28
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
Interpretation of Normal
and Abnormal Cardiac Rhythms
from the ECG
Normal
One important use of the ECG is to enable a
physician to evaluate abnormally slow, rapid,
or irregular cardiac rhythms. Atrial and ven-
tricular rates of depolarization can be deter-
mined from the frequency of P waves and
QRS complexes by recording a rhythm strip.
A rhythm strip is usually generated from a
single ECG lead (often lead II). In a normal
ECG (Fig.
2.14), a consistent, one-to-one
correspondence exists between P waves and
the QRS complex; that is, each P wave is fol-
lowed by a QRS complex. This correspond-
ence indicates that ventricular depolarization
is being triggered by atrial depolarization.
Under these normal conditions, the heart
is said to be in sinus rhythm, because the
SA node is controlling the cardiac rhythm.
Normal sinus rhythm can range from 60 to
100 beats/min. Although the term “beats” is
being used here, strictly speaking, the ECG
gives information only about the frequency of
electrical depolarizations. However, a depo-
larization usually results in contraction and
therefore a “beat.”
Abnormal rhythms (arrhythmias) can be
caused by abnormal formation of action poten-
tials. A sinus rate <60 beats/min is termed
sinus bradycardia. The resting sinus rhythm,
as previously described, is highly dependent
on vagal tone. Some people, especially highly
conditioned athletes, may have normal rest-
ing heart rates that are significantly <60 beats/
min. In other individuals, sinus bradycardia
may result from depressed SA nodal func-
tion. A sinus rate of 100 to 180 beats/min,
sinus tachycardia, is an abnormal condition
for a person at rest; however, it is a normal
response when a person exercises or becomes
excited.
In a normal ECG, a QRS complex follows
each P wave. Conditions exist, however, when
the frequency of P waves and QRS complexes
may be different (Fig. 2.14). For example,
atrial rate may become so high in atrial flut-
ter (250 to 350 beats/min) that not all of the
impulses are conducted through the AV node;
Atrial Flutter
Atrial Fibrillation
First-Degree AV Block
Second-Degree AV Block (2:1)
----------
-------------
------
Third-Degree AV Block
Premature Ventricular Complex
Ventricular Tachycardia
Ventricular Fibrillation
■ FIGURE 2.14 ECG examples of abnormal
rhythms.
AV,
atrioventricular.
therefore, the ventricular rate (as determined
by the frequency of QRS complexes) may
be less than half of the atrial rate. In atrial
fibrillation, the SA node does not trigger the
atrial depolarizations. Instead, depolarization
currents arise from many sites throughout
the
atria,
leading to uncoordinated,
low-
voltage, high-frequency depolarizations with
no discernable P waves. In this condition,
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