b. Electrical Activity In The Heart. The heart's electrical activity begins in the
sinoatrial (SA) node and flows toward the ventricles. See figure 3-2. All the areas of
this conduction system can do the following: initiate impulses; become irritable;
respond to an impulse. Impulses are initiated in each area of the conduction system as
SA node ...........60-100 per minute
AV Junction ......40-60 per minute
Ventricle ...........20-40 per minute
Figure 2-2. Electrical conduction system of the heart.
c. The Pacemaker Site. The common pacemaker site is the SA node because
it initiates electrical impulses at a faster rate than the junction or ventricle. An impulse
from the AV junction can take over if the SA node should fail. If the AV junction also
fails, the ventricle can take over. This is a protective backup system, a system which
helps the heart maintain electrical efficiency. Sometimes the junction or the ventricle
becomes irritable and starts impulses at a faster than normal rate which overrides the
SA node. When this happens, the pacemaker site that is the fastest dominates and
takes over control of the heart.