Type I second-degree AV block

Type I second-degree AV block with bundle branch block (which is far less common than narrow QRS type I block) must not be automatically labeled as AV nodal. Outside of acute myocardial infarction, type I block and bundle branch block (QRS > 0.12 s) occur in the His-Purkinje system in 60-70% of the cases (10) (Fig. 10.5). In such cases exercise is likely to aggravate the degree of AV block. Yet, many still believe that type I blocks are all AV nodal and therefore basically benign. It is believed that the prognosis of infranodal type I block is as serious as that of type II block and a permanent pacemaker... 

Fig. 10.5 Sinus rhythm with second-degree Type 13 2 iniranodal AV block, and RBBB. Note that the AH interval remains constant. TTie HV interval increases from 80 (following first P-wave) to 150ms (following second P-wave). The third P-wave is followed by an H deflection but no QRS complex. AV block occurs in the His-Purkinje system below the site of recording of the His bundle potential (arrow). Note the shorter PR interval after the nonconducted P-wave, a feature typical of Type I second-degree AV block. HBE = His bundle electrogram, A = atrial deflection, H = His bundle deflection, V = ventricular deflection, P = P-wave. TL = time lines 50ms. (Barold SS. Pacemaker treatment of bradycardias and selection of optimal pacing modes. In Zipes DP (Ed.). Contemporary Treatments in Cardiovascular Disease, 1997 1 123, with pamission.)...

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