CARDIOVASCULAR JOURNAL OF AFRICA • Vol 21, No 1, January/February 2010
18
AFRICA
Surveillance System (NDSS), which is owned by the Navrongo
Health Research Centre, a field station of the Health Research
Unit of the Ghana Health Service. NDSS is a continuous popula-
tion registration system that assesses the demographic dynamics
of the entire population of the Kassena–Nankana district. Every
90 days, all compounds in the district are visited to update vital
events such as births, deaths, pregnancy, marriages and migra-
tion in and out of the district.
Blood pressure, anthropometric, time of blood pressure and
room temperature measurements were taken in 574 adult males
and females aged between 18 and 65 years, resident in the study
area and who agreed to be part of the study. Response rate was
95.7% with 207 males and 367 females completing the study
protocol. The imbalance between the genders in the random
sample drawn was due to the fact that females constitute a larger
percentage of the population. The 26 non-respondents, all males,
were either busy on their farms or not interested in participating.
Fieldwork was carried out in the KND from February to April
2007, the period when morning temperatures are low and daily
temperatures reach their maximum.
Ethical approval was obtained from the local Institutional
Review Board of the Navrongo Health Research Centre,
Navrongo, Ghana (reference: NHRC 061). The traditional
chiefs, community leaders, political and opinion leaders were
approached prior to commencement of fieldwork, and communi-
ty meetings were held to explain the purpose of the whole study.
Individual affirmations of informed consent were obtained from
residents willing to participate, using informed consent forms.
The consent forms were signed by those who were literate and
marked with a left thumbprint for those who were not.
Five fieldworkers and a supervisor with prior experience and
training in epidemiological surveys of this nature were recruited
and retrained in the techniques of questionnaire administration,
and anthropometric and blood pressure measurements. The
study team were all locals of the study area and were fluent in
the local languages. Before the start of the fieldwork, each of
the techniques and instruments was tested on a small sample of
the population and shown to be practicable, sufficiently reliable
and valid.
Data were collected by interview using a simple structured
questionnaire, and measurements were recorded on a stand-
ard form. For date of birth, an estimated date was assigned to
those who were unable to give their exact dates of birth, using
the seasons, festivals and some historical indicators as guides.
June 30 was allocated if the exact date and month could not be
decided upon.
Blood pressure measurements followed administration of the
questionnaire and anthropometric measurements. Reliable auto-
matic blood pressure devices (Omron MX3 plus), which were
validated against the reference method of sphygmomanometry
(have passed the validation recommendations of the International
Protocol of the European Society of Hypertension),
11
were used
by trained staff. Measurement was taken in the quiet room
provided, and the room temperature and time of the day was
recorded. Blood pressure was measured in the right arm with
participants in the seated position. Blood pressure and tempera-
ture measurements were taken between 07:00 and 14:00 for the
main survey.
Two weeks after the main survey, a sub-sample, randomly
sampled from the study participants, were invited to have repeat
BP and temperature measurements taken in the morning between
06:00 and 10:00 when temperatures were at their minimum,
and again in the afternoon between 13:00 and 15:00 when the
temperatures were at their maximum. Thirty-seven participants
volunteered for the repeat measurements. In the repeaters, BPs
and time-of-day measurements were taken randomly. Following
theWHOMONICAmethodology,
12
blood pressure was measured
twice and the mean of the two used for the analysis. The same
trained staff took blood pressure measurements in all locations.
Statistical analysis
To determine BP changes with ambient temperature, readings
were grouped based on the temperatures at the time of blood
pressure measurements in the morning, and in the afternoon for
the follow-up survey. Two temperature categories were used.
Category 1 consisted of BP readings with an ambient tempera-
ture range of 28 to 34ºC and category 2 consisted of readings
with temperatures ranging from 39 to 43ºC. BP readings for the
two temperature groups were averaged and compared.
Pearson’s linear correlation was performed to determine the
dose–response relationship between ambient temperature and
BP readings. To further explore the association between tempera-
ture and BP, bivariate regression analyses were used. To assess
the independent association of ambient temperature with BP
variability, systolic and diastolic blood pressure were regressed
on ambient temperature in separate multiple linear regression
models, including and excluding age, gender, body mass index,
waist circumference and time of blood pressure measurement.
P
-values
<
0.05 (two-sided) were considered statistically signifi-
cant. All results were given as mean (standard deviation) unless
indicated otherwise. All statistical analyses were performed
using the Statistical Package for Social Sciences (SPSS) version
15 for MS WINDOWS.
Results
The 574 participants ranged in age from 18 to 65 years. Mean
age was 37.8 (14.0) years with 36.1% being males and 63.9%
females. Mean age of males was 36 (14) years and that of females
was 39 (14) years. The sub-sample of 37 participants ranged in
age from 18 to 65 years. Mean age was 38.4 (14.8) years.
For the follow-up survey, at morning temperatures of between
28 and 34ºC, mean SBP and DBP were 123.1 (18.4) and 71.9
(13.0) mmHg, respectively. At afternoon temperatures between
39 and 43ºC, mean SBP and DBP were 120.9 (19.4) and 70.6
(12.1) mmHg, respectively. There was a moderate but relevant
difference in mean systolic and diastolic BPs for the two ambient
temperature categories.
Mean correlation coefficients for SBP and DBP against
temperature are displayed in (Table 1). A significant inverse
dose–response relationship was demonstrated between ambient
temperature and BP (for both SBP and DBP). The significant
inverse correlation between SBP and ambient temperature is
illustrated graphically in Fig. 1 (
r
=
–0.1,
p
=
0.02).
Linear regression analysis showed that SBP was significantly
and inversely related to ambient temperature (
β
=
–0.98,
p
=
0.02, 95% confidence interval: –1.19 to –0.11). Table 2 shows
results of the multiple regression model which included ambi-
ent temperature. The results show that for the given variables in
the model, SBP was lowered by 5 mmHg per 10ºC increase in
