Incidence, determinants and significance of fixed retrograde conduction in the region of the atrioventricular node. Evidence for retrograde atrioventricular nodal bypass tracts.

Of 104 consecutive patients studied in our laboratory with His bundle electrograms, atrial and ventricular pacing and the atrial and ventricular extrastimulus techniques, 18 patients in whom the existence and utilization of ventriculoatrial (V-A) bypass tracts were excluded demonstrated evidence for fixed and rapid retrograde conduction in the region of the atrioventricular node (A-V) as suggested by the following: (1) short (36 +/- 2 msec [mean +/- standard error of mean]) and constant retrograde H2-A2 intervals during retrograde refractory period studies; (2) significantly (P less than 0.025) better V-A than A-V conduction; (3) significantly (P less than 0.025) shorter retrograde functional refractory period of the V-A conducting system than of the A-V conduction system; and (4) the retrograde effective refractory period of the A=V nodal region was not attainable in any of the 18 patients. Fourteen of the 18 patients (77 percent) had a history of palpitations and 10 (51 percent) had documented paroxysmal supraventricular tachycardia; in 13 (72 percent) single echoes or sustained reentrant supraventricular tachycardia, or both, could be induced during atrial pacing or atrial premature stimulation studies, or both. During tachycardia all these 13 patients had a short (37 +/- 2.4 msec) and constant conduction time in the retrograde limb (H-Ae interval) of the reentrant circuit that was identical to the H2-A2 interval. In conclusion, fixed and rapid retrograde conduction in the region of the A-V node (1) is seen in approximately 17 percent of patients, (2) is associated with a large incidence of reentrant paroxysmal supraventricular tachycardia, and (3) suggests the presence of A-V nodal bypass tracts (intranodal or extranodal functioning in retrograde manner).

Effects of phenytoin on refractoriness and conduction in the human heart.

Using His bundle electrograms and the atrial (A2) and ventricular extrastimulus (V2) techniques, anterograde and retrograde refractory period studies were performed (in 9 and 12 patients, respectively) before and 10 min after intravenous infusion of phenytoin (DPH; mean plasma level, 17.3 micrograms/ml). DPH had no effect on the duration of the QRS complex or the H-V interval of the sinus beats; it had variable but insignificant effects on the sinus rates and the atrial, A-V nodal, and ventricular muscle refractoriness. With the use of the A2 technique, the effective refractory period (ERP) of the His-Purkinje system (HPS) could not be determined in any patient; the relative refractory period (RRP) of the HPS could be determined in 2/9 patients and shortened in both patients after DPH. With the use of the V2 technique, retrograde functional refractory period (FRP) and RRP of the HPS could be determined in all 12 patients and the retrograde ERP of the HPS in 7/12; DPH significantly shortened all these parameters (p less than 0.001, less than 0.001, and less than 0.005, respectively). Functional refractory period of the ventriculo-atrial conduction system (VACS) could be determined in 11/12 patients during control studies (the remaining one patient had complete ventriculo-atrial block). DPH significantly shortened the FRP of the VACS in those (4) patients (Group I) in whom it was determined primarily by the HPS (p less than 0.025), and had variable but insignificant effects on FRP of the VACS in the other seven patients (Group II) in whom it was determined almost exclusively by the A-V node. DPH significantly decreased the retrograde HPS conduction times of the premature impulses (V2H2 intervals) for the same coupling (V1V2) intervals (p less than 0.001). It is concluded that, in the human heart, DPH exerts its most important effects on the HPS where it significantly decreases refractoriness and enhances conduction of the premature impulses. This study also demonstrates that the V2 technique is far superior to the A2 technique for evaluating the effects of drugs on refractoriness and conduction in the HPS.

Effects of digitalis on ventricular myocardial and His-Purkinje refractoriness and reentry in man.

The effects of digitalis on retrograde conduction and refractoriness of the His-Purkinje system, ventricular myocardium and reentry within the His-Purkinje system were studied in 17 patients using the ventricular extrastimulus (V2) technique. Studies were performed, before and 30 minutes after intravenous administration of ouabain, 0.01 mg/kg. After treatment with ouabain, there was a significant decrease in the functional refractory period (266 +/- 19 to 254 +/- 18 msec, P less than 0.001), relative refractory period (253 +/- 17 to 240 +/- 16 msec, P less than 0.001) and effective refractory period (242 +/- 23 to 231 +/- 24 msec, P less than 0.005) of the ventricular muscle. In contrast, there was no significant change in retrograde His-Purkinje conduction and refractoriness. The phenomenon of reentry within the His-Purkinje system characterized by the reentrant beat (V3) at critical retrograde conduction delays in the His-Purkinje system (V2-H2) within a narrow range of V1-V2 intervals was seen in 10 of 17 patients. Ouabain increased and shifted to the left the zone of reentry within the His-Purkinje system in 7 of 10 patients (36 +/- 23 to 55 +/- 23 msec, P less than 0.001) and decreased it by 10 to 30 msec in the remaining 3 patients. The critical V2-H2 (186 +/- 29 to 193 +/- 27 msec, difference not significant [NS]) and V1-V2 (299 +/- 30 to 294 +/- 36 msec, NS) intervals for reentry did not significantly change after ouabain. However, the minimal V1-V2 intervals (266 +/- 26 to 253 +/- 25 msec, P less than 0.025) decreased significantly, whereas the maximal V2-H2 intervals (266 +/- 40 to 239 +/- 37 msec, P less than 0.01) increased significantly. Thus, in the intact human heart, digitalis (1) significantly decreased all measures of ventricular myocardial refractoriness, (2) had no significant effect on retrograde conduction and refractoriness of the His-Purkinje system, and (3) widened the zone of reentry within the His-Purkinje system due to shortening of the functional refractory period of the ventricular muscle with attainment of longer V2-H2 delays.

Study of the temporal effects on conduction and refractoriness of the His-Purkinje system in man.

Temporal effects on refractoriness within the His-Purkinje system (HPS) were studied in 14 patients in whom effective refractory period (ERP) of HPS could be determined, using His bundle electrograms, incremental atrial pacing, and atrial extrastimulus method. His-Purkinje conduction times (H-V interval) and relative (R), effective (E), and functional (F) refractory periods (RP) of HPS were measured during the control period and repeat measurements were made after a 30 minute interval. H-V intervals were unchanged from control in all patients. Although changes of the magnitude of 5 to 25 msec. in either direction from control values commonly occurred, on the average, no statistically significant changes were seen in RRP, ERP, or FRP of HPS. The results of the present study confirm the stability of refractoriness of HPS over a 30 minute period and provide baseline data for future investigations of cardiovascular drugs which act primarily on the distal part of the A-V conduction system.

Modification and abolition of re-entry within the His-Purkinje system in man by diphenylhydantoin.

The phenomenon of macrore-entry (Re) within the His-Purkinje system (HPS) was consistently observed in 10 of 19 patients during retrograde refractory period studies. Effects of intravenous infusion of diphenylhydantoin (DPH) on Re were studied in these 10 patients 10 minutes after completion of infusion (mean plasma level equal to 17.0 microgram/ml). Diphenylhydantoin modified determinants of Re in seven patients (group I) and abolished Re in the remaining three patients (group II). In group I, DPH shortened the critical V1 V2 from 310.0 +/- 30.5 to 292.9 +/- 25.6 msec (P less than 0.025) and critical V2 H2 intervals for Re from 201.4 +/- 18.4 to 185.0 +/- 13.8 msec (P greater than 0.05). In group II, DPH abolished Re in two of three patients by precluding attainment of critical V2 H2 intervals whereas Re was abolished in the remaining one patient despite attainment of critical V2 H2 intervals (vs control). For both groups, DPH significantly shortened functional and effective refractory periods of the HPS (P less than 0.001 and less than 0.01, respectively) without significantly affecting the effective refractory period of the ventricular muscle. Diphenylhydantoin either completely abolished or significantly shortened the retrograde gap zones in the HPS. It is concluded that diphenylhydantoin significantly shortens His-Purkinje system refractoriness, abolishing Re in the patients with higher degree of improvement in refractoriness.

Induction of iatrogenic electrocardiographic patterns during electrophysiologic studies.

Indwelling cardiac catheters by producing local mechanical stimulation or trauma can induce electrocardiographic (ECG) patterns which simulate known electrophysiologic phenomena. Catheter-induced ECG patterns were analyzed in 447 consecutive patients undergoing electrophysiologic studies. Iatrogenic nature of these patterns was suggested by 1) absence prior to placement of catheter; 2) sudden appearance with catheter placement and disappearance with catheter repositioning; 3) reoccurrence with remanipulation of catheters; and 4) simulation (in some cases) by programmed electrical stimulation from the catheter. Common catheter-induced patterns were 1) right bundle branch block (RBBB) lasting less than 24 hours occurred in 19 patients; 2) transient third degree atrioventricular block in His-Purkinje system developed in 3/13 patients with pre-existing left BBB; 3) catheter-induced ventricular pre-excitation which simulated ECG patterns of type B Wolff-Parkinson-White syndrome; 4) fortuitous synchronization of right ventricular excitation from the catheter, and left ventricular excitation from sinus beat resulted in normalization of the QRS complexes in 5/68 patients with pre-existing RBBB; 5) premature beats from the atria, right ventricle, and His bundle, which were common, resulted in complex ECG patterns. These iatrogenic ECG patterns must be identified in order to avoid errors in interpretation.

Ventricular septal motion and left ventriclular dimensions during abnormal ventricular activation.

To determine the effect of abnormal ventricular activation on ventricular septal motion, left ventricular endocardial motion and left ventricular dimensions, 12 patients with normal motion were studied with echocardiography during incremental pacing of the right ventricular apex, outflow and inflow regions. Three types of abnormal ventricular septal motion were seen: The type I pattern was characterized by an early rapid preejection posterior ventricular septal motion followed by another posterior systolic motion that lasted throughout ejection, both of which were associated with septal thickening. In the type II pattern an early rapid preejection posterior ventricular septal motion was followed by an anterior ejection motion; the latter was not accompanied by septal thickening. The type III pattern consisted of an early preejection posterior ventricular septal motion followed by a mid and late systolic posterior motion: the latter motion extended through diastole. During right ventricular apical pacing, 8 of 11 patients showed a type 1 pattern, 1 a type II pattern and 2 a normal septal motion. During right ventricular outflow pacing,seven of nine patients showed a type II pattern, one a type III pattern and one a type I pattern. During right ventricular inflow pacing, eight of nine patients showed a type II pattern and one a type III pattern. At faster pacing rates patterns of types I and III changed to a type II pattern (five patients). End-diastolic dimensions decreased significantly during incremental right ventricular pacing when compared with those during sinus rhythm. End-systolic dimensions decreased significantly only during right ventricular apical and outflow pacing at maximal rates. In the seven patients who had pacing from all three sites, the decrease in left ventricular dimensions did not significantly differ when the three pacing sites were compared. These findings suggest that (1) abnormal ventricular septal motion during right ventricular pacing (induced left bundle branch block patterns) is dependent on the sequence of ventricular activation; (2) ventricular septal motion during right ventricular outflow and inflow pacing is similar to that seen in spontaneous left bundle branch block, whereas the pattern of septal motion during right ventricular apical pacing is different from that of spontaneous left bundle branch block; and (3) changes in left ventricular dimension are dependent on ventricular pacing rate but independent of pacing site.

Mobitz type I atrioventricular block in the ventricular specialized conduction system.

A patient with a history of multiple syncopal episodes had electrocardiographic findings of Wenckebach type of second degree atrioventricular block and left bundle branch block. He was thought to have intermittent complete heart block. His bundle recordings demonstrated the unusual occurrence of Mobitz type I block localized to the ventricular specialized conduction system. Based on the findings of this case and those of previous case reports, it is recommended that electrophysiologic studies should be performed on all patients with Mobitz type I atrioventricular block who also have bundle branch block.