CARDIOVASCULAR JOURNAL OF AFRICA • Vol 23, No 2, March 2012
AFRICA
71
Gender differences
There were significant differences between males and females
for resting HR (
p
<
0.0001), R–R intervals (
p
<
0.0001) and for
the frequency-domain HRV parameters, HFnu (
p
=
0.020) and
LF/HFnu ratio (
p
=
0.003) (Table 3). The female resting heart
rate was 16% higher than that of the males (
p
<
0.0001), while
the IBIs were 21% higher in the males (
p
<
0.0001). The LFnu
(
p
=
0.087) and LF/HFnu ratio (
p
=
0.003) were 13 and 41%
higher in the males, respectively, while the HFnu (
p
=
0.02) was
12% higher in the females. There were no significant differences
between males and females in the HRV time-domain parameters.
Discussion
Despite extensive use and research in both clinical and physi-
ological settings, HRV analysis is still poorly supported by rigor-
ous reliability studies.
3
The purpose of the present study was to
examine the reliability and gender characteristics of standard
parameters of HRV from short-term (five-minute) laboratory
recordings in physically active individuals. The main findings of
this study were that the reliability of HRV recording over consec-
utive days varied depending on the specific reliability index used,
the HRV parameter examined as well as the gender of the popu-
lation. A comparison of HRV frequency-domain parameters for
males and females demonstrated significant gender differences
in the sympathovagal balance.
Although the definition of a categorical rating of relative
reliability based on ICCs is still controversial,
3
the results from
the present study demonstrate substantial to excellent rela-
tive reliability for the majority of time- and frequency-domain
HRV parameters when comparing measurements obtained from
days 3 and 4. For most measurements, the ICC was above 0.80
(‘good’),
28
indicating that the repeated measurements reflect
mostly the true value of HRV parameters relative to random
error. Females demonstrated a higher relative reliability than
males for all frequency-domain parameters. The high ICC of
these short-term recordings indicate a considerable consistency
with time, similar to previous studies.
3,8
However, absolute reliability, indicated by the TEM and
TEM%, revealed the presence of a large random error in all
HRV parameters, particularly for the males. Females demon-
strated better absolute reliability (lower TEM) than males for
all parameters, particularly those in the frequency domain. The
TEM% also indicated a low absolute reliability due to the high
values found specifically for the LF/HFnu ratio. These findings
are similar to those of Pinna
et al
. who found a greater random
error (TEM) in frequency domain measures.
3
These results might place doubts on the use of HRV indices in
assessing interventions or treatment effects in individual partici-
pants, specifically when examining male participants or clinical
populations.
3
Furthermore, these results place doubt on the use of
HRV measurement for monitoring performance changes in well-
trained athletes. The high TEM and TEM% indicated that HRV
would be unable to detect small but significant changes (
≤
1%)
in performance.
29
Considering the absolute reliability was low
(high TEM and TEM% values), the large random error found in
HRV would require significant changes in HRV to occur to deem
them meaningful.
29
Gender differences
Findings of research examining HRV gender differences in
healthy individuals are conflicting.
5-8
Research has demonstrated
that HRV measures are either the same or differ considerably
between the genders and may also be HRV parameter or age
dependant,
5-8
and therefore, further research has been advocated.
3
Umetani
et al.
5
has shown that HRV (for all measures) is signifi-
cantly lower in ‘young’ (10–29 years) females compared to their
age-matched male counterparts. The gender differences subse-
quently decreased and then disappeared with age and at different
rates for the HRV parameters. Their findings suggested a higher
level of parasympathetic activity in males.
5
Conversely, Ryan
et al
.
6
concluded that vagal high-frequency
power was higher in females. They suggested this difference
was most apparent for young (20–39) and middle aged (40–64)
females. Raemaeker
et al.
noted that the LFnu (sympathetic
dominance) was higher in females, however, HFnu (parasym-
pathetic dominance) was not significantly different between the
genders.
7
Sinnreich
et al
.
8
observed that RMSSD and HF compo-
nents (measures reflecting predominantly vagal activity) were
small and non-significant, but that the LF/HF ratio (suggested to
reflect sympathetic/parasympathetic balance) differed substan-
tially due to greater VLF and LF power found in the male
participants. They concluded that their results illustrated rela-
tively higher sympathetic activity in men compared to women.
Our findings demonstrate a similar gender distinction to that of
Ryan
et al.
6
and Sinnreich
et al
.
8
A higher level of parasympa-
thetic activity was found in the female participants compared to
the males. This was demonstrated by a lower LF/HFnu ratio and
higher HF value in the females.
The finding of a higher HRV and contribution of the para-
sympathetic nervous system to HRV in females may help
explain the overall protection of pre-menopausal women from
coronary heart disease (CHD), coronary mortality and sudden
cardiac death, compared to males in this age group.
8,13
Research
has demonstrated that a high HRV is associated with significant
cardiovascular health. Although not measured in the present
study, the effect of oestrogen on HRV and parasympathetic activ-
ity in females may be a key factor in this finding.
Research examining HRV in pre- and postmenopausal women
found a significant difference in HRV.
26
Postmenopausal women
had a significantly reduced HRV, as demonstrated by a higher
relative power of LF and LF/HF ratio, which was related to a
decline in the level of oestrogen. The authors concluded that a
decline in the level of oestrogen from pre-menopausal to post-
menopausal status favours the shifting of autonomic balance
TABLE 3. COMPARISON OF MEAN (± SD) OF DAYS 2, 3 AND
4 OF HEART RATEVARIABILITY DATA BY GENDER
Men
Women
p
Resting HR (b/min)
55.09 (8.1)
65.84 (7.2)
<
0.0001*
IBIs (ms)
1126 (169)
927.7 (101)
<
0.0001*
LFnu
47.95 (17.93) 42.48 (18.43)
0.087
HFnu
52.02 (17.73) 59.17 (17.16)
0.020*
LF/HFnu
1.114 (0.710) 0.792 (0.522)
0.003*
SDNN
103.1 (36.13) 93.39 (57.55)
0.252
RMSSD
101.3 (43.75) 87.57 (59.44)
0.137
pNN50
53.35 (18.71) 48.46 (22.77)
0.182
