Ganong's Review of Medical Physiology, 23rd Edition

498 SECTION VI Cardiovascular Physiology
aVF
FIGURE 30–11 Heart block.
syncope in whom carotid sinus stimulation produces pauses
of more than 3 s between heartbeats.
ECTOPIC FOCI OF EXCITATION
Normally, myocardial cells do not discharge spontaneously,
and the possibility of spontaneous discharge of the His bundle
and Purkinje system is low because the normal pacemaker discharge
of the SA node is more rapid than their rate of spontaneous
discharge. However, in abnormal conditions, the His–
Purkinje fibers or the myocardial fibers may discharge spontaneously.
In these conditions, increased automaticity of the
heart is said to be present. If an irritable ectopic focus discharges
once, the result is a beat that occurs before the expected
next normal beat and transiently interrupts the cardiac
rhythm (atrial, nodal, or ventricular extrasystole or premature
beat). If the focus discharges repetitively at a rate higher
than that of the SA node, it produces rapid, regular tachycardia
(atrial, ventricular, or nodal paroxysmal tachycardia or
atrial flutter).
REENTRY
PR = 0.16 s
Normal complex
Second-degree heart block
(2:1 heart block)
aVF
Complete heart block. Atrial rate, 107; ventricular rate, 43
A more common cause of paroxysmal arrhythmias is a defect in
conduction that permits a wave of excitation to propagate continuously
within a closed circuit (circus movement). For example,
if a transient block is present on one side of a portion of the
conducting system, the impulse can go down the other side. If
PR = 0.38 s
First-degree heart block
aVF
Second-degree heart block
(Wenckebach phenomenon)
V5 V6 Two V leads in left
bundle branch block
the block then wears off, the impulse may conduct in a retrograde
direction in the previously blocked side back to the origin
and then descend again, establishing a circus movement. An example
of this in a ring of tissue is shown in Figure 30–12. If the
reentry is in the AV node, the reentrant activity depolarizes the
atrium, and the resulting atrial beat is called an echo beat. In addition,
the reentrant activity in the node propagates back down
to the ventricle, producing paroxysmal nodal tachycardia. Circus
movements can also become established in the atrial or ventricular
muscle fibers. In individuals with an abnormal extra
bundle of conducting tissue connecting the atria to the ventricles
(bundle of Kent), the circus activity can pass in one direction
through the AV node and in the other direction through
the bundle, thus involving both the atria and the ventricles.
FIGURE 30–12 Depolarization of a ring of cardiac tissue.
Normally, the impulse spreads in both directions in the ring (left) and
the tissue immediately behind each branch of the impulse is refractory.
When a transient block occurs on one side (center), the impulse on
the other side goes around the ring, and if the transient block has now
worn off (right), the impulse passes this area and continues to circle
indefinitely (circus movement).