Atrioventricular Nodal Response to Retrograde Activation in Atrial Fibrillation

Ventricular rate smoothing for atrial fibrillation: a quantitative comparison study

AIMS

To quantitatively compare the ventricular rate-smoothing (VRS) effects of different ventricular pacing (VP) protocols for atrial fibrillation (AF).

METHODS AND RESULTS

Using a recently developed open-source model that can simulate the ventricular response in AF and VP, the performance of fixed-rate pacing and four previously published VRS algorithms were assessed by the mean RR (mRR), the root mean square of successive RR differences (RMSSD), the percentage of ventricular senses (VS%), and the percentage of short RR intervals (sRR%). All pacing protocols cause rate-dependent reduction of RMSSD, VS%, and sRR% with or without shortening of mRR compared to spontaneous AF. Fixed-rate pacing was more sensitive to the intrinsic rate than the VRS algorithms. The performance was generally comparable between different VRS algorithms, although higher mRR and VS% can be achieved at the expense of larger RMSSD and sRR%.

CONCLUSION

The effect of VP on ventricular rhythm in AF depends on both intrinsic rate and the aggressiveness of the pacing protocol. Adequate rate control is necessary for effective operation of the VRS algorithm. Choosing VRS algorithm should balance between the beneficial effects of rate regularization and the negative effects of increasing heart rate and percentage of VP.

On the role of ventricular conduction time in rate stabilization for atrial fibrillation

AIM

Ventricular pacing (VP) could stabilize the ventricular rhythm in atrial fibrillation (AF). This study investigates the role of ventricular conduction time (VCT) in rate stabilization for AF.

METHODS AND RESULTS

A novel computer model was used to generate various patterns of RR intervals in AF. For each model configuration, the rate stabilization effect of VP was compared with respect to different VCTs. In all tested cases, the ventricular rate in AF could be stabilized at pacing intervals longer than the shortest spontaneous RR intervals. For each model configuration, slightly longer pacing interval (difference <100 ms) was needed to achieve 95% VP when the antegrade/retrograde VCT was increased from 10/10 to 110/110 ms, whereas the VCT had less effect at lower pacing rate. Although longer VCT was associated with increased percentage of ventricular fusion, its role was diminished at higher pacing rate when more retrograde waves could conduct to the atrium.

CONCLUSION

Ventricular conduction time has limited effects on rate stabilization, which could be explained by multi-level interactions between antegrade waves induced by AF and retrograde waves induced by VP.

Computer modeling of ventricular rhythm during atrial fibrillation and ventricular pacing

We propose a unified atrial fibrillation (AF)-ventricular pacing (VP) (AF-VP) model to demonstrate the effects of VP on the ventricular rhythm during atrial fibrillation AF. In this model, the AV junction (AVJ) is treated as a lumped structure characterized by refractoriness and automaticity. Bombarded by random AF impulses, the AVJ can also be invaded by the VP-induced retrograde wave. The model includes bidirectional conduction delays in the AVJ and ventricle. Both refractory period and conduction delay of the AVJ are dependent upon its recovery time. The electrotonic modulation by blocked impulses is also considered in the model. Our simulations show that, with proper parameter settings, the present model can account for most principal statistical properties of the RR intervals during AF. We further demonstrate that the AV conduction property and the ventricular rate in AF depend on both AF rate and the degree of electrotonic modulation in the AVJ. Finally, we show that multilevel interactions between AF and VP can generate various patterns of ventricular rhythm that are consistent with previous experimental observations.

A randomized, prospective comparison of anterior and posterior approaches to atrioventricular junction modification of medically refractory atrial fibrillation

To compare the safety and efficacy of anterior versus posterior approach for atrioventricular (AV) junction modification, 40 patients with medically refractory paroxysmal (PAF) or chronic atrial fibrillation (AF) were randomly assigned to receive AV junction modification with an anterior or posterior approach. If the ablation session had taken more than 1 hour without success, the alternative ablation approach was attempted. Among the 18 patients assigned to receive the anterior approach, 14 (78%) had a primary success. One (5%) patient had complete AV block after ablation. Three patients crossed over to the posterior approach and had a successful outcome. Fourteen (64%) of 22 patients initially treated with the posterior approach had primary success. One (4%) patient developed complete AV block. Seven patients crossed over to the anterior approach and had a successful outcome. The primary success rate (14/18 vs 14/22, P = NS), incidence of transient AV block (3/18 vs 3/22, P = NS), and complete AV block (1/18 vs 1/22, P = NS) were similar between the anterior approach and posterior approach. The major differences between the two groups showed more radiofrequency pulses (10 +/- 4 vs 6 +/- 3 pulses, P < 0.01), longer procedure duration (50 +/- 24 vs 28 +/- 18 minutes, P < 0.01), and longer fluoroscopy exposure time (28 +/- 17 vs 16 +/- 8 minutes, P < 0.01) in the patients who had primary success with the posterior approach. In conclusion, this study demonstrated that (1) the two techniques had similar efficacies; (2) if one approach was ineffective, switching to the other approach might be safe; (3) combining these two approaches resulted in overall improvement in the success rate of this procedure, and (4) the posterior approach needed more radiofrequency pulses, longer procedural time, and longer fluoroscopy exposure time.

Electrophysiological mechanisms in successful radiofrequency catheter modification of atrioventricular junction for patients with medically refractory paroxysmal atrial fibrillation

BACKGROUND

Mechanisms and changes of electrophysiological (EP) characteristics in successful radiofrequency (RF) modification of right midseptal and posteroseptal areas for controlling rapid ventricular response to atrial fibrillation (Af) are not clear.

METHODS AND RESULTS

We studied 50 patients with medically refractory paroxysmal Af. Group 1 consisted of 40 patients without dual atrioventricular (AV) node physiology with modification sites located in the mid/posteroseptal area. Of the 40 patients, 36 had successful modification (follow-up of 14 +/- 8 months), and 3 had AV block. Late follow-up electrophysiological study (98 +/- 10 days) showed pattern 1 (67%) with prolongation of AV node effective refractory period (ERP, > or =40 milliseconds) and Wenckebach block cycle length (WBCL, > or =40 milliseconds); pattern 2 (22%) with prolongation of AH interval (> or =20 milliseconds), ERP, and WBCL; and pattern 3 (11%) without any change in AV node conduction parameter. Change in ventricular rate negatively correlated with change of WBCL in patterns 1 (r=-.691, P=.019) and 2 (r=-.90, P=.01). Group 2 consisted of 10 patients with dual AV node pathway; elimination of slow pathway property was performed. Late follow-up electrophysiological study (92+/-7 days) showed that change in ventricular rate negatively correlated with change in AV node ERP (r=-.926, P=.0001) and WBCL (r=-.969, P=.0001). Four patients without significant modification effect had success after RF energy was delivered to higher levels (follow-up, 15+/-7 months).

CONCLUSIONS

RF modification of right mid/posteroseptal area is feasible in 92% of patients with paroxysmal Af. Mechanisms of successful modification might be elimination of posterior input and/or partial injury of the compact node. Furthermore, simple elimination of slow pathway might be inadequate for control of ventricular rate in patients with little difference in conduction properties between fast and slow pathways.

Characterization of atrioventricular nodal behavior and ventricular response during atrial fibrillation before and after a selective slow-pathway ablation

BACKGROUND

The presence of atrioventricular nodal dual-pathway physiology in patients with atrioventricular nodal reentrant tachycardia (AVNRT) provides an opportunity to characterize the effect of a selective slow-pathway ablation on the ventricular rate during atrial fibrillation (AF). This may have important clinical implications for the nonpharmacological management of AF with a rapid ventricular rate.

METHODS AND RESULTS

Selective radiofrequency catheter ablation of the atrioventricular nodal slow pathway was performed with a stepwise approach in patients with documented sustained AVNRT. The AV nodal conduction properties and refractoriness and the ventricular rate during induced AF were assessed at baseline and under autonomic blockade before and after a selective slow-pathway ablation in 18 patients (mean age, 34 +/- 8 years). Sustained AVNRT was induced with a mean cycle length of 339 +/- 58 ms. A slow-pathway ablation was successfully achieved with 5 +/- 4 applications of radiofrequency energy. The shortest cycle length of 1:1 AV conduction and the AV nodal effective refractory period significantly prolonged after ablation (367 +/- 53 versus 403 +/- 55 ms, P < .0001, and 258 +/- 55 versus 292 +/- 74 ms, P < .05, respectively). Selective slow-pathway ablation significantly prolonged the mean (526 +/- 93 versus 612 +/- 107 ms, P < .0001), the shortest (378 +/- 59 versus 423 +/- 73 ms, P < .0001), and the longest (826 +/- 150 versus 969 +/- 226 ms, P < .01) cycle lengths of the ventricular response to AF. Significant slowing of the ventricular rate during AF occurred in 13 patients (72%), including all eight patients in whom AV nodal dual-pathway physiology was abolished. Five patients did not have a significant change in the ventricular rate during AF; a persistent dual AV nodal pathway physiology was demonstrable in four of these patients. Loss of dual-pathway physiology after ablation had a sensitivity of 77%, specificity of 80%, and positive predictive value of 91% for slowing the ventricular rate during AF.

CONCLUSIONS

In patients undergoing a slow-pathway ablation for control of AVNRT, selective slow-pathway ablation may cause a significant decrease in the ventricular rate during AF. These effects are primarily due to the prolongation of AV nodal conduction properties and refractory period of the residual AV nodal transmission system. These findings may have important therapeutic implications for the nonpharmacological treatment of AF, particularly in patients with underlying dual AV nodal physiology.