AJP - Heart Journal of Applied Physiology
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Am J Physiol Heart Circ Physiol 291: H184-H192, 2006. First published February 24, 2006; doi:10.1152/ajpheart.01300.2005
0363-6135/06 $8.00
This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow All Versions of this Article:
291/1/H184    most recent
01300.2005v1
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (7)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Ripplinger, C. M.
Right arrow Articles by Efimov, I. R.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Ripplinger, C. M.
Right arrow Articles by Efimov, I. R.

Mechanisms of unpinning and termination of ventricular tachycardia

Crystal M. Ripplinger, Valentin I. Krinsky, Vladimir P. Nikolski, and Igor R. Efimov

Department of Biomedical Engineering, Washington University in St. Louis, St. Louis, Missouri

Submitted 8 December 2005 ; accepted in final form 8 February 2006


   ABSTRACT
TOP
 ABSTRACT
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
High-energy defibrillation shock is the only therapy for ventricular tachyarrhythmias. However, because of adverse side effects, lowering defibrillation energy is desirable. We investigated mechanisms of unpinning, destabilization, and termination of ventricular tachycardia (VT) by low-energy shocks in isolated rabbit right ventricular preparations (n = 22). Stable VT was initiated with burst pacing and was optically mapped. Monophasic "unpinning" shocks (10 ms) of different strengths were applied at various phases throughout the reentry cycle. In 8 of 22 preparations, antitachycardia pacing (ATP: 8–20 pulses, 50–105% of period, 0.8–10 mA) was also applied. Termination of reentry by ATP was achieved in only 5 of 8 preparations. Termination by unpinning occurred in all 22 preparations. Rayleigh's test showed a statistically significant unpinning phase window, during which reentry could be unpinned and subsequently terminated with E80 (magnitude at which 80% of reentries were unpinned) = 1.2 V/cm. All reentries were unpinned with field strengths ≤2.4 V/cm. Unpinning was achieved by inducing virtual electrode polarization and secondary sources of excitation at the core of reentry. Optical mapping revealed the mechanisms of phase-dependent unpinning of reentry. These results suggest that a 20-fold reduction in energy could be achieved compared with conventional high-energy defibrillation and that the unpinning method may be more effective than ATP for terminating stable, pinned reentry in this experimental model.

reentry; optical mapping; defibrillation; antitachycardia pacing

SUDDEN CARDIAC DEATH (SCD) resulting from cardiac arrest is the leading cause of death in the industrialized world and is normally attributed to ventricular arrhythmias. Up to 90% of ventricular arrhythmias can start as ventricular tachycardia (VT) before degenerating into ventricular fibrillation (VF) (40). A number of clinical trials have demonstrated the success of defibrillation therapy and the superiority of implantable cardioverter-defibrillator (ICD) therapy over pharmacological therapy, especially for those patients at high risk for VF (7, 23). Therefore, ICD therapy remains the only effective treatment for SCD. Yet, application of high-energy defibrillation shocks may have adverse side effects that can include 1) transient ectopy, tachycardia, or induction of VF (9, 39); 2) depression of electrical and mechanical functions (1, 21, 25); 3) bradycardia, complete heart block, and increased pacing thresholds (12, 35, 39); 4) mechanical dysfunction (stunning), which is directly related to the strength of shocks (16, 19, 34); 5) significant elevation of troponin I serum level in patients after spontaneous cardioverter defibrillator shocks (17); and 6) decrease of the myocardial lactate extraction rate by mitochondria (27). In addition to heart muscle damage, defibrillation is also associated with psychological side effects. High-energy shock in a conscious patient is painful and extremely unpleasant. Recent clinical studies (14, 20) have demonstrated that ICD patients have a significantly higher incidence of anxiety, depression, and panic disorders than does the general population. Based on these findings, a significant reduction of defibrillation energy is highly desirable.

The current low-energy alternative to defibrillation shocks is antitachycardia pacing (ATP), applied before defibrillation at an empirically chosen frequency higher than that of the VT. ATP is normally applied to VTs <188–200 beats/min with a success rate of 78–91% (24, 29, 37). Recently, ATP therapy has also been applied to fast VTs (200–250 beats/min) with similar success rates (40), although this therapy is not universally applied. In either scenario, if ATP fails or if the device deems the frequency of VT too high to apply ATP, a defibrillation shock must be applied, even if the VT has not degenerated into VF.

Mechanisms of success and failure of ATP are not fully understood. According to one theory, ATP failure may occur when the pacing electrode is located at a distance from the reentry core or when the VT is anatomical rather than functional in nature (22). To address these limitations, we have proposed a new method of destabilizing and subsequently terminating anatomical reentrant tachyarrhythmias (36) in which a low-voltage shock is applied to unpin reentry from its stationary core, which is pinned or anchored at a myocardial heterogeneity (30). This method relies on the effect of virtual electrode polarization (VEP) (10, 13, 41), which predicts hyperpolarization and depolarization on opposite sides of a functional or anatomical heterogeneity in response to an applied external electrical field. The areas of depolarization can give rise to secondary sources of excitation (13, 33, 38). When shock application is properly timed, these secondary sources are induced at the same heterogeneity that also serves as the core of reentry. Therefore, VEP can be used to destabilize and unpin a reentrant arrhythmia. Because this method relies on the VEP mechanism of excitation, all possible reentry cores are simultaneously excited with the low-voltage shock, regardless of electrode location. This may prove to be a distinct advantage over ATP in which efficacy may depend highly on electrode location relative to the reentry core. Thus far, this method of unpinning has been studied only analytically (32) and numerically with the bidomain model of cardiac tissue (36).

The goal of our study was to experimentally validate the possibility and effectiveness of this new method in an in vitro acute model of the infarction border zone (BZ), which is known to provide the anatomical and functional substrate for reentrant tachycardia due to nonuniform conduction caused by remodeling of the BZ (31). We used isolated preparations of the rabbit right ventricular free wall in which the endocardial superfused surface of the ventricle was a model of the endocardial BZ. This model was able to sustain stable reentrant arrhythmias that were easily visualized with the use of voltage-sensitive dye and fluorescent imaging techniques.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Experimental preparation. The protocol was approved by the Institutional Animal Care and Use Committee at Washington University. Experiments were performed in vitro on hearts obtained from New Zealand White rabbits (n = 22) of both sexes. The experimental procedure has been described previously in detail (26). Briefly, rabbits were anesthetized intravenously with 50 mg/kg pentobarbital sodium and 1,000–2,000 U heparin. The heart was removed and placed onto a Langendorff apparatus, where it was coronary perfused at 20 ml/min with Tyrode solution (26). After being stained with di-4-ANEPPS (D-1999, Molecular Probes, 1.3 µM, 5 min), the heart was removed from the Langendorff apparatus and placed in a bath of ice-cold Tyrode solution. The right ventricular free wall was dissected, stretched, and pinned epicardial side down onto a silicon disk. The preparation was then placed in a temperature-controlled bath (36°C) with Tyrode solution. The excitation-contraction uncoupler 2,3-butanedione monoxime (15 mM, diacetyl monoxime, Sigma, St. Louis, MO) was used to reduce motion artifacts.

The animals were divided into two experimental groups. An extensive unpinning protocol was performed on one group (n = 14). In the other group (n = 8), a protocol to determine the VEP was performed in addition to ATP and an abbreviated unpinning protocol to verify that unpinning could be achieved. Preparations from the unpinning experimental group were superfused at a rate of 80 ml/min. To improve the quality of the optical recordings, preparations from the VEP/ATP experimental group were coronary perfused at a perfusion pressure >40 mmHg in addition to superfusion at 80 ml/min.

Optical mapping and defibrillation setup. The optical mapping system used in this study has been described previously (26). Optical action potentials were collected by a 16 x 16 photodiode array (C4675, Hamamatsu, Japan), amplified, and digitized at 1,500 frames/s. The defibrillation setup included a bipolar electrogram that was used to trigger shock application during the unpinning protocol. A threshold detector determined the time of local activation and a shock was delivered across electrode meshes located in the bath after a programmable delay. Figure 1 shows a block diagram of this setup. Field strength was calibrated with 2 unipolar electrode recordings taken 1 cm apart in the center of the chamber between the meshes.


Figure 1
View larger version (28K):
[in this window]
[in a new window]
  		Fig. 1. Experimental setup. Block diagram of circuitry used to modulate time of shock application. ISO, isolation.

 
Experimental protocol: unpinning protocol. After a 15-min equilibration period, the field excitation threshold was determined for both shock polarities. Twenty basic pacing pulses were applied at a cycle length of 300 ms, followed by a 10 ms square pulse from the electrode meshes at the same coupling interval. The amplitude of the shock was decreased until an action potential was no longer observed in the optical signal after the shock. Stable reentry was then initiated by burst pacing at an interval of 100–130 ms. Shocks consisting of 10-ms monophasic square pulses were applied at an initial field strength of 0.38 V/cm. The timing of shock application was varied throughout the entire period of reentry at 10-ms steps until the full period of reentry was scanned. If the full period of reentry was scanned and the reentry did not terminate, the magnitude of the shocks was increased in 0.1-V/cm increments and scanning of the period was repeated. Once terminated, reentry was immediately reinitiated. If shocks were not applied, the stable reentry would last anywhere from 20 min to 1 h (this was the longest time period we allowed the reentry to continue).

Experimental protocol: VEP/ATP protocol. The VEP protocol consisted of 20-paced beats at a cycle length of 300 ms, followed by a 1.9 V/cm shock (square pulse, 2-ms duration) applied at the same interval. This action potential (AP) was then the "control" AP. After that shock, another 1.9-V/cm, 2-ms shock was applied at the same interval, followed by a plateau shock of the same magnitude. To improve the signal-to-noise ratio (SNR), we averaged 25 VEP measurements. The APs were then scaled from –85 to +15 mV. The VEP was then calculated as the difference between the plateau shock AP and the control AP at the end of the shock. The protocol was then repeated with the plateau shock applied with the opposite polarity. Several reentries were then initiated in these preparations to determine the location of reentry cores with respect to the VEP.

ATP was applied from a bipolar pacing electrode located at the preparation edge. Eight to twenty pacing pulses were applied at a rate of 50–105% of the reentry period. The current was set to 4–6 times the pacing threshold, which corresponded to 0.8–10 mA. Most reentry cores were located near the center of the preparation; therefore, ATP pulses applied from the preparation edge helped to ensure that the pacing electrode was located at a distance from the reentry core, as is often the case with ICDs. A brief unpinning protocol was then applied to verify that reentry in these preparations could be unpinned.

Immunohistochemistry. After three of the unpinning experiments, the preparation was embedded in Tissue-Tek OCT compound and was then frozen and cryosectioned. The 16-µm transmural cryosections were mounted on poly-L-lysine-coated glass slides. Double staining was then performed with a commercially available anti-phosphorylated (Ser368) connexin43 (Cx43) polyclonal antibody raised in rabbit (AB3841, Chemicon) used at a dilution of 1:400 and with a commercially available anti-dephosphorylated (Ser368) Cx43 monoclonal antibody raised in mouse (13–8300, Zymed), used at a dilution of 1:200. Alexa fluor 488 goat anti-mouse IgG1 (A-21121, Molecular Probes) and Alexa fluor 555 goat anti-rabbit IgG (A-21428, Molecular Probes) were used as secondary antibodies at dilutions of 1:1,000. Confocal imaging was then performed by using a Nikon C1/80i confocal microscope.

Data analysis. Phase-plane analysis was performed on reentry data to determine the instantaneous location of the reentry core, which is indicated by a point of phase singularity (PS), as previously described (4, 6, 15). Specifically, the Bray-Wikswo (5) method of pseudoempirical mode decomposition along with the Hilbert transform was used to generate the phase-plane data. To determine lines of block created by the reentry cores, PS trajectories were also obtained by tracking the locations of PS throughout one or more periods of reentry (Fig. 2B). Signals with a low SNR (<5 dB) were not included in phase-plane analysis. Activation maps were created by using two different methods. For paced data (Fig. 2A) or reentry, which included a three-dimensional (3-D) path (Fig. 9), activation maps were created from the local activation time (maximum of the first derivative) corresponding to each photodiode signal. For the reentry data, after phase-plane analysis was performed, isochronal maps were created by tracking the location of the wavefront in the phase plane throughout the entire period of reentry (Fig. 2, C and D).


Figure 2
View larger version (44K):
[in this window]
[in a new window]
  		Fig. 2. Data analysis techniques. A: activation map for pacing at a constant interval of 300 ms from location indicated with a star. Crowded isochrones (arrow) indicate an area of slow conduction. B: preparation photograph overlapped with the trajectory of points of phase singularity (PS), indicating the cores of two different stable reentries for which the isochronal maps are shown in C and D. C: isochronal map for a stable reentry with 10-ms isochrones. Trajectory of points of PS (yellow line) is shown. The core of this reentry corresponds to the area of slow conduction in A. D: isochronal map for a stable reentry with 10-ms isochrones. Trajectory of points of PS (cyan line) is shown. E: optical traces recorded from reentry in D. Numbers in B correspond to the location of each optical signal.

 

Figure 9
View larger version (41K):
[in this window]
[in a new window]
  		Fig. 9. Isthmus mechanism of termination. A: steady-state activation map of reentry that uses a three-dimensional trabecula in the reentry path. Isochrones are 10 ms apart. B: polar plot of successful and unsuccessful shocks applied throughout all phases of reentry. Successful shocks were found to have a very low concentration (0.46) around the angular mean, which was not significantly different from a circular uniform distribution, indicating that the time of shock application may be less critical compared with the traditional unpinning mechanism. C: optical recording from location (cyan box) in A. D: local ECG recording from surface of the preparation.

 
Rayleigh's test (42) was used to determine whether the phase of successful shock application was statistically different from a circular uniform distribution. The concentration and angular mean of successful shocks were also determined. Microscopy images were analyzed by determining the means of the normalized phosphorylated Cx43 and dephosphorylated Cx43 signal intensities averaged over 1.26 x 0.080 mm2 areas along and across the tissue surface. The depth of the phosphorylated layer of tissue for each section was determined to be the distance at which the mean phosphorylated and dephosphorylated signal intensities intersected. The differences between the phosphorylated and dephosphorylated signal intensities were analyzed by one-way repeated-measurements ANOVA. P values <0.05 were considered significant.


   RESULTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Immunohistochemistry. Preparations used in the unpinning protocol were superfused only. Thus oxygen and nutrients can only diffuse ~200–400 µm into the depth of the tissue (8). Therefore, we expected that the midmyocardium would be subjected to ischemia, which is associated with cellular uncoupling and dephosphorylation of Ser368 Cx43 (2, 3). Because ischemia and cellular uncoupling can contribute greatly to arrhythmogenesis, we performed immunohistochemistry experiments to fully quantify the extent of ischemia in this model. Staining with anti-phosphorylated and anti-dephosphorylated Ser368 Cx43 antibodies revealed that only a thin layer of tissue had predominantly phosphorylated Cx43 after 1–2 h of superfusion, whereas the midmyocardium had predominantly dephosphorylated Cx43. Figure 3 shows a summary of these results. A typical confocal microscopy image at x10 magnification is shown in Fig. 3A. The endocardial superfused surface of this preparation is the left edge of the tissue section. The average depth of the phosphorylated layer was then calculated (see METHODS), and the combined results from three animals are shown in Fig. 3B. The phosphorylated and dephosphorylated signals were significantly different throughout the depth of the tissue (P = 0.0036). Although the depth of the phosphorylated layer varied throughout the tissue section, the average depth of tissue at which phosphorylated Cx43 was of higher density compared with dephosphorylated Cx43 was 0.38 ± 0.10 mm.


Figure 3
View larger version (50K):
[in this window]
[in a new window]
  		Fig. 3. Surviving layer of superfused endocardium. A: typical confocal microscopy image showing predominately phosphorylated Ser368 connexin43 (Cx43; red) on the superfused surface of the tissue and predominately dephosphorylated Ser368 Cx43 (green) in the midmyocardium. B: mean normalized signal intensities from three experimental animals in which both phosphorylated (red) and dephosphorylated (green) signals have been normalized to their respective maximum values in the image.

 
Virtual electrodes and reentry core locations. To examine the pattern of shock-induced polarization, VEP maps were created for preparations on which the VEP protocol was applied (n = 8). Locations of the reentry cores, as determined by PS analysis, were then overlaid onto the VEP maps. Figure 4A shows a typical preparation. The VEP (Fig. 4A, inset) corresponds to the area within the dashed square. As can be observed in the map, the reentry cores correspond to areas where positive and negative polarizations are adjacent to one another. These areas are a consequence of underlying anatomical or functional heterogeneity found at the reentry core and can provide the shock-induced polarization needed to unpin reentry. VEPs were visualized in regions containing 21 different reentry cores. Within a 2-mm radius of these cores, the average positive VEP was found to be 3.6 ± 0.5 mV and the average negative VEP was –3.5 ± 0.6 mV, separated by an average distance of 4.3 ± 0.4 mm (mean ± SE).


Figure 4
View larger version (59K):
[in this window]
[in a new window]
  		Fig. 4. Virtual electrodes and reentry core locations. A: preparation photograph showing two different reentry cores. Additional cores were present in this preparation; however, only those corresponding to the dashed square are shown here. A, inset: virtual electrode polarization (VEP) map and corresponding reentry cores from the location indicated with a dashed square. B: optical traces from the numbered locations in VEP map showing slight positive and negative polarization compared with the control action potential (AP) on opposite sides of the reentry cores. Differences between control and plateau-shock APs during shock application are shown in black and indicate the magnitude of VEP.

 
Mechanisms of unpinning. Experimental unpinning results from the rabbit right ventricular free wall confirmed theoretical predictions of the mechanisms of unpinning of anatomical stationary reentry (36). Figures 5 and 6 illustrate the mechanisms of successful and unsuccessful unpinning, respectively. Figure 5D shows the time course of successful unpinning with the time point of each panel indicated in Fig. 5C. When shock application was properly timed, a VEP-induced secondary source of excitation was created near the reentry core (Fig. 5D,2, red arrows) and interacted with the reentrant wavefront, unpinning it from its core (Fig. 5D,3). Although an additional reentrant wave was created by the VEP-induced secondary source that reset the reentry, this new wavefront terminated on hitting the refractory tail of the original reentrant circuit (Fig. 5D,5–7). When shock application was varied by approximately {pi} radians for the same reentrant circuit, the reentry was neither unpinned nor reset. Figure 6 illustrates this example. Immediately after shock application (Fig. 6,1), no secondary sources of excitation were observed as in Fig. 5D (Fig. 6,2) because the tissue in this area was now refractory. Thus the reentry continued to rotate, unaffected by the shock. The phase dependence of the unpinning mechanism was confirmed by statistical analysis of circular uniformity, and the results of two such analyses are shown in Figs. 7F and 8F.


Figure 5
View larger version (57K):
[in this window]
[in a new window]
  		Fig. 5. Mechanisms of successful unpinning of anatomical reentry. A: preparation photograph showing optical imaging field of view (solid black square), location of reentry core (identified as a stationary line of block/trajectory of PS), and outline of phase-plane maps (dashed black line). Monomorphic ventricular tachycardia (VT) was maintained by counterclockwise reentry rotating around the line of block. We imaged the entire ventricular preparation. Tissue above the field of view is atrial and did not have any communication with ventricular preparation. B: VEP map showing positive and negative polarization near the line of block/core of reentry as in Fig. 4. C: optical trace (blue) from site shown with white box in D,1–7. This trace can be used as a reference to compare the timing of the shock between Figs. 5 and 6. VT was terminated by a 0.58 V/cm shock (red trace). D: mechanisms of termination (1–7) show instantaneous phase distributions at time instants marked in C. The unpinning pulse created a VEP-induced secondary source of excitation (D,2, red arrows) that merged with the reentrant wave (D,3), detaching it from the core. This unpinned wavefront terminated on reaching the preparation edge. A remnant of the wave induced by the VEP continued to propagate counterclockwise and self-terminated at a refractory area (D,5–7).

 

Figure 6
View larger version (30K):
[in this window]
[in a new window]
  		Fig. 6. Mechanisms of unsuccessful unpinning when shock is applied at an incorrect phase. Optical trace (top) and instantaneous phase distribution (bottom) as in Fig. 5; only shock time is varied by approximately {pi} radians. Immediately after shock application (bottom,1), no secondary source of excitation appears as in Fig. 5 because tissue immediately to the left of the reentry core is now refractory. Reentry continues to rotate counterclockwise with no apparent changes in core location or reentry period.

 

Figure 7
View larger version (28K):
[in this window]
[in a new window]
  		Fig. 7. Unpinning of reentry by VEP-induced excitation of reentry core that leads to termination. A: steady-state isochronal map with 10-ms isochrones. B: phase-plane map 1 ms before shock application. Trajectory of PS (black line) and instantaneous location of the PS (white circle) are shown. C: phase-plane map 10 ms after shock application showing a new region of shock-induced depolarization near the reentry core. D: optical traces from the locations indicated with corresponding colored boxes in B. Red optical trace appears unaffected by the shock, whereas the third AP in cyan optical trace occurs prematurely due to the shock. This new wave of depolarization then collided with the reentrant wavefront, unpinning it from the reentry core. The unpinned reentry then made two additional rotations with the core located far from the area of pinning before it reached the edge of the preparation and terminated. E: local ECG recording from surface of the preparation. F: polar plot of successful and unsuccessful shocks applied throughout all phases of reentry. Successful shocks were clustered around the angular mean with a concentration of 0.80, which was found to be significantly different from a circular uniform distribution, indicating a phase-dependent mechanism.

 

Figure 8
View larger version (30K):
[in this window]
[in a new window]
  		Fig. 8. Unpinning of reentry by VEP-induced excitation of reentry core without termination. A: steady-state isochronal map with 10-ms isochrones. B and C: phase-plane maps 1 ms before and 10 ms after shock application. Trajectory of PS of the original reentry core (black) and new reentry core (white) are shown. D: optical trace from reentry core (cyan box) in B. E: ECG recording from surface of the preparation. F: polar plot of successful and unsuccessful shocks applied throughout all phases of reentry (see RESULTSfor details).

 
Unpinned reentry either terminates at tissue boundary or repins to another heterogeneity. After unpinning of the reentrant arrhythmia, the predominant result in this isolated ventricular preparation was a termination of the reentry when it reached the preparation edge. Examples of termination by this mechanism are illustrated in Figs. 5 and 7. However, this was not the only observed result, and immediate termination would not necessarily be expected in a whole heart model where much heterogeneity exists that could repin an unpinned reentry. A rare example in this model of an unpinned reentry immediately repinning and anchoring to another heterogeneity on the preparation is presented in Fig. 8.

In this example, reentry is rotating clockwise near the center of the preparation as indicated by the trajectory of PS shown in black in Fig. 8, B and C. Additional points of PS were present elsewhere in the preparation due to transient block of conduction; however, full rotations were not maintained around these PSs. After shock application, a new region of depolarization can be seen near the core in Fig. 8C. This secondary source of excitation then collided with and unpinned the reentry as in previous examples. However, unlike previous examples, the reentry immediately reattached to a new core (indicated by the white trajectory of PS) where it made 1–5 rotations before spontaneously moving back to the original reentry core, which apparently had a stronger pinning force (28). An optical trace from the reentry core is shown in Fig. 8B. Before shock application, a dual-hump morphology was observed that indicated the presence of a reentry core or line of block (11). However, after shock application, three full-magnitude APs were recorded while the reentry was pinned to a new location. The reentry then spontaneously repinned to the original reentry core, and the dual-hump morphology was again observed. Successful shocks were clustered around the angular mean with a concentration of 0.80, which was found to be significantly different from a circular uniform distribution, indicating a phase-dependent mechanism. Such unpinning without termination due to repinning was observed in 1 out of 14 preparations.

Termination of arrhythmia when reentrant path includes an isthmus of tissue. In two experimental preparations, we observed reentrant paths that included a narrow isthmus of tissue. In these cases, the mechanism of unpinning was different from previous examples. For these reentrant circuits, the timing of shock application was not as critical. Rather, termination was achieved over a much wider phase range. This mechanism occurred when the reentry was rotating near the edge of the preparation or when the reentrant path included a 3-D trabecula as in the example presented in Fig. 9. The activation map for this reentry is shown in Fig. 9A, with the reentrant pathway indicated with yellow arrows. In the middle of the preparation, the pathway included a narrow trabecula. As can be observed in the polar plot of successful and unsuccessful shocks in Fig. 9B, termination of this arrhythmia had a threshold-like behavior. Below 0.58 V/cm, termination could not be achieved at any phase of reentry. However, above this value, termination could be achieved over many phases of reentry. This observation was confirmed with Rayleigh's test, which found the successful shocks to have a relatively low concentration (0.46) around the angular mean, which was not found to be statistically different from a uniform circular distribution (P > 0.05).

Combined results of low-voltage unpinning and termination. In this study a total of 192 reentries were initiated and terminated or unpinned in preparations on which the complete unpinning protocol was applied (n = 14). Across these preparations, the average period of reentry was 155.8 ± 38.6 ms. Survival analysis found that E80 (shock strength at which 80% of reentries were unpinned) was 1.2 V/cm. This value corresponds to 4.3 times the average diastolic field excitation threshold, which was found to be 0.28 V/cm. A survival plot of the combined experiments is shown in Fig. 10. All reentries in these experimental animals were unpinned at shock strengths ≤2.4 V/cm.


Figure 10
View larger version (12K):
[in this window]
[in a new window]
  		Fig. 10. Survival analysis of unpinned reentries versus shock strength. Observed unpinning events (black line) and 95% confidence intervals (red dashed lines). E80 (shock strength at which 80% of reentries were unpinned) was 1.2 V/cm. Eo, applied field.

 
Efficacy of ATP compared with unpinning. Both ATP and a brief unpinning protocol were performed in eight experimental preparations. A minimum of 12 ATP attempts were applied to each preparation on a minimum of three different reentry cores. Termination of reentry by ATP was achieved in only five out of eight preparations and often only after several attempts. With all atempts included, the overall success rate of ATP was 6.4%. Termination by unpinning was achieved in all eight preparations in this experimental group. With all preparations included from the unpinning experimental group, termination by unpinning was achieved in a total of 22 preparations, which include all preparations on which unpinning shocks were applied. Unpinning shocks were applied uniformly throughout all phases of reentry, which yielded an overall success rate of 13.1%.


   DISCUSSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
In the present study, a new method of low-voltage destabilization and termination of ventricular reentrant tachyarrhythmias was investigated in a model of the infarction BZ. Two different mechanisms of unpinning and termination of reentrant arrhythmias by this method were observed in this model, one that was predicted by our earlier theoretical investigations (32, 36) and one that was not.

The acute infarction BZ model used in the unpinning experiments was critical for many reasons. First and foremost, immunohistochemistry results indicated predominantly phosphorylated Ser368 Cx43 present in 0.38 ± 0.10 µm of the endocardial superfused surface. This thin layer of superfused tissue provided an essentially two-dimensional sheet of functional endocardium. This assured that the reentrant arrhythmia could be constantly visualized by using optical imaging techniques. Also of great importance was the ability of this preparation to sustain stable reentrant arrhythmias. This was likely due to variations in the depth of the infarction BZ and levels of phosphorylated Ser368 Cx43. Peters and colleagues (31) observed that pathways of reentrant circuits and functional lines of block occurred in regions where the surviving layer of the infarction BZ was thinnest in a canine model of the epicardial BZ. Aside from the infarction BZ providing the substrate for stable reentrant arrhythmias, studying the mechanisms of arrhythmias and defibrillation in an infarction model is also clinically relevant. Previous myocardial infarction is one of the major indications for ICD therapy (18), because the hearts of these high-risk patients contain anatomical substrates or obstacles, such as scars and ischemic zones that are capable of pinning cores for reentrant arrhythmias.

Two different mechansims of low-voltage unpinning and destabilization of reentrant arrhythmias were observed in this model of the infarction BZ. The first of these mechanisms was predicted with bidomain models of anatomically defined reentry by Takagi and colleagues (36) where appropriately timed VEP-induced excitation of the reentry core interacts with the reentry causing unpinning and subsequent termination. This mechanism was observed in all experimental preparations on which the unpinning protocol was applied and is illustrated in Figs. 5 and 7. The accompanying VEP maps (Figs. 4A and 5B) revealed adjacent areas of positive and negative polarization that are a consequence of underlying anatomical and/or functional heterogeneities. It is the depolarized region of these VEPs present at or near the reentry core that can provide the necessary secondary sources responsible for unpinning and destabilizing the reentry.

In this isolated model, unpinning was normally followed by termination of the reentry when it reached the preparation edge. However, in one experimental animal, an unpinned reentry immediately repinned to another heterogeneity on the preparation (Fig. 8). This result is not unexpected, because many areas of heterogeneity may exist in a single heart that can anchor reentry. To avoid immediate repinning and facilitate complete termination of an unpinned reentry, one possible alternative would be the use of antirepinning pacing similar to ATP applied after low-voltage unpinning shocks. It is known that ATP has difficulties in terminating anatomical reentry when the pacing site is located at a distance from the reentry core (36). Our results are in agreement with this finding, because we observed termination by ATP in only five of eight preparations with an overall success rate of 6.4% in this model of stable reentry. However, there are no such difficulties associated with ATP termination of a functional reentry (22). Therefore, once the reentry is unpinned from its anatomical core with a low-voltage unpinning shock, antirepinning pacing could be administered and may be quite effective for terminating these now functional reentries and preventing their reattachment to a new core. However, there is still the risk that if antirepinning pacing fails, unpinning a stable VT may lead to a degeneration of the VT into VF. Therefore, even if clinically implemented, this method will still require full-strength defibrillation as a backup.

The second observed mechanism of low-voltage termination occurred when the reentrant path included a small isthmus of tissue (Fig. 9). These paths were present when the reentry was rotating near the edge of the isolated preparation or when the reentrant path included a 3-D trabecula. In these instances, termination was always immediate and reentry was never effectively "unpinned," freely rotating about the tissue. The timing of shock application in these instances was not as critical and exhibited a threshold-like behavior. Below a certain threshold, termination could not be achieved. However, above this threshold, termination could be achieved over many phases of the reentry period. This mechanism of termination was observed for four different reentries in two different experimental animals.

We hypothesize that both the thresholdlike behavior and the large unpinning window associated with this mechanism are due to the isthmus of tissue itself. The threshold of termination likely corresponds to the field excitation threshold of the isthmus when it is excitable. Shock magnitudes below this threshold will have little or no effect on the reentry regardless of when they are applied. However, shocks applied when the isthmus is excitable, at a magnitude above this threshold, will excite the isthmus. Because the narrow isthmus is essential for maintenance of the reentry, when the reentrant wavefront reaches this now excited or refractory tissue, no other pathway exists for the reentry, and it immediately terminates. This mechanism of termination may result in a larger unpinning window compared with the conventional unpinning mechanism if the isthmus remains excitable for a large portion of the VT cycle.

Regardless of the mechanism of unpinning and/or termination, the destabilization of a reentrant arrhythmia using this method can be achieved at significantly lower voltage gradients than those required for conventional defibrillation. The accepted voltage gradient for conventional defibrillation shocks (to defibrillate 80% of the time) is 5.4 ± 0.8 V/cm for a 10-ms monophasic waveform (43). In the present study, we determined that 80% of initiated reentries could be unpinned with a voltage gradient of 1.2 V/cm (10-ms monophasic waveform) when shocks were applied at the correct phase within the period of reentry. This corresponds to a 20-fold reduction in defibrillation energy. This is likely to reduce tissue damage and myocardial dysfunction that is often associated with conventional defibrillation. Our results also suggest that low-voltage unpinning shocks may be more effective than ATP for terminating stable, anatomical reentrant arrhythmias.

Although further studies need to be conducted on whole heart models to further validate this method of termination, low-voltage unpinning of ventricular tachyarrhythmias may provide an exciting new avenue to be explored in cardioversion/defibrillation research. Unpinning may provide a novel approach to defibrillation by preventing deterioration of VT into VF.

Limitations. A significant limitation of the current study was the ambiguity of phase measurements. Because the phase of reentry was measured with a bipolar electrogram at only one tissue location, all phase measurements are relative to this particular electrode location and the location of the corresponding reentry core. Therefore, for a particular reentry core, the location (angular mean) of the unpinning window will differ by {pi} radians if the recording electrode is moved from a location immediately to the right of the reentry core to a location immediately to the left. Although the angular mean at which the termination window occurs is not absolute, we have demonstrated the existence of an unpinning window by evaluating the concentration of successful shocks and determining whether that concentration is statistically different from a circular uniform distribution, which would indicate that no unpinning window was present.


   ACKNOWLEDGMENTS
 
This work was supported by the National Heart, Lung, and Blood Institute Grant R01-HL-67322 (to I. R. Efimov) and American Heart Association Predoctoral Fellowship (to C. M. Ripplinger).


   FOOTNOTES
 

Address for reprint requests and other correspondence: I. R. Efimov, Campus Box 1097, One Brookings Dr., St. Louis, MO 63130 (e-mail: igor{at}wustl.edu)

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


   REFERENCES
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

  1. Al-Khadra AS, Nikolski V, and Efimov IR. The role of electroporation in defibrillation. Circ Res 87: 797–804, 2000.[Abstract/Free Full Text]
  2. Axelsen LN, Mohammed S, Kjolbye AL, Nielsen MS, Holstein-Rathlou N, Jensen ON, Andersen S, Hennan JK, Rsch W, and Stahlhut M. Serine 297 and serine 306: two new phosphorylation sites involved in the regulation of connexin43 during ischemia (Abstract). Circulation 112: II-114, 2005.
  3. Beardslee MA, Lerner DL, Tadros PN, Laing JG, Beyer EC, Yamada KA, Kleber AG, Schuessler RB, and Saffitz JE. Dephosphorylation and intracellular redistribution of ventricular connexin43 during electrical uncoupling induced by ischemia. Circ Res 87: 656–662, 2000.[Abstract/Free Full Text]
  4. Bray MA, Lin SF, Aliev RR, Roth BJ, and Wikswo JP Jr. Experimental and theoretical analysis of phase singularity dynamics in cardiac tissue. J Cardiovasc Electrophysiol 12: 716–722, 2001.[CrossRef][ISI][Medline]
  5. Bray MA and Wikswo JP. Considerations in phase plane analysis for nonstationary reentrant cardiac behavior. Phys Rev E Stat Nonlin Soft Matter Phys 65: 051902-1–051902-8, 2002.[Medline]
  6. Bray MA and Wikswo JP. Use of topological charge to determine filament location and dynamics in a numerical model of scroll wave activity. IEEE Trans Biomed Eng 49: 1086–1093, 2002.[CrossRef][ISI][Medline]
  7. Buxton AE, Lee KL, Fisher JD, Josephson ME, Prystowsky EN, and Hafley G. A randomized study of the prevention of sudden death in patients with coronary artery disease. Multicenter Unsustained Tachycardia Trial Investigators. N Engl J Med 341: 1882–1890, 1999.[Abstract/Free Full Text]
  8. Cranefield PF and Greenspan K. The rate of oxygen uptake of quiescent cardiac muscle. J Gen Physiol 44: 235–249, 1960.[Abstract/Free Full Text]
  9. Donoso E and Cohn LJ, and Friedberg CK. Ventricular arrhythmias after precordial electric shock. Am Heart J 73: 595–601, 1967.[CrossRef][ISI][Medline]
  10. Efimov IR, Cheng YN, Biermann M, Van Wagoner DR, Mazgalev T, and Tchou PJ. Transmembrane voltage changes produced by real and virtual electrodes during monophasic defibrillation shock delivered by an implantable electrode. J Cardiovasc Electrophysiol 8: 1031–1045, 1997.[ISI][Medline]
  11. Efimov IR, Sidorov VY, Cheng Y, and Wollenzier B. Evidence of 3D scroll waves with ribbon-shaped filament as a mechanism of ventricular tachycardia in the isolated rabbit heart. J Cardiovasc Electrophysiol 10: 1452–1462, 1999.[ISI][Medline]
  12. Eysmann SB, Marchlinski FE, Buxton AE, and Josephson ME. Electrocardiographic changes after cardioversion of ventricular arrhythmias. Circulation 73: 73–81, 1986.[Abstract/Free Full Text]
  13. Fast VG, Rohr S, Gillis AM, and Kleber AG. Activation of cardiac tissue by extracellular electrical shocks: formation of 'secondary sources' at intercellular clefts in monolayers of cultured myocytes. Circ Res 82: 375–385, 1998.[Abstract/Free Full Text]
  14. Godemann F, Butter C, Lampe F, Linden M, Schlegl M, Schultheiss HP, and Behrens S. Panic disorders and agoraphobia: side effects of treatment with an implantable cardioverter/defibrillator. Clin Cardiol 27: 321–326, 2004.[ISI][Medline]
  15. Gray RA, Pertsov AM, and Jalife J. Spatial and temporal organization during cardiac fibrillation. Nature 392: 75–78, 1998.[CrossRef][Medline]
  16. Grimm RA, Stewart WJ, Arheart K, Thomas JD, and Klein AL. Left atrial appendage "stunning" after electrical cardioversion of atrial flutter: an attenuated response compared with atrial fibrillation as the mechanism for lower susceptibility to thromboembolic events. J Am Coll Cardiol 29: 582–589, 1997.[Abstract]
  17. Hasdemir C, Shah N, Rao AP, Acosta H, Matsudaira K, Neas BR, Reynolds DW, Po S, Lazzara R, and Beckman KJ. Analysis of troponin I levels after spontaneous implantable cardioverter defibrillator shocks. J Cardiovasc Electrophysiol 13: 144–150, 2002.[CrossRef][ISI][Medline]
  18. Hayes DL, Lloyd MA, and Friedman PA.Cardiac Pacing and Defibrillation: A Clinical Approach. Armonk, NY: Futura, 2000.
  19. Kam RM, Garan H, McGovern BA, Ruskin JN, and Harthorne JW. Transient right bundle branch block causing R wave attenuation postdefibrillation. Pacing Clin Electrophysiol 20: 130–131, 1997.[CrossRef][Medline]
  20. Kamphuis HC, Verhoeven NW, Leeuw R, Derksen R, Hauer RN, and Winnubst JA. ICD: a qualitative study of patient experience the first year after implantation. J Clin Nurs 13: 1008–1016, 2004.[CrossRef][ISI][Medline]
  21. Kodama I, Shibata N, Sakuma I, Mitsui K, Iida M, Suzuki R, Fukui Y, Hosoda S, and Toyama J. Aftereffects of high-intensity DC stimulation on the electromechanical performance of ventricular muscle. Am J Physiol Heart Circ Physiol 267: H248–H258, 1994.[Abstract/Free Full Text]
  22. Krinsky VI and Agladze K. Interaction of rotating waves in an active chemical medium. Physica D 8: 50–56, 1983.[CrossRef]
  23. Lee DS, Green LD, Liu PP, Dorian P, Newman DM, Grant FC, Tu JV, and Alter DA. Effectiveness of implantable defibrillators for preventing arrhythmic events and death: a meta-analysis. J Am Coll Cardiol 41: 1573–1582, 2003.[Abstract/Free Full Text]
  24. Luceri RM, Habal SM, David IB, Puchferran RL, Muratore C, and Rabinovich R. Changing trends in therapy delivery with a third generation noncommitted implantable defibrillator: results of a large single center clinical trial. Pacing Clin Electrophysiol 16: 159–164, 1993.[CrossRef][Medline]
  25. Neunlist M and Tung L. Dose-dependent reduction of cardiac transmembrane potential by high-intensity electrical shocks. Am J Physiol Heart Circ Physiol 273: H2817–H2825, 1997.[Abstract/Free Full Text]
  26. Nikolski V and Efimov I. Fluorescent imaging of a dual-pathway atrioventricular-nodal conduction system. Circ Res 88: E23–E30, 2001.
  27. Osswald S, Trouton TG, O'Nunain SS, Holden HB, Ruskin JN, and Garan H. Relation between shock-related myocardial injury and defibrillation efficacy of monophasic and biphasic shocks in a canine model. Circulation 90: 2501–2509, 1994.[Abstract/Free Full Text]
  28. Pazo D, Kramer L, Pumir A, Kanani S, Efimov I, and Krinsky V. Pinning force in active media. Phys Rev Lett 93: 168303-1–168303-4, 2004.
  29. Peinado R, Almendral J, Rius T, Moya A, Merino JL, Martinez-Alday J, Perez-Villacastin J, Arenal A, Ormaetxe J, Tercedor L, Medina O, Pastor A, and Delcan J. Randomized, prospective comparison of four burst pacing algorithms for spontaneous ventricular tachycardia. Am J Cardiol 82: 1422–1429, 1998.[CrossRef][ISI][Medline]
  30. Pertsov AM, Davidenko JM, Salomonsz R, Baxter WT, and Jalife J. Spiral waves of excitation underlie reentrant activity in isolated cardiac muscle. Circ Res 72: 631–650, 1993.[Abstract/Free Full Text]
  31. Peters NS, Coromilas J, Severs NJ, and Wit AL. Disturbed connexin43 gap junction distribution correlates with the location of reentrant circuits in the epicardial border zone of healing canine infarcts that cause ventricular tachycardia. Circulation 95: 988–996, 1997.[Abstract/Free Full Text]
  32. Pumir A and Krinsky V. Unpinning of a rotating wave in cardiac muscle by an electric field. J Theor Biol 199: 311–319, 1999.[CrossRef][Medline]
  33. Roth BJ and Krassowska W. The induction of reentry in cardiac tissue. The missing link: how electric fields alter transmembrane potential. Chaos 8: 204–220, 1998.[CrossRef][ISI][Medline]
  34. Sparks PB, Jayaprakash S, Mond HG, Vohra JK, Grigg LE, and Kalman JM. Left atrial mechanical function after brief duration atrial fibrillation. J Am Coll Cardiol 33: 342–349, 1999.[Abstract/Free Full Text]
  35. Stickney RE, Doherty A, Kudenchuk PJ, Morud SA, Walker C, Chapman FW, and Cummins RO. Survival and postshock ECG rhythms for out-of-hospital defibrillation (Abstract). PACE 22: 740, 1999.
  36. Takagi S, Pumir A, Pazo D, Efimov I, Nikolski V, and Krinsky V. Unpinning and removal of a rotating wave in cardiac muscle. Phys Rev Lett 93: 058101-1–058101-4, 2004.[CrossRef][Medline]
  37. Trappe HJ, Klein H, Fieguth HG, Kielblock B, Wenzlaff P, and Lichtlen PR. Clinical efficacy and safety of the new cardioverter defibrillator systems. Pacing Clin Electrophysiol 16: 153–158, 1993.[CrossRef][Medline]
  38. Trayanova N, Skouibine K, and Aguel F. The role of cardiac tissue structure in defibrillation. Chaos 8: 221–233, 1998.[CrossRef][ISI][Medline]
  39. Waldecker B, Brugada P, Zehender M, Stevenson W, and Welens HJ. Ventricular arrhythmias after precordial electric shock. Am J Cardiol 57: 120–123, 1986.[CrossRef][ISI][Medline]
  40. Wathen MS, DeGroot PJ, Sweeney MO, Stark AJ, Otterness MF, Adkisson WO, Canby RC, Khalighi K, Machado C, Rubenstein DS, and Volosin KJ. Prospective randomized multicenter trial of empirical antitachycardia pacing versus shocks for spontaneous rapid ventricular tachycardia in patients with implantable cardioverter-defibrillators: Pacing Fast Ventricular Tachycardia Reduces Shock Therapies (PainFREE Rx II) trial results. Circulation 110: 2591–2596, 2004.[Abstract/Free Full Text]
  41. White JB, Fotuhi PC, Pedoto RW, Chattipakorn N, Rogers JM, and Ideker RE. Reduction in atrial defibrillation threshold by a single linear ablation lesion. J Cardiovasc Electrophysiol 12: 463–471, 2001.[Medline]
  42. Zar JH.Biostatistical Analysis. Upper Saddle River, NJ: Prentice-Hall, 1999.
  43. Zhou X, Daubert JP, Wolf PD, Smith WM, and Ideker RE. Epicardial mapping of ventricular defibrillation with monophasic and biphasic shocks in dogs. Circ Res 72: 145–160, 1993.[Abstract/Free Full Text]



This article has been cited by other articles:


Home page
 Biophys. JHome page
J. Cysyk and L. Tung
Electric Field Perturbations of Spiral Waves Attached to Millimeter-Size Obstacles
Biophys. J., February 15, 2008; 94(4): 1533 - 1541.
[Abstract] [Full Text] [PDF]

Home page
 Am. J. Physiol. Heart Circ. Physiol.Home page
K. Agladze, M. W. Kay, V. Krinsky, and N. Sarvazyan
Interaction between spiral and paced waves in cardiac tissue
Am J Physiol Heart Circ Physiol, July 1, 2007; 293(1): H503 - H513.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow All Versions of this Article:
291/1/H184    most recent
01300.2005v1
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (7)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Ripplinger, C. M.
Right arrow Articles by Efimov, I. R.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Ripplinger, C. M.
Right arrow Articles by Efimov, I. R.

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
Visit Other APS Journals Online
Copyright © 2006 by the American Physiological Society.