Wagner_Marriot's Practical Electrocardiography, 12e

V 1

F I G U R E 2 2 . 1 5 . A lead V1 rhythm strip from an elderly patient on digitalis therapy for conges- tive heart failure. Arrows indicate the varying PR intervals during the third and fourth cycles that prove the capacity for variable conduction times.

The need to consider the variability in AV conduction times to determine the location of an AV block is illustrated in Figure 22.15. There is normal sinus rhythm with second- degree AV block and RBB block. For the initial complete cardiac cycles, the RP intervals are constant (1.36 seconds) and the PR intervals are also constant (0.24 second). It is tempt- ing to locate the AV block below the AV node because the PR intervals do not vary and there is an obvious intraventricular conduction problem. However, the possibility of AV nodal block has not been eliminated because, with a constant RP interval, the AV node would be expected to conduct with a constant PR interval. Only when the conduction ratio changes from 2:1 (P waves 1 to 4) to 3:2 (P waves 5 to 7) is a change produced in the RP interval (from 1.36 to 0.56 seconds). This shorter RP interval is accompanied by a recip- rocally greater PR interval (from 0.24 to 0.36 second), identifying the AV node rather than the ventricular Purkinje system as the location of the AV block.

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SECTION III: Abnormal Rhythms