Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 30Origin of the Heartbeat & the Electrical Activity of the Heart 501

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 myocar-
dial infarcts is ventricular fibrillation.

LONG QT SYNDROME

An indication of vulnerability of the heart during repolariza-
tion is the fact that in patients in whom the QT interval is pro-
longed, cardiac repolarization is irregular and the incidence of
ventricular arrhythmias and sudden death increases. The syn-
drome can be caused by a number of different drugs, by elec-
trolyte abnormalities, and by myocardial ischemia. It can also
be congenital. Mutations of eight different genes have been re-
ported to cause the syndrome. Six cause reduced function of
various K+ channels by alterations in their structure; one in-
hibits a K+ channel by reducing the amount of the ankyrin iso-
form 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 indi-
viduals who are prone to attacks of paroxysmal atrial arrhyth-
mias 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 aber-
rant muscular or nodal tissue connection (bundle of Kent) be-
tween 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 nor-
mal QRS pattern, producing a short PR interval and a pro-
longed 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 prema-
ture 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 move-
ment 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.

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 atri-

oventricular pathways during fetal development.

Attacks of paroxysmal supraventricular tachycardia, usually

nodal tachycardia, are seen in individuals with short PR inter-

vals 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 con-

ducting 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 ec-

topic 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 cathe-

ter 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 ex-

act location of an ectopic focus or accessory bundle that is re-

sponsible for the production of reentry and supraventricular

tachycardia. The pathway can then be ablated by passing

FIGURE 30–16 Accelerated AV conduction. Top: Normal si-

nus 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.)

PJ