Biomedical Engineering Reference
In-Depth Information
Figure 1
. A schematic diagram of the heart. Adapted with permission from Goldberger
and Goldberger (1994) (1).
a normal fluctuation with the respiratory cycle, called respiratory sinus arrhyth-
mia. In addition, there is a modulation of the periodicity of the normal heartbeat
in response to changing demands of the body and the operation of a variety of
feedback and control mechanisms. This normal operation of the heart can be
disturbed, leading to abnormal cardiac rhythms that are collectively called car-
diac arrhythmias. Cardiac arrhythmias are associated with abnormal initiation of
a wave of cardiac excitation, abnormal propagation of a wave of cardiac excita-
tion, or some combination of the two. However, despite the apparent simplicity,
cardiac arrhythmias can manifest themselves in a great many different ways, and
it is still not always possible to figure out the mechanism of an arrhythmia in
any given individual. In this essay, I address some of the advances that have
been made in understanding cardiac arrhythmias theoretically, and indicate sev-
eral directions in which future advances may lead to improved therapies.
In order to appreciate the origin of cardiac arrhythmias, it is first necessary
to have a rudimentary knowledge about the spread of the cardiac impulse in the
heart (1). The heart is composed of four chambers: the right and left atria, and
the right and left ventricles (Figure 1). The atria are electrically connected to
each other, but are insulated from the ventricles everywhere except in a small
region called the atrioventricular (AV) node. The ventricles are also electrically
connected to each other. The rhythm of the heart is set by the sinoatrial node
located in the right atrium, which acts as the pacemaker of the heart. From a
