Atypical antidromic resetting during programmed extrastimulation of reentrant ventricular tachycardia

A patient with reentrant ventricular tachycardia exhibited both the orthodromic and antidromic resetting responses at a single intracardiac recording site during programmed extrastimulation of ventricular tachycardia. The transition from orthodromic to antidromic resetting with extrastimulation demonstrated a sudden shortening in conduction interval to an electrogram recording site and unexpected identical morphology of the spontaneous and captured electrograms at that site, indicating atypical antidromic resetting. This newly observed resetting phenomenon with programmed extrastimulation suggests that the fourth entrainment criterion with overdrive pacing may likely be demonstrated in an atypical form; that is, a sudden shortening in conduction interval to an electrogram recording site may occur without any significant change in the bipolar electrogram morphology at that site when overdrive pacing is performed during tachycardia from a single pacing site at two different constant rates.

Earlier activation of the distal than the proximal site of the coronary sinus may represent retrograde conduction through AV node: significance of recording of far distal coronary sinus

During retrograde conduction through an accessory pathway (AP) or the atrioventricular (AV) node, earlier activation of the distal recording site than a more proximal site of the coronary sinus (CS) generally indicates retrograde conduction via a distally located AP. Thus, after successful ablation of a left-sided AP, if the distal CS recording site is activated earlier than a more proximal site retrogradely, it is considered to suggest-in the absence of His-bundle recording or more frequently in the setting of poor recording of the low septal right atrial electrogram-a conduction via a second AP (located more distally), and not conduction via the AV node. Yet, we hypothesized that retrograde conduction through the AV node may activate the far distal site of the CS (CSD) earlier than a more proximal site, as the anterior atrial wavefront, coming retrogradely from the AV node and traveling along the anterior mitral annulus, could reach the CSD earlier than a more proximal site. To test this we studied 18 patients with intact retrograde conduction via the AV node, but without evidence of an AP. The CSD was recorded by means of a quadripolar catheter (interelectrode distance of 2-5 mm); retrograde activation sequence at the distal (CSD1-2) versus proximal (CSD3-4) bipolar recording site was determined during ventricular stimulation. In 12 of 18 patients the CSD1-2 recording site was activated 5-10 ms earlier than the CSD3-4 recording site, in 3 of 18 patients the CSD1-2 site was activated 5 ms later than the CSD3-4 site; in the remaining 3 patients both recording sites were depolarized simultaneously. The results indicate that the CSD was often depolarized earlier than a more proximal site by impulses that conducted to the atria retrogradely via the AV node while the quadripolar recording catheter was placed at the CSD. This observation, although not well documented previously, suggests that the sequence of retrograde atrial activation in the CS should be studied carefully in consideration of the actual location of the mapping catheter in order to correctly diagnose the presence or absence of conduction via an AP.

Bradycardia-dependent activation delay of the atrial tissue in a patient with sick sinus syndrome

Bradycardia-dependent block or activation delay of the atrial tissue has not been documented in the literature. A patient with sick sinus syndrome in whom bradycardia-dependent, as well as tachycardia-dependent, activation delay in the atria was elucidated is reported on. The phenomenon of bradycardia-dependent activation delay was reproducibly demonstrated, both during atrial extrastimulation and incremental atrial pacing from the high right atrium, in the setting of an abnormally prolonged activation interval from the distal stimulating electrodes to the proximal recording electrodes of a quadripolar catheter, which was localized in the high right atrial region. Supernormal conduction seemed to be an unlikely explanation for the observed phenomenon.

The response of atrioventricular junctional tissue to temperature

To determine the optimal temperature for catheter heat mapping without damaging cardiac tissue, we studied the electrophysiologic and histologic responses of the atrioventricular (AV) conduction system exposed to a specific range of temperatures. In 18 closed-chest dogs, an electrode catheter with a thermistor, tip was positioned transvenously at the AV junction. Radiofrequency current (RFC) was applied in incremental temperature steps until transient 2nd-degree AV block was induced. Catheter tip temperature (CTT) was measured at each step. RFC was immediately discontinued when AV block occurred. AV conduction was evaluated before and 4 weeks after the procedure. Acute transient 2nd-degree AV block was induced in 45 applications, during which the average CTT was 48.7 +/- 2.7 degrees C. In another 40 applications in which 2nd degree AV block was not induced, the average CTT was significantly lower [46.3 +/- 2.5 degrees C] (p < 0.001). Eleven of 16 dogs showed acute 2nd-degree AV block, but had normal AV conduction at 4 weeks (Group A). In the other 5 dogs, 1st-degree AV block was seen at 4 weeks (Group B). The lowest CTTs in Groups A and B were 45 and 49 degrees C, respectively. Histologic findings in 2 dogs from Group A revealed that 10-15% (by area) of the AV node was fibrotic. These findings suggest that the induction of fully reversible AV block can be achieved by titration of RFC, during the application of RFC to the AV junction. In conclusion, RF energy was used to produce a tip temperature of between 45 degrees C and 49 degrees C, which induced reversible and significant interruption of conduction of in tissue in the AV junction, and presumably also in target sites in clinical RF ablation.

Double ventricular responses during extrastimulation of atrioventricular nodal reentrant tachycardia

In patients with dual atrioventricular (AV) nodal pathways, double ventricular responses to a single atrial depolarization have been shown to occur, but virtually only during atrial pacing in sinus rhythm. We report on a patient with a slow-fast form of AV nodal reentrant tachycardia who exhibited double ventricular responses following extrastimulation during AV nodal reentrant tachycardia. The phenomenon of double ventricular responses during the tachycardia was demonstrated by extrastimulation from the proximal coronary sinus. Retrograde unidirectional block in the slow pathway, and an anterograde effective refractory period that was shorter in the fast pathway than that in the slow pathway, are suggested.

"Paradoxical" AH shortening caused by proximal coronary sinus stimulation during orthodromic reciprocating tachycardia

INTRODUCTION

During extrastimulation or entrainment of orthodromic atrioventricular (AV) reciprocating tachycardia (ORT), the atrium-His (AH) interval as measured at the His-bundle recording site is expected to lengthen due to extrastimulation-dependent or pacing rate-dependent slowing of AV nodal conduction by impulses that penetrate the tachycardia circuit. We report 6 patients in whom the AH interval "paradoxically" shortened during ORT in response to extrastimulation and rapid pacing from the proximal coronary sinus.

METHODS AND RESULTS

Accessory pathway location was right anterior (1 patient), right anteroseptal (1 patient), and left anterior (4 patients). Cycle length of ORT was stable (variation < or = 5 msec) and ranged from 325 to 410 msec. During ORT, extrastimulation and rapid pacing were performed from the proximal coronary sinus and the right atrium. Extrastimulation from the proximal coronary sinus late in diastole caused significant shortening of AH interval in all patients by a mean of 18 +/- 3 msec (range 15 to 20 msec). AH shortening was demonstrated without a change of either the timing or morphologic appearance of the low septal right atrium at the His-bundle recording site. This phenomenon was not observed during right atrial extrastimulation. Rapid pacing from the proximal coronary sinus at cycle lengths of 305 to 390 msec (i.e., 15 to 20 msec shorter than the cycle length of each ORT) again demonstrated shortening of AH interval in all patients by a mean of 15 +/- 3 msec (range 10 to 20 msec). By contrast, rapid pacing from the right atrium demonstrated classical AH prolongation at any paced cycle length.

CONCLUSION

AH shortening without a change of either the timing or morphologic appearance of the low septal right atrium at the His-bundle recording site confirms the existence of a distinct posterior atrial input to the AV node. In this setting low septal right atrial activation is not requisite for AV nodal conduction. Whether activation of the low septal right atrium is essential for, or contributes to, AV nodal conduction of atrial impulses from locations other than the proximal coronary sinus needs to be determined.

Nonuniformity of AH intervals during stimulation at different left atrial sites

Studies in humans have found left atrial stimulation via the coronary sinus (CS) to elicit significantly shorter atrium-His (AH) intervals as compared to right atrial stimulation, but whether pacing at different left atrial sites (anterior vs posterior left atrium, i.e., far distal vs proximal CS) affects the AH interval has not been studied. Hence, in 22 patients, we compared the effects of stimulation from various atrial sites, including anterior high right atrium (HRA), distal CS, mid-CS, and proximal CS, on: stimulus-atrium (SA), AH, and stimulus-His intervals on the His bundle electrogram. Paced cycle length differed for each patient (range 900-350 msec, mean 532 +/- 140 msec), but conduction intervals from different atrial sites were compared using identical cycle length in each patient. The mean SA intervals were 34 +/- 10 msec, 57 +/- 10 msec, 44 +/- 11 msec, and 32 +/- 8 msec with stimulation, respectively, from HRA, distal CS, mid-CS, and proximal CS (each significantly different except for HRA vs proximal CS). The mean AH intervals were 123 +/- 23 msec, 104 +/- 28 msec, 95 +/- 15 msec, and 90 +/- 18 msec with stimulation, respectively, from HRA, distal CS, mid-CS, and proximal CS (each significantly different except for mid-CS vs proximal CS). In 13 patients, the discrepancy in AH intervals during distal versus proximal CS stimulation was > or = 15 msec; in 9 patients this difference was only < or = 10 msec, considered within the range of measurement error. Thus, in a significant portion of patients, discrepant AH intervals were demonstrated during stimulation from the distal versus proximal CS. These previously undescribed observations suggest that electrophysiological studies on atrioventricular nodal conduction that involve left atrial stimulation must take into account actual location of the stimulation site (anterior or posterior) in order to properly interpret the findings.

The relationship between early afterdepolarization and the occurrence of torsades de pointes--an in vivo canine model study

The relationship between early afterdepolarization (EAD) and the occurrence of torsades de pointes (TdP) was studied in a canine model. Twelve dogs of both sexes, weighing 9.9-16 Kg, were studied. After reducing the concentration of serum potassium to 3.0-4.0 mEq/l, by administration of calcium polystyrene sulfonate at 15-20 g/day for 1 or 2 weeks, a 6F electrode catheter was introduced via the femoral vein and positioned at the atrioventricular (AV) junction. Complete AV block was produced by catheter ablation using a high frequency current. A Franz 6F catheter was introduced into the right ventricle to record monophasic action potentials (MAPs) using the contact electrode technique. After a stable recording of the MAPs was achieved, cesium chloride (CsCl; 1 mM/Kg) was administered as an intravenous bolus over 15 sec. The MAPs and electrocardiogram (ECG) changes were simultaneously recorded for 30 min after the administration of CsCl. The administration was repeated several times at intervals 30 min. Sustained or non-sustained ventricular tachycardia was produced in all dogs. EAD appeared in 8 of 12 dogs. When EAD developed sufficiently high amplitude, ventricular premature beats occurred near the peak of EAD and TdP was induced in 3 of 8 EAD-positive dogs. TdP was not induced in EAD-negative dogs. Although TdP was comparatively difficult to induce, EAD-triggered activity was suggested to be one of the necessary conditions for TdP, because TdP occurred only when EAD reached a sufficiently high amplitude to produce ventricular premature beats.

Retrograde supernormal conduction, gap phenomenon in concealed accessory atrioventricular pathways

We present four patients with the Wolff-Parkinson-White syndrome who exhibited retrograde supernormal conduction or gap phenomenon in concealed accessory pathways. In the first patient, ventricular extrastimulus testing revealed retrograde block at the coupling interval of 520 msec and reappearance of conduction at the coupling interval of 370 msec. In a second patient, 1:1 retrograde conduction was not present but supernormal conduction was demonstrated at coupling intervals of 360 msec to 310 msec during the ventricular extrastimulus testing when the basic drive consisted of atrioventricular (AV) simultaneous pacing. In a third patient, ventricular extrastimulus testing demonstrated retrograde conduction through the accessory pathway only at coupling intervals of 400 msec to 360 msec. In a fourth patient, retrograde block occurred at the coupling interval of 340 msec and retrograde "slow" conduction reappeared at coupling intervals of 300 msec to 250 msec (gap phenomenon) only when the basic drive consisted of AV simultaneous pacing. Thus, concealed accessory pathways may exhibit retrograde supernormal conduction or gap phenomenon. Ventricular extrastimulus testing consisting of AV simultaneous pacing during the basic drive may facilitate demonstration of these unusual properties.

[Progress in clinical electrophysiologic study (EPS): recent tendency]

About twenty years have passed since the clinical introduction of electrophysiologic study (EPS). The mechanism of arrhythmias can be understood by EPS, and EPS-guided therapy is available theoretically. Of course, not all arrhythmia needs EPS, because other non-invasive tests are available. Clinically, reentry is the most frequently observed mechanism. Therefore, EPS is most suitable for the reentrant arrhythmias which can be repeatedly initiated and terminated by programmed stimulation. In many tachyarrhythmias, slow conduction pathway or area essential for reentry are demonstrated. On the other hand, triggered activity and abnormal automatism have been less frequently observed clinically. Today, application of EPS has been extended to catheter ablation for which precise mapping is required.

Atrioventricular nodal reentrant tachycardia with retrograde block induced by aprindine

Two patients are described who had atrioventricular nodal reentrant tachycardia (AVNRT) with 1:1 relationship in the control state, but in whom a varying degree of VA block during AVNRT was observed during therapy with aprindine. Aprindine, however, did not cause anterograde blockade of conduction over the slow AV nodal pathway during tachycardia. These observations support the conclusion that the bulk of atrial muscle is not a requisite part of the tachycardia circuit in AVNRT and that antiarrhythmic drugs may have disparate effects on conduction in the retrograde and anterograde limbs of the circuit.

Experimental study on the electrophysiological effects of the combination of the antiarrhythmic drugs aprindine and verapamil

The acute electrophysiological effects of the antiarrhythmic drugs aprindine and verapamil, injected intravenously either alone or in combination, were studied in 14 dogs during invasive electrophysiology. The AH and HV intervals during sinus rhythm were significantly prolonged, especially in the aprindine and the aprindine plus verapamil groups. The cycle lengths of the antegrade and retrograde atrio-ventricular block were most prolonged in the combination group. The effective refractory period and the functional refractory period of the atrial tissue, as well as the functional refractory period of the atrio-ventricular node, the effective refractory period of ventricular tissue and the ventriculo-atrial conduction system were most prolonged when the combination of the agents was given. The effective refractory period of the atrio-ventricular node was prolonged in the groups receiving verapamil and verapamil plus aprindine. There was no significant difference in the serum concentration of each agent given alone or in combination. These results suggest that the efficacy of the combination of verapamil and aprindine may be due to additive or synergistic effects of these antiarrhythmic agents.

Demonstration of right and left atrial dissociation by atrial rapid pacing or extrastimulation during fast-slow (uncommon) form of atrioventricular nodal reentrant tachycardia

Some recent works suggest that extranodal atrial fibers may form part of the reentry circuit in the atrioventricular (AV) nodal reentrant tachycardia (AVNRT). This hypothesis is based on the fact that the perinodal dissection successfully abolished AVNRT while preserving intact AV conduction. Apart from the surgical success, the electrophysiological evidence supporting this hypothesis has not been demonstrated, especially in the uncommon (fast-slow) form of AVNRT. We present some electrophysiological evidence suggesting atrial participation in eight patients with the fast-slow form of AVNRT. During the tachycardia, rapid pacing or extrastimulation was done from the orifice of the coronary sinus (CS) and the right atrium (RA), while recording the electrograms of the CS and the low septal RA. In seven patients, right and left atrial dissociation was demonstrated during pacing from the RA, while in the remaining one this was demonstrated from the CS. The interatrial dissociation will be unlikely if the intranodal reentry circuit connects with the atria via a single upper common pathway. This suggests that the upper turnaround of the reentry circuit involves atrial tissue and that the extranodal accessory pathway with long conduction times may form the ascending limb of the circuit (atrionodal reentry). Alternatively, the reentry circuit is entirely intranodal and two or more connecting pathways are present between the atria and the circuit.

Combination therapy with aprindine and verapamil for paroxysmal supraventricular tachycardia as assessed by transesophageal atrial pacing

To assess the efficacy of combination therapy of aprindine (40 mg/day) and verapamil (160 mg/day), transesophageal programmed atrial stimulation was performed on 21 patients with paroxysmal supraventricular tachycardia (including 12 patients with atrioventricular nodal reentrant tachycardia and nine patients with atrioventricular reentrant tachycardia) under four conditions: a) control, b) aprindine alone, c) verapamil alone, and d) aprindine + verapamil.

RESULTS

a) Aprindine, verapamil, and aprindine + verapamil prevented paroxysmal supraventricular tachycardia induction in 2/21, 3/21, and 9/21 patients, respectively; b) aprindine + verapamil prolonged the cycle length of paroxysmal supraventricular tachycardia more than aprindine or verapamil alone; c) aprindine, verapamil, and aprindine + verapamil decreased the AV blocking rate by 15, 23, and 35 beats/min, respectively, in comparison with the control state; d) aprindine, verapamil, and aprindine + verapamil prolonged the effective refractory period of atrioventricular conduction system by 20, 34, and 76 msec, respectively, compared with the control state. In conclusion, aprindine + verapamil appear to be more effective than aprindine or verapamil alone in preventing paroxysmal supraventricular tachycardia with nodal reentry, but there was less benefit in those without nodal reentry (Wolff-Parkinson-White group).