Characterization of return cycle responses predictive of successful pacing-mediated termination of ventricular tachycardia

Antitachycardia pacing at the His bundle is safer than conventional right ventricular antitachycardia pacing in a canine myocardial ischemic injury model

INTRODUCTION

Antitachycardia pacing (ATP) is used to terminate ventricular tachycardia (VT) by delivering rapid, low energy pacing to the right ventricle (RV). Unfortunately, ATP is not effective against all VT episodes and can result in adverse outcomes, such as VT acceleration and degeneration into ventricular fibrillation (VF). Improving ATP is therefore desirable. Our objective was to compare the efficacy and safety of ATP delivered at the His bundle to traditional ATP.

METHODS

Six dogs were anesthetized and pacing leads were implanted in the RV and His bundle. The left anterior descending artery was occluded for 2 h to create an ischemic injury. In a study 4-7 days later, a 128-electrode sock was placed snugly around the ventricles and VT was induced using rapid pacing. ATP was delivered from either the His bundle or RV lead, then attempted at the other location if unsuccessful. Success rates and instances of VT acceleration and degeneration into VF were calculated.

RESULTS

We induced 83 runs of VT and attempted ATP 128 times. RV ATP was successful in 36% of attempts; His ATP was successful in 38% of attempts. RV ATP resulted in significantly more adverse outcomes. RV and His ATP induced VT acceleration in 9% and 3% of trains, respectively, and induced degeneration into VF in 5% and 1% of trains, respectively.

CONCLUSION

His bundle ATP is safer, but not significantly more effective, than RV ATP.

Patient-by-patient basis anti-tachycardia pacing for fast ventricular tachycardia with structural heart diseases

BACKGROUND

Anti-tachycardia pacing (ATP) delivered from an implantable device is an important tool to terminate ventricular tachycardia (VT). But its real-world efficacy for fast VT has not been fully studied.

METHODS

Using the database of Nippon-storm study, effect of patient-by-patient basis ATP programming for fast VT (≥188 bpm) was assessed for the patients with structural heart diseases. Fast VTs were divided into three groups depending on heart rate (HR); Group A was 188-209 bpm, and Group-B and Group-C were 210-239 bpm and ≥240 bpm, respectively.

RESULTS

During a median follow-up of 28 months, 202 fast VT episodes (209 ± 19 bpm) were demonstrated in the 85 patients. ATP terminated 151 of the 202 episodes (74.8%) in total. The success rate of the ATP was not different among the three groups: 73.3% in Group A, 80.6% in Group B, and 66.7% in Group C. ATP success rate of >50% and >70% was 77.6% and 64.7% of the patients, respectively. Left ventricular ejection fraction (LVEF) was significantly higher in the patients with rather than without successful ATP therapy, and receiver operating characteristic (ROC) analysis revealed that LVEF of 23% was the optimal cut-off value. ATP was less effective in patients taking amiodarone, but etiology of the structural heart diseases, indication of the device implantation, and all Electrocardiogram (ECG) parameters were not useful predictors for successful ATP therapy.

CONCLUSIONS

ATP highly terminated fast VT with wide HR ranges in patients with structural heart diseases, and should be considered as the first-line therapy for fast VT except for patients with very low LVEF.

Mark E Josephson: Clinical Investigator

Mark E Josephson entered the world of clinical cardiac electrophysiology (EP) almost at its inception (1972); with so much to learn and so many directions one could take, he dived into the field with unbridled enthusiasm and an uncommon - perhaps almost unique - aptitude for asking questions and finding ways to answer them. Few aspects of EP escaped his indelible influence. In this short paper, I will attempt to touch on some of the high points of his astounding career as a clinical investigator.

Collision-based spiral acceleration in cardiac media: roles of wavefront curvature and excitable gap

We have previously shown in experimental cardiac cell monolayers that rapid point pacing can convert basic functional reentry (single spiral) into a stable multiwave spiral that activates the tissue at an accelerated rate. Here, our goal is to further elucidate the biophysical mechanisms of this rate acceleration without the potential confounding effects of microscopic tissue heterogeneities inherent to experimental preparations. We use computer simulations to show that, similar to experimental observations, single spirals can be converted by point stimuli into stable multiwave spirals. In multiwave spirals, individual waves collide, yielding regions with negative wavefront curvature. When a sufficient excitable gap is present and the negative-curvature regions are close to spiral tips, an electrotonic spread of excitatory currents from these regions propels each colliding spiral to rotate faster than the single spiral, causing an overall rate acceleration. As observed experimentally, the degree of rate acceleration increases with the number of colliding spiral waves. Conversely, if collision sites are far from spiral tips, excitatory currents have no effect on spiral rotation and multiple spirals rotate independently, without rate acceleration. Understanding the mechanisms of spiral rate acceleration may yield new strategies for preventing the transition from monomorphic tachycardia to polymorphic tachycardia and fibrillation.

Can early timed internal atrial defibrillation shocks reduce the atrial defibrillation threshold?

The defibrillation threshold is markedly reduced very early following the initiation of ventricular fibrillation. The purpose of this study was to determine if the same finding holds true for atrial defibrillation. Sustained, reproducible AF was induced with programmed atrial pacing using acetyl-beta-methylcholine chloride (40-640 microL/min) in six adult sheep (heart weight 245-300 g). Seven timing intervals (125 ms, 200 ms, 1 s, 3 s, 10 s, 30 s, and 5 min after AF induction) and two lead configurations: (1) RA as cathode and CS as anode; and (2) RA as cathode and RV apex as anode were tested. Single capacitor biphasic waveforms (3/1 ms) were delivered and atrial defibrillation thresholds (ADFTs) were determined in random order. No significant differences in leading edge voltage and total energy were detected for the RA-CS configuration for the seven timing intervals. For the RA-RV configuration, a significant difference was detected comparing the voltage for 125 ms to the 5-minute timing interval. For all times except 125 ms, the RA-RV threshold was significantly higher than the RA-CS level. In contrast to ventricular defibrillation, the ADFT does not change significantly within the first 5 minutes after the initiation of AF for the RA-CS configuration. However, if the shock is given very early (125 ms after AF induction) with the RA-RV configuration, the ADFT is lowered almost to the RA-CS level.

The effects of ventricular fibrillation duration and site of initiation on the defibrillation threshold during early ventricular fibrillation

OBJECTIVES

The purpose of this study was to determine if the defibrillation threshold (DFT) is lower during the first few cycles of ventricular fibrillation (VF) than after 10 s of VF and, if so, if the effect is caused by local or global factors.

BACKGROUND

The DFT may be low very early during VF because: (1) for the first few cycles VF arises from a localized region close to a defibrillation electrode where the shock field is strong (local factors), or (2) during early VF the effects of ischemia and sympathetic discharge have not yet fully developed and the heart has not yet completely dilated (global factors).

METHODS

Protocol 1 included seven pigs in which a defibrillation electrode and a pacing catheter were both placed in the right ventricular apex. VF was induced by delivering a high current premature stimulus from the pacing catheter that should have caused reentry confined to the right ventricular apex for the first few cycles of VF. A bipolar electrogram was recorded from the tip of the defibrillation catheter. Using a three reversal up-down protocol, the DFT was determined for biphasic shocks delivered after 1, 2, 3, 4, 5, 7, 10, 15, 20 and 25 activations in this electrogram and after 10 s (control). Protocol 2 included seven pigs undergoing the same procedure as in protocol 1 except that an additional pacing catheter was placed in the left ventricle. Defibrillation thresholds were determined after 1, 2, 3, 4 and 5 VF activations following VF induction from the right ventricle (RV) or the left ventricle (LV) and after 10 s (control).

RESULTS

In protocol 1, the mean +/- SD DFrs were lower during the first three cycles than after 10 s of VF (3.0 +/- 4.1 J for the first VF cycle vs 15.8 +/- 6.6 J after 10 s of VF, p < 0.05). In protocol 2, the DFF for the first few cycles of VF induced away from the defibrillation electrode in the LV (6.9 +/- 1.4 J for the first VF cycle) was significantly lower than that after 10 s of VF (16.0 +/- 2.2 J), whereas the DFF for the first few cycles induced near the defibrillation electrode in the right ventricular apex was significantly lower (2.3 +/- 2.7 J for the first VF cycle) than that induced from the LV.

CONCLUSIONS

This study demonstrates that the DFT is significantly lower during the first few VF cycles of VF than after 10 s of VF and that this decrease may be caused by both local factors and global factors. These results provide an impetus for exploring earlier shock delivery in implantable devices.

The variable contribution of functional and anatomic barriers in human ventricular tachycardia: an analysis with resetting from two sites

OBJECTIVES

This study sought to investigate the influence of stimulation site on the properties of the circuit in ventricular tachycardia.

BACKGROUND

A fully excitable gap can be demonstrated in most human ventricular tachycardias. This requires the presence of an arc of block so that the entire circuit can recover from refractoriness within the period of the cycle length. Resetting characterizes the conduction properties of the tissue within the ventricular tachycardia circuit. Previous studies have not investigated the possibility of site-dependent differences in the resetting response.

METHODS

Resetting was performed from the right ventricular apex and outflow tract in 23 patients. Two characteristics of the resetting response were analyzed: 1) the total duration of the flat portion, and 2) the slope of the increasing portion.

RESULTS

A flat portion of the resetting response was observed in 18 tachycardias; in 8 of the 18, there was a significant site-dependent difference (> or = 40 ms) in the duration of the flat portion. A significant site-dependent difference in the slope of the increasing portion of the resetting curve was seen in 6 of 22 tachycardias. In all, a stimulation site-dependent change in at least one characteristic of the resetting response was seen in 12 (52%) of the 23 tachycardias.

CONCLUSIONS

A stimulation site-dependent change in the flat portion of the resetting response is compatible with an arc of block that is at least partially functional in nature. A change in the slope of the increasing portion is compatible with either partially functional circuit barriers or variation in properties of conduction and refractoriness at different locations within the circuit, or both. These observations suggest that a spectrum of circuit properties may exist in humans, with a variable contribution of anatomic and functional characteristics.