Ganong's Review of Medical Physiology, 23rd Edition

498 SECTION VICardiovascular Physiology

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 dis-
charge of the SA node is more rapid than their rate of sponta-
neous discharge. However, in abnormal conditions, the His–
Purkinje fibers or the myocardial fibers may discharge spon-
taneously. In these conditions, increased automaticity of the
heart is said to be present. If an irritable ectopic focus dis-
charges once, the result is a beat that occurs before the expect-
ed next normal beat and transiently interrupts the cardiac
rhythm (atrial, nodal, or ventricular extrasystole or prema-
ture beat). If the focus discharges repetitively at a rate higher
than that of the SA node, it produces rapid, regular tachycar-
dia (atrial, ventricular, or nodal paroxysmal tachycardia or
atrial flutter).

REENTRY

A more common cause of paroxysmal arrhythmias is a defect in
conduction that permits a wave of excitation to propagate con-
tinuously within a closed circuit (circus movement). For exam-
ple, 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

the block then wears off, the impulse may conduct in a retro-

grade direction in the previously blocked side back to the origin

and then descend again, establishing a circus movement. An ex-

ample 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 ad-

dition, the reentrant activity in the node propagates back down

to the ventricle, producing paroxysmal nodal tachycardia. Cir-

cus movements can also become established in the atrial or ven-

tricular muscle fibers. In individuals with an abnormal extra

bundle of conducting tissue connecting the atria to the ventri-

cles (bundle of Kent), the circus activity can pass in one direc-

tion through the AV node and in the other direction through

the bundle, thus involving both the atria and the ventricles.

FIGURE 30–11 Heart block.

PR = 0.16 s

Normal complex

PR = 0.38 s

First-degree heart block

Second-degree heart block

(2:1 heart block)

Complete heart block. Atrial rate, 107; ventricular rate, 43

Second-degree heart block

(Wenckebach phenomenon)

aVF aVF

aVF

Two V leads in left

bundle branch block

V 6

V 5

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 refracto-

ry. 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).