Page 324 - Cardiac Nursing

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CHAPTER
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E E E E Electrocardiography
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Carol Jacobson
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Electrocardiographhy iis thhe gra hiic display of the changingg poten- 3 3. It screens out rapidd attriiaal iimpulses to protect the venttricles
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tials of the electrical field generated by the heart as recorded by fr from dangerously fastt ratess when the atrial rate is very rapid.
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electrodes placed on the body surface. Recording off he 12 l-lead
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el electrocardiogramm (ECG) is the mostt frequently used procedure e Bundle of His
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for the diagnosis of heart disease. It is noninvasive, safe, simple to The bundle of His iss a short bundlee of fibers att thee bottom of thhe
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perform, reprodu ible, a dnd r latiiv ly iinexpensive. The 12-lead AV node leading to the bundle branches. Conduction velocity ac-
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ECG can record changes indicative of primary myocardial disease celerates in the bundle of His, and the impulse is transmitted to
such as coronary artery disease, cardiomyopathy, hypertension, or both bundle branches.
infiltrative diseases. It can also reflect changes associated with elec-
trolyte abnormalities, metabolic disorders, drug effect, and other Bundle Branches
disease processes such as pulmonary embolism or pulmonary hy- The bundle branches are bundles of fibers that rapidly conduct
pertension, renal failure, and central nervous system disease. The the impulse into the right and left ventricles. The right bundle
ECG is the gold standard for noninvasive diagnosis of cardiac ar- branch travels along the right side of the interventricular septum
rhythmias and conduction abnormalities (see Chapter 16) and is and carries the impulse into the right ventricle. The left bundle
a useful tool in evaluating function of implanted devices such as branch has two main divisions, the anterior fascicle and the poste-
pacemakers and implantable cardioverter defibrillators. 1 rior fascicle, which carry the impulse into the left ventricle.
This chapter discusses the electrocardiographic features of var-
ious cardiac conditions and other disease processes that may cause Purkinje Fibers
changes on the ECG. Specific information on the pathophysiol- The Purkinje fibers are hairlike fibers that spread out from the
ogy and treatment of cardiac disease and other medical conditions bundle branches along the endocardial surface of both ventricles
that may affect the ECG can be found in other chapters in this and rapidly conduct the impulse to the ventricular muscle cells.
book or in medical textbooks. Cells in the Purkinje system have automaticity at a rate of 20 to
40 beats per minute and can function as a backup pacemaker if all
other pacemakers fail.
ELECTRICAL CONDUCTION
THROUGH THE HEART Origin and Spread of the Electrical
Impulse Through the Heart
The electrical impulse of the heart is the stimulus for cardiac con-
traction. The conduction system (Fig. 15-1) is responsible for the The impulse normally begins in the SA node, located in the high
initiation of the electrical impulse and its sequential spread right atrium, because the SA node has the fastest rate of automatic-
through the atria, atrioventricular (AV) junction, and ventricles. ity of all potential pacemaker cells in the heart. The impulse spreads
from the SA node through both atria in an inferior and leftward di-
The Cardiac Conduction System rection, resulting in depolarization of the atrial muscle. When the
impulse reaches the AV node, its conduction velocity is slowed be-
The conduction system of the heart consists of the following fore it continues into the ventricles. The slowing in the AV node is
structures.
necessary to allow time for the atria to contract and empty their
Sinus Node blood into the ventricles before the ventricles contract. The atrium’s
The sinus or sinoatrial (SA) node is a small group of cells in the high contribution to ventricular filling is referred to as “atrial kick.”
right atrium that functions as the normal pacemaker of the heart When the impulse emerges from the AV node, it travels rapidly
because it has the fastest rate of automaticity. The SA node normally through the bundle of His and down the right and left bundle
depolarizes between 60 and 100 times per minute. branches into the Purkinje network of both ventricles, and results in
depolarization of the ventricular muscle. The spread of this wave of
AV Node depolarization through the heart produces the classic surface ECG,
The AV node is a small group of cells in the low right atrium near which can be recorded by an electrocardiograph (ECG machine) or
the tricuspid valve. The AV node has three main functions: monitored continuously on a bedside cardiac monitor.
1. Its major job is to slow conduction of the impulse from the
atria to the ventricles to allow time for the atria to contract and Waves, Complexes, and Intervals
empty their blood into the ventricles. of the Cardiac Cycle
2. The area around the AV node (junction) has automaticity at an
impulse rate of 40 to 60 beats per minute and can function as The ECG waves, complexes, and intervals are illustrated in Figure
a backup pacemaker if the SA node fails. 15-2.
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