMI–Body mass index.
Table II Results of Stepwise Regression Analysis of SBP and DBP on Independent Variables
SBP-systolic blood pressure DBP-Diastolic blood pressure.
Table III Height Percentiles by Age in the Reference Sample
Regression coefficients from age specific blood pressure regression models in reference sample to calculate the 50th, 90th, 95th and 99th percentile values of systolic and diastolic blood pressure by age and height are given in Table IV.
Regression Coefficients from Age Specific Blood Pressure Regression Models in Reference Sample
Blood pressure (age)=a1+b (Z height)+ X.s, where s2 was estimated from the residual mean square from regression model and X=2.326, 1.64 or 1.280 for the 99th, 95th and 90th percentiles, respectively.
The percentile BP levels by age and height percentiles are reported in Table V. The mean blood pressure increased with age (exception 9, 13 and 17 years for systolic blood pressure and 13 and 18years for diastolic blood pressure). The rate of increase was gradual with a spurt of 4 to 5 mm Hg in systolic blood pressure at the of age 8 years and 14 years.
Blood Pressure Levels for the 50th, 90th, 95th and 99th Percentiles of Systolic and Diastolic Blood Pressure by Percentiles of Height in Boys and Girls of Age 3 to 18 years
Table V shows that the systolic blood pressure of 11th and 17th year and diastolic blood pressure of 8th, 11th and 13th year children with height between 5th through 95th percentiles is lower than those of previous age groups with corresponding height percentiles. We recommend that the higher values be used for all above mentioned age group children to avoid any possibility of mislabeling. To interpret the blood pressure of a child, his/her height percentile has to be determined from Table III, and then Table V has to be consulted to estimate different percentile values of systolic and diastolic blood pressure. The BP level <90th percentile is normal. The BP measurements between the 90th and 95th percentiles indicate prehypertension. The BP level > 120/80 mm Hg in an adolescent is considered prehypertension. The BP level >95th percentile is hypertension. Stage 1 hypertension is the designation for BP levels that range from the 95th percentile to 5 mm Hg above the 99th percentile. Stage 2 hyper-tension is the designation for BP levels that are >5 mm Hg above the 99th percentile.
In this study age and height specific, 50th, 90th, 95th and 99th percentile values of systolic and diastolic blood pressure are reported in tabular form in Table V based on the first BP measurement taken on 5208 children(3). This table helps clinician to decide, whether observed BP values are normal or abnormal. The child is normotensive if the BP is below 90th percentile. If the BP is >90th percentile, the BP measurement should be repeated at that visit to verify an elevated BP. The average BP measurements between the 90th and 95th percentiles are high normal or prehypertension. Adolescents with BP levels >120/80 mm Hg should be considered to be prehypertensive even if the level is <90th percentile as with adults according to Seventh Report of the Joint National Committee on the Prevention, Detection, Evaluation, and Treatment of High Blood Pressure(11). If the child’s BP is >95th percentile, the child may be hypertensive and repeated measurements are indicated. Though the precise characterization of a person’s BP level is an average of multiple BP measurements taken over weeks to months, it is recommended to take BP measurement on at least 2 additional separate occasions to confirm hypertension. Hypertension is defined as average SBP or DBP that is >95th percentile for age and height on at least 3 separate occasions. The 50th percentile values provide the clinician with the BP level at the midpoint of the normal range. The 99th percentile helps to determine the degree or severity of hypertension by staging of BP into stages 1 and 2(8).
Categorization and staging of BP, is helpful in planning a specific treatment that is most appropriate for an individual. Prehypertension warrants reassessment and consideration of risk factors and for this, treatment is by lifestyle modification. Stage 1 hypertension needs limited evaluation where as in stage 2 hypertension immediate evaluation and therapeutic intervention is indicated.
In order to prepare a normative data, the significant factors influencing systolic and diastolic blood pressure were analyzed. The Table II shows that there is a significant effect of age and height (p <0.001, p <0.001) for systolic and (p <0.005, p <0.001) for diastolic blood pressure respectively. The age year groups shows the increasing trend from 3 years through 18 years with the spurt of 4-5 mm of Hg in systolic blood pressure at the age of 10 year and 15 year. Diastolic blood pressure also shows similar increasing trend. Similar observations have been made by other workers and have found a spurt in systolic blood pressure at different age group(12-15). The spurt may be possibly due to age related hormonal and physical changes occurring in the body during puberty.
The reference table reveals that the 90th percentile of systolic and diastolic blood pressure for 5th through 95th percentiles of height from age group 12 and 10 years onwards respectively exceeds 120/80 mm of Hg. It is now recommended that, as with adults, adolescents with BP levels >120/80 mm Hg but <90th percentile should be considered prehypertensive according to the new reports from US(8). Similar trends have been observed in US normative tables. This sudden increase in BP level may be attributed to the rapid growth associated with rapid weight gain from late childhood into adolescence (16).
In the present study, the reference table (Table V) shows that, for children of particular age, the 95th percentile values of blood pressure varied from 3 to 12 mm of Hg (for systolic blood pressure) and 1 to11 mm of Hg (for diastolic blood pressure) between 5th and 95th percentiles of height. The difference in the blood pressure values for different height percentiles indicates that the height plays substantial role in determining blood pressure value of an individual. Hence height has to be considered as a factor before classifying a child as hypertensive. This approach of developing BP standards that are based on height provide a more precise classification of BP according to body size and avoids misclassifying children who are very tall or very short.
It is difficult to compare the present study with others because of difference in methodologies such as age group studied, phase of Korotkoff sound used to determine DBP, pooling of sex group etc. One major observation made in this study is that, in Karnataka, a southern Indian state, the mean value of blood pressure is found to be higher in sample for given age and also for a given height compared to previous Indian reference values in which sample derived is mainly from northern states of India(5) and also to that of US young population(8). This may possibly indicate an increase in the risk of this young individuals becoming hypertensive when they become adults. The high prevalence of hypertension has been related to rising mean systolic blood pressure in adults(17). The higher blood pressure level in this population may be because of many other factors that determine blood pressure level in an individual such as genetic inheritance, low level of physical activity, mental stress and dietary habits and environment which the authors have not studied. The difference in the ethnicity also may be one of the factors. The increased level of normative values of blood pressure may be because of factors such as body size, plasma high-density lipoprotein cholesterol, plasma triglyceride and abnormal glucose tolerance, dietary habits, life style and many other factors stated above that determine BP, which have to be ascertained by further studies.
In our study sample the undernourished children have shown significantly reduced (P<0.05) levels of systolic and diastolic blood pressure. This might be one of the reasons for increased level of normative values as compared to previous Indian standards in which children with <5th percentile of BMI (Undernourished) are not excluded(5). Further more, blood pressure is found to be associated with BMI both in normal and obese children(18-21). The higher level of BMI observed in our sample (except for 6 year) might be a contributing factor for our study population having higher values of blood pressure because body size appears to be a major determinant of BP and study by Ramachandran et al., reports that there is a high prevalence of overweight in adolescent children in India(22,23).
The socioeconomic status of the subjects in this study was not considered as the reference values are developed on the basis of BMI of the individuals, which was between 5th-95th percentiles of reference sample.
Limitations of the study
Though the sample was collected from various schools, sample sizes were very low for extreme ages 3, 5 and 17, 18 years to derive the normative values and making it applicable to general population. The sample size for age year 5 was 70 and for age years 3,17, and 18 the sample size was about 100 only, hence care should be taken in interpreting results of these age years.
While it is recommended to use average of multiple blood pressure measurements taken for weeks to months to characterize individual’s blood pressure level, high cost, limited resources and time restricted the authors from doing so. However studies on effect of sequential blood pressure reading in normal and hypertensive adults have indicated that the difference between the subsequent BP measurements depends on age, BMI and initial BP level(24,25). This may be the one of the reasons why no significant difference was observed between the first and second reading of BP in 275 young subjects of our study. But efficacy of single measurement protocol in young individuals needs to be established.
Even though the blood pressure measurements were measured as exact as possible there may be some digit preference in our data. This effect is unlikely to be large as reported in the previous studies(7,26). Blood pressure is also influenced by various other factors such as time of the day, ambience, fasting vs. non-fasting state of the subject etc., which could not be controlled in the study.
We would like to thank Dr. Anura Kurpad and Dr. M. Vaz, Department of Nutrition, St. John’s Medical College, for critical review of the data and Dr. Rajeev Sharma, Head, Department of Physiology, MSRMC, for the support.
Contributors: Concept, design and final approval by PKM and ND; acquisition, interpretation and drafting by PK; PK will act as the guarantor of the study; Analysis of data by TK.
Funding: Dr. Thennarasu was supported by Fogarty International Training Grant Number D43 TW00 5811.
Competent interests: None.