Page 402 - Cardiac Nursing

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378 P A R T III / Assessment of Heart Disease
I aVR V1 V4
II aVL V2 V5
III aVF V3 V6
A
I aVR V1 V4
II aVL V2 V5
III aVF V3 V6
B B
■ Figure 16-27 (A) Twelve-lead ECG of VT with RBBB morphology. Note monophasic r wave with taller
left rabbit ear in V 1 and QS complex in V 6 . The indeterminate QRS axis also favors VT. (B) Twelve-lead ECG
V
of VT with LBBB morphology. Note wide R waves in V 1 and V 2 , and qR pattern in V 6 .
separate pacemakers. Capture beats occur when, in the presence of Cycle Length Variations. Ashman’s phenomenon states that
AV dissociation, a supraventricular impulse manages to conduct a beat that terminates a short cycle after a long cycle tends to be
into the ventricles and “capture” them, resulting in a normally con- aberrantly conducted. Because the refractory period of the con-
ducted QRS complex. Thus, the presence of fusion or capture duction system varies with preceding cycle length, a beat that ter-
beats in a run of wide QRS tachycardia is diagnostic of VT, but, minates a long cycle has a long refractory period, causing the next
unfortunately, capture beats are rare and cannot be counted on to
make the diagnosis (Fig. 16-30).

■ Figure 16-28 The Netherlands clues (so named because these ■ Figure 16-29 The San Francisco clue (so named because these
clues originated in the Netherlands from research done by Brugada clues originated from research done in San Francisco by Drew and
and colleagues). In a wide QRS tachycardia, if no precordial lead dis- Scheinman). In wide QRS tachycardias of either right or LBBB mor-
plays an RS complex, or if any precordial lead displays an RS complex phology, if measurement from beginning of QRS to tallest peak or to
that measures greater than 100 milliseconds from onset to nadir, VT nadir of S wave is less than 50 milliseconds in V 6 or MCL 6 , aberra-
is the favored diagnosis. AV dissociation and the morphology clues fa- tion is favored. If the measurement is more than 70 milliseconds, VT
voring VT in V 1–2 and V 6 are also helpful. is favored.