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4.4.2.1 Sex and age

Mean SBP and DBF levels in the study are presented in Table 4.3. On average SBP was 1.60 mmHg (95% Cl 0.87, 2.33) higher in the girls compared to the boys and this difference is highly significant (P= 0.0001); there was no

difference in DBP between the sexes. Figures 4.2 and 4.3 show the relationship between SBP and DBP at different ages in this cross-sectional cohort. Both SBP and DBP rose fairly linearly with age across the study population.

Regression coefficients showing the slopes for the BP rise with age, estimated from this cross-sectional data, are shown in Table 4.7. For the children all together SBP rose by a mean of 3.10 mmHg per year increase in age (95% Cl 2.55, 3.65, P value for slope 0.003) and DBP rose by 1.00 mmHg (95% Cl 0.65, 1.35, P value 0.04). Rates of BP rise with age appeared to be

significantly higher for both SBP and DBP in girls compared to boys; and formal tests of sex interaction tended to significance (Table 4.7).

4.4.2.2 Body size

The relationships between height and weight and BP are shown in Figures 4.4 to 4.7. These graphs show linear, positive relationships between the two measures of body size and SBP or DBP. Linear regression coefficients for the relationship between height and weight and blood pressure, both unadjusted and adjusted for age, are shown in Table 4.4. Unadjusted, both variables showed strong positive associations with both SBP and DBP, and this

association was barely altered by the addition of age into the model. However, the association between height and SBP or DBP disappeared once weight was included in the model, whilst the relationship between weight and BP appeared to be independent of height.

In addition to univariate measures o f size, such as height and weight, which were strongly intercorrelated, three measures of weight-for-height (BMI, PI and Chinn Index, see methods) were explored. Benn has advocated using a weight-for-height measure that is independent of height and exhibits the maximum correlation with weight (Benn 1971). A correlation matrix for the

various measures of body size and age is shown in Table 4.5. Weight and the three composite measures of weight-for-height were very strongly correlated. BMI was also relatively strongly correlated with age, while PI was much less strongly correlated with age; the Chinn index was not correlated with age. BMI and the Chinn Index were strongly correlated with height, while PI was very weakly correlated with height. As a result of these findings, PI was adopted as the preferred measure of weight-for-height in all the subsequent analyses on this cohort. Multiple regression models of BP on height or PI, with and without simultaneous adjustment for the other measure of body size,

demonstrate that both measures are independently positively associated with BP (Table 4.6). A linear regression model including both height and PI predicted 19% of the variation in SBP and 5% of the variation in DBP.

4.4.2.3 The relationships between age, body size and blood pressure

The correlation matrix. Table 4.5, demonstrates that age, height and weight were strongly correlated at 8-11 years old, although it has already been shown that the relationships between height or weight and BP were virtually

independent of age (Table 4.4). The addition of age to regression models of BP on height and PI had no effect on the relationship between these measures of body size and SBP or DBP (Table 4.6). This table shows that the

association between age and BP, whilst virtually independent of PI,

disappeared once height was added into the model. These results suggest that, at this age, the relationship between age and BP could be attributed to the association between height and BP. When both height and PI were included in the regression model, there was only a weak positive association between age and SBP and no apparent association between age and DBP (Table 4.6). This probably arises because of the negative correlation between age and PI (Table 4.5). When adjusted for PI alone, the slope for SBP or DBP on age is slightly steeper than the unadjusted slope.

When these data were examined separately by sex, the relationships between height and BP appeared to be very similar in girls and boys (data not shown). The relationship between PI and SBP was also very similar in boys and girls, however the slope for DBP on PI adjusted for height and age was significantly

greater in the boys {b = 0.47 mmHg/kg m'^, 95% Cl 0.33, 0.61, P for slope = 0.0001, and b = 0.77 mmHg/kg m'^, 95% Cl 0.63, 0.91, P for slope = 0.0001 in the girls and boys respectively, P value for sex interaction test = 0.007).

The relationships between age and BP in boys and girls separately are shown in Table 4.7. There is a positive association between age and SBP or DBP in both sexes. These associations are unaltered or slightly strengthened by

simultaneous adjustment for PI and the association between age and DBP disappears with simultaneous adjustment for height. There is some suggestion that the effect of age on SBP may be greater in the girls in this study (test for sex interaction P = 0.04 after adjustment for height and PI). Although the slope for the association between age and DBP is also greater in the girls, there is no convincing evidence of any sex interaction in these data (P for sex interaction test after adjustment for body size = 0.08).

4.4.2.4 Sexual maturation amongst the girls

Information on whether or not menarche had been reached was available for 1447 (81%) of the girls with a full set of study data. Only 29 (2%) were reported as having reached menarche. Mean SBP simultaneously adjusted for age was significantly higher in those who had reached menarche than in those who had not (data not shown). However additional adjustment for current PI and height abolished this difference and there were no differences in DBP with or without additional adjustment for current body size (data not shown).

4.4.2.5 Pulse rate

Pulse rate rose linearly with increase in SBP or DBP (see Figures 4.8 and 4.9). The linear regression coefficients (Table 4.8) suggests that a rise in pulse rate of 5 beats per min would be associated with in an increase in SBP of 1.25 mmHg (95% Cl 1.15, 1.35, after adjustment for age, height and PI) and in DBP of 0.85 mmHg (95% Cl 0.75, 0.95). Table 4.8 demonstrates that the association between pulse rate and BP was largely independent of age and current body size.