CARDIOVASCULAR JOURNAL OF AFRICA • Vol 23, No 2, March 2012
72
AFRICA
towards sympathetic dominance.
26
In support of this finding,
research has demonstrated that physiological levels of oestrogen
increase vagal tone and suppress sympathetic modulation of
heart rate in females.
30
A limitation of this study was that respiratory rate was not
controlled when measuring the IBIs over the five-minute record-
ing period. Research has shown that the reliability of HRV
measures was increased when respiratory rate was regulated.
12
Furthermore, part of intra-subject variability was also due to
the natural change of HRV parameters that occurs under the
influence of factors such as mood, alertness and mental activity,
which is very difficult to control for in any study.
3
Conclusion
Heart rate variability measures are a popular, non-invasive tool
used to monitor autonomic function. Short-term recordings are
easy to perform and are suitable for both clinical and physi-
ological research. However, the findings of the present study
have demonstrated a high relative reliability but low absolute
reliability for HRV. In particular, HRV random error was higher
in males. For clinical or sport/exercise science practice, these
results place in doubt the use of HRV indices for assessing
small (
<
5%) intervention or treatment effects, specifically when
examining male or clinical populations.
3
Furthermore, these results place doubt on the use of HRV
measurement for monitoring performance changes in well-
trained or elite athletes, who typically require assessment tech-
niques that can detect physiological or performance changes
below 1%.
29
Finally, specific HRV parameters differed between
males and females, indicating a greater parasympathetic modula-
tion of HRV in females. This finding suggests a possible mecha-
nism for why pre-menopausal females have a lower incidence of
coronary heart disease compared to males in the same age group,
as well as compared to postmenopausal women.
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