activation are suppressed when strong vagal activity simultane-
ously occurs, whereas vagal deceleration effects are augmented
during high sympathetic background levels.
In the concept of autonomic space (Berntson et al., 1993;
Berntson, Cacioppo, Quigley, & Fabro, 1994), functional states of
the ANS are described in a two-dimensional space bound by sym-
pathetic and parasympathetic axes. In this space, multiple modes of
autonomic control can be characterized. These comprise (a) a
coupled reciprocal mode, in which activities of the two branches
are inversely related; (2) a coupled nonreciprocal mode, in which
activities of both branches are positively correlated (coactivation);
and (3) an uncoupled mode, in which activity changes of the two
divisions occur independently of one another. The assumptions of
the Pagani group may only hold under the assumption that the ANS
is working in a reciprocal mode most of the time, which is unlikely
to be the case (see Berntson, Cacioppo, & Quigley, 1991, for a
review).
Pagani and colleagues chose normalized LF and HF power and
the LF/HF ratio as mathematical expression of HRV. As summa-
rized above, in most studies supporting their theory these transfor-
mations were applied (cf. Montano et al., 2009). Their model is
supported mainly by the effects of orthostatic tilt (head-up tilt
increases sympathetic and inhibits vagal activity). In the Montano
et al. (1994) study, absolute LF power was widely unrelated to the
magnitude of tilt (r = .17), whereas HF power correlated signifi-
cantly with tilt incline (r = -.41). In contrast, tilt highly correlated
with LF and HF power expressed in normalized units (r = .78 and
-.72, respectively, for LFnu and HFnu) and with the LF/HF ratio
(r = .68). Normalization procedures thus lead to a nearly linear
relation between LFnu and tilt angle. However, as can be seen in
their figures, the close associations for LFnu and the LF/HF ratio
resulted from the inclusion of HF in the equation applied to
compute LFnu and the LF/HF ratio. The effects of tilt on the
transformed measures are therefore mainly due to the reduction in
HF as tilt angle increases.
Regarding the transformation of LF and HF into normalized
tions produce artificially high correlations. This may lead to the
wrong conclusion that a variable is related to sympathetic cardiac
tone such as in the case of tilt angle in the Montano et al. (1994)
study. In our pharmacological blockade study, atropine administra-
tion increased LFnu (from 49 15 to 87 12 for baseline and
from 44 12 to 75 23 for mental load), decreased HFnu (from
50 14 to 13 12 for baseline and from 56 12 to 25 23 for
mental load), and produced a very strong increase in the LF/HF
ratio (from 1.15 0.71 to 11.89 6.52 for baseline and from
0.87 0.42 to 7.47 8.35 for mental load). In this context of
small residual HRV (i.e., nearly constant HR) the suggested trans-
formations can lead to substantial distortion of the data and thus
wrong conclusions.
Conclusions
Evidence presented in this article challenges the suitability of the
LF component of HRV as an index of cardiac sympathetic control
with a Dependent Variable (DV) for 10 Subjects
Subject
SPD
VLF
LF
HF
LF/HF
LFnu
HFnu
DV
1
21
5
10
6
1.67
63
38
1
2
20
4.50
9
6.50
1.38
58
42
2
3
25
6
12
7
1.71
63
37
3
4
19.20
3
9
7.20
1.25
56
44
4
5
25.50
5
13
7.50
1.73
63
37
5
6
19
3
8
8
1
50
50
6
7
29.50
6
15
8.50
1.76
64
36
7
8
21
5
7
9
0.78
44
56
8
9
29.50
6
14
9.50
1.47
60
40
9
10
21
5
6
10
0.60
38
63
10
Note. SPD = spectral power density in the range 0 to 0.40 Hz, that is, total
power; VLF = very low frequency; LF = low frequency; HF = high fre-
quency. Normalized units were multiplied by 100.
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