Cardiovascular Journal of Africa: Vol 32 No 4 (JULY/AUGUST 2021)

CARDIOVASCULAR JOURNAL OF AFRICA • Volume 32, No 4, July/August 2021

210

AFRICA

almost unchanged (50.7% in the third occasion and 51.4% in

the fourth occasion), even though the percentage of obesity

increased by more than 15.6 points (50.4 to 66.0%) between the

two occasions.

Table 3 shows the mean change in BP and BMI between two

observation periods. Between the first two time points, average

SBP and DBP per individual changed by about 11.3 and 4.1

mmHg, respectively but that change decreased substantially

between the third and fourth time points to about 1.7 and

1.1 mmHg, respectively. The majority remained in their BP

status between observation times but with time (age), the

percentage remaining in the hypertension state increased, while

the percentage remaining in the normotensive state decreased.

Between the first two occasions when the average age at both

times was below 45 years, the percentage of those who were

normotensive who transited to the hypertensive state was 21.5%,

while the percentage for the counter-transition was 33.3%, or a

net recovery of 11.8%. By the fourth occasion when the average

age was about 50 years, those initially in the non-hypertensive

state who transited to hypertensive state were 40.6%, but the

counter-transition (recovery) rate was 15.6%, or a net increase of

25.0% to the hypertensive state.

Fig. 1 and Table 4 show how the progression of BP evolves.

A crest (inverted U-shape) marks a negative change (decrease)

while a trough (U-shape) marks a positive change (increase).

Fig. 1 however shows that the 95% confidence band for ages

above 70 years are quite large and therefore the shape in that

age group could be spurious. The first crest occurs around 25

years of age and is indicated by negative coefficients (–1.8 for

SBP and –1.5 for DBP) for age (Table 4), and the second around

60 years, and is shown by negative coefficients (–0.004 for SBP

and –0.003 for DBP) for age cubed. Similarly, the troughs are

identifiable around ages 30 and 70 years and marked by the

positive coefficients for age squared and age raised to the power

of four (Table 4).

Model 1 describes the BP trajectories without accounting

for the effect of BMI, while models 2 and 3 show the adjusted

Table 3. Average BP changes and shifts between categories

at the four data-collection time points

Time

points

Average

years

apart

BP category

Mean change

(SD)

Non-hyper-

tensive

Hyper-

tensive SBP DBP

2003 &

2005/6

2.4 Non-hypertensive 824 647 (78.5) 177 (21.5) 11.3

(19.3)

4.1

(12.1)

Hypertensive 159 53 (33.3) 106 (66.7)

Total

983 700 (71.2) 283 (28.8)

2005/6

2007/9

2.0 Non-hypertensive 512 354 (69.1) 158 (30.9) 4.5

(20.4)

6.4

(11.9)

Hypertensive 237 72 (30.4) 165 (69.6)

Total

749 426 (56.9) 323 (43.1)

2007/9

2011/14

5.3 Non-hypertensive 234 162 (69.2) 72 (30.8) 1.7

(19.0)

1.1

(11.2)

Hypertensive 154 39 (25.3) 115 (74.7)

Total

388 201 (51.8) 187 (48.2)

2003 &

2007/9

4.4 Non-hypertensive 493 303 (61.5) 190 (38.5) 16.4

(20.9)

10.9

(12.6)

Hypertensive 99 26 (26.3) 73 (73.7)

Total

592 329 (55.6) 263 (44.4)

2005/6

2011/14

7.3 Non-hypertensive 460 291 (63.3) 169 (36.7) 8.1

(22.3)

8.1

(12.6)

Hypertensive 185 49 (26.5) 136 (73.5)

Total

645 340 (52.7) 305 (47.3)

2003 &

2011/14

9.7 Non-hypertensive 549 326 (59.4) 223 (40.6) 19.1

(20.4)

12.6

(12.3)

Hypertensive 96 15 (15.6) 81 (84.4)

Total

645 341 (52.9) 304 (47.1)

Table 4. Coefficients (95% confidence interval) for the three models predicting SBP and DBP

trajectories with standard deviations (SD) for the random effects

Variables

Model 1

Model 2

Model 3

Systolic

Fixed effects

Intercept

116.2 (106.7, 125.7)

105.5 (95.7, 115.3)

105.0 (95.1,114.9)

Age

–1.7 (–3.3, –0.1)

–1.7 (–3.4, –0.1)

–1.8 (–3.4, –0.2)

Age

0.18 (0.08, 0.27)

0.17 (0.08, 0.27)

0.17 (0.08 ,0.27)

Age

–0.004 (–0.006, –0.002)

Age

0.00003 (0.00001, 0.00005)

BMI

0.39 (0.29, 0.49)

0.41 (0.30, 0.52)

Random effects

SD (intercept)

14.5 (13.7, 15.2)

14.4 (13.7, 15.1)

24.3 (18.4, 32.2)

SD (BMI)

0.65 (0.43, 0.96)

Correlation (BMI, intercept)

–0.82 (–0.91, –0.66)

SD (residual)

15.5 (15.1, 15.9)

15.4 (15.0, 15.8)

15.2 (14.8, 15.7)

ICC

0.47 (0.43, 0.50)

0.72 (0.59, 0.82)

Diastolic

Fixed effects

Intercept

75.7 (69.8, 81.5)

64.5 (58.5, 70.4)

63.3 (57.2, 69.3)

Age

–1.4 (–2.4, –0.4)

–1.5 (–2.4, –0.5)

Age

0.15 (0.09, 0.21)

0.14 (0.08, 0.20)

Age

–0.004 (–0.005, –0.002)

–0.003 (–0.005, –0.002)

Age

0.00003 (0.00001, 0.00004)

0.00002 (0.00001, 0.00004)

BMI

0.41 (0.35, 0.47)

0.46 (0.39, 0.53)

Random effects

SD (intercept)

9.4 (8.9, 9.9)

9.1 (8.6, 9.5)

17.5 (14.2, 21.7)

SD (BMI)

0.52 (0.41, 0.67)

Correlation (BMI, intercept)

–0.87 (–0.92, –0.79)

SD (residual)

9.4 (9.1, 9.6)

9.3 (9.0, 9.5)

9.0 (8.8, 9.3)

ICC

0.50 (0.47, 0.53)

0.49 (0.46, 0.52)

0.79 (0.71, 0.85)

SBP, systolic blood pressure; DBP, diastolic blood pressure; ICC, intraclass correlation coefficient.