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CARDIOVASCULAR JOURNAL OF AFRICA • Volume 32, No 4, July/August 2021
AFRICA
209
appropriate timing. The aim of this study was therefore to find
out the association of age with long-term BP observations. The
study had three specific objectives: (1) to identify groups of
women with similar SBP and DBP between 22 and 89 years of
age; (2) to find out how the trajectories are affected by body mass
index (BMI); and (3) identify critical ages when intervention
measures would be more appropriate in slowing down a steep
upward trajectory.
Methods
Ethical approval (M170866) was granted by the Human Research
and Ethics Committee of the University of the Witwatersrand,
Johannesburg, South Africa.
The study consisted of four measurement time points of
data collection on African women dwelling in urban Soweto,
Johannesburg, who were care givers of children in the Birth to
Twenty Plus cohort study. The majority of the caregivers were
the mothers, however a few of the participants included other
close relatives such as sisters, aunts and grandmothers.
Data included four waves collected between 2003 and 2014.
The four waves provided a sample of 1 969 individuals who had
at least one wave of measurements for SBP, DBP, body weight
and height, resulting in 4 554 observations. BP measurements
were taken in a seated position after 30 minutes of seated rest.
The SBP and DBP were measured twice on the right arm using
a standard mercury sphygmomanometer and appropriately
sized cuff. A final systolic/diastolic BP was calculated by taking
the average of the BPs at each time point. Hypertension was
defined as systolic/diastolic BP of more than 140/90 mmHg, and
BMI was classified according to World Health Organisation as
underweight (< 18.5 kg/m
2
), normal weight (≥ 18.5– < 25 kg/m
2
),
overweight (≥ 25– < 30 kg/m
2
) and obese (≥ 30 kg/m
2
).
Statistical analysis
Multilevel (ML) growth-curve models (a technique to describe
and explain an individual’s change over time) were used to
describe the intra-individual BP trajectories, and the inter-
individual differences in the intra-individual changes with age
were used as the time metric. Three models were used to describe
the patterns of change. The first (model 1) had time effect as the
only covariate, model 2 described the changes by adjusting for
BMI, and model 3 built on model 2 by allowing effect of BMI to
also vary randomly between individuals.
To estimate the mean SBP and DBP trajectories as a function
of age, quadratic and cubic non-linear models were used in the
analysis, as growth trajectories are known to take a variety of
shapes other than linear,
19
mostly characterised by increases or
decreases. Age was centred at the minimum age of 22 years to
help in the interpretation of the models’ intercepts. As BMI
is generally known to increase with age, it was included as a
time-varying covariate in the second stage of modelling to find
out its effect on the BP trajectories. The intra-class correlation
coefficient (ICC) was used to measure the degree of dependency
among observations within an individual. A group-based
trajectory model was used to identify distinct groups for the BPs.
Results
Summary measures for age, BP and BMI at the four time points
are shown in Table 1. The mean age at the first occasion (2003)
was 41.1 years, and 49.6 years in the fourth occasion. Systolic
and diastolic BPs increased from one time point to another, but
the increases became smaller with time, and remained almost
unchanged, especially for SBP, between the third and fourth
occasions. Mean BMI for the four occasions was high, at obesity
level, and remained almost constant between the first and third
waves, but increased by at least 2.5 kg/m
2
by the fourth occasion.
There is an indication of constant variation in BP and BMI
across the measurement occasions.
Table 2 shows the percentage of hypertension by BMI
category at each data-collection time point. The percentage
of subjects who had hypertension in 2003 was 16.7% and by
2014, this had increased to 47.1%. At each time point it was
highest for those with an obese BMI, as expected. The greatest
percentage increase was between the first and the second
occasions (after approximately 2.4 years), where it almost
doubled. For those with normal or overweight BMI statuses,
the percentage of hypertension almost tripled between the
first and fourth occasions. Between the third and fourth time
points (approximately 5.3 years), the percentage of those with
hypertension among participants in the obese category remained
Table 1. Mean and standard deviation for age, SBP,
DBP and BMI at the four data-collection time points
Time point Number Age (years) SBP (mmHg) DBP (mmHg) BMI (kg/m
2
)
2003
1358 41.1 (7.9) 116.0 (20.1)
76.0 (12.4)
29.9 (6.4)
2005/6
1343 43.4 (8.0) 128.9 (21.6)
80.6 (13.5)
30.7 (7.0)
2007/9
854 45.6 (8.2) 133.5 (21.0)
86.8 (12.9)
30.5 (7.1)
2011/14
999 49.6 (5.7) 134.1 (21.9)
88.7 (12.6)
33.2 (7.2)
SBP, systolic blood pressure; DBP, diastolic blood pressure; BMI, body mass
index.
Table 2. Hypertension percentages by BMI categories
at the four data-collection time points
Time
point
Blood pressure category
Non-
hypertensive
n
(%)
Hyper-
tensive
n
(%) Total
n
(%)
BMI
Category
n
(%)
2003 Underweight
24 (1.8)
22 (91.7)
2 (8.3)
24 (100.0)
Normal
300 (22.1)
263 (87.7)
37 (12.3) 300 (100.0)
Overweight 405 (29.8)
349 (86.2)
56 (13.8) 405 (100.0)
Obese
629 (46.3)
497 (79.0) 132 (21.0) 629 (100.0)
Total
1358 (100.0) 1131 (83.3) 227 (16.7) 1358 (100.0)
2005/6 Underweight
19 (1.4)
12 (63.2)
7 (36.8)
19 (100.0)
Normal
269 (20.0)
202 (75.1)
67 (24.9) 269 (100.0)
Overweight 343 (25.5)
252 (73.5)
91 (26.5) 343 (100.0)
Obese
712 (53.0)
467 (65.6) 245 (34.4) 712 (100.0)
Total
1343 (100.0) 933 (69.5) 410 (30.5) 1343 (100.0)
2007/9 Underweight
10 (1.2)
5 (50.0)
5 (50.0)
10 (100.0)
Normal
188 (22.0)
125 (66.5)
63 (33.5) 188 (100.0)
Overweight 226 (26.5)
146 (64.6)
80 (35.4) 226 (100.0)
Obese
430 (50.4)
212 (49.3) 218 (50.7) 430 (100.0)
Total
854 (100.0)
488 (57.1) 366 (42.9) 854 (100.0)
2011/14 Underweight
7 (0.7)
4 (57.1)
3 (42.9)
7 (100.0)
Normal
116 (11.6)
77 (66.4)
39 (33.6) 116 (100.0)
Overweight 217 (21.7)
127 (58.5)
90 (41.5) 217 (100.0)
Obese
659 (66.0)
320 (48.6) 339 (51.4) 659 (100.0)
Total
999 (100.0)
528 (52.9) 471 (47.2) 999 (100.0)
BMI, body mass index. Due to approximation to one decimal place, some
percentages may fall below or exceed 100% by 0.1.