Chapter 10. Optical Mapping of Electrical Activity

Current diagnostic criteria of cardiac arrhythmias are based mostly on the electrocardiogram (ECG) recorded with body surface electrodes, which register local far-field potentials generated by all cardiac myocytes. The remote body surface placement and low number of electrodes limit the information content of ECG recordings and the details in the mechanisms of arrhythmias, the regions of electrophysiologic abnormalities, and the paths of reentry that can be identified.

Recently, high spatial resolution optical-mapping systems have been developed and used in research laboratories to investigate the mechanisms of cardiac arrhythmias in animal models under controlled conditions.1 Optical mapping uses fluorescent dyes that are sensitive to transmembrane potential or to ion concentrations as probes, optics to collect the signal-carrying fluorescence, and photon-sensing devices (eg, camera or photodiode array) to convert fluorescence to electrical signals, which are then amplified, digitized, processed, and analyzed (Fig. 10–1). The advantages of camera-based optical-mapping systems are (1) recording high spatial resolution movies of electrophysiologic activity in cardiac muscle; (2) reporting transmembrane potential, which reflects cellular electrophysiology more directly than the extracellular field potential registered by electrode-based body surface and intracardiac mapping systems; and (3) capability of recording cytosolic ion concentrations (eg, Ca2+ transient) by using ion-sensitive fluorescent dyes. The current trend in optical mapping has been moving toward dual-camera simultaneous recording of both membrane potential and Ca2+ transient within the same view field. High spatial resolution optical mapping provides details in electrophysiologic heterogeneity, focal activation, and abnormal conduction in the heart and has contributed significantly to the insight and mechanistic understanding of cardiac arrhythmias in recent years.

Most of the existing optical-mapping systems use a design that optically focuses the to-be-observed tissue surface onto the sensor array of a stationary camera, thus requiring direct view of the tissue surface. The mapping area can range from a single isolated cell to the entire heart, depending on the optics mounted in front of the camera. In this design, muscle contraction can change the relative position of tissue surface, introducing motion artifacts into the recorded electrophysiologic signal. The undesired motion artifacts are commonly eliminated by tissue immobilization with pharmacologic excitation-contraction uncoupling agents, such as cytochalasin D2,3 or blebbistatin,4 in isolated heart or tissue preparations. Alternative methods (eg, ratiometric imaging or signal processing) can also be used to extract useful data from signals containing motion artifacts obtained in beating hearts. In creating models for investigating the mechanisms of arrhythmias in diseased human hearts, hearts from large animals are preferred over those from small animals due to their similarities to human hearts in terms of ion currents, electrophysiologic heterogeneity, conduction phenomena, anatomy, and coronary perfusion, among other characteristics. All studies presented in this chapter were performed by using a 256-channel (16 × 16 array) optical-mapping system to record electric activity on the surface (mapping field: 19.5 × 19.5 mm2) of isolated and arterially perfused canine ventricular tissues that were stained with a membrane potential–sensitive dye, 4(beta-[2-(di-n-butylamino)-6-napathy]vinyl)pyridinium (di-4-ANEPPS).

Clinically, cardiac arrhythmias occur commonly in diseased hearts and are triggered by acute events. Although the basic theory and mechanisms of the initiation and maintenance of ventricular tachycardia (VT) have been investigated intensively (eg, early and delayed afterdepolarizations [EADs] and [DADs],5 dispersion of repolarization, reentry), their manifestation, interactions, and roles in arrhythmias in diseased hearts are less clear. This chapter presents some recent developments in understanding the mechanisms of arrhythmias in tissue models of ischemia/reperfusion; Brugada, long QT, and Andersen-Tawil syndromes; and atrioventricular (AV) nodal reentry unveiled by optical mapping in our laboratory.

We used optical mapping to elucidate the functional nature of M-cell behavior in the ventricular wall. Ventricular action potentials (APs) are heterogeneous.6 The AP duration (APD) is longest in the mid myocardium and shortest in the epicardium across the ventricular free wall. Compared with the endocardium, myocytes in the epicardium have a larger transient outward current (Ito); a stronger ischemia-induced KATP current; and an AP that has a larger spike and dome morphology, more pronounced rate dependency, and a higher sensitivity to ischemia. The slow delayed rectifier K+ current, IKs, is lowest in the mid myocardium, whereas the density of the rapid delayed rectifier K+ current, IKr, is similar across the canine left ventricle wall. The heterogeneity in ion currents causes dispersion of APs and of repolarization, which normally serves a physiologic function but can be proarrhythmic when it occurs excessively or interacts with the abnormal sequence of activation in a diseased heart.

The mid myocardium in the ventricular free wall has been shown to differ significantly from the endocardium and epicardium, with exceedingly long APDs during long cycle length (CL) activation. It was postulated that a special group of mid-myocardial cells, so-called M cells, is responsible for the distinctive mid-myocardial electrophysiologic properties.6-9 The M cells have been proposed to underlie the T and U waves in ECG and play major roles in the dispersion of repolarization and in the initiation of VT.6,7 However, some studies failed to find evidence of M cells.8,10-12 Using optical mapping, we demonstrated that instead of a separate group of cells, the M-cell behavior was functional extremes of ventricular heterogeneity that became manifest under specific conditions.13 The mid-myocardial preferential prolongation of APDs also contributed to the formation of a transmurally asymmetric distribution of APDs in a tissue model of long QT syndrome14 with the shortest APD in the epicardium and longest APD in the mid myocardium, upon which epicardial but not endocardial premature stimulation can easily initiate VT.15

We isolated and arterially perfused wedges of canine left ventricular free wall and optically mapped APDs on their cut-exposed transmural surfaces, before and after treatment with anemone toxin II (ATX-II), which delays the inactivation of membrane Na+ current (INa). Before ATX-II, the row of recording sites having the longest mean APD (APDL layer) was in the (sub)endocardium. ATX-II prolonged APD heterogeneously (mid myocardium > endocardium > epicardium) and "moved" the APDL layer progressively from the endocardium into mid myocardium (Fig. 10–2). We detected M-cell behavior (the APDL layer located at sites other than in the endocardium and epicardium with statistical significance) in 18 wedges exposed to ≥5 nmol/L of ATX-II, but not in the other 18 wedges exposed to ≤2.5 nmol/L of ATX-II. We also repeated the same experiments in wedges isolated from the canine ventricular septum. In contrast to the ventricular free wall, none of the 22 septum wedges showed M-cell behavior under the same conditions.16,17 Therefore, M cell is a functional phenomenon in the left ventricular free wall, but not in the septum, that can become manifest under specific nonphysiologic conditions.

Acute ischemia and reperfusion are strong triggers of VT. Most ischemia-induced VTs occur during the first 10 to 15 minutes of ischemia and are due to focal activation and transmural reentry.18-20 Both the differences in the coronary blood flow between the healthy and ischemic regions and heterogeneic tissue responses to ischemia contribute to the initiation of VT. Using optical mapping, we investigated the contributions to VT by heterogeneic tissue responses to acute global ischemia. Global ischemia eliminated the heterogeneity in coronary perfusion, thus isolating the tissue responses to ischemia.

Acute ischemia suppresses excitability faster in the epicardium than endocardium, increasing transmural dispersion of excitability in the ventricular wall.18,21 Ischemia also slows conduction after a transient initial increase. The rapidly changing transmural dispersion of tissue excitability and velocity of conduction form a proarrhythmic substrate in which reentry can be initiated, sustained, and then abolished during acute global ischemia and reperfusion.

We induced global ischemia and optically mapped APs on the transmural surfaces of 36 isolated wedges of canine left ventricular free wall. Before ischemia, the transmural response of the tissues was 1:1 to endocardial pacing (CL = 300 ms). Ischemia increased the transmural gradient of the refractory period, which blocked conduction unidirectionally and initiated reentry and VT after 535 ± 146 seconds (31 wedges). Multiple zones of responsiveness (1:1, 2:1, and 4:1) to endocardial pacing developed after 570 ± 165 seconds of ischemia (34 wedges). Further ischemia inactivated the epicardium and eliminated reentry and VT in 24 wedges. Therefore, heterogeneic prolongation of the refractory period and conduction slowing by early ischemia provided a proarrhythmic substrate in which reentry and VT could be induced by rapid endocardial activation (Fig. 10–3).

In contrast to endocardial pacing, epicardial pacing initiated reentry during acute ischemia by a different mechanism.20 The transmural gradient of sensitivity to ischemia caused decremental conduction to occur first in the epicardium. At this time, epicardial stimulation failed to conduct laterally along the rim of epicardium and, instead, conducted to the more excitable endocardium, then laterally along the rim of endocardium, and finally reentered the subepicardium (in 9 of 18 wedges after 719 ± 399 seconds of ischemia). In contrast, endocardial stimulation applied right before or after the above epicardial stimulations initiated activation that conducted rapidly along the rim of endocardium, then slowly from the endocardium toward the epicardium without initiating reentry (Fig. 10–4). In addition to these mechanisms of reentry initiation after several minutes of ischemia, we also showed that a transient prolongation of APD during the first 2 minutes of ischemia contributed to arrhythmogenesis, especially when combined with delayed INa inactivation by ATX-II.22

After 25 minutes of ischemia, the majority of the epicardium and mid myocardium was inexcitable, whereas the endocardium was still excitable during endocardial pacing. Reperfusion restored the perfusion flow uniformly. However, the recovery of tissue excitability was faster in the subendocardium and mid myocardium than in the deeply depressed subepicardium. During the first minute of reperfusion, the reappearance of excitability in the subepicardium was associated with APDs that were much longer than in the subendocardium. The transmural differences in reperfusion recovery produced an initial steep transmural gradient of APD, which was progressively reduced during further reperfusion. Unidirectional block of conduction, focal activations, and reentry occurred during the first minute of reperfusion. Sustained VT and ventricular fibrillation occurred after 93 ± 49 seconds of reperfusion (in two of the eight wedges reperfused at a quarter of the preischemia flow rate).

We concluded that transmural heterogeneity in the tissue responses to ischemia contributed to arrhythmogenesis during acute ischemia by providing steep transmural gradients of excitability and refractory period in which unidirectional block and reentry could occur easily. Transmural heterogeneity in reperfusion recovery contributed to reperfusion VT by providing a steep transmural gradient of APD and by promoting focal activation. The clinical implication is that the heterogeneic tissue responses provide a proarrhythmic substrate in which reentry appears to be a predominant mechanism of ventricular arrhythmias during and after global ischemia.

Using optical mapping, we demonstrated a potential mechanism of arrhythmias in postischemia ventricular tissue. It is well known that myocardial ischemia can induce persistent proarrhythmic remodeling even after sufficient reperfusion. We investigated mechanisms of postischemia arrhythmogenesis in 10 isolated wedges of canine left ventricular free wall.23 Among the 10 wedges, 8 recovered well with return of both APD and velocity of conduction back to normal after 40 minutes of ischemia and 60 minutes of reperfusion. Subsequent perfusion with ATX-II (20 nmol/L) prolonged APDs significantly, induced EADs in all of the recovered wedges, and induced spontaneous VTs in seven of the eight recovered wedges. Focal EADs and reentry were responsible for 73% and 18% of the activations, respectively, in the VTs (Fig. 10–5). Changes in the EAD foci and conduction pathways altered the contour of the transmural ECG during VT. In comparison, a control group of eight ischemia-free wedges had less APD prolongation, less dispersion of repolarization, and no EADs or VT after the same ATX-II treatment. The exaggerated APD responses to ATX-II indicated a low repolarization reserve24 in the postischemia wedges, which provided a proarrhythmic substrate. Therefore, a brief period of ischemia, even after sufficient reperfusion recovery of AP and conduction velocity, can persistently suppress repolarization reserve with arrhythmic consequences. Clinical implications of this study would be that patients recovered from an episode of ischemia have an increased risk of arrhythmic episodes, which can be triggered easily by an APD-prolonging event.

Brugada syndrome (BS) is characterized by ST-segment elevation and negative T waves in the right precordial ECG leads in patients.25-28 Mutations in genes that encode the Na+ channel and the Ca2+ channel have been identified in 18% to 30% and 8% of patients with BS, respectively.29-31 BS is temperature sensitive, and temperature elevation can lead to T wave alternans (TWAs), VT, and sudden death, especially in adult male patients. We investigated the mechanisms of VT and genotype-phenotype relationships with a series of optical-mapping studies in in vitro models of BS.32

In patients with BS, the right ventricular outflow tract (RVOT) is a critical area associated with prominent electrophysiologic differences and is a frequent origin of arrhythmias.33-36 We reproduced the electrophysiologic characteristics of BS in two models, one with Na+ channelopathy (Na-model) and the other with Ca2+ channelopathy (Ca-model), using isolated canine RVOT tissues. The Na-model was induced with combinations of terfenadine, pilsicainide, and pinacidil. Pilsicainide and terfenadine reduce INa, simulating the Na+ channelopathy. Pinacidil deepens the phase 1 notch of the AP, which is a major mediator of the adult male predominance of BS.37-41 The Ca-model was induced with only verapamil, a blocker of ICaL, because the involvement of other currents is still unclear in patients with the Ca2+ channelopathy type of BS.

We demonstrated the critical role of RVOT heterogeneity in the arrhythmogenesis in BS using the Na-model.42 Compared with the paired right ventricular anteroinferior (RVAI) tissues, RVOT tissues have a deeper phase 1 notch of AP, especially in the epicardium, larger J wave, and higher sensitivity to the BS-inducing drugs. Induction of BS progressively deepened the phase 1 notch and delayed and enhanced the phase 2 dome initially (suprathreshold of ICaL activation) and then abbreviated (due to subthreshold of ICaL activation) the phase 2 dome in the RVOT epicardial AP. These changes led to the simultaneous presence of APs with and without the phase 2 dome. This created a large dispersion of repolarization in association with deep phase 1 notches in the epicardium, but not in the mid myocardium and endocardium, resulting in opposite transmural gradient of repolarization in adjacent regions (having supra- and subthreshold of ICaL activation) within the RVOT. Conduction of the phase 2 dome from the epicardial region having a prominent phase 2 dome to regions without a dome led to phase 2 reentry, premature activation, and VT (Fig. 10–6). Fluctuations in the depth of the phase 1 notch around the threshold of ICaL activation caused alternans in the phase 2 dome of the AP and in the T wave in the ECG, which was further promoted by bradycardia.43

In contrast, the same drugs had much less effect in the RVAI tissues. VTs occurred in 47% of RVOT preparations but only in 7% of the paired RVAI tissues. Blockade of Ito reduced the heterogeneity of APs and prevented VT in the RVOT tissues, because Ito regulates the depth of phase 1 notch and the size of the phase 2 dome of AP.44 Therefore, a high Ito-mediated heterogeneity in the RVOT provided a proarrhythmic substrate for the initiation of phase 2 reentry and VT in BS. Hypothermia enhanced the heterogeneity of the AP and promoted the origination of phase 2 reentry in the epicardium of the RVOT, but the prolonged APD reduced the possibility of sustained VT.45 Hyperthermia abbreviated the AP and facilitated the maintenance of reentry and VTs, especially when combined with rapid activations.45 Such responses occur frequently in patients with fever.46-48 In addition, delayed epicardial activation (eg, by slowed or blocked transmural conduction in association with a dysfunctional INa), as we simulated experimentally using separate stimulations in an additional piece of epicardial tissue staggered on the epicardium of a transmural wedge,49 led to fragmented QRS in ECG, which is a risk marker of prognosis in BS.50 Body surface mapping in patients with BS supported these experimental findings (see Fig. 10–6).51 In conclusion, the AP heterogeneity within the epicardium of the RVOT contributes to the ECG characteristics, temperature sensitivity, TWA, and arrhythmias in BS.

We evaluated the efficacy of catheter ablation in eliminating VT in 17 isolated RVOT models of BS.52 Epicardial ablation at the earliest activation site of premature ventricular complexes (PVCs) disconnected the short and long APD regions and eliminated all PVCs and VTs, although APD heterogeneity was still present in the epicardium. In contrast, endocardial ablation failed to eliminate VT. These results suggest that the most effective ablation to eliminated VT in patients with BS should be at the earliest activation sites in the epicardium of the RVOT.

We also evaluated the phenotypic differences and therapeutic effects between the Na+ and Ca2+ channelopathies in BS (Na-model and Ca-model in 11 and 7 canine RVOT tissues, respectively).53 At a pacing CL of 1000 ms, both models had coved-type ST-segment elevation in the ECG, longer APDs in the epicardium than in the endocardium, and a similar incidence of spontaneous VTs. However, the Ca-model had a higher incidence of TWA than the Na-model. At a CL of 2000 ms, the Ca-model had saddle-back–type ST-segment elevation in ECGs and shorter APDs in the epicardium than in the endocardium, whereas the Na-model had coved-type ST-segment elevation in the ECG and extensive AP heterogeneity in the epicardium. VT occurred less frequently with the Ca2+ than with the Na+ channelopathy at a CL of 2000 ms. Isoproterenol normalized the ECG morphology and transmural APD gradient in both the Na+ and Ca2+ channelopathy models of BS, but quinidine failed to normalize the ECG and prevent TWAs in the Ca2+ model, especially at the CL of 1000 ms. Both quinidine and isoproterenol eliminated epicardial AP heterogeneity and prevented VTs. We conclude that although both Na+ and Ca2+ dysfunctions produced similar BS characteristic ECGs and arrhythmogenesis at 60 beats per minute (bpm), Ca2+ dysfunction was associated with a higher incidence of TWAs at 60 bpm, less ST-segment elevation, and fewer arrhythmias at 30 bpm compared with Na+ dysfunction.

Our experiments suggest that patients with Ca2+ channelopathy–type BS are likely to have rate-dependent changes of ST-T morphology and arrhythmogenesis. The rate-dependent ECG responses and arrhythmogenicity in the Ca-model suggest that patients with Ca2+ channelopathy may have more arrhythmic events during the daytime, whereas cardiac events may be infrequent at night due to lower heart rates. This study demonstrated that although both quinidine and isoproterenol restored the dome in the AP and abolished arrhythmias in both Na+ and Ca2+ channelopathy models, quinidine did not prevent J-ST elevation and TWA at a CL of 1000 ms in the Ca-model. Therefore, quinidine and isoproterenol can be effective in BS with either Na+ or Ca2+ channelopathies, but quinidine may not fully prevent arrhythmias in patients with BS caused by Ca2+ channelopathy. Naturally, results from animal experiments must be applied clinically with caution.

Patients with Andersen-Tawil syndrome (ATS; also known as type 7 long QT syndrome) have mild QT prolongation, large U waves, and frequent VT.54,55 Mutations in the gene encoding the inward rectifier K+ channel (current: IK1) have been identified in type 1 ATS (ATS1). IK1 plays major roles in maintaining the resting potential and in the final repolarization of the AP.54,56,57 Patients with ATS1 were reported to have frequent bigeminal PVCs that were increased by hypokalemia58,59 and QRS alternans in association with bidirectional VT.60

Using optical mapping, we investigated the mechanism of arrhythmogenesis and formation of U waves in an ATS1 model induced by IK1 blockade with cesium chloride (CsCl; 5-10 mmol/L) in isolated left ventricular free wall wedges.61 To investigate the effects of sympathetic activation, we added isoproterenol (ISP; 0.05, 0.15 μmol/L) to the perfusate of 11 and 7 CsCl-treated preparations with low and normal extracellular K+ concentration ([K+]o; 2.5 and 4.69 mmol/L), respectively. To evaluate the role of ICaL in the arrhythmias in ATS1, we added verapamil (3.0 μmol/L) to the CsCl-treated preparations having VT. We mapped APs on the transmural surface of the preparations at control, after CsCl, and after adding the previously mentioned drugs.

IK1 blockade delayed late phase 3 repolarization and prolonged the AP, more so during low [K+]o perfusion. The preparations were stable and free of spontaneous activity. Rapid pacing (CL = 1000 ms) induced slow phase 4 diastolic depolarization (DADs) in all low [K+]o preparations and in 71% of normal [K+]o preparations after CsCl treatment. Sympathetic activation (ISP: 0.05 and 0.15 μmol/L) increased the occurrence of DADs (in 86% of the normal [K+]o preparations treated with CsCl). Prominent DADs initiated new activations. Repetitive DAD-initiated activations generated VT. VT occurrences increased with sympathetic activation (VT occurrences: 0%, 14%, and 71% at ISP concentrations of 0, 0.05, and 0.15 μmol/L, respectively). Hypokalemia promoted VT in the ATS1 preparations. DADs occurred in 0%, 25%, 100%, and 100% and VT occurred in 0%, 0%, 38%, and 100% of the low [K+]o preparations before and after CsCl (0, 5, and 10 mmol/L) and after subsequent ISP treatment, respectively. Multiple migrating foci (with endocardial preference) caused polymorphic VT. Alternating DADs at two foci with coordinated timing resulted in bidirectional VT (Fig. 10–7), which occurred in 64% of low [K+]o preparations having ISP, but not in the normal [K+]o preparations. ICaL blockade with verapamil shortened APD, eliminated the DADs and VT, and prevented their induction by burst pacing in all preparations. U waves appeared following the T waves in the ECG, forming a TU complex, after CsCl treatment in 75% and 29% of the low and normal [K+]o preparations, respectively. These U waves were associated with a significant delay in the late phase 3 repolarization of the AP and with the occurrence of DADs.5

These results suggest the critical roles of [K+]o, heart rate, resting membrane potential, autonomic activity, and ICaL in the arrhythmogenesis of ATS1. IK1 reduction causes instability in the resting potential and delays repolarization, which promotes ICaL. ISP also increases ICaL.62 Both an unstable resting potential and an increased ICaL facilitate DADs and initiation of VTs. The elimination of DADs and VT by verapamil suggested that Ca2+ overload was a major trigger of VT in ATS1. U waves in this ATS1 model had two origins, one from the delayed phase 3 repolarization and the other from the DADs. The dual origins provided an explanation of the polymorphism of the U wave (eg, the biphasic or enlarged U waves) observed clinically.60 The endocardial preferences of DADs and of VT foci could be related to the longer APDs in the endocardium than epicardium. The results of this study support therapeutic strategies to manage VT in patients having ATS1 by reducing cytosolic Ca2+ overload, preventing and limiting excessive heart rate, controlling autonomic activity, elevating plasma K+ concentration, and increasing the conductivity of membrane at rest.

Although dual conduction pathways have been suggested to underlie the AV nodal reentrant echo beats (EBs) and sustained AV nodal reentrant tachycardia,63-65 newer data suggest that multiple AV nodal input pathways might exist outside the compact AV node because anterograde AV conduction remained after ablation of both the fast pathway (FP) and slow pathway (SP).66

Optical mapping was used to unveil the AV nodal conduction pathways and mechanisms of AV nodal reentry in arterially perfused preparations containing the AV node and surrounding atrial tissue isolated from canine hearts.67 The isolated preparations were paced at the anterior limbus of the fossa ovalis near the FP. Electrical activity in the Koch triangle (KT) and surrounding atrial tissue was mapped optically. His bundle electrograms were also recorded.

Optical mapping identified three major nondiscrete AV nodal input pathways having fast (FP; 192.6 ± 17.4 cm/s), intermediate (intermediate pathway [IP]; 87.4 ± 10.2 cm/s), and slow (SP; 40.5 ± 5 cm/s) velocity of conduction in all preparations with continuous (n = 16) and discontinuous (n = 6) AV nodal function curves (AVNFCs; pacing CL = 800 ms). AV nodal EBs were induced in 12 preparations. The reentrant circuit of the slow/fast EB (n = 8) started as block in the FP, delay in the SP conduction to the compact AV node, and then exit from the AV node to the FP and rapid return to the SP through the atrial tissue at the base of KT (Fig. 10–8). The reentrant circuit of the fast/slow EB (n = 2) was in an opposite direction. In the slow/slow EB (n = 2), anterograde conduction was over the IP, and retrograde conduction was over the SP. Unidirectional conduction block occurred during short coupling intervals at the junction between the AV node and its input pathways where local transitional cells had long APDs. Conduction over the IP smoothed the transition from the FP to the SP, resulting in a continuous AVNFC. A "jump" in atrium-His bundle interval resulted from shifting of anterograde conduction from the FP to the SP (n = 4) or abrupt conduction delay within the AV node through the FP (n = 2). Transection of the SP (n = 8) eliminated all EBs. In conclusion, there were three AV nodal anterograde pathways outside the AV node without an upper common pathway; unidirectional block could occur in the transition zone between the AV node and its input pathways when the coupling interval was shorter than the local APD, and the IP could mask the FP and SP, producing continuous AVNFCs.

This study has several clinical implications. First, the involvement of the SP in all reentrant circuits provides a mechanistic explanation for the clinical success of SP ablation in terminating various types of AV nodal reentry tachycardia (AVNRT). Second, the results suggest the presence of multiple retrograde atrial exit sites in atypical AVNRT because multiple connections exist between the SP and atrial tissue near the coronary sinus ostium. Finally, the noninvolvement of FP in the reentrant circuit of slow/slow EBs explains the ineffectiveness of FP ablation in some patients with atypical AVNRT.

Although optical mapping is a powerful method to observe the spatiotemporal dynamics of electrophysiologic activity in cardiac muscle, there are several technical limitations that need to be overcome before its direct clinical applications. Most of the current optical-mapping systems use conventional lens-based optics, which require direct optical viewing of the to-be-mapped tissue surface. Miniaturization of the optics (eg, based on flexible fiber optic image conduits) and/or miniature sensor arrays need to be developed for clinical applications. Currently, most optical-mapping experiments are carried out in immobilized tissues to avoid motion artifacts. However, clinical applications require mapping in a beating heart, in which the undesired contractual distortion in the electrophysiologic signal needs to be eliminated or managed either by ratiometric imaging, which subtracts motion artifacts from the signal-carrying fluorescence, or by data processing algorithms to extract physiologic information from signals containing motion artifacts. Currently, commercially available fluorescent dyes are mostly working in the visible light spectrum, which limits the depth of tissue from where the fluorescence signal can be recorded. Long-wavelength infrared membrane potential–sensitive fluorescent dyes that enable deep tissue recording are now in active development in research laboratories. Furthermore, the safety of the dyes needs to be evaluated before clinical application. The developments in optics and in fluorescent dyes will facilitate the applications of optical mapping in clinical cardiac electrophysiology to provide real-time movies of electrical activity in heart. Optical mapping can be very powerful when the recorded real-time movies of electrical activity are combined with anatomic and pathophysiologic imaging technologies, such as computed tomography, positron emission tomography, and magnetic resonance imaging. The complementary electrical movies and anatomic and pathophysiologic information will enable identification of the causes and their electrophysiologic consequences in a diseased heart and thus will contribute to making a more precise diagnosis and more effective treatment of cardiac arrhythmic diseases.

Optical mapping has played a critical role in understanding the electrophysiologic heterogeneity in the heart and in illustrating the sites of focal activations and the mechanisms of initiation and maintenance of VT. Our optical-mapping studies demonstrated the following mechanisms. (1) Ventricular M cells are a functional expression of electrophysiologic heterogeneities under extreme conditions. (2) Transmural differences in the suppression of excitability and in the prolongation of refractoriness provide a proarrhythmic substrate in which transmural reentry can be initiated easily with either endocardial or epicardial stimulation. (3) A prior episode of ischemia can leave long-lasting suppression of repolarization reserve, providing a proarrhythmic substrate in which VT can be triggered by a repolarization delaying intervention. (4) Arrhythmogenicity in BS is caused by the following electrophysiologic sequences: a Na+/Ca2+ channelopathy and an age and sex enhancement of Ito, a deep phase 1 notch of the AP, dispersion in the phase 2 dome and repolarization of the AP, phase 2 reentry, and VT. The presence of a large Ito underlies the arrhythmogenicity of the RVOT in BS. Factors that promote the increase in Ito and dispersion of repolarization (eg, temperature and CL) can be the triggers of arrhythmias in BS. Ablation that disconnects the long and short APD regions in the epicardium of RVOT can be effective treatment of arrhythmias in BS. (5) Arrhythmogenicity in ATS1 is an electrophysiologic consequence of IK1 reduction, resting membrane potential instability and repolarization delay, ICaL enhancement, DADs, and VT. Both delayed repolarization and DADs cause the U wave in the ECG. Multiple DAD foci lead to polymorphism of VT and U wave in ATS1 and to bidirectional VT. (6) AV nodal reentry involves three anterograde pathways in the atrial tissue outside of the AV node.