Tracking of Blood Pressure Among Adolescents and Young Adults in an Urban Slum of Puducherry

Background: Early diagnosis of hypertension (HT) is an important strategy in its control. Tracking of blood pressure (BP) has been found useful in identifying persons with potential HT, particularly in youngsters. A cohort of 756 subjects (with baseline information as a cross-sectional study in 2002) was followed up in 2006 to comment on the distribution of BP and its attributes. Objectives: To track BP distribution in a cohort of adolescents and young adults, and assess the persistence of high/low normotensives; to measure the incidence of HT and study the relationship of BP with age, sex, socioeconomic status, BMI, physical exercise, salt intake, smoking and alcohol consumption. Materials and Methods: The baseline study cohort (2002) of 756 subjects (19-24 years) in urban field area of Department of Preventive and Social Medicine, JIPMER, was followed up between May and November 2006 by house visits for measurement of sociodemographic variables, anthropometry, salt intake, physical activity and BP. Results: A total of 555 subjects from the 2002 cohort were contacted (73.4%), in that 54.5% subjects who were below 5 th percentile, 93.6% subjects between 5 th and 95 th percentiles and 72% of those above 95 th percentile previously persisted in the same cut-offs for systolic blood pressure (SBP). The corresponding figures for diastolic blood pressure (DBP) were 46.2, 92.2 and 74.1%, respectively. Shift from one cut-off to another was not significant for both SBP and DBP, proving the tracking phenomenon. Annual incidence of HT was 9.8/1000. Baseline BP was the significant predictor of current BP for the entire cohort; BMI and salt intake were significant predictors only in certain sections of the study cohort. Conclusions: Early diagnosis of hypertension even among adolescents/young adults is an important preventive measure, as tracking exists in the

Coronary risk factors such as hypertension, smoking, physical inactivity, obesity and improper diet are fairly widespread. (1) Cardiovascular diseases, particularly hypertension, account for high mortality in the form of cardiovascular strokes in countries like India, Taiwan and Japan. (2) In Indian adolescent school children, there is a high prevalence of obesity, hypertension, hypercholesterolemia and high fat diet. (3) Studies (4,5) from Boston and Pennsylvania had commented that the role of hypertension as a risk factor is clear, and familial aggregation of blood pressure and tracking phenomenon support the concept that children with hypertension are likely to be hypertensive as adults and will be at risk for early CHD. In Puducherry, several studies on distribution of blood pressure and prevalence of hypertension (including its determinants) have been conducted in 1991, 1996 (among adults) and 2002 (6) among adolescents. As a follow up of the latest study on adolescents, this work was undertaken with the following objectives: 1. To track BP distribution among the cohort of adolescents/young adults (of the earlier 2002 study) and assess if persons who were high normal/low normal/hypertensive continue to have similar trends. 2. To assess the incidence of hypertension in the study cohort. 3. To asses the relationship between BP and hypertension in the study cohort with certain factors like age, sex, education, occupation, income, BMI, physical exercise, salt intake, smoking and alcohol consumption. Height -recorded to nearest 0.1 cm with stadiometer, weight to nearest 100 g with solar weighing machine, (8) measurement of BP: the conditions followed for measuring BP were as described by Dasgupta. (8) The subject was asked to rest for 5-10 min if he/she had engaged in physical activity. The WHO criteria (9) were followed in recording the BP and the average of two readings recorded 3 min apart was taken as BP. Systolic BP more than 160 mmHg and/or DBP more than 100 mmHg was severe hypertension, SBP

Tracking of BP and incidence of hypertension
Comparing the SBP of the study and reference cohorts, persons in extreme percentiles in 2002 continued in respective categories (below 5 th percentile, 5 th to 95 th percentiles and above 95 th percentile). The shift from one cut-off to another was not significant. Of those in 5 th to 95 th percentile of reference cohort, 93.6% continued within the same cut-offs. More than 70% of individuals of reference cohort who were in >95 th percentile continued to be so in study cohort. Similarly, for DBP, 92.2% individuals of reference cohort remained in 5 th to 95 th percentile. In the extreme categories (<5 th percentile and >95 th percentile) 74.1% of subjects continued in the same. The shift from one cut-off to another was not statistically significant [ Table 2]. Similarly the shift from one cut-off to another was not significant at various levels like deciles, quintiles, quartiles and tertiles. Thus, tracking is demonstrated to be significant, viz., high/low normotensives tend to persist in their respective percentiles -thereby enabling early identification.

Relation between BP and other variables
Demography and parental history: There was a significant .70 mmHg, respectively (P < 0.001). Incidences of hypertension among underweight, normal and overweight persons were 2, 12.3 and 13.5/1000, respectively, which was statistically significant (P < 0.001). There was no significant effect of type of food intake, smoking and alcohol consumption on blood pressure among these adolescents and young adults.
Lifestyle: There was significant association between physical activity and BP (P = 0.016), with sedentary and mild physical activity merged into a single class interval.
Here, physical activity denoted only the deliberate physical exercise by subjects (like playing, etc.) and not the whole range of activities performed by them. Mean   Table 5].

Discussion
This study comments on the tracking of BP in a cohort representative of urban slum dwellers of Puducherry. Several studies have highlighted the importance of tracking BP. In the present study, more than 70% of those who were hypertensives in the reference cohort (>95 th percentile) continued in the same range, which is similar to a Spanish study, (10) which reported that more than 70% children in the upper quartile of SBP at any previous examination remained so. The Shimane heart study (11) showed 43.5% boys and 59.1% girls in cohort 1, and 25.0% boys and 56.5% girls in cohort 2 remained in extreme quintiles.
The incidence of HT in adolescents and young adults in the present study was 0.98%. The Framingham (12) study showed an incidence of 3.3% among men aged 30-39, and Menghett (13) et al. showed a high incidence  of 6.5% among 293 children in the age group of 11-14. In the present study on adolescent and young adults, a significant increasing trend of BP was seen only among males. This is similar to the Turkish (14) and Zambian (15) studies on school children showing rise of BP with age. The latter study showed a significantly elevated mean SBP and DBP among males than females. This is comparable to other studies on populations of 13-18 years, (16) 15-24 years (17) and 15-25 years. (18) Although the present study did not find a significant association of mean BP with social class, the findings in a South Indian community (19) showed such significance.
There was a gradient of significant association of BP and HT with physical activity in the present study on adolescents as also observed by Pittsburgh study. (16) Physical fitness appears to be a graded, independent, long-term predictor of mortality from cardiovascular causes in healthy, middle-aged men. (21) The present study found significant rise in both SBP and DBP with increasing BMI in both genders, comparable with the findings from adolescents 17 years in Jerusalem, (20) whereas another study (16) reported weight-dependent rise in BP only among males with respect to SBP alone. Several studies reported the association of BP with both weight and height. (16,(21)(22)(23) Prevalence of HT was 0.5 to 3 times higher among the overweight. (24) Taiwan study (25) which followed 7685 males for over 30 years recorded an increase in incidence of HT with increasing ponderosity. The Framingham study (12) showed increased prevalence of obesity in subjects with HT as well increase in BP in established obesity. Similar findings were reported among adolescent populations in India, (26) Hungary (27) and France. (28) Such association in early childhood with SBP alone was reported by Minneapolis children's BP study (29) and British cohort. (30) Pittsburgh (16) study reported similar association only in SBP among males. Other studies on populations of 4-18 years (14) and 7-16 years (15) also identified weight as a major determinant of BP. Childhood weight gain was positively associated with adult BP. (31) Another study (13) showed that elevated BMI in childhood predicted risk of hypertension in young adulthood.
Subjects with history of parental hypertension had higher BP and higher incidence of hypertension. Zimbabwean study (34) showed that parental history before age 60 was related to offspring's hypertension. This relationship was stronger when compared to both parental histories versus none similar to the present study. Another Zimbabwean (35) study showed parental history of hypertension influenced both resting and reactivity BP.
In this study, BP was significantly predicted by baseline salt intake only in 17-18 years group in males. There is evidence that high salt intake increases BP. (32) Low sodium was reported to lower BP. (33) Likewise there was a significant predictability of BP by baseline BMI only in 17-18 years in both genders.

Limitations
1. Since the cohort follow-up was done after 4 years, the annual incidence of hypertension was calculated from the 4 years incidence, assuming uniform yearly incidence. About 201 (26.3%) subjects (out of the 2002 cohort of 756 subjects) could not be approached even after 4 visits. Although the age composition of this 201 was comparable with the 555 of the present cohort, it is still possible that this loss to follow-up might have some effect on the results depending on the distribution of other variables. The incidence of HT was calculated individually for adolescents/young adults as two different groups and the combined incidence for the whole group was taken as addition of these two incidences. 3. Likewise, any differential drop outs with respect to age, BMI, social class, alcohol intake, smoking, physical activity and family history of hypertension might have further limitations in interpretation of results; these aspects are specifically addressed in a paper from a subsequent study. 4. In addition measurement of physical activity which included only exercise might have limited accuracy but was used to ensure comparability with the earlier cohort. Similarly the per capita salt intake was calculated as an average of the family intake since a specific dietary measurement was not within the scope of this study.

Conclusion
Tracking is a very useful tool in early diagnosis of prehypertension and hypertension even among adolescents/ young adults (19-24 years). The overall mean SBP and DBP were 117.17 mm Hg (range 96-150) and 78.12 (range 52-100), respectively, and annual incidence of hypertension was 9.8/1000. Baseline BP (both SBP and DBP) was the significant independent predictor of BP. Other variables like BMI and salt intake were significant predictors of BP only in a section of the study population.
Hence, pertaining to adolescents/young adults, these may be termed as early risk factors.