Power spectral analysis of the beat-to-beat variations of heart rate or the heart
period (R–R interval) has become widely used to quantify cardiac autonomic regulation
(Appel et al., 1989; Task Force of the European Society of Cardiology and the North
American Society of Pacing and Electrophysiology, 1996; Berntson et al., 1997; Denver
et al., 2007; Thayler et al., 2010; Billman, 2011). This technique partitions the
total variance (the “power”) of a continuous series of beats into its frequency components,
typically identifying two or three main peaks: Very Low Frequency (VLF) <0.04 Hz,
Low Frequency (LF), 0.04–0.15 Hz, and High Frequency (HF) 0.15–0.4 Hz. It should be
noted that the HF peak is shifted to a higher range (typically 0.24–1.04 Hz) in infants
and during exercise (Berntson et al., 1997). The HF peak is widely believed to reflect
cardiac parasympathetic nerve activity while the LF, although more complex, is often
assumed to have a dominant sympathetic component (Task Force of the European Society
of Cardiology and the North American Society of Pacing and Electrophysiology, 1996;
Berntson et al., 1997; Billman, 2011). Based upon these assumptions, Pagani and co-workers
proposed that the ratio of LF to HF (LF/HF) could be used to quantify the changing
relationship between sympathetic and parasympathetic nerve activities (i.e., the sympatho-vagal
balance) (Pagani et al., 1984, 1986; Malliani et al., 1991) in both health and disease.
However, this concept has been challenged (Kingwell et al., 1994; Koh et al., 1994;
Hopf et al., 1995; Eckberg, 1997; Houle and Billman, 1999; Billman, 2011). Despite
serious and largely under-appreciated limitations, the LF/HF ratio has gained wide
acceptance as a tool to assess cardiovascular autonomic regulation where increases
in LF/HF are assumed to reflect a shift to “sympathetic dominance” and decreases in
this index correspond to a “parasympathetic dominance.” Therefore, it is vital to
provide a critical assessment of the assumptions upon which this concept is based.
The hypothesis that LF/HF accurately reflects sympatho-vagal balance rests upon several
interrelated assumptions as follows (modified from Eckberg, 1997): (1) cardiac sympathetic
nerve activity is a major, if not the exclusive, factor responsible for the LF peak
of the heart rate power spectrum; (2) cardiac parasympathetic is exclusively responsible
for the HF peak of the heart rate power spectrum; (3) disease or physiological challenges
provoke reciprocal changes in cardiac sympathetic and parasympathetic nerve activity
(i.e., increases in cardiac parasympathetic nerve activity are always accompanied
with corresponding reductions in cardiac sympathetic nerve activity and vice versa);
and (4) there is a simple linear interaction between the effects of cardiac sympathetic
and cardiac parasympathetic nerve activity on heart rate variability (HRV).
As previously noted, frequency domain analysis of HRV usually reveals two or more
peaks, a lower frequency (<015 Hz) and a higher frequency peak (>0.15 Hz) that are
often assumed to correspond to cardiac sympathetic and cardiac parasympathetic neural
activity, respectively (Pagani et al., 1984, 1986; Malliani et al., 1991). However,
accumulating evidence clearly demonstrates that this assumption is naïve and greatly
oversimplifies the complex non-linear interactions between the sympathetic and the
parasympathetic divisions of the autonomic nervous system (Berntson et al., 1997;
Eckberg, 1997; Parati et al., 2006; Billman, 2009, 2011). This is particularly true
with regards to the relationship between LF power and cardiac sympathetic regulation
(Randall et al., 1991; Ahmed et al., 1994; Kingwell et al., 1994; Hopf et al., 1995;
Eckberg, 1997; Houle and Billman, 1999; Parati et al., 2006; Billman, 2009, 2011).
The LF peak of the heart rate power spectrum is reduced by at least 50% by either
cholinergic antagonists or selective parasympathectomy (Akselrod et al., 1981; Randall
et al., 1991; Houle and Billman, 1999). Importantly, this peak is not completely eliminated
by the combination of selective denervation and beta-adrenoceptor blockade (Randall
et al., 1991); ~25% of the peak remains after this treatment. As a consequence, LF/HF
often actually increases from baseline values when both parasympathetic and adrenergic
nerve activity have been blocked. For example, using the data reported by Randall
and co-workers (Randall et al., 1991), LF/HF increased from a baseline value of 1.1–8.4
when selective parasympathetic denervation was combined with beta-adrenergic receptor
blockade, falsely suggesting a major shift to sympathetic dominance! In a similar
manner, interventions that would be expected to increase cardiac sympathetic activity,
such as acute exercise or myocardial ischemia, not only failed to increase LF power
but actually provoked significant reductions in this variable (Houle and Billman,
1999), once again yielding LF/HF values that are difficult to interpret. Indeed, despite
large increases in heart rate, LF/HF ratio was largely unaffected by either acute
myocardial ischemia, exercise, or the cholinergic antagonist atropine sulfate (Houle
and Billman, 1999). Finally, direct recording of sympathetic nerve activity failed
to correlate with LF power in either healthy subjects or patients with heart failure
(Hopf et al., 1995; Notarius and Floras, 2001; Jardine et al., 2002; Moak et al.,
2007; Piccirillo et al., 2009), a condition known to increase cardiac sympathetic
drive (Hasking et al., 1986; Saul et al., 1988; Watson et al., 2007). Thus, the LF
component of HRV does not provide an index of cardiac sympathetic drive but rather
reflects a complex and not easily discernible mix of sympathetic, parasympathetic,
and other unidentified factors with parasympathetic factors accounting for the largest
portion of the variability in this frequency range. As a consequence, the physiological
basis for LF/HF is difficult to discern.
Although the vast majority of the clinical and the experimental studies demonstrate
a strong association between HF power and cardiac parasympathetic activity (Katona
et al., 1970; Appel et al., 1989; Billman and Hoskins, 1989; Billman and Dujardin,
1990; Task Force of the European Society of Cardiology and the North American Society
of Pacing and Electrophysiology, 1996; Billman, 2009, 2011; Thayler et al., 2010),
this concept has also been challenged (Kollai and Mizsei, 1990; Goldberger et al.,
1994; Hedman et al., 1995; Taylor et al., 2001; Parati et al., 2006). Unlike LF power
and sympathetic nerve activity, a strong correlation between HF power and direct recordings
of cardiac parasympathetic activity has been reported (Chess et al., 1975; Piccirillo
et al., 2009). However, just as parasympathetic activation exerts profound influences
on the LF component of HRV, sympathetic neural activity may modulate the HF component
of the R–R interval variability (Taylor et al., 2001; Cohen and Taylor, 2002). Taylor
et al. (2001) found that cardioselective beta-adrenergic receptor blockade (drugs
that should not indirectly alter vagal outflow via action within the central nervous
system) increased the amplitude of the respiratory sinus arrhythmia over a wide range
of respiratory frequencies (i.e., the increases were not restricted to lower frequencies,
<0.15 Hz). They concluded that “cardiac sympathetic outflow can oppose vagally mediated
R-R interval oscillations and sympathetic blockade removes this effect” (Cohen and
Taylor, 2002). Based upon these data, sympathetic nerve activation may alter the HF
peak by perhaps as much as 10%. Thus, differences in cardiac sympathetic activation
during a physiological challenge (e.g., exercise or postural changes) in healthy subjects
or that occur as the consequence of cardiovascular disease (following myocardial infarction)
could restrain vagally mediated changes in HRV. These data suggest that HF power cannot
be solely attributed to changes in cardiac vagal efferent nerve traffic, further compromising
an accurate interpretation of the LF/HF ratio.
Accurate interpretation of LF/HF ratio also depends upon the assumption that physiological
interventions always elicit reciprocal changes in parasympathetic and sympathetic
nerve activity. However, following the termination of exercise sympathetic activation
remains high despite the rapid re-activation of cardiac parasympathetic drive (Smith
et al., 2005; Billman and Kukielka, 2007; Billman, 2009). Furthermore, chemoreceptor
activation by carbon dioxide provokes parallel reductions in sympathetic and parasympathetic
nerve activity (Eckberg, 1997) while facial emersion in cold water (activating the
so-called “diving reflex”) increased sympathetic nerve activity yet elicited a profound
bradycardia (Eckberg et al., 1984; Fagius and Sundlof, 1986). The observation that
heart rate declines, despite increases in sympathetic nerve activity, highlights the
complex non-linear interactions of the sympathetic and parasympathetic nervous system,
providing an example of “accentuated antagonism” (Levy, 1971; Stramba-Badiale et al.,
1991; Uijtdehaage and Thayer, 2000), the dominance of parasympathetic over sympathetic
influences on cardiac rate. Finally, reciprocal changes in parasympathetic and sympathetic
nerve activity do not always occur even during the activation of the baroreceptor
reflex (Eckberg, 1997). Eckberg and co-workers have shown that, although small changes
in arterial pressure typically provoke reciprocal changes sympathetic and parasympathetic
nerve activity, large increases in arterial pressure only provoke increases in parasympathetic
nerve activity without altering the prevailing sympathetic activity (Eckberg, 1980;
Rea and Eckberg, 1987). Furthermore, autonomic response to baroreceptor reflex activation
depends on whether the pressure changes occur near the threshold or the saturation
point of the response curve; the same change in pressure can elicit larger or smaller
autonomic responses depending on how close the prevailing pressure lies to the threshold
(larger) or saturation (smaller) portion of the stimulus-response curve (Eckberg,
1980, 1997). As previously noted, changes in heart rate do not result from the simple
algebraic summation of the sympathetic and parasympathetic nerve activity. Rather,
parasympathetic nerve activation can completely override even maximal sympathetic
nerve stimulation, provoking large reductions in heart rate in the face of sympathetic
nerve activation as was previously noted for the diving reflex. Thus, physiological
interventions can elicit either complex non-linear reciprocal or parallel changes
in either division of the autonomic nervous system. These complex interactions can
profoundly influence the calculation and the interpretation of LF/HF.
Mathematical considerations can also influence LF/HF values. Similar LF/HF values
can be obtained via either exclusive changes in the numerator (i.e., LF), or the dominator
(i.e., HF), or by some combination of the two, as is illustrated in Table 1. For example,
a doubling of parasympathetic activity against maintained sympathetic nerve activation
yields the identical LF/HF value as a 50% reduction in sympathetic nerve activity
against a constant background parasympathetic regulation. Based upon the literature,
one can conclude that parasympathetic nerve activation contributes to at least 50%
of the LF variability while sympathetic activity, at best, only contributes 25% to
this variability (Randall et al., 1991). A substantial portion of the variability
in the LF band also results from other unidentified factors. In a similar fashion,
sympathetic nerve activity could contribute to perhaps as much as 10% of the HF variability
(Taylor et al., 2001; Cohen and Taylor, 2002). As a consequence, the effects of changing
sympathetic and parasympathetic activity on the LF/HF are quite variable and not intuitively
obvious, as is illustrated in Figure 1. This figure was constructed using the following
formula that was based upon a synthesis of the literature (particularly, Randall et
al., 1991; Taylor et al., 2001; Cohen and Taylor, 2002), LF = 0.5 parasympathetic
+ 0.25 sympathetic activity while HF = 0.9 parasympathetic + 0.1 sympathetic nerve
activity. The nerve activity was varied from baseline (1 arbitrary unit each) increasing
or decreasing by up to a factor of 10. Due to the substantial contribution (accounting
for up to 25% of the variability) (Randall et al., 1991) from non-neural factors to
LF power, very distorted values of LF/HF can be obtained when both sympathetic and
parasympathetic nerve activity are minimal. If for example, one assumes that LF =
0.5 parasympathetic + 0.25 sympathetic + 0.25 other factors and both parasympathetic
and sympathetic nerve activity are reduced to 1/100 the baseline values, the calculated
LF/HF becomes (0.005 + 0.0025 + 0.25)/(0.009 + 0.001) = 25.75! Despite the almost
complete absence of cardiac autonomic regulation, this value could be inappropriately
interpreted as a major shift toward sympathetic dominance. Furthermore, LF/HF cannot
be determined if both sympathetic activity and parasympathetic nerve activity were
to be abolished completely (i.e., when the dominator is zero). Finally, mathematical
complications also arise due to the non-linear relationship between R–R interval and
heart rate; similar changes in heart rate elicit much greater variability in R–R interval
at lower than at higher heart rates (Sacha and Pluta, 2008). As a consequence of this
non-linear relationship, it is difficult to separate the changes in HRV that arise
from direct action of cardiac autonomic nerves from those changes that result indirectly
from neurally induced changes in average heart rate. This observation led Sacha and
Pluta (2008), to propose that HRV has both physiological and mathematical influences
that can be corrected by the division of HRV by average R–R interval. Thus, the physiological
basis for changes in LF/HF is not readily discernible and spurious values for LF/HF
can result as a consequence of the mathematical manipulations of the data.
Table 1
Examples of the effects of varying cardiac sympathetic and parasympathetic nerve activity
on LF/HF.
Parasympathetic nerve activity
Sympathetic nerve activity
LF
HF
LF/HF
1
1
0.75
1
0.75
2
1
1.25
1.9
0.66
0.5
1
0.5
0.55
0.91
1
2
1
1.1
0.91
1
0.5
0.625
0.95
0.66
2
2
1.5
2
0.75
2
0.5
1.125
1.85
0.61
0.5
2
0.75
0.65
1.15
0.5
0.5
0.375
0.5
0.75
These numbers were generated using the following formula (derived from a synthesis
of the literature, particularly Randall et al., 1991; Taylor et al., 2001; Cohen and
Taylor, 2002): LF/HF = (0.5 parasympathetic + 0.25 sympathetic nerve activity)/(0.9
parasympathetic + 0.1 sympathetic nerve activity). The nerve activity is reported
as arbitrary units where at baseline sympathetic and parasympathetic nerve activity
were normalized as 1 arbitrary unit each. The data shown are for baseline and various
combinations of doubling (2 × baseline) or halving (0.5 × baseline) the autonomic
nerve activity.
Figure 1
An illustration of the possible non-linear effects of varying cardiac sympathetic
and cardiac parasympathetic nerve activity on LF/HF. This graph was constructed using
the following formula (derived from a synthesis of the literature, particularly Randall
et al., 1991; Taylor et al., 2001; Cohen and Taylor, 2002): LF/HF = (0.5 parasympathetic
+ 0.25 sympathetic nerve activity)/(0.9 parasympathetic + 0.1 sympathetic nerve activity).
The nerve activity was varied from baseline (1 arbitrary unit each) increasing or
decreasing by up to a factor of 10 (i.e., from 0.1 to 10 units).
It should also be noted, that HRV (and thereby LF/HF) is affected by respiratory parameters
and mechanical events independent of changes in cardiac autonomic nerve activity.
The contribution of mechanical factors (due to stretch of the atria that results from
both changes in cardiac filling and the changing thoracic pressure that occur during
respiration) to changes in HRV was first proposed by Bainbridge (1930). This conclusion
is supported by the observation that heart transplant patients, despite the absence
of cardiac nerves, still exhibit small (~2–8% of normal) change in R–R interval associated
with the respiratory cycle (Bernardi et al., 1989). Taylor et al. (2001) further demonstrated
that atrial stretch can exert significant influences on R–R interval in subjects with
complete autonomic blockade. They found that after combined cholinergic and adrenergic
receptor blockade slow deep breathing could still provoke oscillations of ~120 ms
in healthy human subjects (Taylor et al., 2001). In similar manner, mechanical distortion
(stretch) of the sinoatrial nodal stretch in pigs without functional autonomic innervation
(vagal nerve section combined with propranolol treatment) reduced the HF component
of HRV (Horner et al., 1996).
Respiratory parameters can also profoundly alter heart rate and R–R interval variability
independent of changes in cardiac autonomic regulation (i.e., against a constant background
level of automatic regulation) (Angelone and Coulter, 1964; Davies and Neilson, 1967;
Hainsworth, 1974; Melcher, 1976; Hirsch and Bishop, 1981; Brown et al., 1993; Van
De Borne et al., 2001). It is now well established that increases in respiratory frequency
reduce the amplitude of heart rate oscillations (Angelone and Coulter, 1964; Melcher,
1976; Hirsch and Bishop, 1981; Brown et al., 1993) while either increases in tidal
(Davies and Neilson, 1967; Melcher, 1976; Hirsch and Bishop, 1981; Eckberg, 1983;
Kollai and Mizsei, 1990; Brown et al., 1993) or static lung volume (Hainsworth, 1974)
provoke increases in the R–R interval variability. The facts are in direct opposiiton
to the assumptions. Conversely, reductions in respiratory frequency increase HRV (Angelone
and Coulter, 1964; Melcher, 1976; Hirsch and Bishop, 1981; Brown et al., 1993) while
decreases in tidal volume lead to reductions in the R–R interval variability (Davies
and Neilson, 1967; Melcher, 1976; Hirsch and Bishop, 1981; Eckberg, 1983; Kollai and
Mizsei, 1990; Brown et al., 1993). Thus, it is critical to control breathing (paced
or timed breathing) in order to interpret HRV data accurately. For obvious reasons,
it is much more difficult to control respiratory parameters in conscious animal than
in human studies. However, these respiratory parameters frequently are not controlled
even in human studies (Brown et al., 1993). Brown and co-workers (Brown et al., 1993),
reviewed the human literature and found that only about 51% controlled respiratory
rate, and even fewer studies controlled for tidal volume (11%). They further reported
that respiratory parameters not only altered HF power but also strongly influenced
the LF components of the R–R interval power spectrum, a component that previously
was viewed to vary independently of changes in respiration (Brown et al., 1993).
Finally, prevailing heart rate can also influence HRV. There are a number of studies
that report a strong positive correlation between mean R–R interval and various time
domain indices of HRV (e.g., the standard deviation of normal beats, SDNN) (Kleiger
et al., 1987; Van Hoogenhuyze et al., 1991; Fleiss et al., 1992) such that HRV was
greater during longer mean R–R intervals (slower heart rates) than at shorter mean
R–R intervals (faster heart rates). Frequency domain analysis of HRV is similarly
affected by mean heart rate. Sacha and Pluta (2005) found that LF was directly related,
while HF was indirectly related, to the average heart rate of the subject. As a consequence,
they further report that LF/HF varied depending on heart rate, lower at slower and
higher at faster heart rates. Thus, heart rate per se can influence LF/HF independent
of changes cardiac autonomic nerve activity.
As we have seen, the hypothesis that LF/HF quantifies “sympatho-vagal balance” depends
upon four interrelated assumptions, all of which can be proven to be false. The facts
are in direct opposition to the assumptions. In particular, the complex nature of
LF power, its exceedingly poor relationship to sympathetic nerve activation, and the
non-linear (and often non-reciprocal) interactions between sympathetic and parasympathetic
nerve activity that are confounded by the mechanical effects of respiration and prevailing
heart rate, make it impossible to delineate the physiological basis for LF/HF with
any degree of certainty. Thus, the LF/HF sympatho-vagal balance hypothesis has been
disproven—the preponderance of evidence confirms that LF/HF data cannot accurately
quantify cardiac “sympatho-vagal balance” either in health or disease.