Page 338 - Cardiac Nursing

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314 P A R T III / Assessment of Heart Disease
ead I
Le
Anterior fascicle I I I
Posterior
fascicle
Lead III
III III III I
B
A
I I I
■ Figure 15-21 (A) Normal conduction through left ventricle. Im-
pulse travels through both fascicles and depolarizes ventricle in supe-
rior, leftward, and inferior directions simultaneously as illustrated by
small arrows. Large arrow represents mean QRS axis. Lead I and lead
III usually show upright QRS. (B) Anterior fascicular block. Impulse
III III III I I
depolarizes left ventricle in downward and rightward direction first
through posterior fascicle (small arrows), then travels upwards and to
the left (large arrows), resulting in LAD, Q wave in lead I, and S wave
in lead III. (C) Posterior fascicular block. Impulse depolarizes left ven-
tricle in upward and leftward direction first through anterior fascicle
(small arrows), then travels downward and rightward (large arrows), re-
C sulting in RAD, S wave in lead I, and Q wave in lead III.

and can be due to a variety of causes other than ischemia. Other Myocardial injury is most often indicated by ST-segment ele-
indicators of ischemia include horizontal or downsloping ST- vation of 1 mm or more above the baseline in leads with positive
segment depression of 0.5 mm or more; an ST segment that re- electrodes facing the infracted area. Other signs of acute injury in-
mains on the baseline longer than 0.12 second; an ST segment clude a straightening of the ST segment that slopes up to the peak
that forms a sharp angle with the upright T wave; tall, wide-based of the T wave without spending any time on the baseline; tall,
T waves; and inverted U waves. 5–8,10–13 Display 15-1 lists several peaked T waves; and symmetric T-wave inversion 5–8,12–14 (Fig.
causes of ST-segment and T-wave changes. 15-26).