Wagner_Marriot's Practical Electrocardiography, 12e

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F I G U R E 2 2 . 1 1 . Three examples of atrial tachyarrhythmias with third-degree AV block and lower escape rhythms. In A and B , the QRS duration of 0.12 second indicates escape from the com- mon bundle, but in C, the QRS duration of 0.16 second indicates either escape from the common bundle accompanied by LBB block or escape from the RBB. Arrows indicate the varying P-QRS (A) and F-QRS (B) intervals, and asterisks indicate the constant ventricular rates in all three examples.

Differentiation between second- and third-degree AV block is accomplished by consid- ering the relationship among the ventricular waveforms in a series of cycles (RR intervals) and the relationship between the atrial and ventricular waveforms in each of these cycles (PR or flutter-R intervals). If the RR interval is irregular, some AV conduction can be as- sumed, and second-degree block is present. If the RR interval is regular, a constant PR or flutter-R interval indicates second-degree AV block, whereas a varying PR or flutter-R interval indicates third-degree block, with an escape rhythm generated by a lower site. Figure 22.11 presents examples of AV block occurring in the presence of three different atrial tachyarrhythmias: sinus tachycardia (see Fig. 22.11A), atrial flutter (see Fig. 22.11B), and atrial fibrillation (see Fig. 22.11C). Consecutive RR intervals are constant in all of the examples, at 2.84, 1.40, and 1.96 seconds, respectively. In Figure 22.11A and B, it is obvi- ous that there is third-degree AV block because the adjacent PR relationships in Figure 22.11A and flutter wave-R relationships in Figure 22.11B are quite variable. In Figure 22.11C, third-degree AV block can be assumed because the absence of regular atrial activ- ity in atrial fibrillation prohibits any constancy in AV conduction relationships.

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SECTION III: Abnormal Rhythms