CARDIOVASCULAR JOURNAL OF AFRICA • Vol 21, No 1, January/February 2010
AFRICA
19
temperature. The results are in favour of significant changes in
blood pressure when exposed to different ambient temperatures.
Discussion
The results presented in this study show a significant influence
of ambient temperature on blood pressure measurements. An
inverse dose–response relationship was demonstrated for both
SBP and DBP with ambient temperature, but this was more
significant for SBP. Jansen
et al
.
13
in their study of the effect of
change in ambient temperature on BP demonstrated an inverse
dose–response relationship between BP and temperature but
more especially for SBP. The regression analysis also showed
significant changes in SBP with ambient temperature.
Other factors that could have caused the variation in BP
during specific periods of the day should be considered when
interpreting the results. A difference in stress levels could have
been a possible confounder but this could not have been so for
this rural population because their stress levels are usually high
in the daytime when they go to the farm and it is expected that
their blood pressures would have been high around this time.
Daily variation in BP is also a potential confounder but this is
unlikely, considering the significant variation in BP with temper-
ature. Other potential confounding sources include lifestyle
factors such as diet, but these are highly unlikely considering the
cross-sectional nature of the study. Ambient temperature appears
to be the main factor causing the variation in BP.
The seasonal variation in SBP and DBP has been documented
in many studies,
5-10
with ambient temperature being implicated in
this variation.
14,15
Jansen and colleagues
13
demonstrated a moder-
ate but significant influence of ambient temperature on BP. A
significant increase in both SBP and DBP was seen when moving
from higher to lower ambient temperature. Komulainen and
colleagues
16
in their study also reported that BP seems to react
in three minutes to changes in ambient temperature. Chifamba
et
al
.
17
studied the effect of variation in environmental temperature
on blood pressure in subjects in Zimbabwe and showed that
SBPs and DBPs were significantly higher when recorded at 15ºC
than at 25ºC (a mean difference of 32.2
±
4.2 mmHg and 19.5
±
3.0 mmHg for SBPs and DBPs, respectively).
It has been observed that people with high blood pressure
require less medication in the summer compared with the
winter.
18
Various studies have also reported increased prevalence
of strokes and myocardial infarctions and higher cardiovascular
disease mortality during the winter months.
19,20
The phenomenon
of BP variation with ambient temperature demonstrated in this
study is a possible explanation for these observations.
Several mechanisms have been suggested to explain this
phenomenon. One is the activation of the sympathetic nerv-
ous system. Cold temperature increases sympathetic tone and
increased secretion of catecholamines, which in turn causes
increased heart rate and BP. The BPs associated with warm
temperature is due to cutaneous vasodilatation, which in turn
reduces the peripheral vascular resistance and BP.
Study limitations
Our study was subject to multiple potential biases. The use of the
automated blood pressure device (Omron MX3 Plus) opposed
to the random-zero sphygmomanometer recommended by the
WHO MONICA protocol
12
may have introduced measurement
biases. However, the Omron MX3 Plus has been validated against
the reference method of sphygmomanometry and has passed the
validation recommendations of the International Protocol of the
European Society of Hypertension, making it eligible for use in
epidemiological surveys.
11
Validated automated electronic BP
devices may actually improve reliability and reproducibility of
BP measurements.
21
The devices were also regularly standardised
against mercury sphygmomanometers to check for any drifts.
Ideally 24-hour ambulatory, clinic or home blood pressure moni-
toring would have been the best way to demonstrate ambient
temperature variations in blood pressure.
Conclusion
Awareness of the phenomenon that there is a correlation between
BP and ambient temperature is quite important, considering the
fact that high blood pressures are recorded during low ambient
temperatures. The significant differences in blood pressure at
different ambient temperatures may have some implications,
such as under- or overestimating blood pressure levels in various
populations.
TABLE 1. CORRELATION OF BLOOD PRESSUREWITH
AMBIENT TEMPERATURE
Variable
Pearson’s correlation (r)
p
-value
SBP (mmHg)
–0.098*
0.019
DBP (mmHg)
–0.088*
0.036
*Correlation significant at
p
=
0.05.
TABLE 2. MULTIPLE REGRESSIONANALYSIS OF SBP
WITHAGE,WAIST CIRCUMFERENCE, ALCOHOL
CONSUMPTION, SMOKINGAND TEMPERATURE
Model
β
SE
p
-value
(Constant)
100.812
13.096
0.000
Age (years)
0.332
0.062
0.000
Waist circumference (cm)
0.411
0.090
0.000
Alcohol (yes or no)
–3.003
1.758
0.088
Smoking (yes or no)
–0.362
2.832
0.898
Temperature (ºC)
–0.521
0.262
0.047
r
²
=
0.119;
β
: regression coefficient; SE: standard error; dependent
variable: SBP.
Fig. 1. Regression of SBP with ambient temperature.
200.00
175.00
150.00
125.00
100.00
75.00
SBP (mmHG)
30
33
36
39
42
45
Temperature (°C)
