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including LF HRV (Cook et al., 1991; Jokkel et al., 1995). The
latter effect was actually interpreted as indicating that LF consti-
tutes a marker of vagal activity (Cook et al., 1991). Montano et al.
(1994) stated that for the assessment of sympatholytic action of
b-adrenergic blocking agents, HRV has to be expressed in normal-
ized units or in terms of the LF/HF ratio. In our study, however,
metoprolol did not significantly alter LFnu (from 47 10 to
52 20 during baseline and from 52 20 to 47 11 during cog-
nitive load), HFnu (from 53 10 to 48 19 during baseline and
from 48 20 to 53 11 during cognitive load) and the LF/HF
ratio (from 0.95 0.36 to 1.14 0.39 during baseline and from
1.75 2.04 to 0.93 0.38 during cognitive load).
Studies using nonpharmacological blockade procedures also
raise doubts about the sympathetic origin of the HF band. Hopf,
Skyschally, Heusch, and Peters (1995) and Introna et al. (1995)
investigated the effects of preganglionic cardiac sympathetic
blockade by segmental thoracic epidural anesthesia in humans.
Both in a supine position and during tilt, LF and HF spectral power
remained unchanged. Similarly, partial cardiac sympathetic dener-
vation by means of bilateral thoracic sympathectomy affected
neither LF power nor the LF/HF ratio (Noppen et al., 1996;
Tedoriya, Sakagami, Ueyama, Thompson, & Hetzer, 1999).
In sum existing studies do not yield evidence of specific effects
BL (pre drug)
Task (pre drug)
BL (post drug)
Task (post drug)
h2 (LB), h2 (Task)
IBI
Atropine
938 (126)
871 (122)
563 (29)
550 (38)
.906, .870
b-blocker
876 (136)
845 (134)
1129 (155)
1109 (156)
.980, .992
VLF
Atropine
266 (273)
77 (44)
14 (23)
5 (2)
.494, .752
b-blocker
332 (579)
102 (105)
266 (156)
123 (75)
.018, .062
LF
Atropine
555 (421)
152 (122)
7 (7)
2 (1)
.675, .643
b-blocker
642 (1052)
305 (283)
509 (293)
366 (279)
.031, .254
HF
Atropine
547 (531)
175 (71)
1 (1)
1 (1)
.559, .876
b-blocker
578 (789)
206 (82)
502 (322)
443 (364)
.011, .342
Note. BL = baseline, IBI = interbeat interval, VLF = very low frequency, LF = low frequency, HF = high frequency. Effect sizes for the comparison pre–post
drug (h2 BL, h2 Task) are also shown.
loaded from https://onlinelibrary.wiley.com/doi/10.1111/psyp.12027 by University of Canterbury Library Access and Collections, Wiley Online Library on [17/05/2023]. See the Terms and Conditions (https