Escape beats (atrial, junctional, or ventricular) may occasionally occur during the pause in the ventricular rhythm, and may obscure the diagnosis because they interrupt the group beating pattern ( Figure 8-22). The overall appearance of the rhythm demonstrates group beating (groups of beats separated by pauses) and is a distinguishing characteristic of Mobitz I. After each dropped beat the cycle repeats itself. The missing QRS complex (dropped beat) causes the ventricular rhythm to be irregular. This rhythm is reflected on the ECG by P waves that occur at regular intervals across the rhythm strip and PR intervals that progressively lengthen from beat to beat until a P wave appears that is not followed by a QRS complex, but instead by a pause. In Mobitz I, the sinus impulse is normally conducted to the AV node, but each successive impulse has increasing difficulty passing through the AV node, until finally an impulse does not pass through (isn’t conducted). This rhythm ( Figures 8-20, 8-21, 8-22 and 8-23 and Box 8-6) is characterized by a failure of some of the sinus impulses to be conducted to the ventricles. Second-degree AV block, type I is commonly known as Mobitz I or Wenckebach (for the early 20th century physician who discovered it). PJCs are less common than PACs or premature ventricular contractions (PVCs) (discussed in Chapter 9). Figure 8-4 shows a PJC with the P wave before the QRS complex Figure 8-5 shows a PJC with the P wave after the QRS complex and in Figure 8-6 the P wave is hidden within the QRS. The PR interval will be short (0.10 second or less). The inverted P waves will occur immediately before or after the QRS, or will be hidden within the QRS complex. Because atrial depolarization occurs in a retrograde fashion with the PJC, the P wave associated with the premature beat will be negative in lead II (a positive lead). Some differences exist, however, between the two premature beats. Like the premature atrial contraction (PAC), the premature junctional beat is characterized by a premature, abnormal P wave and a premature QRS complex that’s identical or similar to the QRS complex of the normally conducted beats, and is followed by a pause that is usually noncompensatory. A premature junctional contraction (PJC) ( Figures 8-3, 8-4, 8-5, 8-6, 8-7 and 8-8 and Box 8-1) is an early beat that originates in an ectopic pacemaker site in the AV junction.
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