Cardiovascular Journal of Africa: Vol 31 No 2 (March/April 2020)

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CARDIOVASCULAR JOURNAL OF AFRICA • Volume 31, No 2, March/April 2020 92 AFRICA
The study design has previously been described.33,34 Nuclear
families of black African descent with siblings older than 16
years were randomly recruited from the South West Township
(SOWETO) of Johannesburg, South Africa. To ensure that
relationships noted were independent of haemodynamic factors
that could be altered by therapy, 24-hour and aortic BP and
carotid–femoral pulse-wave velocity (PWV) and index of aortic
stiffness were determined.
Of the 1 010 participants studied, 896 participants had
aortic PWV measurements and 688 had 24-hour ambulatory BP
measurements that met with pre-specified quality-control criteria
(longer than 20 hours and more than 10 and five readings for the
computation of day and night means, respectively). Of the 1 010
participants, 872 did not have diabetes mellitus, and 779 of these
had aortic PWV measurements and 600 had 24-hour ambulatory
BP measurements.
Demographic and clinical data were obtained using a
standardised questionnaire.33,34 Regular tobacco use was defined
as daily cigarette smoking, and regular alcohol consumption as
one beer a day or a bottle of wine (750 ml) a week or 250 ml of
spirits a week. Height, weight, waist circumference (WC) and hip
circumference were measured using standard approaches and
participants were identified as being overweight if their body
mass index (BMI) was ≥ 25 kg/m2 and obese if their BMI was ≥
30 kg/m2. Central obesity was defined as an enlarged WC (≥ 88
cm in women and ≥ 102 cm in men).
Fasting laboratory blood tests of renal function, blood glucose
levels, lipid profiles and percentage glycated haemoglobin (HbA1c)
(Roche Diagnostics, Mannheim, Germany) were performed.
Fasting plasma insulin concentrations were determined from
an insulin immulite, solid-phase, two-site chemiluminescent
immunometric assay (Diagnostic Products Corporation, Los
Angeles, CA, USA) and insulin resistance was estimated by the
homeostasis model assessment of insulin resistance (HOMA-IR)
using the formula [insulin (µU/ml) × glucose (mmol/l)]/22.5.
Diabetes mellitus (DM) was defined as the use of insulin or
oral hypoglycaemic agents or an HbA1c value greater than 6.5%.
The metabolic syndrome was defined as a combination of the
presence of WC ≥ 88 cm in women and ≥ 102 cm in men, fasting
blood glucose ≥ 5.6 mmol/l, triglycerides ≥ 1.7 mmol/l, high-
density lipoprotein (HDL) cholesterol < 1.04 mmol/l in men
and < 1.30 mmol/l in women, and systolic BP ≥ 130 mmHg or
diastolic BP ≥ 85 mmHg or treatment for hypertension.
Nurse-derived conventional BP was measured after 10 minutes
of rest in the seated position, as previously described,33,34 within a
half hour of obtaining blood samples and in the opposite arm to
that subjected to venesection. These measurements were performed
to an accuracy of 2 mmHg by a trained nurse using a mercury
sphygmomanometer and an appropriate-sized cuff according to
guidelines. Five consecutive BP readings were obtained 30 to 60
seconds apart. The average of the five readings was taken as the BP.
Hypertension was diagnosed in those receiving antihypertensive
therapy or having a conventional BP ≥ 140/90 mmHg.
Ambulatory 24-hour BP was determined using SpaceLabs
monitors (model 90207; Spacelabs, Redmond, Washington,
USA), as previously described.34 The size of the cuff was the
same as that used for conventional BP measurements. Monitors
were programmed to measure 24-hour BP at 15-minute intervals
from 06:00 to 22:00 hours and at 30-minute intervals from
22:00 to 06:00 hours. Intra-individual means of the ambulatory
measurements were weighted by the time interval between
successive readings.34 The average (± SD) number of BP readings
obtained was 60.7 ± 12.2 (range = 24–81) for the 24-hour period.
Central aortic haemodynamics were determined as previously
described.33,34 After participants had rested for 15 minutes in the
supine position, arterial waveforms at the radial (dominant arm),
carotid and femoral artery pulses were recorded by applanation
tonometry. Pressure waveforms were recorded during an eight-
second period using a high-fidelity SPC-301 micromanometer
(Millar Instrument, Inc, Houston, Texas) interfaced with a
computer employing SphygmoCor, version 9.0 software (AtCor
Medical Pty, Ltd, West Ryde, New South Wales, Australia).
To determine aortic BP the pulse wave obtained from the radial
tonometer recordings was calibrated by manual measurement
(auscultation) of brachial BP taken immediately before the
recordings. The radial pressure waveform was converted into a
central (aortic) waveform using a validated generalised transfer
function incorporated in SphygmoCor software. Central aortic
systolic BP was derived from the aortic waveform. Aortic PWV
was determined from sequential waveform measurements at the
carotid and femoral sites.
The time delay in the pulse waves between the carotid and
femoral sites was determined using an electrocardiograph-
derived R wave as a fiducial point. Pulse transit time was taken
as the average of 10 consecutive beats. The distance that the
pulse wave travels was determined as the difference between
the distance from the femoral sampling site to the suprasternal
notch, and the distance from the carotid sampling site to the
suprasternal notch. Aortic PWV was calculated as the ratio of
the distance to the transit time (m/s).
Serum creatinine concentrations were measured using the
Advia Chemistry systems (Siemens) with calibration traceable to
isotope dilution mass spectrometry (IDMS). The four-variable
Modification of Diet in Renal Disease Study (MDRD) equation
and the Chronic Kidney Disease Epidemiology Collaboration
(CKD-EPI) equation were employed to estimate GFR. The
ethnicity factor as recommended in African Americans when
calculating the MDRD and CKD-EPI eGFR was not applied in
this study as the use results in overestimation of kidney function
in black Africans. 35,36
Blood samples were centrifuged and immediately stored at
–80°C. Plasma concentrations of human resistin and high-
sensitivity C-reactive protein (range 0.01–50 ng/ml) (hs-CRP)
concentrationsweremeasuredusingenzyme-linkedimmunosorbent
assays (Quantikine, R&D Systems Inc, Minneapolis, MN, USA).
Resistin was selected as an adipocytokine with possible adverse
effects on renal function beyond that of obesity per se and CRP
as a marker of general inflammation. The resistin assay had a
lower detection limit of 0.026 ng/ml and intra- and inter-assay
coefficients of variation ranging from 3.8 to 5.3% and 7.8 to 9.2%,
respectively. The CRP assay had a mean lower detection limit of
0.010 ng/ml and intra- and inter-assay coefficients of variation
ranging from 3.8 to 8.3% and 6.0 to 7.0%, respectively.
Statistical analysis
Database management and statistical analyses were performed
with SAS software, version 9.4 (SAS Institute Inc, Cary, NC,
USA). Continuous data are reported as mean ± SD or SEM,
or median and interquartile range. Unadjusted means and