Wagner_Marriot's Practical Electrocardiography, 12e

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INFRANODAL (PURKINJE) BLOCK

PR PR

F I G U R E 2 2 . 1 6 . Lead II rhythm strips from a woman with recurrent presyncopal episodes (A) and another patient with an acute inferior myocardial infarction (B) . Brackets indicate the vari- able RP/constant PR pattern typical of type II AV block in A and the variable RP/variable PR pattern typical of type I AV block in B .

Although infranodal (i.e., occurring in the Purkinje system) block is much less common than AV nodal block, it is a much more serious condition. It is almost always preceded by a bundle-branch block pattern for the conducted beats, with the nonconducted beats resulting from intermittent block in the other bundle branch. 4,5 Continuous block in the other bundle branch results in syncope or heart failure if ventricular escape occurs and sudden death if there is no ventricular escape. Infranodal block is almost always due to bilateral bundle-branch block (level C in Fig. 22.1) rather than His-bundle block (level B in Fig. 22.1). First-degree AV block may or may not accompany the bundle-branch block, but there is usually no stable period of second-degree AV block. Infranodal block is typically characterized by a sudden progression from no AV block to third-degree (complete) AV block. Because it occurs in the distal part of the pacemaking and conduction system, the escape rhythm may be too slow or too unreliable to support adequate circulation of blood, thereby causing serious and even fatal clinical events. Unlike the cells in the AV node, those in the Purkinje system have an extremely short relative refractory period. Therefore, they either conduct at a particular speed or not at all. Infranodal block is characterized by a lack of lengthening of the PR interval preceding the nonconducted P wave and a lack of shortening of the PR interval in the following cycle. This is termed Mobitz type II or simply type II AV block. It should be diagnosed whenever there is sec- ond-degree AV block with a constant PR interval despite a change in the RP interval. Indeed, the distinction between type I and type II blocks does not require the presence of a noncon- ducted P wave and can therefore be made in the presence of first-degree AV block alone. The cardiac rhythm shown in Figure 22.16A should be comparedwith that in Figure 22.16B. The consistent 3:2 AV ratio provides varying RP intervals. However, in Figure 22.16A, the PR intervals remain constant at 0.20 second, in contrast with Figure 22.16B in which the varying PP intervals result in varying PR intervals. Therefore, the AV block producing the rhythm shown in Figure 22.16A is in a location that is incapable of varying its conduction time even when it receives impulses at varying intervals. The PR intervals are independent of, rather than reciprocal to, their associated RP intervals. This type II block in Figure 22.16A is indica- tive of an infranodal (Purkinje) site of failure of AV conduction, in contrast to type I block in Figure 22.16B, which is indicative of an AV nodal site.

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CHAPTER 22: Atrioventricular Block

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