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natural pacemaker) receives signals from the autonomic (involuntary) portion of the
nervous system, which has two major branches: the parasympathetic, whose stimulation
decreases the firing rate of the sinus node, and the sympathetic, whose stimulation
increases the firing rate of the sinus node's pacemaker cells. The influence of these
two branches of the nervous system produces a continual tug-of-war on the sinus node,
one decreasing and the other increasing the heart rate. It has been suggested that it is this
tug-of-war that produces the fluctuations in the heart rate of healthy subjects, and we
note the similarity to the competitive mechanism postulated by Taylor and Taylor [
79
].
Consequently, these fluctuations in the interbeat interval are termed heart-rate variabil-
ity (HRV) and strongly influence the heart's ability to respond to everyday disturbances
that can affect its rhythm. The clinician focuses on retaining the balance in regula-
tory impulses from the vagus nerve and sympathetic nervous system and in this effort
requires a robust measure of that balance. A quantitative measure of HRV such as the
fractal dimension serves this purpose.
The beating of the human heart is typically recorded on a strip chart called an elec-
trocardiogram, where the maximum of the pulse is denoted by R and the interbeat time
interval is denoted by the time between these maxima. The RR intervals therefore define
the time series of interest. A typical HRV time series for a young healthy adult male is
depicted in Figure
2.19
. The variability in the time between heartbeats is clear from
this figure; the variation in the time intervals between heartbeats is relatively small, the
mean being 1.0 s and the standard deviation being 0.06 s.
In Figure
2.20
the logarithm of the standard deviation is plotted versus the logarithm
of the average value for the HRV time series depicted in Figure
2.19
. At the left-most
position the data point indicates the standard deviation and average, using all the data
1.1
1.05
1
0.95
0.9
0.85
0.08
0
100
200 300
Interval Number
400
500
Figure 2.19.
A time series of heartbeat intervals of a healthy young adult male is shown. It is clear that the
variation in the time interval between beats is relatively modest but not negligible [
100
].
Reproduced with permission.
