Excitable gap in canine fibrillating ventricular myocardium: effect of subacute and chronic myocardial infarction

Interaction of phase singularities on the spiral wave tail: reconsideration of capturing the excitable gap

The action mechanism of stimulation toward spiral waves (SWs) owing to the complex excitation patterns that occur just after point stimulation has not yet been experimentally clarified. This study sought to test our hypothesis that the effect of capturing excitable gap of SWs by stimulation can also be explained as the interaction of original phase singularity (PS) and PSs induced by the stimulation on the wave tail (WT) of the original SW. Phase variance analysis was used to quantitatively analyze the postshock PS trajectories. In a two-dimensional subepicardial layer of Langendorff-perfused rabbit hearts, optical mapping was used to record the excitation pattern during stimulation. After a SW was induced by S1–S2 shock, single biphasic point stimulation S3 was applied. In 70 of the S1-S2-S3 stimulation episodes applied on 6 hearts, the original PS was clearly observed just before the S3 point stimulation in 37 episodes. Pairwise PSs were newly induced by the S3 in 20 episodes. The original PS collided with the newly induced PSs in 16 episodes; otherwise, they did not interact with the original PS. SW shift occurred most efficiently when the S3 shock was applied at the relative refractory period, and PS shifted in the direction of the WT. In conclusion, quantitative tracking of PS clarified that stimulation in desirable conditions induces pairwise PSs on WT and that the collision of PSs causes SW shift along the WT. The results of this study indicate the importance of the interaction of shock-induced excitation with the WT for effective stimulation.

NEW & NOTEWORTHY The quantitative analysis of spiral wave dynamics during stimulation clarified the action mechanism of capturing the excitable gap, i.e., the induction of pairwise phase singularities on the wave tail and spiral wave shift along the wave tail as a result of these interactions. The importance of the wave tail for effective stimulation was revealed.

Capture of activation during ventricular arrhythmia using distributed stimulation

Results of previous studies suggest that pacing strength stimuli can capture activation during ventricular arrhythmia locally near pacing sites. The existence of spatio-temporal distribution of excitable gap during arrhythmia suggests that multiple and timed stimuli delivered over a region may permit capture over larger areas.

OBJECTIVE OF THE STUDY

Our objective in this study was to evaluate the efficacy of using spatially distributed pacing (DP) to capture activation during ventricular arrhythmia.

METHODS

Data were obtained from rabbit hearts which were placed against a lattice of parallel wires through which biphasic pacing stimuli were delivered. Electrical activity was recorded optically. Pacing stimuli were delivered in sequence through the parallel wires starting with the wire closest to the apex and ending with one closest to the base. Inter-stimulus delay was based on conduction velocity. Time-frequency analysis of optical signals was used to determine variability in activation. A decrease in standard deviation of dominant frequencies of activation from a grid of locations that spanned the captured area and a concurrence with paced frequency were used as an index of capture.

RESULTS

Results from five animals showed that the average standard deviation decreased from 0.81 Hz during arrhythmia to 0.66 Hz during DP at pacing cycle length of 125 ms (p = 0.03) reflecting decreased spatio-temporal variability in activation during DP. Results of time-frequency analysis during these pacing trials showed agreement between activation and paced frequencies.

CONCLUSIONS

These results show that spatially distributed and timed stimulation can be used to modify and capture activation during ventricular arrhythmia.

Optical recording-guided pacing to create functional line of block during ventricular fibrillation

Low-energy defibrillation is very desirable in cardiac rhythm management. We previously reported that ventricular fibrillation (VF) can be synchronized with a novel synchronized pacing technique (SyncP) using low-energy pacing pulses. This study sought to create a line of block during VF using SyncP. SyncP was performed in six isolated rabbit hearts during VF using optical recording to control the delivery of pacing pulses in real time. Four pacing electrodes with interelectrode distances of 5 mm were configured in a line along and across the myocardial fiber direction. The electrodes were controlled independently (independent mode) or fired together (simultaneous mode). Significant wavefront synchronization was observed along the electrode line as indicated by a decrease in variance. With the independent SyncP protocol, the decrease in the variance was 19.3 and 13.7% (P<0.001) for the along-, and across-fiber configurations, respectively. With the simultaneous SyncP protocol, the variance was reduced by 24.2 and 10.7% (P<0.001) in the along- and across-fiber configurations. The effect of synchronization dropped off with distance from the line of pacing. We conclude that SyncP can effectively create a line of functional block that isolates regions of VF propagation. Further optimization of this technique may prove useful for low-energy ventricular defibrillation.

Method of post-shock synchronized pacing in the excitable gaps

Ventricular fibrillation (VF) can be synchronized with a novel synchronized pacing technique (SyncP) using low-energy pacing pulses, which causes pace termination of VF. Synchronized pacing (SyncP) is defined as optical recording guided real-time detection and stimulation of spatiotemporal excitable gaps. In this paper, we investigate the effect of post-shock SyncP strategy on improvement of defibrillation efficacy. After a near-threshold defibrillation shock, when the reference site detected the earliest activation of the reinitiated VF, a 5-mA electric stimulus was delivered from the post-shock pacing electrode to depolarize the excitable gap. This area of wavefront synchronization may lead to a change in the timing of VF propagation, which is important for VF termination. Here, we implemented the concept of post-shock synchronized pacing by a real-time feedback mechanism and demonstrated a successful VF termination by the post-shock SyncP strategy. Further optimization of this technique may prove effective in improving the defibrillation efficacy for low-energy ventricular defibrillation.

Mechanisms of ventricular fibrillation in canine models of congestive heart failure and ischemia assessed by in vivo noncontact mapping

BACKGROUND

Much of the research performed studying the mechanism of ventricular fibrillation (VF) has been in normal ventricles rather than under a pathological condition predisposing to VF. We hypothesized that different ventricular substrates would alter the mechanism and characteristics of VF.

METHODS AND RESULTS

Three groups of dogs were studied: (1) control (n=8), (2) pacing-induced congestive heart failure (n=7), and (3) acute ischemia produced by 30 minutes of mid left anterior descending artery ligation (n=5). A noncontact mapping catheter (Ensite 3000, ESI) was placed via transseptal into the left ventricle (LV), along with an electrophysiology catheter. A multielectrode basket catheter (EP Technologies) was placed in the right ventricle, along with an electrophysiology catheter. Several episodes of VF were recorded in each animal. In addition to constructing isopotential and isochronal maps of the VF episodes, signals underwent frequency domain analysis as a fast Fourier transform was performed over a 2-second window every 1 second. From the fast Fourier transform, the dominant frequency was determined, and the organization was calculated. In control dogs, meandering, reentrant spiral wave activity was the main feature of the VF. The congestive heart failure group showed evidence of a stable rotor (n=3), evidence of a focal source (n=3), or no evidence of a driver in the LV (n=1). The ischemic group showed evidence of an initial focal mechanism that transitioned into reentry. In the control and ischemic groups, the LV always had higher dominant frequencies than the right ventricle.

CONCLUSIONS

Different ventricular substrates produced by the different animal models altered the characteristics of VF. Thus, different mechanisms of VF may be present in the LV, depending on the animal model.

Is there a correlation between ventricular fibrillation cycle length and electrophysiological and anatomic properties of the canine left ventricle?

We hypothesized that myocardial infarction-related alterations in ventricular fibrillation (VF) cycle length (VFCL) would correlate with changes in local cardiac electrophysiological and anatomic properties. An electrophysiological study was performed in normal, subacute, and chronic infarction mongrel dogs. VF was induced by programmed electrical stimulation and mean and minimum early and late VFCL was determined and correlated with local electrophysiological and anatomic properties. Effective refractory period (ERP), activation recovery time (ART), ERP/ART ratio, threshold, and ERP and ART dispersion were determined at 112 sites on the anterior left ventricle. Wave front progression was analyzed over a 2-s period. The extent of local tissue necrosis and of myocardial fiber disarray was also evaluated. The early mean VFCL was significantly longer in the subacute infarction (149 +/- 35 ms) and chronic infarction dogs (129 +/- 18 ms) compared with control dogs (102 +/- 15 ms; P < 0.0001 for both comparisons) as was the early minimum VFCL with similar trends seen during late VF. Complete epicardial reentrant circuits were significantly more common in normal dogs (4.3 +/- 2.4, 22.4% of cycles) than in subacute (0.75 +/- 0.96, 5.3% of cycles, P < 0.05 vs. normal) and chronic infarction dogs (1.3 +/- 1.3, 7.5% of cycles, P < 0.05 vs. normal). There was a poor correlation between the mean and minimum early and late VFCL and local electrophysiological and anatomic properties (R(2) < 0.2 for all comparisons) with a much better correlation between average mean and minimum VFCL (over the entire plaque) and global ERP and ART dispersion during early and late VF. In conclusion, VFCL in normal and infarcted myocardium shows a poor correlation with local ventricular electrophysiological and anatomic properties measured in sinus rhythm. However, there was a much better correlation between the average VFCL with global dispersion of repolarization. The lack of correlation between local VFCL and refractoriness and the infrequent occurrence of epicardial reentry suggests that intramural reentry may be the primary mechanism of VF in this model.

Improvement of Defibrillation Efficacy with Preshock Synchronized Pacing

INTRODUCTION

We previously demonstrated that wavefront synchronization by spatiotemporal excitable gap pacing (Sync P) is effective at facilitating spontaneous termination of ventricular fibrillation (VF). Therefore, we hypothesized that a spatiotemporally controlled defibrillation (STCD) strategy using defibrillation shocks preceded by Sync P can improve defibrillation efficacy.

METHOD AND RESULTS

We explored the STCD effects in 13 isolated rabbit hearts. During VF, a low-voltage gradient (LVG) area was synchronized by Sync P for 0.92 second. For Sync P, optical action potentials (OAPs) adjacent to four pacing electrodes (10 mm apart) were monitored. When one of the electrodes was in the excitable gap, a 5-mA current was administered from all electrodes. A shock was delivered 23 ms after the excitable gap when the LVG area was unexcitable. The effects of STCD was compared to random shocks (C) by evaluating the defibrillation threshold 50% (DFT(50); n = 35 for each) and preshock coupling intervals (n = 208 for STCD, n = 172 for C). Results were as follows. (1) Sync P caused wavefront synchronization as indicated by a decreased number of phase singularity points (P < 0.0001) and reduced spatial dispersion of VF cycle length (P < 0.01). (2) STCD decreased DFT(50) by 10.3% (P < 0.05). (3) The successful shocks showed shorter preshock coupling intervals (CI; P < 0.05) and a higher proportion of unexcitable shock at the LVG area (P < 0.001) than failed shocks. STCD showed shorter CIs (P < 0.05) and a higher unexcitable shock rate at LVG area (P < 0.05) than C.

CONCLUSION

STCD improves defibrillation efficacy by synchronizing VF activations and increasing probability of shock delivery to the unexcitable LVG area.

Synchronization of ventricular fibrillation with real-time feedback pacing: implication to low-energy defibrillation

Wavefront synchronization is an important aspect preceding the termination of ventricular fibrillation (VF). We evaluated the defibrillation efficacy of a novel multisite pacing algorithm using optical recording-guided synchronized pacing (SyncP) in the excitable gaps. We compared the effects of SyncP with traditional overdrive pacing (ODP) at 90% of the VF cycle length (VFCL) and high-frequency pacing (HFP; 43-215 Hz) on spontaneous VF termination in isolated rabbit hearts. For SyncP, the pacing current was triggered by the activation of a reference site and was delivered when the optical potential of the pacing site was in an excitable gap. We measured VFCL and the spatial dispersion of VFCL (SDCL) from five points (3 points in the paced area and 2 points in the nonpaced area) and the distribution of phase singularities during the prepacing, pacing, and postpacing periods. The results showed that 1) the VF termination rate of SyncP (16.0%, n = 106) was higher than that of ODP (2.1%, n = 48, P < 0.01) or HFP (1.6%, n = 129, P < 0.0001); 2) energy consumption for SyncP (7.6 +/- 9.3 mJ) was significantly lower than that of ODP (14.0 +/- 14.8 mJ, P < 0.0001); and 3) SyncP, but not ODP or HFP, decreased SDCL in the paced area during the pacing (P < 0.01) and postpacing (P < 0.05) periods compared with the prepacing period. We conclude that SyncP is effective in inducing wavefront synchronization and is more effective at facilitating spontaneous VF termination than non-SyncP.