Ganong's Review of Medical Physiology, 23rd Edition

ectopic foci or a circus movement. The fibrillating ventricles,
like the fibrillating atria, look like a quivering “bag of worms.”
Ventricular fibrillation can be produced by an electric shock
or an extrasystole during a critical interval, the vulnerable
period. The vulnerable period coincides in time with the
midportion of the T wave; that is, it occurs at a time when
some of the ventricular myocardium is depolarized, some is
incompletely repolarized, and some is completely repolarized.
These are excellent conditions in which to establish reentry
and a circus movement. The fibrillating ventricles cannot
pump blood effectively, and circulation of the blood stops.
Therefore, in the absence of emergency treatment, ventricular
fibrillation that lasts more than a few minutes is fatal. The
most frequent cause of sudden death in patients with myocardial
infarcts is ventricular fibrillation.
LONG QT SYNDROME
An indication of vulnerability of the heart during repolarization
is the fact that in patients in whom the QT interval is prolonged,
cardiac repolarization is irregular and the incidence of
ventricular arrhythmias and sudden death increases. The syndrome
can be caused by a number of different drugs, by electrolyte
abnormalities, and by myocardial ischemia. It can also
be congenital. Mutations of eight different genes have been reported
to cause the syndrome. Six cause reduced function of
various K + channels by alterations in their structure; one inhibits
a K + channel by reducing the amount of the ankyrin isoform
that links it to the cytoskeleton; and one increases the
function of the cardiac Na + channel.
ACCELERATED AV CONDUCTION
An interesting condition seen in some otherwise normal individuals
who are prone to attacks of paroxysmal atrial arrhythmias
is accelerated AV conduction (Wolff–Parkinson–White
syndrome). Normally, the only conducting pathway between
the atria and the ventricles is the AV node. Individuals with
Wolff–Parkinson–White syndrome have an additional aberrant
muscular or nodal tissue connection (bundle of Kent) between
the atria and ventricles. This conducts more rapidly than
the slowly conducting AV node, and one ventricle is excited
early. The manifestations of its activation merge with the normal
QRS pattern, producing a short PR interval and a prolonged
QRS deflection slurred on the upstroke (Figure 30–16),
with a normal interval between the start of the P wave and the
end of the QRS complex (“PJ interval”). The paroxysmal atrial
tachycardias seen in this syndrome often follow an atrial premature
beat. This beat conducts normally down the AV node but
spreads to the ventricular end of the aberrant bundle, and the
impulse is transmitted retrograde to the atrium. A circus movement
is thus established. Less commonly, an atrial premature
beat finds the AV node refractory but reaches the ventricles via
the bundle of Kent, setting up a circus movement in which the
impulse passes from the ventricles to the atria via the AV node.
CHAPTER 30 Origin of the Heartbeat & the Electrical Activity of the Heart 501
P J
FIGURE 30–16 Accelerated AV conduction. Top: Normal sinus
beat. Middle: Short PR interval; wide, slurred QRS complex; normal
PJ interval (Wolff–Parkinson–White syndrome). Bottom: Short PR
interval, normal QRS complex (Lown–Ganong–Levine syndrome).
(Reproduced with permission from Goldschlager N, Goldman MJ: Principles of Clinical
Electrocardiography, 13th ed. Originally published by Appleton & Lange. Copyright ©
1989 by McGraw-Hill.)
In some instances, the Wolff–Parkinson–White syndrome
is familial. In two such families, there is a mutation in a gene
that codes for an AMP-activated protein kinase. Presumably,
this kinase is normally involved in suppressing abnormal atrioventricular
pathways during fetal development.
Attacks of paroxysmal supraventricular tachycardia, usually
nodal tachycardia, are seen in individuals with short PR intervals
and normal QRS complexes (Lown–Ganong–Levine
syndrome). In this condition, depolarization presumably
passes from the atria to the ventricles via an aberrant bundle
that bypasses the AV node but enters the intraventricular conducting
system distal to the node.
ANTIARRHYTHMIC DRUGS
Many different drugs have been developed that are used in the
treatment of arrhythmias because they slow conduction in the
conduction system and the myocardium. This depresses ectopic
activity and reduces the discrepancy between normal
and reentrant paths so that reentry does not occur. However,
it has now become clear that in some patients any of these
drugs can be proarrhythmic rather than antiarrhythmic—
that is, they can also cause various arrhythmias. Therefore,
they are increasingly being replaced by radiofrequency catheter
ablation for the treatment of arrhythmias.
RADIOFREQUENCY CATHETER
ABLATION OF REENTRANT PATHWAYS
Catheters with electrodes at the tip can now be inserted into the
chambers of the heart and its environs and used to map the exact
location of an ectopic focus or accessory bundle that is responsible
for the production of reentry and supraventricular
tachycardia. The pathway can then be ablated by passing