Biology Reference
In-Depth Information
ECG - Full
Time (sec)
−
5.0
0.0
FIGURE 6-2.
An EKG recording. An EKG records the electrical activity of the heart muscle as it contracts.
The triangular markers indicate the peak of each beat. (Courtesy of Dr. J. Randall Moorman,
University of Virginia.)
Figure 6-2 presents 5 seconds of EKG data. It is important to note that
even in healthy individuals the duration of sequential RRIs is not
constant (and, as we see below, is normally even more variable than in
some diseases). This natural HR variability (HRV) arises from the
interplay between the two arms of the autonomic nervous system—the
system responsible for those necessary bodily functions not under
voluntary control. The autonomic nervous system controls HR through
the sympathetic pathway, which increases HR, and the parasympathetic
pathway, which slows it.
In general, some variability in the HR is a healthy sign, because
internal fluctuations give the organism greater freedom to adapt to
external challenges. In contrast, in newborn infants, as in adults,
HRV is substantially reduced during periods of severe illness (see
Burnard, [1959]; Rudolph et al. [1965]; Cabal et al. [1980]; Griffin et al.
[1994]).
There are various theories concerning the mechanisms of reduced
HRV aimed at explaining and quantifying the differences in the
mathematical characteristics of RRIs for normal and low HRV. We focus
on the theory, developed at the University of Virginia by Drs. J. Randall
Moorman and M. Pamela Griffin, which explains the mechanism of
observed HRV abnormalities using signal transduction cascades (see
Nelson et al. [1998]). We begin with the physiological mechanisms
causing and regulating heartbeat.
The heart is a muscular pump consisting of four chambers (two atria and
two ventricles) that contract in a coordinated fashion to push blood
through the circulatory system. Blood returning from the body enters the
right atrium, passes through to the right ventricle, and is then pumped
to the lungs. Blood returning from the lungs enters the left atrium,
passes through to the left ventricle, and is then pumped to the body. The
heartbeat (the rhythmic contraction of the heart muscle) is controlled by
