bundle branches, impulses travel along Purkinje
fibersto the rest of the ventricular myocardium and
bring about ventricular systole. The electrical activity
of the atria and ventricles is depicted by an electrocar-
diogram (ECG); this is discussed in Box 12–3:
Electrocardiogram.
If the SA node does not function properly, the AV
node will initiate the heartbeat, but at a slower rate (50
to 60 beats per minute). The AV bundle is also capa-
ble of generating the beat of the ventricles, but at a
much slower rate (15 to 40 beats per minute). This
may occur in certain kinds of heart disease in which
transmission of impulses from the atria to the ventri-
cles is blocked.
Arrhythmiasare irregular heartbeats; their effects
range from harmless to life-threatening. Nearly
everyone experiences heart palpitations(becoming
aware of an irregular beat) from time to time. These
are usually not serious and may be the result of
too much caffeine, nicotine, or alcohol. Much more
serious is ventricular fibrillation, a very rapid and
uncoordinated ventricular beat that is totally inef-282 The Heart
BOX12–3 ELECTROCARDIOGRAM
throughout the ventricular myocardium. The T
wave represents repolarization of the ventricles
(atrial repolarization does not appear as a separate
wave because it is masked by the QRS complex).
Detailed interpretation of abnormal ECGs is
beyond the scope of this book, but in general, the
length of each wave and the time intervals between
waves are noted. An ECG may be helpful in the
diagnosis of coronary atherosclerosis, which de-
prives the myocardium of oxygen, or of rheumatic
fever or other valve disorders that result in enlarge-
ment of a chamber of the heart and prolong a spe-
cific wave of an ECG. For example, the enlargement
of the left ventricle that is often a consequence of
hypertension may be indicated by an abnormal
QRS complex.A heartbeat is a series of electrical events, and the
electrical changes generated by the myocardium
can be recorded by placing electrodes on the body
surface. Such a recording is called an electrocar-
diogram (ECG)(see Fig. 12–6).
A typical ECG consists of three distinguishable
waves or deflections: the P wave, the QRS complex,
and the T wave. Each represents a specific electrical
event; all are shown in Fig. 12–6 in a normal ECG
tracing.
The P wave represents depolarization of the
atria, that is, the transmission of electrical impulses
from the SA node throughout the atrial myocar-
dium.
The QRS complex represents depolarization of
the ventricles as the electrical impulses spreadBOX12–4 ARRHYTHMIAS
rillating ventricles are not pumping, and cardiac
output decreases sharply.
Ventricular fibrillation may follow a non-fatal
heart attack (myocardial infarction). Damaged car-
diac muscle cells may not be able to maintain a nor-
mal state of polarization, and they depolarize
spontaneously and rapidly. From this ectopic focus,
impulses spread to other parts of the ventricular
myocardium in a rapid and haphazard pattern, and
the ventricles quiver rather than contract as a unit.
It is often possible to correct ventricular fibrilla-
tion with the use of an electrical defibrillator. This
instrument delivers an electric shock to the heart,
which causes the entire myocardium to depolarize
and contract, then relax. If the first part of the heart
to recover is the SA node (which usually has the
most rapid rate of contraction), a normal heartbeat
may be restored.Arrhythmias(also called dysrhythmias) are irreg-
ular heartbeats caused by damage to part of the
conduction pathway, or by an ectopic focus,
which is a beat generated in part of the
myocardium other than the SA node.
Flutteris a very rapid but fairly regular heart-
beat. In atrial flutter, the atria may contract up to
300 times per minute. Because atrial pumping is
not crucial, however, blood flow to the ventricles
may be maintained for a time, and flutter may not
be immediately life-threatening. Ventricular flutter
is usually only a brief transition between ventricular
tachycardia and fibrillation.
Fibrillation is very rapid and uncoordinated
contractions. Ventricular fibrillation is a medical
emergency that must be quickly corrected to pre-
vent death. Normal contraction of the ventricles is
necessary to pump blood into the arteries, but fib-