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SUMMARY The purpose of this study was to determine the site of conduction delay or block in situ during antegrade and retrograde functional right bundle branch block (FRBBB) and whether reentrant excitation can occur in both directions in the blocked bundle branch, depending on whether the site of block occurred proximally or distally. Studies were performed in 20 adult mongrel dogs during cardiopulmonary bypass, using direct endocardial extracellular bipolar recordings of activaation in the His-Purkinje system. In 20 dogs subjected to premature atrial stimulation, 30 sites of antegrade conduction delay or block were documented: 17 were in the proximal portion of the right bundle branch (RBB), seven were in the distal portion and six were diffusely delayed throughout. In nine dogs subjected to premature right ventricular stimulation, nine sites of retrograde conduction delay or block were noted: eight were in the distal portion of the RBB and one was in the proximal portion of the RBB.

ATRIAL TACHYCARDIA WITHOUT P WAVES/Zipes et al.pathologic anatomy is seemingly so varied, one would expect the physiology to be somewhat varied and that prognosis might be related to a number of factors hitherto unrecognized in these patients.It is our feeling that a number of potential risk factors have been recognized in this study -all requiring long-term follow-up to see which, if any, do indeed increase risk. The abnormalities identified in this study, most of which were totally unsuspected from routine ECG studies and which are suggested as possible risk factors include: 1) nocturnal bradycardia mostly due to junctional exit block, noted in 7/20; 2) tachyarrhythmias (12/20) and especially major tachyarrhythmias (3/20); and 3) unresponsive or "lazy" junctional pacemakers which did not vary discharge rate. Several patients had more than one of these potential risk factors but the only patient with all three suffered a serious syncopal attack. In order to determine whether or not these really are risk factors, we would recommend that follow-up of all patients with congenital heart block should include, in addition to a His study, electromagnetic ECG tape recordings at fairly frequent intervals. The meaning of these described abnormalities is still not sufficiently understood to change recommendations for insertion of permanent pacemakers. These indications, at present, are symptomatic block or evidence of distal block.

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