26
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
THE ELECTROCARDIOGRAM
The ECG is a crucial diagnostic tool in clini-
cal practice. It is especially useful in diagnos-
ing rhythm disturbances, changes in electrical
conduction, and myocardial ischemia and
infarction.
The remaining sections of this
chapter describe how the ECG is generated
and how it can be used to examine changes in
cardiac electrical activity.
ECG Tracing
As cardiac cells depolarize and repolarize,
electrical
currents
spread
throughout
the
body because the tissues surrounding the
heart are able to conduct electrical currents
generated by the heart. When these electri-
cal currents are measured by an array of elec-
trodes placed at specific locations on the body
surface, the recorded tracing is called an ECG
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Time (sec)
■ FIGURE 2.13 Components of the ECG trace. An enlargement of one of the repeating waveform units
in the rhythm strip shows the P wave, QRS complex, and T wave, which represent atrial depolarization,
ventricular depolarization, and ventricular repolarization, respectively. The PR interval represents the tim e
required for the depolarization wave to transverse the atria and the AV node; the QT interval represents
the period of ventricular depolarization and repolarization; and the ST segment is the isoelectric period
when the entire ventricle is depolarized. Each small square is 1
mm.
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