Reference values for blood pressure of healthy Sri Lankan Tamil children in the Jaffna district

Introduction: Increasing trend of childhood obesity leads to high blood Pressure (BP) in children and adolescents. Ethnic differences in BP have been reported. Normal BP values of Sri Lankan Tamil children are not available. Objective: To measure BP of healthy Sri Lankan Tamil children in the Jaffna district to get normal BP values and to correlate them with anthropometric measurements. Method: A population based descriptive cross sectional study was carried out among children and adolescents (950 boys, 972 girls) aged 6 to 18 years in schools in Jaffna district. Cluster sampling was applied to classrooms in the schools. The classes were selected by systematic random sampling. Age, height, weight, waist circumference and hip circumference were taken. Body mass index, waist hip ratio and waist height ratio were calculated. BP was measured with a mercury sphygmomanometer. Pubertal stage was assessed with a self-administered Tanner staging scale. Results: Mean of the systolic blood pressure (SBP) and diastolic blood pressure (DBP) of boys and girls increased from 98/70 mm Hg and 99/70 mm Hg to 107/73 mm Hg and 107/73 mm Hg until 10 years, decreased slightly up to 13 years (101/64 mm Hg and 102/63 mm Hg) and increased until 18 years to 119/76 mm Hg and 111/70 mm Hg. From the age of 15 years, boys had higher SBP and DBP than girls (p<0.05). SBP had significant (p<0.001) positive correlations with pubertal staging and all measured anthropometric parameters. Highest correlation was observed with weight (0.522). A normogram for BP was constructed with age and height. _________________________________________ Lecturer, Demonstrator, Associate Professor, Department of Physiology, Faculty of Medicine, University of Jaffna, Sri Lanka *Correspondence: mathu1481@yahoo.com (Received on 11 December 2017: Accepted after revision on 19 January 2018) The authors declare that there are no conflicts of interest Funding: University Research Grant, 2015 Open Access Article published under the Creative Commons Attribution CC-BY License Statistically significant (p<0.05) increases in SBP and DBP were observed from pubertal stage 3 to pubertal stage 4 in both boys and girls. Jaffna boys up to 15 years and girls up to 10 years have higher BP than Sinhalese children at a purana village. After these ages BP values were lower than in Sinhalese children. Conclusion: BP of healthy Sri Lankan Tamil children in the Jaffna district has significant correlations with anthropometric measurements and Tanner staging. DOI: http://dx.doi.org/10.4038/sljch.v47i3.8548 (

Objective: To measure BP of healthy Sri Lankan Tamil children in the Jaffna district to get normal BP values and to correlate them with anthropometric measurements.
Method: A population based descriptive cross sectional study was carried out among children and adolescents (950 boys, 972 girls) aged 6 to 18 years in schools in Jaffna district.Cluster sampling was applied to classrooms in the schools.The classes were selected by systematic random sampling.Age, height, weight, waist circumference and hip circumference were taken.Body mass index, waist hip ratio and waist height ratio were calculated.BP was measured with a mercury sphygmomanometer.Pubertal stage was assessed with a self-administered Tanner staging scale.
Results: Mean of the systolic blood pressure (SBP) and diastolic blood pressure (DBP) of boys and girls increased from 98/70 mm Hg and 99/70 mm Hg to 107/73 mm Hg and 107/73 mm Hg until 10 years, decreased slightly up to 13 years (101/64 mm Hg and 102/63 mm Hg) and increased until 18 years to 119/76 mm Hg and 111/70 mm Hg.From the age of 15 years, boys had higher SBP and DBP than girls (p<0.05).SBP had significant (p<0.001)positive correlations with pubertal staging and all measured anthropometric parameters.Highest correlation was observed with weight (0.522).A normogram for BP was constructed with age and height._________________________________________ Introduction High blood pressure (BP) is a powerful, constant, and independent risk factor for cardiovascular and renal diseases 1 .The definition of hypertension is based on normative distribution of BP 2 .Increasing trend of childhood obesity leads to high BP in children and adolescents 3 .Nowadays behavioural patterns of communities are changing with urbanization and industrialization leading to sedentary life styles and consumption of high fat and energy dense diets which cause obesity.Hypertension is the common childhood condition seen in obese and overweight individuals.Hereditary and environmental factors also influence body mass index (BMI) and body fat which may influence the BP 4 .The biological rationale for relating measures of central adiposity to cardiovascular disease (CVD) risk is that the abdominal adipose tissue is positively associated with waist circumference (WC) and waist-hip ratio (WHR).As the WHR increases, BP increases 5 .Further, BP is important in childhood conditions such as acute glomerular nephritis and dehydration.
Unlike the practice of measuring BP in all adult patients and advising to monitor it regularly, BP in children is measured only when there is a suspicion of alteration in BP.In such instances, interpretation of the measured BP can be of value only if reference norms for the age, sex, height and ethnicity are available.Therefore, normal values of the Jaffna population should be known to be able to diagnose hypertension among children and adolescents in this region.The only report available on BP values of Sri Lankan children is from a study on inhabitants of a "purana" village of Sri Lanka in 1991 6 .Data on the BP of healthy Sri Lankan Tamil children are not available.Mean BP values were higher in Afro-Caribbean and South Asian men than their white counterparts 7 .Among South Asians, Indians have a slightly higher BP, Pakistanis have a slightly lower BP and Bangladeshis have a much lower BP 8 .This shows that ethnic differences in BP exist.

Objectives
This study aimed at measuring BP in healthy Tamil children in Jaffna to get normal BP values and to correlate them with anthropometric measurements.

Method
This was a population based descriptive cross sectional study.Children aged 6 to 18 years were recruited from schools in Jaffna district as education is compulsory in Sri Lanka.Cluster sampling was applied to classrooms in schools of Jaffna District.Each cluster was a classroom consisting of around 30 students.Number of clusters in each grade was selected according to the percentage of total students in that grade.Classes were selected by systematic random sampling.
Ethical clearance for the study was obtained from the Ethical Review Committee of the Faculty of Medicine, University of Jaffna.Permission was obtained from the Provincial Director of Education, each Zonal Director of Education, School Principals and Class Teachers prior to the study.Informed written consent was obtained from all parents and assent from adolescents 14 years or older.The information sheet and consent form were given to the participants on the day before data collection to get the consent from the parents.
A self-administered questionnaire, which included general information and disease history of the participant, was issued to one parent of each participant.Children with a history of diabetes and children diagnosed to have any type of CVD were excluded.Anyone who had signs and symptoms suggestive of anaemia, cardiac or renal disorders on medical examination during data collection and children having the habit of smoking and alcohol intake were also excluded.
Age was calculated in years as on the last birthday.Body height and weight were measured according to a standardized protocol to the nearest 0.1cm and 0.1kg respectively by using a portable device (Seca).BMI was calculated from measured body weight and height.WC was measured by placing the non-elastic measuring tape midway between the uppermost border of the iliac crest and lower border of costal margin while subject was standing.
Measurement was taken at the end of expiration.Hip circumference (HC) was measured by placing a non-elastic tape over the widest part of the hip 9 .
The procedure was explained thoroughly to the participants before recording blood pressure and sufficient time was given to allay anxiety and fear.Participants were in a sitting position with the legs uncrossed.BP was measured in right arm by the auscultatory method using a standard mercury sphygmomanometer.The cuff bladder was wide enough to cover at least 80% of the arm and long enough to encircle the arm.The cuff was inflated to at least 30mm Hg above the point at which radial pulse disappeared.The cuff was deflated at a rate of 2-3mm Hg/second.The first Korotkoff sound was taken as indicative of systolic pressure and the disappearance of the sound was the diastolic blood pressure.Three readings were taken at 5 minute intervals.As the first reading was usually higher, the average of second and third readings was considered for analysis.
Participants aged above 8 years were given the self-administered questionnaire with the Tanner staging scale which consisted of pictures of genital organs and breasts at different stages.Participants had to compare their genital organs and breasts with that on the Tanner staging scale and identify and mark the pubertal stage in the questionnaire.This was done in a confidential place separately for males and females after giving clear instructions.One investigator of the same sex was available to clear doubts regarding marking the pubertal stage.
Data analysis was done by Statistical Package for Social Science.Independent t-test was used to compare significance of difference between means.Pearson's correlation coefficient (r) was used to evaluate relationship between BP and height, weight, BMI, WC, HC, WHR and Tanner staging.

Results
There were 950 boys and 972 girls.Anthropometric measurements of the study population are summarized in Table 1.
Mean height of boys and girls of each age showed statistically significant (p<0.05)differences beyond 13 years of age.Differences in the mean weight of boys and girls were statistically significant (p<0.05)beyond 16 years of age.
Pearson correlation of height, weight, BMI, WC and HC with age were 0.931, 0.817, 0.558, 0.620 and 0.784 respectively in boys and 0.886, 0.810, 0.629, 0.625 and 0.814 respectively in girls.All these were statistically significant at p<0.001.Blood pressure values of boys and girls in each age group are summarised in Table 2. From the age of 15 years, boys had observably higher SBP than girls of the same age which was statistically significant (p<0.05).Considering the DBP, girls in the ages of 8 to 9 years had significantly higher values than boys of the same age.
When the difference between mean DBP of each age group of boys was analysed, statistically significant (p<0.05)increase in DBP was observed from 13 to 14 year and from 17 to 18 years.In girls, statistically significant (p<0.05)increase from one age to the next is observed in 7 to 8 years and 13 to 14 years of age groups.However, from 6 to 10 years of age DBP increases and then there is a reduction until 13 years of age and increases again until 18 years of age.This pattern was observed in both boys and girls.
Pearson correlation between anthropometric measurements and BP are summarised in Table 3.As can be seen, the regression analysis of the entire data seems to be unreliable because of the very low value of R 2 .This may be due to the variation of blood pressure of different age groups.
A normogram was constructed with age and height to keep in line with the current practice of diagnosing hypertension in children and adolescents using the data of each age group separately.However, the present study showed better correlation between BP and body weight than with height.
Height percentile values (25 th , 50 th , 75 th ) of each age group are summarised in Table 4.
The 50 th , 90 th , 95 th values of both SBP and DBP were analysed according to height percentiles (25 th , 50 th , 75 th ) of each age group and summarised in Table 5.Both 90 th and 95 th percentiles of BP were derived as the definitions for hypertension and prehypertension are based on those percentiles respectively.
Tanner scale staging was analysed and summarised in Table 6.

Discussion
In this study population, mean BP increases from 6 to 10 years of age and then there is a reduction until 13 years followed by an increase up to the age of 18 years.A similar pattern can be noticed in the SBP and DBP of both boys and girls.A study done in Madhya Pradesh, India showed a continuous increase in SBP from 7 to 14 years of age 10 .Children from Calcutta, India showed a different pattern 11 .In boys, there was a decrease in SBP from 6 to 9 years and then a gradual increase whilst in girls SBP gradually increased with drops in 7 and 10 years 11 .This variation in SBP among children from Calcutta and Jaffna suggests that there may be some factor influencing SBP during these ages.This could be due to pubertal or environmental factors.There is no documented reason for this fluctuation in BP.There is a coincidental factor affecting children of 9-10 years in Sri Lanka because they have to sit for a highly competitive scholarship examination which determines grade 6 admission in many schools.Intense coaching for this examination starts even when they are in grade 5.This factor was not anticipated at the time of planning the study.
Another study may be needed to see whether the stress of this examination affects the BP.
When the mean SBP was compared with that of Indian children from two studies 10,11 , it is seen that up to the age of 11 years, SBP was higher in the present population than in Indian children.However, from 12 years of age, the SBP of Sri Lankan Tamil children was lower than that of the Indian children.This was observed in both boys and girls.DBP was higher in Sri Lankan boys than in Indian boys.However, after 10 years of age, DBP was lower in the present study than in the children from Calcutta.In girls, DBP was lower in the present study after 12 years of age than in the children of both Indian studies.These observations appear to confirm the ethnic differences in BP.