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Optical mapping of the functional reentrant circuit of ventricular tachycardia in acute myocardial infarction
Optical mapping of the functional reentrant circuit of ventricular tachycardia in acute myocardial infarction Tamana Takahashi, MD, Pascal van Dessel, MD, PhD, John C. Lopshire, MD, PhD, William J. Groh, MD, John Miller, MD, Jianyi Wu, MD, Douglas P. Zipes, MD From the Krannert Institute of Cardiology, Department of Medicine, Indiana University School of Medicine, Indianapolis, Indiana. OBJECTIVES We used optical mapping to characterize the reentrant circuit of ventricular tachycardia (VT) during acute myocardial infarction (MI) in isolated canine left ventricular preparations. BACKGROUND The nature of the reentrant circuit that underlies VT during acute MI is not well understood. METHODS Using optical mapping in isolated canine left ventricular preparations, we characterized the reentrant circuit of monomorphic VT (mean cycle length 245.3 ⫾ 15.6 ms, n ⫽ 7) induced by programmed stimulation during acute MI. RESULTS Optical mapping during VT revealed a functional reentrant circuit consisting of four components: (1) a protected isthmus located between the infarction area and the functional line of block; (2) an entrance site located at one end of the isthmus; (3) an exit site located at the other end of the isthmus; and (4) an outer loop consisting of nonischemic normal tissue, connecting the exit and entrance sites. Rate-dependent slow conduction within the border zone was associated with significant changes (n ⫽ 6) in action potential amplitude (99.1 ⫾ 0.4 vs 71.4 ⫾ 0.6 mV, P ⬍ .01), maximal diastolic potential (⫺80.6 ⫾ 0.2 vs ⫺65.4 ⫾ 0.6 mV, P ⬍ .05), action potential duration at 90% repolarization (APD90; 188.4 ⫾ 1.0 vs 164.3 ⫾ 3.1 ms, P ⬍ .05), and dV/dt (302.4 ⫾ 7.9 vs 168.5 ⫾ 3.6 V/s, P ⬍ .05). Compared to preparations with no inducible VT (n ⫽ 7), formation of a functional line of block was the key mechanism for initiation of functional reentry in preparations with VT. When comparing preparations with sustained and nonsustained VT, preservation of slow conduction over the isthmus was the key component for maintenance of sustained VT. CONCLUSIONS The reentrant circuit of monomorphic VT in the setting of acute MI involved both the infarction border zone and nonischemic normal tissue. The underlying mechanism is related to the presence of rate-dependent slow conduction and the development of a functional line of block in the border zone. KEYWORDS Ventricular tachycardia; Reentry; Optical mapping © 2004 Heart Rhythm Society. All rights reserved.
Sustained monomorphic ventricular tachycardia (VT) occurs during the acute (⬍48 hours after occlusion), late (days to weeks), and chronic (months to years) phases of myocardial infarction (MI).1,2 Reentry is believed to be the underlying mechanism responsible for VT in each of these phases.1,2 VT developing in the late and chronic phases of MI has been extensively studied. In these phases, the reentrant circuit has been mapped to a protected isthmus of This manuscript was processed by a guest editor. Address reprint requests and correspondence: Dr. Jianyi Wu, Krannert Institute of Cardiology, Indiana University School of Medicine, 1800 North Capital Avenue, Indianapolis, Indiana 46202. E-mail address: [email protected]. (Received February 16, 2004; accepted May 10, 2004.)
surviving fibers with slow conduction located at the infarction border zone or within the infarction region.3– 8 However, the mechanisms underlying VT that occurs during the acute phase of MI is not well understood. We used optical mapping to directly visualize the reentrant circuit of VT during the acute phase of MI in isolated canine left ventricular preparations. Accordingly, this study was performed to characterize changes in action potentials (AP) and conduction velocity (CV) to address three specific questions related to reentrant VT (1) What is the anatomic orientation of the reentrant circuit? (2) What is the mechanism underlying sustained versus nonsustained VT? (3) What are the differences between preparations with and without inducible VT?
1547-5271/$ -see front matter © 2004 Heart Rhythm Society. All rights reserved. doi:10.1016/j.hrthm.2004.05.005
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Methods Isolated left ventricular preparation Adult mongrel dogs (weight 20 – 40 kg) were anesthetized, and the hearts were rapidly excised and perfused through the aorta with cardioplegic solution as described elsewhere.9,10 As shown in Figure 1, the preparation was isolated from the left ventricular lateral free wall. The endocardial and mid-layer tissues were trimmed to limit potential intramural reentry. The mean dimensions of the preparations were 58.5 ⫾ 0.7 ⫻ 33.6 ⫾ 0.9 ⫻ 11.6 ⫾ 0.4 mm (n ⫽ 16). The preparation was placed in a double-walled circulated heated tissue bath chamber and perfused with Tyrode’s solution at 37°C through the left circumflex artery at a flow rate of 35 mL/min.9,10 In addition, the preparation was superfused with the same Tyrode’s solution maintained at 37 °C. During the initial 30 minutes of stabilization and the period of tissue staining with optical dye, the tissue was submerged in the bath solution. Cytochalasin-D was added to the perfusate for 30 minutes, after which the level of the bath solution was reduced to just beneath the upper tissue surface to facilitate optical mapping. In the setting of perfusion, it was possible to obtain optical AP signals from both sides of the preparation, suggesting that the entire preparation was viable. No spontaneous activity or arrhythmias were observed after perfusion was established. The tissue then was paced at a cycle length of 700 ms.
Experimental protocol After 30 minutes of stabilization in the perfusion tissue bath, the preparation was stained with 0.2 mg of di-4ANEPPS at a concentration of 2 mol/L. The tissue then was perfused with a solution containing cytochalasin-D at 30 mol/L to eliminate contraction.9,10 Optical mapping and microelectrode recordings were performed on the epicardial side of the preparation before and after MI as described in the following. In four initial experiments, optical mapping also was performed on the other cut surface side. Optical APs obtained from the side where tissue was trimmed away suggested that the entire preparation was perfused well through the coronary cannulae. The programmed stimulation protocols were similar to those commonly used in the electrophysiology laboratory. Briefly, the tissue was paced at a driving cycle length (S1S1) of 700 ms for eight beats, followed by single (S1S2), double (S1S2S3), or triple (S1S2S3S4) extrastimuli. Depending on tissue refractoriness, the coupling intervals of the single or multiple extrastimuli were varied from 400 to 120 ms in decreasing steps of 10 ms.
Optical mapping and microelectrode recording The method of optical mapping and microelectrode recording was similar to that previously described.9,10 The
Figure 1 Optical mapping and microelectrode recordings. A: Isolated left ventricular epicardial preparation. The stimulation electrode (Stim) and recording electrode (Rec) for the extracellular electrogram are located in the top portion of the picture. The square box represents the optical mapping area. The proximal second acute marginal (OM2) artery was ligated (L) to create an acute myocardial infarction (MI) as indicated arbitrarily by the oval area. B: Optical action potentials (APs) recorded from the same preparation. For clarity, only alternate APs are shown in absolute amplitude. C: Optical (OP, top) and intracellular APs (ME) recorded from nonischemic normal tissue (NL), the border zone (BZ), and the infarction area (MI) are superimposed and displayed at a fast sweep speed. The circle in each region as shown in panel B indicates the locations of the recording sites for selected APs. The horizontal dashed lines represent the level of 0 mV for each intracellular AP. The vertical dashed line indicates the beginning of pacing at a cycle length of 700 ms. The activation time (AT) is indicated by the solid vertical lines. D: Optical APs recorded from one of the control experiments without MI. E: Corresponding optical activation map. F: Selected optical APs from sites A and B as shown in panel E are displayed at a fast sweep speed to demonstrate the method used to estimate conduction velocity (see Methods section for details).
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Table 1 Comparison of conduction velocity (n ⫽ 14) and action potential parameters (n ⫽ 6) obtained from nonischemic region and border zone at a pacing cycle length of 700 ms Nonischemic region CV (mm/s) APA (mV) MDP (mV) APD90 (ms) dV/dt (V/s)
512.7 99.1 ⫺80.6 188.4 302.4
⫾ ⫾ ⫾ ⫾ ⫾
11.0 0.4 0.2 1.0 7.9
Border zone 185.0 71.4 ⫺65.4 164.3 168.5
⫾ ⫾ ⫾ ⫾ ⫾
7.6 0.6 0.6 3.1 3.6
P value ⬍.01 ⬍.01 ⬍.01 ⬍.05 ⬍.01
APA ⫽ action potential; APD90 ⫽ action potential duration at 90% repolarization; CV ⫽ conduction velocity; dV/dt ⫽ maximum rate of rise of action potential upstroke; MDP ⫽ maximum diastolic potential.
locations of the pacing site, bipolar recording electrode, and optical mapping are shown in Figure 1A. The optical mapping (16 ⫻ 16 channels) area (19.5 ⫻ 19.5 mm) could be moved to focus on specific sites of interest, such as the reentrant circuit, border zone, or infarction area. The time of activation was determined by the maximum rate of rise from the optical AP. Because anisotropic conduction likely was present in both the nonischemic normal region and the infarction border zone, it was difficult to define CV precisely. The method used to estimate CV in the present study was similar to that previously reported.11 As shown in Figure 1E and 1F, it was calculated from the time interval between 3 and 5 optical sites and the corresponding distance traveled by the activation wavefront detected on the twodimensional optical map, that is: Velocity ⫽ Optical mapping distance ⫼ Conduction time. (1) The distance of each optical mapping site is 1.218 mm (19.5/16). The conduction time is determined by measuring the difference in activation time of the mapping sites. For example (as shown in Figure 1E), the distance between sites A and B was equal to 5 optical mapping sites (1.218 ⫻ 5 ⫽ 6.09 mm). The conduction time between sites A and B was measured as 11.5 ms (Figure 1F). The corresponding CV was 529.9 mm/s (6.09/11.5). These measurements represented “apparent” CVs given that the precise pathway of the wavefront was approximated to that determined from the two-dimensional optical map.
453 Thus, CV might be underestimated if the pathway spread in depth across the thickness of the tissue or traveled at an angle. The rate dependency of conduction was determined in tissues paced at a baseline cycle length of 700 ms for eight cycles, with application of an extrastimulus at intervals ranging from 450 to 180 ms after the eighth cycle. Microelectrode recordings were obtained in six preparations to validate optical AP recordings10 and to obtain intracellular AP data (Tables 1 and 2).
Creation of MI and initiation of VT After obtaining baseline data, the second acute marginal branch of the circumflex artery was completely ligated to create an MI. The infarction area developed gradually, as confirmed by the disappearance of optical AP (Figure 1B) over 3 to 4 hours. After this period, VT became inducible (mean time 252 ⫾ 15 min after ligation) by programmed stimulation at a pacing cycle length of 700 ms with double or triple extrastimuli at shorter cycle lengths.
Statistical analysis Data are expressed as mean ⫾ SEM. Statistical analyses were performed using Student’s t-test for data in Table 1 and 2. P ⬍ .05 was considered statistically significant.
Results Experiments were performed in 16 dogs. Two experiments were performed without creation of MI. No VT or other arrhythmias were observed during perfusion for 6 hours in either of these preparations. In preparations with acute MI (n ⫽ 14), sustained monomorphic VT was induced by programmed stimulation in 50% of the experiments. All of the seven preparations with VT also had nonsustained VT or two to three beats of premature ventricular complexes induced at relatively longer coupling intervals. Polymorphic VT and ventricular fibrillation (VF) were not observed in any of the 14 preparations.
Table 2 Comparison of conduction velocity (n ⫽ 14) and action potential parameters (n ⫽ 6) between pacing at 700 ms and 250 ms in nonischemic region and border zone Nonischemic region 700 ms CV (mm/s) APA (mV) MDP (mV) APD90 (ms) dV/dt (V/s)
512.7 99.1 ⫺80.6 188.4 302.4
Border zone 250 ms
⫾ ⫾ ⫾ ⫾ ⫾
11.0 0.4 0.2 1.0 7.9
475.4 98.8 ⫺80.1 189.3 275.3
⫾ ⫾ ⫾ ⫾ ⫾
10.0 0.3 0.2 1.2 9.9
P value
700 ms
⬍.05 ⬎.05 ⬎.05 ⬎.05 ⬍.05
185.0 71.4 ⫺65.4 164.3 168.5
250 ms ⫾ ⫾ ⫾ ⫾ ⫾
7.6 0.6 0.6 3.1 3.6
142.4 73.5 ⫺65.5 166.2 152.7
P value ⫾ ⫾ ⫾ ⫾ ⫾
7.8 0.5 0.5 3.6 3.2
⬍.05 ⬎.05 ⬎.05 ⬎.05 ⬍.05
APA ⫽ action potential; APD90 ⫽ action potential duration at 90% repolarization; CV ⫽ conduction velocity; dV/dt ⫽ maximum rate of rise of action potential upstroke; MDP ⫽ maximum diastolic potential.
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Characterization of the infarction region and its border zone during acute MI Initial experiments were performed to assess the properties of the isolated ventricular preparations in the setting of acute MI. Three to four hours after ligation of the second acute marginal artery, the preparation developed into three regions: an infarction area with no excitability, a nonischemic region with normal AP configuration, and a transitional border zone. Results obtained from one of the experiments are shown in Figure 1. The infarction area located around the occluded artery was defined by optical mapping as having no recordable AP (panel B) and further confirmed by microelectrode recording (panel C). From the nonischemic region to the infarction area, the optical AP amplitude decreased gradually (panel B), with delayed activation (panel C). Table 1 compares CV and intracellular AP parameters obtained from nonischemic tissue and the border zone. During pacing at a cycle length of 700 ms, a significant decrease in CV (P ⬍ .01, n ⫽ 14) was observed in the border zone. Intracellular recordings (n ⫽ 6) also showed decreased AP amplitude (P ⬍ .01), maximum diastolic potential (P ⬍ .01), APD90 (P ⬍ .05), and dV/dt (P ⬍ .01) compared to normal nonischemic tissue. In two control experiments without creation of MI, CV (515.3 ⫾ 17.1 mm/s), AP amplitude (99.8 ⫾ 2.3 mV), maximal diastolic potential (⫺80.7 ⫾ 0.1 mV), APD90 (188.9 ⫾ 0.5 ms), and dV/dt (314.8 ⫾ 4.6 V/s) were similar to the values obtained in the nonischemic region (Table 1).
Enhancement of rate dependency of delayed conduction in the border zone In addition to slow conduction, a greater rate dependence of delayed activation in the border zone was present compared to nonischemic tissue. Compared to pacing at a cycle length of 700 ms, conduction delay at a shorter coupling interval of 250 ms over the border zone was greater than that in normal tissue (Table 2). That is, velocity in the nonischemic region decreased from 512.7 ⫾ 11.0 to 475.4 ⫾ 10.0 mm/s (7.6 ⫾ 0.1%), whereas velocity in the border zone decreased from 185.0 ⫾ 7.6 to 142.4 ⫾ 7.8 mm/s (23.1 ⫾ 0.3 %). The change of percentiles in CV (23.1% vs 7.6%) was statistically significant (n ⫽ 14, P ⬍ .05). The phenomenon of enhancement of rate-dependent delayed conduction in the border zone also was observed in optical activation maps (Figure 2). The orientation of the optical mapping is similar to that shown in Figure 1A. The mapping area was focused on the junction between normal tissue and the border zone. This region was arbitrarily divided into three zones: zone 1 (Z1) next to the nonischemic tissue, zone 3 (Z3) close to the infarction region, and zone 2 (Z2) between zones 1 and 3. Original recordings of alternate optical APs are shown in Figure 2A in maximum scale of amplitude. Intracellular APs
Figure 2 Rate-dependent slow conduction in the border zone (BZ). A: Original optical action potentials (APs). B,C: Activation maps obtained during pacing at 700 ms and premature coupling interval of 200 ms. The time in each map was referred to the onset of pacing. The BZ was arbitrarily divided into three zones as shown in panel B. D: Optical (top) and intracellular APs obtained from border zones 1, 2, and 3 are superimposed and displayed at a fast sweep speed. The horizontal dashed lines represent the level of 0 mV for each intracellular AP. The vertical dashed line indicates the beginning of pacing at 700/200 ms. Conduction block at the short coupling interval of 200 ms was observed in some of the cells at border zone 3.
recorded in border zones 1, 2, and 3 from the same preparation are superimposed with the corresponding optical APs at a fast sweep speed in Figure 2D. Corresponding activation maps at a pacing cycle length of 700 ms (Figure 2B) with extrastimuli at 200 ms (Figure 2C) are shown. Compared to the nonischemic region during pacing at 700 ms, conduction over the border zone was markedly delayed and was associated with a decrease in AP amplitude and dV/dt, similar to that shown in Figure 1. At the shorter coupling interval of 200 ms, rate-dependent conduction delay was observed in both normal tissue and the border zone. However, delayed conduction over the border zone appeared to be enhanced, as shown in the activation maps in Figure 2B and 2C. When comparing the activation patterns in nonischemic regions between the 20-ms isochronal line in Figure 2B and the 40-ms isochronal line in Figure 2C, the activation patterns were relatively uniform during pacing at a drive cycle length of 700 ms and a short coupling interval of 200 ms. In contrast, the activation patterns in the infarction border
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zone between the 40- and 109-ms isochronal lines in Figure 2B and the 60- and 140-ms isochronal lines in Figure 2C exhibited greater conduction delay that occurred at a shorter coupling interval of 200 ms. Conduction block also was observed in zone 3 by microelectrode recording, as shown in Figure 2D. As a result of delayed conduction, delayed activation also postponed the time of recovery of individual cells in the border zone. For this reason, a premature impulse would be expected to encounter the refractory period of the cell in the border zone from the activation of previous impulse, leading to conduction delay or block. The disproportional reduction in CV suggested that rate-dependent delayed conduction in the border zone was at least associated with a decrease of dV/dt. However, other factors, such as cell uncoupling, may contribute as well.12
Anatomic location of the reentrant circuit In the setting of acute MI, sustained monomorphic VT was induced in seven preparations (VT cycle length 245.3 ⫾ 15.6 ms) at 252 ⫾ 15 minutes after artery occlusion. The reentrant circuit could be visualized directly with optical mapping as functional reentry. Results obtained from a typical study (same preparation shown in Figure 2) are shown in Figure 3. In this preparation, sustained monomorphic VT at a cycle length of 207 ms was induced by triple extrastimuli (700/190/170/120 ms). Original recordings of optical APs at different sites of the reentrant circuit are shown in Figure 3A. Selected sections of the third extrastimuli (S4) and initiation of VT are displayed at fast sweep speed in Figure 3B. The activation maps of S3 and the first beat of VT are shown in Figure 3C and 3D, respectively. The activation maps of S2 and S3 were similar to that shown in Figure 2C. The reentrant circuit of VT and the corresponding anatomic orientation are summarized in Figure 3E. The circuit consisted of four components: the entrance, the protected isthmus, the exit, and the outer loop of conduction in the normal zone. The reentrant circuits of VT induced in six other preparations exhibited the same four components, with the isthmus located between a functional line of block and infarction area. Our experimental methods provided additional data to define the anatomic orientation of the reentrant circuit. First, the reentrant circuit was located outside the infarction area and was not entirely confined to the border zone. The outer loop involved normal tissue located in the nonischemic region. Second, the protected isthmus was located between the infarction area and the line of functional block, rather than between two functional lines of block or two infarction areas. Finally, the functional line of block occurred at the junction between the border zone and the nonischemic tissue.
Figure 3 Sustained ventricular tachycardia (VT) induced by triple extrastimuli (S1S2S3S4: 700/200/170/120 ms). A: Selected optical action potentials from exit, out loop (UL), entrance (Entr), and isthmus (Isth) during initiation of VT are superimposed with the extracellular electrogram. B: Selected portion of the same data shown in panel A is displayed at a fast sweep speed. C, D: Corresponding activation maps obtained during the last beat of pacing (S4) and first beat of VT are shown in panels C and D, respectively. The curved solid line in panel C indicates the spatial extent of the border zone. The solid thick straight line represents the functional line of block. The numbers in each map represent the activation time referred to the onset of S4. E: The four components of the reentrant circuit are superimposed with the activation map shown in panel D.
Initiation and maintenance of VT To assess the mechanisms underlying the initiation of VT, we analyzed the activation sequence of the optical movies during the transition of the last premature pacing stimulus (S4) and the first beat of VT. As shown in Figure 4, three optical movie frames were selected from the same episode of VT shown in Figure 3. The timing of each frame was referred to the onset of S4. Because of rate-dependent delayed conduction, the impulse of S3 was slowly propagated over part of the border zone (Figure 4A, at the lower right corner of the mapping field) when S4 was delivered to the region of nonischemic tissue (Figure 4A, at the left upper corner of the mapping field). With the opposite direction of conduction of S3 and S4, a functional line of
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Heart Rhythm, Vol 1, No 4, October 2004 circuit of sustained VT in Figure 3D, the location of the functional line of block at the exit site was 1.2 mm (one mapping site) closer to the infarction area and resulted in a relatively small isthmus and a shorter reentrant cycle length. Subsequently, the activation wavefront of the last premature ventricular complex was terminated in the isthmus region with a shorter cycle length at 150 ms and no change in the activation pattern. The different locations of the functional line of block at various coupling intervals likely were due to rate-dependent delayed conduction as described earlier.
Impulse propagation in preparations with no inducible VT
Figure 4 Formation of the functional line of block. Selected optical activation movies frames during ventricular tachycardia (VT) initiation are displayed from A–I. These data were obtained from the same VT episode shown in Figure 3. The curved solid line in A–F indicates the spatial extent of the border zone. The thick solid line in G–I represents the functional line of block (line of block). SP ⫽ slow propagation of the impulse over the protective isthmus; UC ⫽ unidirectional conduction propagating in a retrograde fashion through the functional line of block. The opposite direction of impulse propagation initiated by S3 and S4 (A) creates the functional line of block.
In preparations where VT was not inducible, optical mapping showed that programmed extrastimuli did not lead to a line of block at the junction between the nonischemic tissue and border zone. Results from one such experiment are shown in Figure 6. The preparation was paced at basic cycle length of 700 ms and triple extrastimuli (200/180/160 ms) were provided. VT was not inducible at the end of the stimulation as shown in Figure 6A. The optical activation wavefronts of S2 (200 ms) are shown in Figure 6B and 6C. At 37 ms after the onset of stimulation (Figure 6B), the impulse uniformly conducted over the nonischemic region. As the impulse approached the border zone at 57 ms (Figure 6C), the activation sequence separated into multiple wavefronts (Figure 6E), prohibiting the formation of a functional
block line of block developed at the junction between the border zone and the nonischemic tissue (Figure 4G). This resulted in the formation of a protected isthmus between the functional line of block line of block and the nonexcitable infarction tissue. Delayed conduction at the lower end of the line of block allowed distal tissue to be reactivated. The impulse of S4 curved around the lower end of the line of block and slowly proceeded up along the protective isthmus (Figure 4G and 4H), creating unidirectional slow conduction at the upper part of isthmus and subsequently leading to initiation of VT (Figure 4I).
Termination of nonsustained VT In preparations with inducible sustained VT, two to three beats of premature ventricular complexes and nonsustained VT were induced by fewer extrastimuli or a longer coupling interval. Results from the same preparation shown in Figure 4 are shown in Figure 5. Two premature ventricular complexes at cycle lengths of 156 and 150 ms were induced by double extrastimuli (S1S2 200/S2S3 140 ms). The activation patterns of both premature ventricular complexes were similar, as shown in Figure 5B. Compared to the reentrant
Figure 5 Premature ventricular complexes (PVCs) induced by double extrastimuli (S1S2S3: 700/200/140 ms). A: Selected optical action potentials from exit, out loop (UL), entrance (Entr), and isthmus (Isth) during the initiation of PVCs are superimposed with the extracellular electrogram. B: Activation map of the last PVC. The numbers in each map represent the activation time referred to the onset of PVC. C: Termination of slow conduction at the isthmus region as indicated by the thick dash line.
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457 of reentrant VT and support the concept of anisotropic reentry.1,4,5,12
Anatomic orientation of the reentrant circuit
Figure 6 Multiple wavefronts of propagation in preparation with no inducible ventricular tachycardia (VT). A: Selected optical action potentials obtained from a nonischemic region (site 1) and border zone (BZ sites 2 and 3) are superimposed with the extracellular electrogram. VT was not induced by programmed triple extrastimuli (700/200/180/160 ms). The vertical dashed lines indicate the onset of stimulation of S2 at coupling of 200 ms. B,C: Optical activation wavefront of S2 at 37 ms (B) and 57 ms (C).The curved solid line indicates the spatial extent of the BZ. D: Multiple wavefronts are arbitrarily indicated by the arrows, which prevented the formation of a line of block as they approached the BZ. E: The corresponding activation by S2 in panel C are superimposed with the location of sites 1–3 as shown in panel A.
line of block. The irregular shape of infarction border zone (Figure 6D) likely reflected the distribution of the occluded artery. The activation sequences of S3 and S4 were very similar, with multiple wavefronts moving toward the infarction border zone.
Discussion To our knowledge, this study reports for the first time the use of optical mapping to directly visualize the reentrant circuit of monomorphic VT during the acute phase of MI. Our data provide additional insight into our understanding
The reentrant circuit of VT during acute MI involves interaction among the inexcitable infarction area, surviving border zone, and nonischemic normal tissue. The protected isthmus and a functional line of block located at the junction between the nonischemic tissue and the border zone are critical components of this circuit. This reentrant model, theoretically, is neither exclusively anatomic nor functional reentry, but a combination of an anatomic boundary (infarction area) and a functional line of block. The infarction area is not an anatomic obstacle around which the reentry circulates. It acts more like a boundary flanking the circuit. Compared to the classic model of leading circle functional reentry,13,14 the core of the reentrant wavefront within the isthmus is excitable but not excited due to a functional line block.15 This supports the concept of functional spiral wave reentry.16 However, the reentrant pattern is not a smooth and symmetric spiral rotor. Instead, conduction over the nonischemic region is relatively fast and uniform but then becomes slow and nonuniform over the isthmus. Our data are compatible with a model of anisotropic reentrant VT during the late phase of MI.4,5 Anisotropic conduction refers to directional variation in CV and has been characterized as uniform and nonuniform.17 In normal cardiac tissue, uniform anisotropic conduction is due to the transition from faster longitudinal propagation to slower transverse propagation, creating teardrop-shaped isochrones somewhat similar to those shown in Figure 1F. However, in abnormal cardiac tissue, such as in the infarction border zone (Figure 2B and 2C), nonuniform anisotropic conduction likely results from the mismatch of current sink and source due to the combinations of decreased AP amplitude and dV/dt, cell uncoupling, and tissue cellular orientation. Cellular orientation likely plays a minor role in slow conduction in the infarction border zone. As shown in Figure 3C and 3D, conduction over the isthmus during VT was relatively slower than that in the nonischemic region even though tissue orientations were similar in both regions.
Sustained versus nonsustained VT Optical mapping showed that both sustained and nonsustained VTs have similar reentrant circuits in the same preparation. The difference is that nonsustained VT terminated abruptly in the isthmus region, with no change in activation sequence. This observation is very similar to that seen in the subacute epicardial infarction border zone18 and suggests that the underlying mechanism for maintenance of VT is preservation of slow conduction over the isthmus region.
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Inducible versus noninducible VT after acute MI Like other types of functional reentry, a functional line of block, slow conduction, and unidirectional conduction provide reentrant substrates for stable VT. The substrates for slow and rate-dependent delayed conduction are present in all preparations with acute MI. However, compared to preparations with no inducible VT, formation of the functional line of block at a shorter coupling interval is the key mechanism for unidirectional conduction and subsequent initiation of reentrant VT. The dynamic interaction of impulse propagation at the junction between the nonischemic tissue and the border zone determines the development of a line of block for reentry versus multiple wavefronts with no reentry.
Heart Rhythm, Vol 1, No 4, October 2004 slow and rate-dependent delayed conduction exist in all preparations after acute MI. Whether VT can be induced is dependent on whether a line of block develops. Our study demonstrated the formation of a small reentrant circuit and VT within an optical mapping area that was only 2 cm2. Multiple reentrant circuits potentially could be present in the intact heart, leading to the development of polymorphic VT and VF. The significance of this type of functional reentry in late and chronic postinfarction VT is not clear. Despite different substrates, some cases of postinfarction VT might have a similar functional reentrant circuit using the protected isthmus between the functional line of block and the infarction area.8 These data suggest that the phenomenon of sustained monomorphic VT occurring during acute MI warrants further diagnostic study and therapeutic interventions.
Study limitations There are several limitations in our study. First, the depth of optical mapping is 2 to 3 mm,11,19 which is significantly less than the preparation thickness (11.6 mm). It is possible that intramural reentry might not be seen by optical mapping. However, the complete reentrant circuit of VT was mapped continuously, suggesting that VT observed in the present study likely was not the result of intramural reentry. Second, because of the presence of anisotropic conduction in cardiac tissue, CVs measured on the two-dimensional optical map might be underestimated if the activation pathway spreads in depth or at an angle. Third, the preparations used in the present study were relatively small. This likely would prevent the development of multiple reentrant circuits and the more typical polymorphic VT and VF seen with acute MI. For that reason, VT was induced in 50% of the preparations, but polymorphic VT and VF were not observed. It also might restrict the development of anatomic reentry around the infarction area6,7 and figure-of-eight reentry in the infarction border zone.4,5 Consequently, the findings of the present study do not preclude other potential reentrant circuits proposed by other investigators. Finally, the low signal-to-noise ratio in the infarction border zone, as shown in Figures 1 and 2, made it difficult to determine gap junctional conduction using optical mapping in our preparation.20 However, as shown in many previous studies, remodeling of gap junctions in the infarction border zone contributes to the development of anisotropic conduction and reentrant VT.21–24
Clinical implications The incidence of spontaneous sustained monomorphic VT in the acute phase of MI is reported as 1.9%, and its recurrence rate is high as 17%.25 Although sustained VT occurring in the setting of acute MI once was thought to have no prognostic value,26,27 more recent data suggest that it is an independent predictor for high mortality both inhospital and at 1 year post-MI.25,28,29 The substrates for
References 1. Janse MJ, Wit AL. Electrophysiological mechanisms of ventricular arrhythmias resulting from myocardial ischemia and infarction. Physiol Rev 1989;69:1049 –1169. 2. Callans DJ, Josephson ME. Ventricular tachycardia in patients with coronary artery disease. In: Zipes DP, Jalife J, eds. Cardiac electrophysiology: from cell to bedside, 3rd ed. Philadelphia: WB Saunders, 2000:530 –536. 3. de Bakker JM, van Capelle FJ, Janse MJ, Wilde AA, Coronel R, Becker AE, Dingemans KP, van Hemel NM, Hauer RN. Reentry as a cause of ventricular tachycardia in patients with chronic ischemic heart disease: electrophysiologic and anatomic correlation. Circulation 1988;77:589 – 606. 4. Costeas C, Peters NS, Waldecker B, Ciaccio EJ, Wit AL, Coromilas J. Mechanisms causing sustained ventricular tachycardia with multiple QRS morphologies: results of mapping studies in the infarcted canine heart. Circulation 1997;96:3721–3731. 5. Peters NS, Coromilas J, Hanna MS, Josephson ME, Costeas C, Wit AL. Characteristics of the temporal and spatial excitable gap in anisotropic reentrant circuits causing sustained ventricular tachycardia. Circ Res 1998;82:279 –293. 6. Stevenson WG, Friedman PL, Kocovic D, Sager PT, Saxon LA, Pavri B. Radiofrequency catheter ablation of ventricular tachycardia after myocardial infarction. Circulation 1998;98:308 –314. 7. El-Shalakany A, Hadjis T, Papageorgiou P, Monahan K, Epstein L, Josephson ME. Entrainment/mapping criteria for the prediction of termination of ventricular tachycardia by single radiofrequency lesion in patients with coronary artery disease. Circulation 1999;99:2283– 2289. 8. de Chillou C, Lacroix D, Klug D, Magnin-Poull I, Marquie C, Messier M, Andronache M, Kouakam C, Sadoul N, Chen J, Aliot E, Kacet S. Isthmus characteristics of reentrant ventricular tachycardia after myocardial infarction. Circulation 2002;105:726 –731. 9. Wu J, Zipes DP. Transmural reentry during acute global ischemia and reperfusion in canine ventricular muscle. Am J Physiol Heart Circ Physiol 2001;280:H2717–H2725. 10. Wu J, Wu J, Olgin J, Miller JM, Zipes DP. Mechanisms underlying the reentrant circuit of atrioventricular nodal reentrant tachycardia in isolated canine atrioventricular nodal preparation using optical mapping. Circ Res 2001;88:1189 –1195. 11. Choi B, Salama G. Optical mapping of atrioventricular node reveals a conduction barrier between atrial and nodal cells. Am J Physiol 1998; 274:H829 –H845. 12. Pinto JM, Boyden PA. Electrical remodeling in ischemia and infarction. Cardiovasc Res 1999;42:284 –297.
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13. Allessie MA, Bonke FI, Schopman FJ. Circus movement in rabbit atrial muscle as a mechanism of tachycardia. III. The “leading circle” concept: a new model of circus movement in cardiac tissue without the involvement of an anatomical obstacle. Circ Res 1977;41:9 –18. 14. Schalij MJ, Boersma L, Huijberts M, Allessie MA. Anisotropic reentry in a perfused 2-dimensional layer of rabbit ventricular myocardium. Circulation 2000;102:2650 –2658. 15. Athill CA, Ikeda T, Kim YH, Wu TJ, Fishbein MC, Karagueuzian HS, Chen PS. Transmembrane potential properties at the core of functional reentrant wave fronts in isolated canine right atria. Circulation 1998; 98:1556 –1567. 16. Pertsov AM, Davidenko JM, Salomonsz R, Baxter WT, Jalife J. Spiral waves of excitation underlie reentrant activity in isolated cardiac muscle. Circ Res 1993;72:631– 650. 17. Spach MS, Heidlage FJ, Dolber PC. The dual nature of anisotropic discontinuous conduction in the heart. In: Zipes DP, Jalife J, eds. Cardiac electrophysiology: from cell to bedside, 3rd ed. Philadelphia: WB Saunders, 2000:213–222. 18. Schmitt H, Wit AL, Coromilas J, Waldecker B. Mechanisms for spontaneous termination of monomorphic sustained ventricular tachycardia: results of activation mapping of reentrant circuit in the epicardial border zone of subacute canine infarcts. J Am Coll Cardiol 1998;31:460 – 470. 19. Baxter WT, Mironov SF, Zaitsev AV, Jalife J, Pertsov AM. Visualizing excitation waves inside cardiac muscle using transillumination. Biophys J 2001;80:516 –530. 20. Akar FG, Roth BJ, Rosenbaum DS. Optical measurement of cell-tocell coupling in intact heart using subthreshold electrical stimulation. Am J Physiol Heart Circ Physiol 2001;281:H533–H542. 21. Dillon SM, Allessie MA, Ursell PC, Wit AL. Influences of anisotropic tissue structure on reentrant circuits in the epicardial border zone of subacute canine infarcts. Circ Res 1988;63:182–206.
459 22. Peters NS, Coromilas J, Severs NJ, Wit AL. Disturbed connexin 43 gap junction distribution correlates with the location of reentrant circuits in the epicardial border zone of healing canine infarcts that cause ventricular tachycardia. Circulation 1997;95:988 –996. 23. Beardslee MA, Lerner DL, Tadros PN, Laing JG, Beyer EC, Yamada KA, Kleber AG, Schuessler RB, Saffitz JE. Dephosphorylation and intracellular redistribution of ventricular connexin 43 during electrical uncoupling induced by ischemia. Circ Res 2000;87:656 – 662. 24. Yao JA, Hussain W, Patel P, Peters NS, Boyden PA, Wit AL. Remodeling of gap junction channel function in epicardial border zone of healing canine infarcts. Circ Res 2003;92:437– 443. 25. Mont L, Cinca J, Blanch P, Blanco J, Figueras J, Brotons C, SolerSoler J. Predisposing factors and prognostic value of sustained monomorphic ventricular tachycardia in the early phase of acute myocardial infarction. J Am Coll Cardiol 1996;28:1670 –1676. 26. Bigger JT Jr, Fleiss JL, Rolnitzky LM. Prevalence, characteristics and significance of ventricular tachycardia detected by 24-hour continuous electrocardiographic recordings in the late hospital phase of acute myocardial infarction. Am J Cardiol 1986;58:1151–1160. 27. Tofler GH, Stone PH, Muller JE, Rutherford JD, Willich SN, Gustafson NF, Poole WK, Sobel BE, Willerson JT, Robertson T. Prognosis after cardiac arrest due to ventricular tachycardia or ventricular fibrillation associated with acute myocardial infarction (the MILIS Study). Multicenter Investigation of the Limitation of Infarct Size. Am J Cardiol 1987;60:755–761. 28. Newby KH, Thompson T, Stebbins A, Topol EJ, Califf RM, Natale A. Sustained ventricular arrhythmias in patients receiving thrombolytic therapy: incidence and outcomes. The GUSTO Investigators. Circulation 1998;98:2567–2573. 29. Sala MF, Barcena JP, Rota JI, Roca JV, Lopez CA, Puigdevall JM, Soldevilla JG, de Luna AB. Sustained ventricular tachycardia as a marker of inadequate myocardial perfusion during the acute phase of myocardial infarction. Clin Cardiol 2002;25:328 –334.
Sustained monomorphic ventricular tachycardia (VT) occurs during the acute (⬍48 hours after occlusion), late (days to weeks), and chronic (months to years) phases of myocardial infarction (MI).1,2 Reentry is believed to be the underlying mechanism responsible for VT in each of these phases.1,2 VT developing in the late and chronic phases of MI has been extensively studied. In these phases, the reentrant circuit has been mapped to a protected isthmus of This manuscript was processed by a guest editor. Address reprint requests and correspondence: Dr. Jianyi Wu, Krannert Institute of Cardiology, Indiana University School of Medicine, 1800 North Capital Avenue, Indianapolis, Indiana 46202. E-mail address: [email protected]. (Received February 16, 2004; accepted May 10, 2004.)
surviving fibers with slow conduction located at the infarction border zone or within the infarction region.3– 8 However, the mechanisms underlying VT that occurs during the acute phase of MI is not well understood. We used optical mapping to directly visualize the reentrant circuit of VT during the acute phase of MI in isolated canine left ventricular preparations. Accordingly, this study was performed to characterize changes in action potentials (AP) and conduction velocity (CV) to address three specific questions related to reentrant VT (1) What is the anatomic orientation of the reentrant circuit? (2) What is the mechanism underlying sustained versus nonsustained VT? (3) What are the differences between preparations with and without inducible VT?
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Methods Isolated left ventricular preparation Adult mongrel dogs (weight 20 – 40 kg) were anesthetized, and the hearts were rapidly excised and perfused through the aorta with cardioplegic solution as described elsewhere.9,10 As shown in Figure 1, the preparation was isolated from the left ventricular lateral free wall. The endocardial and mid-layer tissues were trimmed to limit potential intramural reentry. The mean dimensions of the preparations were 58.5 ⫾ 0.7 ⫻ 33.6 ⫾ 0.9 ⫻ 11.6 ⫾ 0.4 mm (n ⫽ 16). The preparation was placed in a double-walled circulated heated tissue bath chamber and perfused with Tyrode’s solution at 37°C through the left circumflex artery at a flow rate of 35 mL/min.9,10 In addition, the preparation was superfused with the same Tyrode’s solution maintained at 37 °C. During the initial 30 minutes of stabilization and the period of tissue staining with optical dye, the tissue was submerged in the bath solution. Cytochalasin-D was added to the perfusate for 30 minutes, after which the level of the bath solution was reduced to just beneath the upper tissue surface to facilitate optical mapping. In the setting of perfusion, it was possible to obtain optical AP signals from both sides of the preparation, suggesting that the entire preparation was viable. No spontaneous activity or arrhythmias were observed after perfusion was established. The tissue then was paced at a cycle length of 700 ms.
Experimental protocol After 30 minutes of stabilization in the perfusion tissue bath, the preparation was stained with 0.2 mg of di-4ANEPPS at a concentration of 2 mol/L. The tissue then was perfused with a solution containing cytochalasin-D at 30 mol/L to eliminate contraction.9,10 Optical mapping and microelectrode recordings were performed on the epicardial side of the preparation before and after MI as described in the following. In four initial experiments, optical mapping also was performed on the other cut surface side. Optical APs obtained from the side where tissue was trimmed away suggested that the entire preparation was perfused well through the coronary cannulae. The programmed stimulation protocols were similar to those commonly used in the electrophysiology laboratory. Briefly, the tissue was paced at a driving cycle length (S1S1) of 700 ms for eight beats, followed by single (S1S2), double (S1S2S3), or triple (S1S2S3S4) extrastimuli. Depending on tissue refractoriness, the coupling intervals of the single or multiple extrastimuli were varied from 400 to 120 ms in decreasing steps of 10 ms.
Optical mapping and microelectrode recording The method of optical mapping and microelectrode recording was similar to that previously described.9,10 The
Figure 1 Optical mapping and microelectrode recordings. A: Isolated left ventricular epicardial preparation. The stimulation electrode (Stim) and recording electrode (Rec) for the extracellular electrogram are located in the top portion of the picture. The square box represents the optical mapping area. The proximal second acute marginal (OM2) artery was ligated (L) to create an acute myocardial infarction (MI) as indicated arbitrarily by the oval area. B: Optical action potentials (APs) recorded from the same preparation. For clarity, only alternate APs are shown in absolute amplitude. C: Optical (OP, top) and intracellular APs (ME) recorded from nonischemic normal tissue (NL), the border zone (BZ), and the infarction area (MI) are superimposed and displayed at a fast sweep speed. The circle in each region as shown in panel B indicates the locations of the recording sites for selected APs. The horizontal dashed lines represent the level of 0 mV for each intracellular AP. The vertical dashed line indicates the beginning of pacing at a cycle length of 700 ms. The activation time (AT) is indicated by the solid vertical lines. D: Optical APs recorded from one of the control experiments without MI. E: Corresponding optical activation map. F: Selected optical APs from sites A and B as shown in panel E are displayed at a fast sweep speed to demonstrate the method used to estimate conduction velocity (see Methods section for details).
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Table 1 Comparison of conduction velocity (n ⫽ 14) and action potential parameters (n ⫽ 6) obtained from nonischemic region and border zone at a pacing cycle length of 700 ms Nonischemic region CV (mm/s) APA (mV) MDP (mV) APD90 (ms) dV/dt (V/s)
512.7 99.1 ⫺80.6 188.4 302.4
⫾ ⫾ ⫾ ⫾ ⫾
11.0 0.4 0.2 1.0 7.9
Border zone 185.0 71.4 ⫺65.4 164.3 168.5
⫾ ⫾ ⫾ ⫾ ⫾
7.6 0.6 0.6 3.1 3.6
P value ⬍.01 ⬍.01 ⬍.01 ⬍.05 ⬍.01
APA ⫽ action potential; APD90 ⫽ action potential duration at 90% repolarization; CV ⫽ conduction velocity; dV/dt ⫽ maximum rate of rise of action potential upstroke; MDP ⫽ maximum diastolic potential.
locations of the pacing site, bipolar recording electrode, and optical mapping are shown in Figure 1A. The optical mapping (16 ⫻ 16 channels) area (19.5 ⫻ 19.5 mm) could be moved to focus on specific sites of interest, such as the reentrant circuit, border zone, or infarction area. The time of activation was determined by the maximum rate of rise from the optical AP. Because anisotropic conduction likely was present in both the nonischemic normal region and the infarction border zone, it was difficult to define CV precisely. The method used to estimate CV in the present study was similar to that previously reported.11 As shown in Figure 1E and 1F, it was calculated from the time interval between 3 and 5 optical sites and the corresponding distance traveled by the activation wavefront detected on the twodimensional optical map, that is: Velocity ⫽ Optical mapping distance ⫼ Conduction time. (1) The distance of each optical mapping site is 1.218 mm (19.5/16). The conduction time is determined by measuring the difference in activation time of the mapping sites. For example (as shown in Figure 1E), the distance between sites A and B was equal to 5 optical mapping sites (1.218 ⫻ 5 ⫽ 6.09 mm). The conduction time between sites A and B was measured as 11.5 ms (Figure 1F). The corresponding CV was 529.9 mm/s (6.09/11.5). These measurements represented “apparent” CVs given that the precise pathway of the wavefront was approximated to that determined from the two-dimensional optical map.
453 Thus, CV might be underestimated if the pathway spread in depth across the thickness of the tissue or traveled at an angle. The rate dependency of conduction was determined in tissues paced at a baseline cycle length of 700 ms for eight cycles, with application of an extrastimulus at intervals ranging from 450 to 180 ms after the eighth cycle. Microelectrode recordings were obtained in six preparations to validate optical AP recordings10 and to obtain intracellular AP data (Tables 1 and 2).
Creation of MI and initiation of VT After obtaining baseline data, the second acute marginal branch of the circumflex artery was completely ligated to create an MI. The infarction area developed gradually, as confirmed by the disappearance of optical AP (Figure 1B) over 3 to 4 hours. After this period, VT became inducible (mean time 252 ⫾ 15 min after ligation) by programmed stimulation at a pacing cycle length of 700 ms with double or triple extrastimuli at shorter cycle lengths.
Statistical analysis Data are expressed as mean ⫾ SEM. Statistical analyses were performed using Student’s t-test for data in Table 1 and 2. P ⬍ .05 was considered statistically significant.
Results Experiments were performed in 16 dogs. Two experiments were performed without creation of MI. No VT or other arrhythmias were observed during perfusion for 6 hours in either of these preparations. In preparations with acute MI (n ⫽ 14), sustained monomorphic VT was induced by programmed stimulation in 50% of the experiments. All of the seven preparations with VT also had nonsustained VT or two to three beats of premature ventricular complexes induced at relatively longer coupling intervals. Polymorphic VT and ventricular fibrillation (VF) were not observed in any of the 14 preparations.
Table 2 Comparison of conduction velocity (n ⫽ 14) and action potential parameters (n ⫽ 6) between pacing at 700 ms and 250 ms in nonischemic region and border zone Nonischemic region 700 ms CV (mm/s) APA (mV) MDP (mV) APD90 (ms) dV/dt (V/s)
512.7 99.1 ⫺80.6 188.4 302.4
Border zone 250 ms
⫾ ⫾ ⫾ ⫾ ⫾
11.0 0.4 0.2 1.0 7.9
475.4 98.8 ⫺80.1 189.3 275.3
⫾ ⫾ ⫾ ⫾ ⫾
10.0 0.3 0.2 1.2 9.9
P value
700 ms
⬍.05 ⬎.05 ⬎.05 ⬎.05 ⬍.05
185.0 71.4 ⫺65.4 164.3 168.5
250 ms ⫾ ⫾ ⫾ ⫾ ⫾
7.6 0.6 0.6 3.1 3.6
142.4 73.5 ⫺65.5 166.2 152.7
P value ⫾ ⫾ ⫾ ⫾ ⫾
7.8 0.5 0.5 3.6 3.2
⬍.05 ⬎.05 ⬎.05 ⬎.05 ⬍.05
APA ⫽ action potential; APD90 ⫽ action potential duration at 90% repolarization; CV ⫽ conduction velocity; dV/dt ⫽ maximum rate of rise of action potential upstroke; MDP ⫽ maximum diastolic potential.
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Characterization of the infarction region and its border zone during acute MI Initial experiments were performed to assess the properties of the isolated ventricular preparations in the setting of acute MI. Three to four hours after ligation of the second acute marginal artery, the preparation developed into three regions: an infarction area with no excitability, a nonischemic region with normal AP configuration, and a transitional border zone. Results obtained from one of the experiments are shown in Figure 1. The infarction area located around the occluded artery was defined by optical mapping as having no recordable AP (panel B) and further confirmed by microelectrode recording (panel C). From the nonischemic region to the infarction area, the optical AP amplitude decreased gradually (panel B), with delayed activation (panel C). Table 1 compares CV and intracellular AP parameters obtained from nonischemic tissue and the border zone. During pacing at a cycle length of 700 ms, a significant decrease in CV (P ⬍ .01, n ⫽ 14) was observed in the border zone. Intracellular recordings (n ⫽ 6) also showed decreased AP amplitude (P ⬍ .01), maximum diastolic potential (P ⬍ .01), APD90 (P ⬍ .05), and dV/dt (P ⬍ .01) compared to normal nonischemic tissue. In two control experiments without creation of MI, CV (515.3 ⫾ 17.1 mm/s), AP amplitude (99.8 ⫾ 2.3 mV), maximal diastolic potential (⫺80.7 ⫾ 0.1 mV), APD90 (188.9 ⫾ 0.5 ms), and dV/dt (314.8 ⫾ 4.6 V/s) were similar to the values obtained in the nonischemic region (Table 1).
Enhancement of rate dependency of delayed conduction in the border zone In addition to slow conduction, a greater rate dependence of delayed activation in the border zone was present compared to nonischemic tissue. Compared to pacing at a cycle length of 700 ms, conduction delay at a shorter coupling interval of 250 ms over the border zone was greater than that in normal tissue (Table 2). That is, velocity in the nonischemic region decreased from 512.7 ⫾ 11.0 to 475.4 ⫾ 10.0 mm/s (7.6 ⫾ 0.1%), whereas velocity in the border zone decreased from 185.0 ⫾ 7.6 to 142.4 ⫾ 7.8 mm/s (23.1 ⫾ 0.3 %). The change of percentiles in CV (23.1% vs 7.6%) was statistically significant (n ⫽ 14, P ⬍ .05). The phenomenon of enhancement of rate-dependent delayed conduction in the border zone also was observed in optical activation maps (Figure 2). The orientation of the optical mapping is similar to that shown in Figure 1A. The mapping area was focused on the junction between normal tissue and the border zone. This region was arbitrarily divided into three zones: zone 1 (Z1) next to the nonischemic tissue, zone 3 (Z3) close to the infarction region, and zone 2 (Z2) between zones 1 and 3. Original recordings of alternate optical APs are shown in Figure 2A in maximum scale of amplitude. Intracellular APs
Figure 2 Rate-dependent slow conduction in the border zone (BZ). A: Original optical action potentials (APs). B,C: Activation maps obtained during pacing at 700 ms and premature coupling interval of 200 ms. The time in each map was referred to the onset of pacing. The BZ was arbitrarily divided into three zones as shown in panel B. D: Optical (top) and intracellular APs obtained from border zones 1, 2, and 3 are superimposed and displayed at a fast sweep speed. The horizontal dashed lines represent the level of 0 mV for each intracellular AP. The vertical dashed line indicates the beginning of pacing at 700/200 ms. Conduction block at the short coupling interval of 200 ms was observed in some of the cells at border zone 3.
recorded in border zones 1, 2, and 3 from the same preparation are superimposed with the corresponding optical APs at a fast sweep speed in Figure 2D. Corresponding activation maps at a pacing cycle length of 700 ms (Figure 2B) with extrastimuli at 200 ms (Figure 2C) are shown. Compared to the nonischemic region during pacing at 700 ms, conduction over the border zone was markedly delayed and was associated with a decrease in AP amplitude and dV/dt, similar to that shown in Figure 1. At the shorter coupling interval of 200 ms, rate-dependent conduction delay was observed in both normal tissue and the border zone. However, delayed conduction over the border zone appeared to be enhanced, as shown in the activation maps in Figure 2B and 2C. When comparing the activation patterns in nonischemic regions between the 20-ms isochronal line in Figure 2B and the 40-ms isochronal line in Figure 2C, the activation patterns were relatively uniform during pacing at a drive cycle length of 700 ms and a short coupling interval of 200 ms. In contrast, the activation patterns in the infarction border
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zone between the 40- and 109-ms isochronal lines in Figure 2B and the 60- and 140-ms isochronal lines in Figure 2C exhibited greater conduction delay that occurred at a shorter coupling interval of 200 ms. Conduction block also was observed in zone 3 by microelectrode recording, as shown in Figure 2D. As a result of delayed conduction, delayed activation also postponed the time of recovery of individual cells in the border zone. For this reason, a premature impulse would be expected to encounter the refractory period of the cell in the border zone from the activation of previous impulse, leading to conduction delay or block. The disproportional reduction in CV suggested that rate-dependent delayed conduction in the border zone was at least associated with a decrease of dV/dt. However, other factors, such as cell uncoupling, may contribute as well.12
Anatomic location of the reentrant circuit In the setting of acute MI, sustained monomorphic VT was induced in seven preparations (VT cycle length 245.3 ⫾ 15.6 ms) at 252 ⫾ 15 minutes after artery occlusion. The reentrant circuit could be visualized directly with optical mapping as functional reentry. Results obtained from a typical study (same preparation shown in Figure 2) are shown in Figure 3. In this preparation, sustained monomorphic VT at a cycle length of 207 ms was induced by triple extrastimuli (700/190/170/120 ms). Original recordings of optical APs at different sites of the reentrant circuit are shown in Figure 3A. Selected sections of the third extrastimuli (S4) and initiation of VT are displayed at fast sweep speed in Figure 3B. The activation maps of S3 and the first beat of VT are shown in Figure 3C and 3D, respectively. The activation maps of S2 and S3 were similar to that shown in Figure 2C. The reentrant circuit of VT and the corresponding anatomic orientation are summarized in Figure 3E. The circuit consisted of four components: the entrance, the protected isthmus, the exit, and the outer loop of conduction in the normal zone. The reentrant circuits of VT induced in six other preparations exhibited the same four components, with the isthmus located between a functional line of block and infarction area. Our experimental methods provided additional data to define the anatomic orientation of the reentrant circuit. First, the reentrant circuit was located outside the infarction area and was not entirely confined to the border zone. The outer loop involved normal tissue located in the nonischemic region. Second, the protected isthmus was located between the infarction area and the line of functional block, rather than between two functional lines of block or two infarction areas. Finally, the functional line of block occurred at the junction between the border zone and the nonischemic tissue.
Figure 3 Sustained ventricular tachycardia (VT) induced by triple extrastimuli (S1S2S3S4: 700/200/170/120 ms). A: Selected optical action potentials from exit, out loop (UL), entrance (Entr), and isthmus (Isth) during initiation of VT are superimposed with the extracellular electrogram. B: Selected portion of the same data shown in panel A is displayed at a fast sweep speed. C, D: Corresponding activation maps obtained during the last beat of pacing (S4) and first beat of VT are shown in panels C and D, respectively. The curved solid line in panel C indicates the spatial extent of the border zone. The solid thick straight line represents the functional line of block. The numbers in each map represent the activation time referred to the onset of S4. E: The four components of the reentrant circuit are superimposed with the activation map shown in panel D.
Initiation and maintenance of VT To assess the mechanisms underlying the initiation of VT, we analyzed the activation sequence of the optical movies during the transition of the last premature pacing stimulus (S4) and the first beat of VT. As shown in Figure 4, three optical movie frames were selected from the same episode of VT shown in Figure 3. The timing of each frame was referred to the onset of S4. Because of rate-dependent delayed conduction, the impulse of S3 was slowly propagated over part of the border zone (Figure 4A, at the lower right corner of the mapping field) when S4 was delivered to the region of nonischemic tissue (Figure 4A, at the left upper corner of the mapping field). With the opposite direction of conduction of S3 and S4, a functional line of
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Heart Rhythm, Vol 1, No 4, October 2004 circuit of sustained VT in Figure 3D, the location of the functional line of block at the exit site was 1.2 mm (one mapping site) closer to the infarction area and resulted in a relatively small isthmus and a shorter reentrant cycle length. Subsequently, the activation wavefront of the last premature ventricular complex was terminated in the isthmus region with a shorter cycle length at 150 ms and no change in the activation pattern. The different locations of the functional line of block at various coupling intervals likely were due to rate-dependent delayed conduction as described earlier.
Impulse propagation in preparations with no inducible VT
Figure 4 Formation of the functional line of block. Selected optical activation movies frames during ventricular tachycardia (VT) initiation are displayed from A–I. These data were obtained from the same VT episode shown in Figure 3. The curved solid line in A–F indicates the spatial extent of the border zone. The thick solid line in G–I represents the functional line of block (line of block). SP ⫽ slow propagation of the impulse over the protective isthmus; UC ⫽ unidirectional conduction propagating in a retrograde fashion through the functional line of block. The opposite direction of impulse propagation initiated by S3 and S4 (A) creates the functional line of block.
In preparations where VT was not inducible, optical mapping showed that programmed extrastimuli did not lead to a line of block at the junction between the nonischemic tissue and border zone. Results from one such experiment are shown in Figure 6. The preparation was paced at basic cycle length of 700 ms and triple extrastimuli (200/180/160 ms) were provided. VT was not inducible at the end of the stimulation as shown in Figure 6A. The optical activation wavefronts of S2 (200 ms) are shown in Figure 6B and 6C. At 37 ms after the onset of stimulation (Figure 6B), the impulse uniformly conducted over the nonischemic region. As the impulse approached the border zone at 57 ms (Figure 6C), the activation sequence separated into multiple wavefronts (Figure 6E), prohibiting the formation of a functional
block line of block developed at the junction between the border zone and the nonischemic tissue (Figure 4G). This resulted in the formation of a protected isthmus between the functional line of block line of block and the nonexcitable infarction tissue. Delayed conduction at the lower end of the line of block allowed distal tissue to be reactivated. The impulse of S4 curved around the lower end of the line of block and slowly proceeded up along the protective isthmus (Figure 4G and 4H), creating unidirectional slow conduction at the upper part of isthmus and subsequently leading to initiation of VT (Figure 4I).
Termination of nonsustained VT In preparations with inducible sustained VT, two to three beats of premature ventricular complexes and nonsustained VT were induced by fewer extrastimuli or a longer coupling interval. Results from the same preparation shown in Figure 4 are shown in Figure 5. Two premature ventricular complexes at cycle lengths of 156 and 150 ms were induced by double extrastimuli (S1S2 200/S2S3 140 ms). The activation patterns of both premature ventricular complexes were similar, as shown in Figure 5B. Compared to the reentrant
Figure 5 Premature ventricular complexes (PVCs) induced by double extrastimuli (S1S2S3: 700/200/140 ms). A: Selected optical action potentials from exit, out loop (UL), entrance (Entr), and isthmus (Isth) during the initiation of PVCs are superimposed with the extracellular electrogram. B: Activation map of the last PVC. The numbers in each map represent the activation time referred to the onset of PVC. C: Termination of slow conduction at the isthmus region as indicated by the thick dash line.
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457 of reentrant VT and support the concept of anisotropic reentry.1,4,5,12
Anatomic orientation of the reentrant circuit
Figure 6 Multiple wavefronts of propagation in preparation with no inducible ventricular tachycardia (VT). A: Selected optical action potentials obtained from a nonischemic region (site 1) and border zone (BZ sites 2 and 3) are superimposed with the extracellular electrogram. VT was not induced by programmed triple extrastimuli (700/200/180/160 ms). The vertical dashed lines indicate the onset of stimulation of S2 at coupling of 200 ms. B,C: Optical activation wavefront of S2 at 37 ms (B) and 57 ms (C).The curved solid line indicates the spatial extent of the BZ. D: Multiple wavefronts are arbitrarily indicated by the arrows, which prevented the formation of a line of block as they approached the BZ. E: The corresponding activation by S2 in panel C are superimposed with the location of sites 1–3 as shown in panel A.
line of block. The irregular shape of infarction border zone (Figure 6D) likely reflected the distribution of the occluded artery. The activation sequences of S3 and S4 were very similar, with multiple wavefronts moving toward the infarction border zone.
Discussion To our knowledge, this study reports for the first time the use of optical mapping to directly visualize the reentrant circuit of monomorphic VT during the acute phase of MI. Our data provide additional insight into our understanding
The reentrant circuit of VT during acute MI involves interaction among the inexcitable infarction area, surviving border zone, and nonischemic normal tissue. The protected isthmus and a functional line of block located at the junction between the nonischemic tissue and the border zone are critical components of this circuit. This reentrant model, theoretically, is neither exclusively anatomic nor functional reentry, but a combination of an anatomic boundary (infarction area) and a functional line of block. The infarction area is not an anatomic obstacle around which the reentry circulates. It acts more like a boundary flanking the circuit. Compared to the classic model of leading circle functional reentry,13,14 the core of the reentrant wavefront within the isthmus is excitable but not excited due to a functional line block.15 This supports the concept of functional spiral wave reentry.16 However, the reentrant pattern is not a smooth and symmetric spiral rotor. Instead, conduction over the nonischemic region is relatively fast and uniform but then becomes slow and nonuniform over the isthmus. Our data are compatible with a model of anisotropic reentrant VT during the late phase of MI.4,5 Anisotropic conduction refers to directional variation in CV and has been characterized as uniform and nonuniform.17 In normal cardiac tissue, uniform anisotropic conduction is due to the transition from faster longitudinal propagation to slower transverse propagation, creating teardrop-shaped isochrones somewhat similar to those shown in Figure 1F. However, in abnormal cardiac tissue, such as in the infarction border zone (Figure 2B and 2C), nonuniform anisotropic conduction likely results from the mismatch of current sink and source due to the combinations of decreased AP amplitude and dV/dt, cell uncoupling, and tissue cellular orientation. Cellular orientation likely plays a minor role in slow conduction in the infarction border zone. As shown in Figure 3C and 3D, conduction over the isthmus during VT was relatively slower than that in the nonischemic region even though tissue orientations were similar in both regions.
Sustained versus nonsustained VT Optical mapping showed that both sustained and nonsustained VTs have similar reentrant circuits in the same preparation. The difference is that nonsustained VT terminated abruptly in the isthmus region, with no change in activation sequence. This observation is very similar to that seen in the subacute epicardial infarction border zone18 and suggests that the underlying mechanism for maintenance of VT is preservation of slow conduction over the isthmus region.
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Inducible versus noninducible VT after acute MI Like other types of functional reentry, a functional line of block, slow conduction, and unidirectional conduction provide reentrant substrates for stable VT. The substrates for slow and rate-dependent delayed conduction are present in all preparations with acute MI. However, compared to preparations with no inducible VT, formation of the functional line of block at a shorter coupling interval is the key mechanism for unidirectional conduction and subsequent initiation of reentrant VT. The dynamic interaction of impulse propagation at the junction between the nonischemic tissue and the border zone determines the development of a line of block for reentry versus multiple wavefronts with no reentry.
Heart Rhythm, Vol 1, No 4, October 2004 slow and rate-dependent delayed conduction exist in all preparations after acute MI. Whether VT can be induced is dependent on whether a line of block develops. Our study demonstrated the formation of a small reentrant circuit and VT within an optical mapping area that was only 2 cm2. Multiple reentrant circuits potentially could be present in the intact heart, leading to the development of polymorphic VT and VF. The significance of this type of functional reentry in late and chronic postinfarction VT is not clear. Despite different substrates, some cases of postinfarction VT might have a similar functional reentrant circuit using the protected isthmus between the functional line of block and the infarction area.8 These data suggest that the phenomenon of sustained monomorphic VT occurring during acute MI warrants further diagnostic study and therapeutic interventions.
Study limitations There are several limitations in our study. First, the depth of optical mapping is 2 to 3 mm,11,19 which is significantly less than the preparation thickness (11.6 mm). It is possible that intramural reentry might not be seen by optical mapping. However, the complete reentrant circuit of VT was mapped continuously, suggesting that VT observed in the present study likely was not the result of intramural reentry. Second, because of the presence of anisotropic conduction in cardiac tissue, CVs measured on the two-dimensional optical map might be underestimated if the activation pathway spreads in depth or at an angle. Third, the preparations used in the present study were relatively small. This likely would prevent the development of multiple reentrant circuits and the more typical polymorphic VT and VF seen with acute MI. For that reason, VT was induced in 50% of the preparations, but polymorphic VT and VF were not observed. It also might restrict the development of anatomic reentry around the infarction area6,7 and figure-of-eight reentry in the infarction border zone.4,5 Consequently, the findings of the present study do not preclude other potential reentrant circuits proposed by other investigators. Finally, the low signal-to-noise ratio in the infarction border zone, as shown in Figures 1 and 2, made it difficult to determine gap junctional conduction using optical mapping in our preparation.20 However, as shown in many previous studies, remodeling of gap junctions in the infarction border zone contributes to the development of anisotropic conduction and reentrant VT.21–24
Clinical implications The incidence of spontaneous sustained monomorphic VT in the acute phase of MI is reported as 1.9%, and its recurrence rate is high as 17%.25 Although sustained VT occurring in the setting of acute MI once was thought to have no prognostic value,26,27 more recent data suggest that it is an independent predictor for high mortality both inhospital and at 1 year post-MI.25,28,29 The substrates for
References 1. Janse MJ, Wit AL. Electrophysiological mechanisms of ventricular arrhythmias resulting from myocardial ischemia and infarction. Physiol Rev 1989;69:1049 –1169. 2. Callans DJ, Josephson ME. Ventricular tachycardia in patients with coronary artery disease. In: Zipes DP, Jalife J, eds. Cardiac electrophysiology: from cell to bedside, 3rd ed. Philadelphia: WB Saunders, 2000:530 –536. 3. de Bakker JM, van Capelle FJ, Janse MJ, Wilde AA, Coronel R, Becker AE, Dingemans KP, van Hemel NM, Hauer RN. Reentry as a cause of ventricular tachycardia in patients with chronic ischemic heart disease: electrophysiologic and anatomic correlation. Circulation 1988;77:589 – 606. 4. Costeas C, Peters NS, Waldecker B, Ciaccio EJ, Wit AL, Coromilas J. Mechanisms causing sustained ventricular tachycardia with multiple QRS morphologies: results of mapping studies in the infarcted canine heart. Circulation 1997;96:3721–3731. 5. Peters NS, Coromilas J, Hanna MS, Josephson ME, Costeas C, Wit AL. Characteristics of the temporal and spatial excitable gap in anisotropic reentrant circuits causing sustained ventricular tachycardia. Circ Res 1998;82:279 –293. 6. Stevenson WG, Friedman PL, Kocovic D, Sager PT, Saxon LA, Pavri B. Radiofrequency catheter ablation of ventricular tachycardia after myocardial infarction. Circulation 1998;98:308 –314. 7. El-Shalakany A, Hadjis T, Papageorgiou P, Monahan K, Epstein L, Josephson ME. Entrainment/mapping criteria for the prediction of termination of ventricular tachycardia by single radiofrequency lesion in patients with coronary artery disease. Circulation 1999;99:2283– 2289. 8. de Chillou C, Lacroix D, Klug D, Magnin-Poull I, Marquie C, Messier M, Andronache M, Kouakam C, Sadoul N, Chen J, Aliot E, Kacet S. Isthmus characteristics of reentrant ventricular tachycardia after myocardial infarction. Circulation 2002;105:726 –731. 9. Wu J, Zipes DP. Transmural reentry during acute global ischemia and reperfusion in canine ventricular muscle. Am J Physiol Heart Circ Physiol 2001;280:H2717–H2725. 10. Wu J, Wu J, Olgin J, Miller JM, Zipes DP. Mechanisms underlying the reentrant circuit of atrioventricular nodal reentrant tachycardia in isolated canine atrioventricular nodal preparation using optical mapping. Circ Res 2001;88:1189 –1195. 11. Choi B, Salama G. Optical mapping of atrioventricular node reveals a conduction barrier between atrial and nodal cells. Am J Physiol 1998; 274:H829 –H845. 12. Pinto JM, Boyden PA. Electrical remodeling in ischemia and infarction. Cardiovasc Res 1999;42:284 –297.
Takahashi et al
Optical Mapping of Reentrant Circuit
13. Allessie MA, Bonke FI, Schopman FJ. Circus movement in rabbit atrial muscle as a mechanism of tachycardia. III. The “leading circle” concept: a new model of circus movement in cardiac tissue without the involvement of an anatomical obstacle. Circ Res 1977;41:9 –18. 14. Schalij MJ, Boersma L, Huijberts M, Allessie MA. Anisotropic reentry in a perfused 2-dimensional layer of rabbit ventricular myocardium. Circulation 2000;102:2650 –2658. 15. Athill CA, Ikeda T, Kim YH, Wu TJ, Fishbein MC, Karagueuzian HS, Chen PS. Transmembrane potential properties at the core of functional reentrant wave fronts in isolated canine right atria. Circulation 1998; 98:1556 –1567. 16. Pertsov AM, Davidenko JM, Salomonsz R, Baxter WT, Jalife J. Spiral waves of excitation underlie reentrant activity in isolated cardiac muscle. Circ Res 1993;72:631– 650. 17. Spach MS, Heidlage FJ, Dolber PC. The dual nature of anisotropic discontinuous conduction in the heart. In: Zipes DP, Jalife J, eds. Cardiac electrophysiology: from cell to bedside, 3rd ed. Philadelphia: WB Saunders, 2000:213–222. 18. Schmitt H, Wit AL, Coromilas J, Waldecker B. Mechanisms for spontaneous termination of monomorphic sustained ventricular tachycardia: results of activation mapping of reentrant circuit in the epicardial border zone of subacute canine infarcts. J Am Coll Cardiol 1998;31:460 – 470. 19. Baxter WT, Mironov SF, Zaitsev AV, Jalife J, Pertsov AM. Visualizing excitation waves inside cardiac muscle using transillumination. Biophys J 2001;80:516 –530. 20. Akar FG, Roth BJ, Rosenbaum DS. Optical measurement of cell-tocell coupling in intact heart using subthreshold electrical stimulation. Am J Physiol Heart Circ Physiol 2001;281:H533–H542. 21. Dillon SM, Allessie MA, Ursell PC, Wit AL. Influences of anisotropic tissue structure on reentrant circuits in the epicardial border zone of subacute canine infarcts. Circ Res 1988;63:182–206.
459 22. Peters NS, Coromilas J, Severs NJ, Wit AL. Disturbed connexin 43 gap junction distribution correlates with the location of reentrant circuits in the epicardial border zone of healing canine infarcts that cause ventricular tachycardia. Circulation 1997;95:988 –996. 23. Beardslee MA, Lerner DL, Tadros PN, Laing JG, Beyer EC, Yamada KA, Kleber AG, Schuessler RB, Saffitz JE. Dephosphorylation and intracellular redistribution of ventricular connexin 43 during electrical uncoupling induced by ischemia. Circ Res 2000;87:656 – 662. 24. Yao JA, Hussain W, Patel P, Peters NS, Boyden PA, Wit AL. Remodeling of gap junction channel function in epicardial border zone of healing canine infarcts. Circ Res 2003;92:437– 443. 25. Mont L, Cinca J, Blanch P, Blanco J, Figueras J, Brotons C, SolerSoler J. Predisposing factors and prognostic value of sustained monomorphic ventricular tachycardia in the early phase of acute myocardial infarction. J Am Coll Cardiol 1996;28:1670 –1676. 26. Bigger JT Jr, Fleiss JL, Rolnitzky LM. Prevalence, characteristics and significance of ventricular tachycardia detected by 24-hour continuous electrocardiographic recordings in the late hospital phase of acute myocardial infarction. Am J Cardiol 1986;58:1151–1160. 27. Tofler GH, Stone PH, Muller JE, Rutherford JD, Willich SN, Gustafson NF, Poole WK, Sobel BE, Willerson JT, Robertson T. Prognosis after cardiac arrest due to ventricular tachycardia or ventricular fibrillation associated with acute myocardial infarction (the MILIS Study). Multicenter Investigation of the Limitation of Infarct Size. Am J Cardiol 1987;60:755–761. 28. Newby KH, Thompson T, Stebbins A, Topol EJ, Califf RM, Natale A. Sustained ventricular arrhythmias in patients receiving thrombolytic therapy: incidence and outcomes. The GUSTO Investigators. Circulation 1998;98:2567–2573. 29. Sala MF, Barcena JP, Rota JI, Roca JV, Lopez CA, Puigdevall JM, Soldevilla JG, de Luna AB. Sustained ventricular tachycardia as a marker of inadequate myocardial perfusion during the acute phase of myocardial infarction. Clin Cardiol 2002;25:328 –334.