Page 337 - Cardiac Nursing

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C HAPTER 1 5 / Electrocardiography 313
LBBB is recognized by a wide QS complex in V 1 and wide R small Q wave in lead III. The forces then travel inferiorly and
waves with no Q waves in V 6 , lead I, or aVL. Figure 15-20B rightward, causing a deep S wave in lead I and a tall R wave in lead
shows two commonly seen LBBB patterns, the most common be- III. Before diagnosing LPFB, the clinician must rule out RVH be-
ing the wide QS in lead V 1 , and a less common rS complex in V 1 . cause RVH can cause the identical frontal plane picture. Figure
The ECG in Figure 15-20C illustrates LBBB. 15-16B is an example of LPFB. The ECG characteristics of LPFB
are:
Fascicular Blocks 1. RAD (
100 degrees)
2. Small R in leads I and aVL, small Q in leads II, III, and aVF
The term fascicular block or hemiblock is used to describe block in (SI, QIII), or an rS pattern in leads I and aVL
either division of the left bundle branch. In fascicular block, both 3. Normal QRS duration (not 0.11 second)
ventricles depolarize simultaneously so the QRS remains narrow, 4. Increased QRS voltage in limb leads due to loss of cancellation
but the direction of left ventricular depolarization is altered. The of QRS forces
most useful ECG leads for recognizing fascicular block are leads I 5. No evidence of RVH
and aVF for the QRS axis, and leads I and III for the typical pat-
tern of fascicular block.
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Figure 15-17A illustrates the normal intraventricular conduc- Bifascicular Block
tion system and the relationship between the anterior and poste- Bifascicular block means that two of the three major fascicles are
rior divisions of the left bundle. When the left ventricular free wall blocked. Because block in both divisions of the left bundle branch
is activated normally, the anterior fascicle carries the electrical im- presents as complete LBBB, the term bifascicular block is usually
pulse in a superior and leftward direction, and the posterior fasci- used to refer to block in the right bundle branch along with block
cle carries it downward and rightward. Because free wall activation in either the anterior or posterior divisions of the left bundle
proceeds in both directions simultaneously, most of the forces can- branch. The ECG displays the typical RBBB morphology (wide
cel each other and result in the normal QRS shape seen in leads I QRS and rSR pattern, or one of its variants) along with an axis
and III and a normal QRS axis as the combined forces proceed deviation consistent with the fascicular block. Figure 15-22 is an
downward and leftward through the left ventricle (Fig. 15-21A). example of RBBB and LAFB. Figure 15-23 shows RBBB and
When fascicular block occurs, left ventricular activation proceeds LPFB.
from one site instead of both simultaneously, removing the can-
cellation and altering the shape of the QRS in leads I and III. Be-
cause the left ventricle is depolarized in an abnormal direction, an
axis deviation always results from fascicular block, but the QRS ACUTE CORONARY SYNDROME
duration remains normal or is very slightly prolonged.
Myocardial ischemia is the result of an imbalance between my-
Left Anterior Fascicular Block ocardial O 2 supply and demand and is a reversible process if blood
In LAFB (also called anterior hemiblock), the impulse conducts flow is restored before permanent cellular damage occurs. Is-
through the posterior fascicle and begins depolarizing the ventri- chemia can result from increased myocardial O 2 demands or from
cle in an inferior and rightward direction. It then travels through decreased myocardial O 2 supply. If ischemia is severe and blood
the left ventricular free wall in a superior and leftward direction, flow is not restored relatively soon, cellular injury and eventually
resulting in an LAD (Fig. 15-21B). The degree of LAD requiredB necrosis (cell death) result.
9
to diagnose LAFB is at least 45 degrees. The initial forces are The term acute coronary syndrome (ACS) is used to refer to the
directed inferiorly and rightward, causing a small Q wave in lead pathophysiologic continuum that begins with plaque rupture in a
I and a small R wave in lead III. The forces then travel superiorly coronary artery and ultimately results in permanent cell damage
and leftward, causing a normal R wave in lead I and an abnor- (infarction) if the process is not arrested (see Chapter 22). ACS en-
mally deep S wave in lead III. There may or may not be a Q wave compasses three distinct phases of this continuum: (1) unstable
in lead I, depending on whether initial septal activation is directed angina (UA), (2) non-ST elevation MI (NSTEMI), and (3) ST el-
to the left or to the right. Figure 15-16A is an example of LAFB. evation MI (STEMI). Once an infarction has occurred, as indicated
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The ECG characteristics of LAFB are: by elevated biochemical cardiac markers, it is classified electrocar-
diographically as either a Q-wave or a non–Q-wave MI based on
1. LAD ( 45 degrees or more) the presence or absence of Q waves on the ECG.
2. Small Q in lead I, large S in lead III (QI, SIII), or an rS pattern MI can occur because of blockage of a coronary artery with
in leads II, III, and aVF thrombus or from severe and prolonged ischemia due to coronary
3. QRS duration not prolonged more than 0.11 second artery spasm or unrelieved obstruction of a coronary artery. When
4. Increased QRS voltage in limb leads due to loss of cancellation infarction does occur, there are varying degrees of damage to cells
of forces in left ventricle involved in the process, ranging from ischemia to injury to cell
death. MI has traditionally been described as having three “zones”
Left Posterior Fascicular Block of tissue damage, each of which produces characteristic changes
In LPFB (also called posterior hemiblock), the impulse conducts on the ECG (Fig. 15-24). Although this drawing is an oversim-
through the anterior fascicle and begins depolarizing the ventricle plification of what actually happens, the concept is still useful in
in a superior and leftward direction. It then travels through the understanding the ECG changes that occur with MI.
left ventricular free wall in an inferior and rightward direction, re- Myocardial ischemia can result in several changes on the ECG
sulting in an RAD (Fig. 15-21C ). The initial forces are directed (Fig. 15-25). The most familiar pattern of ischemia is T-wave in-
superiorly and leftward, causing a small R wave in lead I and a version, although T-wave inversion is often a nonspecific finding