CARDIOVASCULAR JOURNAL OF AFRICA • Vol 24, No 9/10, October/November 2013
AFRICA
345
prevalence of hypertension in urban areas.
4-6
Urban lifestyles,
characterised by sedentary living, increased salt intake, obesity
and stress contribute to these differences.
5
With the urban
population in sub-Saharan Africa projected to increase, a greater
risk of hypertension is anticipated.
Studies on the association between ethnicity and hypertension
in high-income countries have documented a higher prevalence
of hypertension in black ethnic groups compared to white
ethnic groups.
7-9
Reasons for this association are complex,
unclear and much debated, reflecting genetic and biochemical
mechanisms, and environmental and socio-economic factors.
10,11
There is limited evidence regarding differences in the prevalence
of hypertension between ethnic groups within the broader
classification of black ethnicity.
6,12,13
Studies in Nigeria and sub-Saharan Africa have mainly
involved specific geographical areas or have focused on
sub-groups of the population.
5,14
Surveys from Nigeria report
prevalence estimates ranging from 20.2 to 36.6%, but all have
involved participants with different age ranges.
15-18
To plan
services for hypertension in Nigeria, it is essential to have
accurate prevalence estimates for the whole population and to
identify populations at risk.
Nigeria, which is the most populous country in sub-Saharan
Africa, is home to over 250 different ethnic groups. Nigeria is
experiencing rapid urbanisation of the population, which is likely
to increase the population at risk for hypertension.
19
The present
study is one of the largest population-based surveys in the region
and is able to provide a nationally representative estimate of
hypertension for Nigeria.
Methods
As part of the Nigerian national blindness and visual impairment
survey of adults aged 40 years and older, data were collected
on blood pressure. A detailed description of the sampling,
enumeration, visual acuity and ocular examination procedures
has been published previously.
20
Nigeria is divided into six administrative zones, which are
called geo-political zones (GPZ), 36 states and the federal capital
territory of Abuja. Each state is subdivided into local government
authorities (LGA), which are the smallest administrative unit.
There are 774 LGAs in the country.
A nationally representative sample was achieved through
a multi-stage, stratified (by urban/rural location and GPZ),
cluster random sampling with probability proportional to size
procedures. A total of 310 clusters were identified, 226 were in
rural areas and 84 in urban populations. The sample covered all
36 states. Fifty adults aged 40 years or older, who were normal
residents (defined as being continually resident for at least the
last three months) were randomly identified in each cluster.
These people were identified by the enumeration team
who, having found the centre of the cluster, spun a bottle and
approached the first household in the direction the bottle pointed.
The team, using an established protocol, travelled from house to
house to identify eligible adults, until the quota was achieved.
If fewer than 50 were identified, the search continued into the
next village. Five clusters were not included due to civil unrest
or refusal to participate. Basic demographic data and informed
consent were taken from each person who agreed to take part.
Respondents were invited to attend a clinical station which
was set up in each cluster. Enumerated individuals not reporting
to the clinical station were followed up three times and offered
an examination at their house. If they still did not take part they
were deemed non-respondents and were not replaced.
At the clinical station, individuals were interviewed to collect
data on socio-demographic variables, including ethnic group,
and history of disease and medication. All had anthropometric
measures taken and their blood pressure was measured prior to
ophthalmic examination. Each of the two survey teams had two
qualified ophthalmologists and two ophthalmic nurses, who were
recruited from each GPZ, so that they would know the main local
languages.
Hypertension was measured by a qualified ophthalmic nurse
trained in the procedure, using an Omron wrist instrument
(UB322, Omron Healthcare Ltd, Milton Keynes, England), with
the occluding cuff being placed around the volar surface of the
wrist. The instrument was calibrated every morning. A total
of three readings were taken by a trained nurse, at least five
minutes apart after at least 10 minutes’ rest in a sitting position.
The nurses’ performance was regularly monitored by the senior
investigators. The mean of the three readings was calculated as
the individual’s blood pressure.
Initial training was undertaken over two weeks and training
sessions were repeated for each GPZ (two weeks each). A
pilot study was conducted before the field work started in
each GPZ. Inter-observer agreement studies were conducted
periodically throughout the study for the ophthalmic nurses
and the ophthalmologists. Data were collected over a 30-month
period from January 2005 to July 2007.
Height and weight were measured using a Tanita measurement
scale (Model 1536, Tanita Corporation, Tokyo, Japan). The
weight measure was calibrated every morning and checked for
error using a standard weight.
The World Health Organisation’s (WHO) classification of
hypertension was used. Hypertension is defined as diastolic
blood pressure (DBP) of 90 mmHg or greater, or a systolic blood
pressure (SBP) of 140 mmHg or greater. The WHO grading
system of hypertension to profile risk was also used. This
defines grade 1 hypertension as SBP 140–159 mmHg or DBP
90–99 mmHg; grade 2 as SBP 160–179 mmHg or DBP 100–109
mmHg; and grade 3 as SBP
≥
180 or DBP
≥
110 mm Hg. These
grades are used to assess the risk in individuals for a cardiac
event in the context of other additional risk factors.
21
Body mass index (BMI) was calculated from weight (kg)
divided by height (m) squared. World Health Organisation
categories of BMI were used in the analysis.
22
Socio-economic status (SES) was calculated by assigning one
of eight occupational categories, ranging from 0 (not in gainful
employment) to 7 (professional), and a grade for the highest
level of school attended, from 0 (no schooling) to 4 (university)
for each person. The sum of the scores was calculated, with a
higher score signifying a more affluent socio-economic status.
The ranked SES scores were then divided into tertiles.
Data on ethnic group were categorised such that groups
with more than 100 participants were analysed separately, with
all other ethnic groups combined into an ‘other’ group. Ethnic
groups were classified based on the father’s ethnic status.
Ethical approval for the study was provided by the London
School of Hygiene and Tropical Medicine and the Federal
Government of Nigeria. The study adhered to the tenets of the
