Nature of the gap phenomenon in man

The atrioventricular (AV) gap phenomenon occurs when the effective refractory period of a distal site is longer than the functional refractory period of a proximal site and when closely coupled stimuli are delayed enough at the proximal site to allow distal site recovery. According to previous studies, in type 1 gap, the distal site of block is distal to the His bundle (ventricular specialized conduction system) and the proximal site of block is in the AV node. In type 2 gap, both the proximal and the distal sites of conduction block are within the ventricular specialized conduction system. Using His bundle recordings and atrial extra-stimulus techniques in man, we observed three previously undescribed types of gaps between (1) the AV node (distal) and the atrium (proximal), (2) the His bundle (distal) and the AV node (proximal), and (3) the ventricular specialized conduction system or a bundle branch (distal) and the His bundle (proximal). The delays at the His bundle in the second and third types of gaps seen in this study were demonstrated as splitting of His bundle potentials. Gaps between the AV node or the His bundle and the ventricular specialized conduction system were more easily demonstrated at long cycle lengths, but gaps between the atrium and the AV node were more easily demonstrated at short cycle lengths. Therefore, the previous subdivision of gaps into two types is an oversimplification, because gaps can occur between multiple sites in the conduction system. The gap phenomenon may be potentiated by both long and short cycle lengths; long cycle lengths increase the effective refractory period of a distal site, e.g., the His bundle and the ventricular specialized conduction system, and the short cycle lengths decrease the functional refractory period of a proximal site, e.g., the atrium and the AV node.

Pathophysiologic correlations in two cases of split His bundle potentials

This is a pathophysiological correlation in two cases showing split His bundle potentials. The first case had a history of previous complete heart block and the electrophysiological studies revealed split His potentials with intact A-V conduction. Case two had split His potentials with complete heart block. Serial sections of the conduction system in both cases revealed calcific impingement on, and degenerative changes within the bundle of His with healthy bundle of His proximal and distal to the lesion.

The significance of second degree atrioventricular block and bundle branch block. Observations regarding site and type of block

His bundle (H) electrograms were recorded in 15 patients with second degree atrioventricular (A-V) block and bundle branch block and these patients were prospectively followed. Site of block was proximal to H in four (BPH), distal to H in nine (BDH), and undetermined in two (studied during 1:1 conduction). Surface electrocardiographic features were retrospectively examined to determine the value of these recordings in predicting the site of block. Patients with type I block, with or without type II or 2:1 block, had BPH. Patients with type II block, 2:1 block, or type II combined with 2:1 block had BDH. Heart failure was more common in those with BPH (three of four patients as compared to three of nine patients with BDH). Syncope developed more commonly in patients with BDH (six of nine patients) as compared to those with BPH (one of four patients). Permanent pacing was indicated in three of four patients with BPH, nine of nine patients with BDH, and one of two patients with block at undetermined site because of syncope or heart failure. Five of nine patients with BDH required pacemakers within ten days of initial admission.

Most patients with second degree A-V block and bundle branch block will need permanent pacing. In patients with 2° BDH, pacemakers are indicated whether or not symptoms are present because of high risk of syncope and potential risk of sudden death. In asymptomatic patients with 2° BPH, careful observation is indicated.

The effects of cycle length on cardiac refractory periods in man

The effects of pacing-induced changes in cycle length on the refractory periods of the atrium, A-V node and His-Purkinje system were studied in 24 patients using the extra stimulus technique. Refractory period determinations were made at two or more cycle lengths in all patients. Slopes relating cycle length and refractory periods were calculated using the least squares method.

Both the effective and functional refractory periods (ERP and FRP) of the atrium shortened with decreasing cycle lengths, with a mean slope of +0.155 and +0.129 respectively. A-V nodal ERP lengthened (mean slope, –0.177) while A-V nodal FRP shortened slightly (mean slope, +0.126). Bundle branch refractory periods as well as relative refractory periods of the His-Purkinje system also decreased, with mean slopes of +0.270 and +0.360, respectively. The ERP of the A-V node at any cycle length was related to the A-H at that cycle length ( r = +0.646, P < 0.001).

The responses of the human heart to changes in cycle length are generally similar to those previously described in the animal laboratory. Such information contributes to our understanding of electrocardiographic phenomena such as aberrant conduction.

Demonstration of dual A-V nodal pathways in patients with paroxysmal supraventricular tachycardia

Electrophysiological evidence suggestive of dual atrioventricular (A-V) nodal pathways is presented in two patients with normal P-R interval and reentrant paroxysmal supraventricular tachycardia (PSVT). His bundle recordings and atrial stimulation were used to obtain this electrophysiological evidence.

Refractory periods were measured with the atrial extra-stimulus technique. Plotting of H 1 -H 2 responses against A 1 -A 2 coupling intervals revealed that as A 1 -A 2 decreased, H 1 -H 2 decreased appropriately. At a critical A 1 -A 2 , a sudden marked increase in H 1 -H 2 occurred, suggesting failure of fast pathway, (defining the fast pathway effective refractory period ERP). Further shortening of A 1 -A 2 defined a second H 1 -H 2 curve. The longest A 1 -A 2 with no H 2 response was defined as the slow pathway ERP. Echo zones coincided with A 1 -A 2 intervals equal to or less than the fast pathway ERP.

These results provide the first electrophysiological demonstration of dual A-V nodal pathways in patients with normal P-R interval and PSVT, as manifest by dual A-V nodal conduction times and refractory periods. Antegrade failure of the fast pathway with subsequent availability for retrograde conduction could allow A-V nodal reentry. These findings provide a basis for reentrance in some patients with reentrant PSVT.