Characterization of the excitable gap in a functionally determined reentrant circuit. Studies in the sterile pericarditis model of atrial flutter

Effect of carvedilol on atrial remodeling in canine model of atrial fibrillation

AIMS

We evaluated the effect of carvedilol, a beta-blocker with anti-oxidative action, against the atrial fibrillation (AF) inducibility, the development of atrial remodeling and the oxidative stress markers in a canine AF model.

METHODS AND RESULTS

AF model was produced by performing 6-week rapid atrial stimulation in 15 dogs. The animals were divided into the following three groups: (I) pacing + carvedilol group (n=5); (II) pacing control group (n=5); and (III) non-pacing group (n=5). AF inducibility was gradually increased along the time course in the pacing control group. In the pacing + carvedilol group, the AF inducibility was suppressed especially in the latter phase of protocol in comparison with the pacing control group. Although carvedilol has beta-blocking effect, pacing control and pacing + carvedilol groups did not exhibit difference in the heart rate (177±13 vs. 155±13 bpm, P=0.08). On 8-hydroxy-2'-deoxyguanosine (8-OHdG), dihydroethidium and dichlorodihydrofluorescein diacetate staining, enhanced oxidative stress was observed in the atrial tissue in the pacing control, but not in the pacing + carvedilol group.

CONCLUSIONS

Carvedilol suppressed AF inducibility and oxidative stress in the canine AF model.

Angiotensin II-mediated up-regulation of connective tissue growth factor promotes atrial tissue fibrosis in the canine atrial fibrillation model

AIMS

Remodelling of the extracellular matrix (ECM) plays an important role in the production of arrhythmogenic substrate for atrial fibrillation (AF), and is considered to be promoted by the connective tissue growth factor (CTGF). Our objective was to assess the relationship between CTGF and ECM synthesis, and the effect of olmesartan on these processes.

METHODS AND RESULTS

Fifteen canine AF models were produced by rapid atrial stimulation. They were divided into three groups: pacing control (n = 5): 6-week pacing, pacing + olmesartan (n = 5): pacing with olmesartan (2 mg/kg/day), and non-pacing group (n = 5). In the pacing control group, messenger ribonucleic acid expressions of CTGF and collagen types 1 and 3 were up-regulated in comparison with the non-pacing group (P < 0.05) while transforming growth factor-β (TGF-β) did not exhibit a significant difference. In the pacing + olmesartan group, these up-regulations were suppressed (P < 0.05). In fluorescent immunostaining, the expression of CTGF was localized in the cytoplasm. The protein level of collagen type 3 was increased in the pacing control and it was suppressed in the pacing + olmesartan group.

CONCLUSIONS

CTGF and associated genes were up-regulated in the atria with the appearance of fibrosis. Because this up-regulation was independent of TGF-β and suppressed by olmesartan, CTGF up-regulation was considered to be mediated by angiotensin II.

Repetitive evaluation of fibrillation cycle length predicts the efficacy of bepridil for interruption of long-lasting persistent atrial fibrillation

Although bepridil is effective for conversion of long-lasting persistent atrial fibrillation (AF) to sinus rhythm, it sometimes takes a long time to interrupt AF and there is no feasible index to predict its efficacy.In 60 patients with long-lasting persistent AF, bepridil (100-200 mg/day) was administered and continued for 8 weeks while body surface ECG was recorded every 2 weeks. The fibrillation cycle length (FCL) was evaluated using the spectral analysis of the fibrillation waves in each ECG. AF was interrupted in 32 patients receiving bepridil. The conversion was observed at 2 weeks in 4, at 4 weeks in 7, at 6 weeks in 7, and at 8 weeks in 14 patients. When comparing these responders and nonresponders, clinical background characteristics other than the dosage of bepridil did not show any difference and neither did temporal changes in QT parameters and heart rate. In contrast, the FCL and ΔFCL (prolongation in FCL from baseline) became significantly larger in responders than in nonresponders at later observation points (FCL: 177 ± 17 versus 164 ± 19 ms, P = 0.018, and ΔFCL: 38 ± 16 versus 22 ± 12 ms, P < 0.001, at 4-week point; FCL: 188 ± 17 versus 169 ± 19 ms, P = 0.004, and ΔFCL: 49 ± 18 versus 27 ± 14 ms, P < 0.001, at 6-week point).Repetitive evaluation of FCL using spectral analysis of fibrillation waves can be a feasible index to predict the efficacy of bepridil therapy.

Predicting the efficacy of antiarrhythmic agents for interrupting persistent atrial fibrillation according to spectral analysis of the fibrillation waves on the surface ECG

BACKGROUND

Spectral analysis of the fibrillation waves was performed in patients with persistent atrial fibrillation (PAF) to clarify the usefulness of this method of predicting the efficacy of antiarrhythmic agents.

METHODS AND RESULTS

The 59 patients with PAF were randomly assigned to pilsicainide (150 mg/day) or bepridil (200 mg/day) therapy for 4 weeks under optimal anticoagulation therapy. When the first therapy did not interrupt PAF, the drugs were changed in a cross-over manner. The fibrillation cycle length (FCL) was calculated using spectral analysis of the fibrillation waves on the body-surface ECG every 2 weeks. Pilsicainide and bepridil were effective in 19 and 20 patients, respectively. The FCL at the basic state was longest in the pilsicainide-effective group, moderate in the bepridil-effective group and shortest in the failure group (P<0.05). The change in FCL with drug administration (DeltaFCL) was larger in the effective than in the ineffective cases (P<0.01). Successful interruption of the atrial fibrillation (AF) with pilsicainide could be expected for patients with a FCL >148 ms (sensitivity =0.917, specificity =0.612, P=0.007) and DeltaFCL >41 ms (sensitivity =0.875, specificity =0.833, P=0.001).

CONCLUSIONS

The FCL reflects the electrophysiological properties of the AF substrate and is considered useful for predicting the efficacy of antiarrhythmic agents.

Inhomogenic effect of bepridil on atrial electrical remodeling in a canine rapid atrial stimulation model

BACKGROUND

The antiarrhythmic or reverse remodeling effects of bepridil, a multi-ion channel blocker, have been recently reported, but inhomogeneity of the electrical remodeling and effects of bepridil have been observed in previous reports. In this study, the effect of long-term administration of bepridil on atrial electrical remodeling was evaluated in a comparison of the right and left atrium (RA and LA) in a canine rapid atrial stimulation model.

METHODS AND RESULTS

In 10 beagle dogs, rapid atrial pacing (400 beats/min) was delivered for 6 weeks and the atrial effective refractory period (AERP), conduction velocity (CV) and inducibility of atrial fibrillation (AF) were evaluated every week. In 5 of the pacing dogs, bepridil (10 mg . kg(-1) . day(-1)) was administered orally, starting 2 weeks after the initiation of the rapid pacing. At the end of the protocol, the hemodynamic parameters and extent of tissue fibrosis were evaluated and the mRNA of SCN5A, Kv4.3, the L-type Ca2+ channel (LCC) and connexin (Cx) 40, 43, and 45 in both atria were examined by quantitative real-time reverse transcriptase-polymerase chain reaction. In the pacing control group, AERP shortening, decreased CV, increased AF inducibility and downregulation of the expression of SCN5A and LCC were observed. In the bepridil group, the AERP exhibited a relatively quick recovery after bepridil was started in the first week and continued to recover gradually until the end of the protocol, but that recovery was smaller in the LA than in the RA. The CV was not affected by bepridil administration. AF inducibility was well suppressed in the RA in the bepridil group, but the induction of short-duration AF could not be suppressed in the LA. The mRNA downregulation of the LCC and SCN5A was negated by bepridil administration in the RA; but not in the LA; however, the data showed similar tendencies. There were no significant differences in the hemodynamic parameters or tissue fibrosis and the mRNA expression of Kv4.3, Cx40, 43, and 45 between the pacing control and bepridil groups.

CONCLUSION

Bepridil exhibited an anti-electrical remodeling effect in this study as previously reported, but the effect was inhomogeneous between the RA and LA, with the LA appearing to be more resistant to the effect of bepridil.

Termination of a tachyarrhythmia by flunarizine is not a specific marker for a triggered mechanism

BACKGROUND

Prior studies have indicated that tachyarrhythmia termination by flunarizine demonstrates a triggered mechanism. This concept was not confirmed in atrial tachyarrhythmias.

OBJECTIVE

The purpose of this study was to test the hypothesis that flunarizine will not terminate reentrant atrial flutter (AFL).

METHODS

We administered flunarizine (2 mg/kg intravenously over 2 minutes) in 11 episodes of reproducibly inducible, sustained AFL in eight canines with sterile pericarditis. If flunarizine terminated AFL, we studied AFL reinducibility. We also studied pacing thresholds, refractoriness, and intra-atrial conduction time during closed-chest studies and pacing at selected cycle lengths (CLs) from selected sites before and after flunarizine administration. Atrial mapping (510 electrodes) assessed the epicardial activation sequence during AFL and its termination in six episodes. Four AFL episodes were studied in the closed-chest state.

RESULTS

Flunarizine increased AFL CL in all episodes (mean 21 ms; range 7-49 ms), which is explained by slowing conduction in the AFL reentrant circuit, principally in the area of slow conduction. AFL was terminated in 10/11 episodes after drug initiation (mean 3.7 minutes; range 0.5-6.5 minutes) by block in the area of slow conduction. AFL was then not immediately reinducible until >20 minutes after drug administration. Flunarizine had no meaningful effect on atrial pacing thresholds for capture or refractoriness and only affected conduction time in the area of slow conduction in the reentrant circuit.

CONCLUSIONS

Flunarizine (1) causes progressive slowing and block in the area of slow conduction of the AFL reentrant circuit in the canine sterile pericarditis model and (2) is effective in terminating reentrant AFL and so is not a specific marker for a triggered mechanism.

Arrhythmogenic Difference Between the Left and Right Atria During Rapid Atrial Activation in a Canine Model of Atrial Fibrillation

BACKGROUND

Continuous rapid atrial stimulation causes atrial remodeling, but little is known about the difference in the arrhythmogenicity of the left (LA) and right atria (RA).

METHODS AND RESULTS

In 14 beagle dogs, continuous rapid pacing (400 beats/min) was delivered from the right (n=7) or left (n=7) atrial appendage (RAA or LAA) for 2 weeks. The atrial effective refractory period (ERP), ERP dispersion, and inducibility of atrial fibrillation (AF) were evaluated along the time course from 4 atrial sites: (1) RAA, (2) area close to the inferior vena cava (IVC), (3) Bachmann's bundle (BB) and (4) LA. The ERP exhibited progressive shortening at all sites, but the degree of shortening differed among them. In the RA stimulation group, ERP shortening was more prominent in the RAA and LA than in the IVC or BB. In contrast, in the LA stimulation group, ERP shortening was more prominent in the LA than in the other sites. As a result, ERP dispersion was larger in the LA stimulation group than in the RA stimulation group and the AF inducibility was higher in the LA stimulation group than in the RA stimulation group, especially at the LA site (p<0.05).

CONCLUSION

LAA stimulation was more arrhythmogenic than RAA stimulation in this model. This result may partly explain the importance of premature contractions occurring from the pulmonary veins in clinical cases of AF.

Bepridil inhibits sub-acute phase of atrial electrical remodeling in canine rapid atrial stimulation model

Background The effect of bepridil, a multichannel blocker, on atrial electrical remodeling was evaluated in a canine rapid atrial stimulation model. Methods and Results In 10 beagle dogs, the right atrial appendage (RAA) was paced at 400 beats/min for 2 weeks. The atrial electrophysiological parameters, including effective refractory period (AERP), were evaluated at three atrial sites: RAA, the right atrium close to the inferior vena cava (IVC) and the left atrium (LA), during the time course of rapid pacing. Five of the dogs were given bepridil (10 mg . kg (-1) . day(-1) po). In the control group, AERP was significantly shortened at all atrial sites and the AERP shortening (DeltaAERP) was larger for the RAA and LA than at the IVC site (p<0.05). In the bepridil group, DeltaAERP was smaller than that of the controls at all atrial sites, and the AERP started to return slowly to the pre-pacing level in the second week, regardless of the continuation of rapid pacing. Conclusions In a canine rapid atrial stimulation model, bepridil suppressed AERP shortening. Bepridil might have a reverse electrical remodeling effect, at least for AERP shortening, because it showed slow recovery of AERP in the subacute phase of rapid atrial pacing. (Circ J 2006; 70: 206 - 213).

Verapamil suppresses the inhomogeneity of electrical remodeling in a canine long-term rapid atrial stimulation model

Verapamil is known to suppress shortening of the atrial effective refractory period (AERP) during relatively short-term atrial pacing, although the effect of a long-term stimulation model is unclear. The effect of verapamil on electrical remodeling was evaluated in a canine rapid atrial stimulation model. The right atrial appendage (RAA) was continuously paced (400 beats/min) for 2 weeks. Four pairs of electrodes were sutured at four atrial sites; the RAA, right atrium close to the inferior vena cava, Bachmann's bundle, and LA. AERP, AERP dispersion (AERPd), conduction time, and inducibility of AF were evaluated during the pacing phase and the recovery phase. The same protocol was performed under the administration of verapamil. In five control dogs, the AERP shortening was inhomogeneous and the shortening of the AERP was most prominent in the LA. AERPd increased during the rapid pacing phase by 5 +/- 2 ms, but recovered quickly in the recovery phase. The max AERPd was 46 +/- 4 ms in the control group and was larger than that in the verapamil group (31 +/- 3 ms, P = 0.001). At the LA site, the shortening of the AERP was decreased by verapamil administration (-19 +/- 3 vs -5 +/- 2 ms, P = 0.04). However, the AF inducibility was not significantly different between the two groups. The effect of verapamil on electrical remodeling was inhomogeneous, depending on the anatomic portion. As a result, AERPd widening during the rapid pacing phase was suppressed by verapamil, while the AF inducibility was unchanged.

Effect of pilsicainide on atrial electrophysiologic properties in the canine rapid atrial stimulation model

The heterogeneous process of atrial electrical remodeling (AER) in the canine rapid atrial stimulation model has been previously reported although it has been reported that a sodium channel blocker might suppress the shortening of the atrial effective refractory period (AERP), its effect on long-term electrical remodeling is unknown. In the present study, the effect of pilsicainide on AER was evaluated. The right atrial appendage (RAA) was paced at 400 beats/min for 2 weeks. In the RAA, Bachmann's bundle (BB), the right atrium near the inferior vena cava (IVC) and in the left atrium (LA), AERP, AERP dispersion (AERPd) and the inducibility of atrial fibrillation (AF) were evaluated at several time points of the pacing phase and the recovery phase (1 week). The same protocol was performed during the administration of pilsicainide (4.5 mg/kg per day) and the parameters were compared with the controls. In the control dogs, the AERP was significantly shortened by rapid pacing at all atrial sites studied and the AERP shortening (DeltaAERP) was larger at the RAA and LA sites (p<0.03). However, pilsicainide decreased these DeltaAERPs at all 4 atrial sites. AERPd was increased during the pacing phase whereas it was decreased during the recovery phase in the control dogs. In contrast, this pacing-induced AERPd was attenuated by the administration of pilsicainide. The AF inducibility was highest at the LA site in both groups, and the inducibility was lower in the pilsicainide group than the control group at all atrial sites. During the rapid pacing phase, the ventricular heart rate was significantly lower in the pilsicainide group than the control because of intra-atrial conduction block. In a canine rapid right atrial stimulation model, pilsicainide suppressed the shortening of the AERP at all atrial sites, possibly through the improvement of the hemodynamics as well as the action of the Na - Ca exchanger.

Monitoring the progression of the atrial electrical remodeling in patients with paroxysmal atrial fibrillation

It is important to clarify how electrical remodeling develops in clinical cases of paroxysmal atrial fibrillation (PAF), because it has been suggested that this electrophysiological phenomenon promotes an increase in the frequency of PAF. In the present study, the f-f interval during PAF was analyzed from the ambulatory ECG recordings of 21 patients with PAF (total PAF duration >2/24 h with normal atrial size) to monitor the atrial electrophysiological changes. The patients were clinically followed-up for 6 months without any antiarrhythmic drugs. Before and after the follow-up period 24-h Holter monitoring was carried out and the duration of both the PAF and the f-f intervals during the PAF episode were evaluated. In selected cases, the atrial effective refractory period (ERP) was evaluated in an electrophysiologic study before and after the follow-up period. The total PAF duration was prolonged from 187+/-50 to 223+/-79 min (p=0.034) and the f-f interval was shortened from 0.14+/-0.03 to 0.12+/-0.02 ms (p=0.003). There was an inverse relationship between the changes in total PAF duration and f-f interval (p=0.027). The ERP was shortened from 214+/-15 to 194+/-5 ms (n=5, p=0.025) and there was a direct correlation between the changes in ERP and f-f interval (p=0.048). In clinical cases, the prolongation of the PAF was related to the shortening of the f-f interval during the PAF episodes and to the shortening of the atrial ERP. Electrical remodeling plays a role in promoting the development of the atrial fibrillation in patients with PAF.

Electrical remodeling of the ventricular myocardium in myocarditis: studies of rat experimental autoimmune myocarditis

The purpose of this study was to evaluate the electrical remodeling of the ventricular myocardium in the experimental autoimmune myocarditis (EAM) model in Lewis rats. EAM was induced by immunization with cardiac myosin. During the active myocarditis phase, the effective refractory period (ERP), the duration of the monophasic action potential (MAPD) was extracted from the left ventricular free wall, and the mRNA levels of Kv1.4, 4.2, 4.3 and L type Ca2+ channel were determined by RNase protection assays. The inducibility of ventricular arrhythmia was higher in EAM rats than in the control rat, and the direct relationship between the coupling intervals of the premature stimulus and the ventricular arrhythmia in EAM rats. The ERP was prolonged in EAM rats compared with the control group. The MAPDs determined as 20% and 90% repolarization time, were both longer in EAM rats than in the controls. The level of expression of Kv4.2 mRNA was reduced in EAM rats in comparison with the controls, whereas those of Kv1.4, 4.3 and the L type Ca2+ channel were unchanged. Ventricular vulnerability was higher in EAM rats than in the control rats, and some of the ventricular arrhythmias observed in the EAM group seemed to be based on triggered activity. The level of expression of Kv4.2 mRNA was significantly reduced, and this change was compatible with prolongation of the action potential duration.

Inhomogeneity in the appearance of electrical remodeling during chronic rapid atrial pacing: evaluation of the dispersion of atrial effective refractoriness

In the present study, the long-term process of progression of electrical remodeling at various atrial sites, which is not well understood, was compared while monitoring continuously the electrophysiologic parameters at multirecording sites in canine atria during continuous atrial burst pacing. A rapid pacing device was implanted in 5 dogs, and continuous atrial burst pacing (400 beats/min) was delivered at the right atrial appendage (RAA). Four pairs of epicardial wire electrodes were sutured on (1) the RAA, (2) Bachmann's bundle (BB), (3) the right atrium close to the inferior vena cava (IVC), and (4) the left atrium (LA). The distal ends of those wires were exteriorized posteriorly and used for pacing and recording. The atrial effective refractory period (AERP), AERP dispersion (AERPd), atrial conduction time (CT) and inducibility of atrial fibrillation (AF) were evaluated during burst pacing for 14 days and during the subsequent 7 days' recovery. The AERP at the LA pacing site was shorter than that at the other sites on day 0. The AERP shortening was greater in the RAA and LA sites than in the BB and IVC sites. The AERPd increased during pacing and reached the maximum level on day 3, and then decreased during the recovery phase. Prolongation of CT tended to be longer between the RAAand IVC sites than that between the other sites. The incidence of AF induction became higher in accordance with the time course of the rapid pacing phase. There was another peak of AF induction on days 7-10. In a canine chronic rapid atrial stimulation model, the progression of electrical remodeling (ie, the shortening of the AERP and the prolongation of the CT) was not homogeneous in both atria, the AERPd showed a temporal increase between days 3 and 7 and matched the increase in AF inducibility at the LA pacing site, the increase in the AERPd was mainly caused by more rapid AERP shortening at the RAA or LA sites, and the LA site always showed a shorter AERP than the other atrial sites in the control state and during the rapid pacing phase, whereas AF inducibility was higher at the LA site than the other sites.

Evaluation of post-repolarization refractoriness for conduction block in cardiac muscle: studies in an artificial isthmus in the canine right atrium

Post-repolarization refractoriness (PRR) is an important factor in determining conduction block and is the difference between the effective refractory period (ERP) and the duration of the monophasic action potential (MAPD). In the present study, conduction block in an artificial isthmus in the canine atrium was evaluated and the coupling interval of a premature beat, which caused the block, was defined as the block coupling interval (BCI). The usefulness of this value was also evaluated. Radiofrequency linear ablation was performed on the right atrial surface parallel to the atrioventricular groove in 5 mongrel dogs, and an artificial isthmus (8-10mm wide and 25-30mm long) was created. Fourteen simultaneous unipolar recordings were performed in the isthmus with a resolution of 1.2 mm. Single extra-stimuli with basic drive train were delivered to induce conduction block in the isthmus and when it occurred, the coupling interval at the recording site just proximal to the site of the block was defined as the BCI. At the site of the block, the ERP and MAPD at each drive cycle length were measured. The PRR was calculated using 2 different formulae: (1) [ERP-MAPD], and (2) [BCI-MAPD]. It was found that each value was shortened in accordance with the shortening of the basic drive cycle length. In all basic drive trains, BCI>ERP>MAPD, and [ERP-MAPD] was always shorter than [BCI-MAPD]. In the shorter cycle length of basic drives, the difference between [ERP-MAPD] and [BCI-MAPD] was more prominent. In the artificial isthmus model in the canine atrium, BCI was always longer than the ERP measured at the same site as the block. Because the ERP may not directly reflect the block phenomenon, the electrophysiologic evaluation should use the BCI instead, as in the PRR evaluation.

Quantitative evaluation of a directly depolarized area induced by high-output pacing on the cardiac muscle

Quantitative information is needed on the directly depolarized area (DDA) induced by high-output energy during a precise mapping procedure to detect the origin of a tachycardia. In the present study, a DDA caused by high-output energy was quantitatively evaluated in the exposed canine heart. In 8 dogs, the right atrial and ventricular surfaces were exposed through a right thoracotomy and pacing with various outputs was delivered from the epicardial surface. A comb-shaped 16 polar electrode array and/or a 224 polar mat electrode array were used for recording the epicardial electrograms. The local activation time was measured at each electrode site, and the relationship of the distance between the electrode location from the pacing site and the local activation time was plotted and fitted to a primary regression line. The intercept of the regression line on the horizontal axis was defined as the radius of the 'DDA' and this was evaluated at each pacing output. The radius of the DDA was 0.6+/-0.1 mm with a 2 V and 3.8+/-0.2 mm with a 10 V output when it was evaluated in a direction perpendicular to the fiber orientation of the pectinate muscle, 0.8+/-0.1 mm with a 2 V and 4.1+/-0.3 mm with a 10 V output in a direction parallel to the pectinate muscle fiber orientation, and 0.9+/-0.3 mm with a 2 V and 3.6+/-0.5 mm with a 10 V output in the right ventricle. The DDA extended according to the increase in stimulation outputs at all sites, and there was no significant difference in the pacing site or the direction of the stimulation propagation. The DDA caused by high-output energy is a purely physical phenomenon that depends only on stimulation output and tissue resistance. The diameter of the DDA exceeded 4 mm (ie, the size of a standard tip electrode for catheter ablation) when pacing was delivered with an output greater than 6 V.

Comparison of arrhythmogenicity of atrial pacing at several right atrial pacing sites: evaluation of canine atrial electrograms during atrial pacing and arrhythmogenicity for atrial fibrillation

The changes in the duration of atrial electrograms and the appearance of AF during atrial pacing were compared among five atrial pacing sites in dogs to clarify the arrhythmogenicity of atrial pacing at different atrial pacing sites. In seven mongrel dogs (15-20 kg), the right atrial surface was exposed by right thoracotomy. Atrial electrograms were recorded via bipolar electrodes with an interelectrode distance of 1.2 mm at four right atrial sites: (1) the high right atrium (HRA), (2) the mid-right atrium (MRA), (3) the low right atrium (LRA), and (4) the center of the pectinate muscle (PM). The duration of the atrial electrograms at these four recording sites were measured during atrial pacing with fixed cycle lengths of 200, 150, and 120 ms delivered at five atrial sites: (1) the HRA, (2) the inferior vena cava (IVC), (3) the right atrial appendage (RAA), (4) Bachman's bundle (BB), and (5) the atrial septum (AS). In each dog, the atrial pacing with the 120-ms cycle length was performed five times at each pacing site to evaluate the inducibility of AF. When AF was induced, the atrial recording site which first showed a fragmented atrial electrogram was considered the initiation site of the AF. AF was induced during 9 of 35 episodes of atrial pacing at the HRA site, 11 of 35 at the IVC site, 5 of 35 at the RAA site, 3 of 35 at the BB site, and none at the AS site. The initiation site of AF was in the HRA site in 11 of 28 episodes of induced AF, in the MRA site in 9 of 28, and in the LRA site in 8 of 28. At each recording site, the shorter the paced cycle length, the longer the duration of the atrial electrogram regardless of the pacing site. During the atrial pacing with the 200-ms cycle length, the HRA pacing resulted in the shortest duration of the atrial electrogram at each recording site in comparison with the other pacing sites. However, during atrial pacing at the two shorter paced cycle lengths, the duration of the atrial electrogram was shorter during the pacing at the BB or AS sites in comparison with the other three pacing sites, i.e., the HRA, IVC, and RAA sites. These results were the same for all atrial recording sites, but the prolongation of the atrial electrogram was most prominent at the HRA and MRA recording sites, which are most likely initiation sites of the induced AF. In the canine atria, (1) the initiation sites of AF were likely to be the HRA, MRA, or LRA sites in comparison with the PM site; and (2) the atrial pacing at the BB or AS sites was considered less arrhythmogenic for AF than the pacing at the HRA, LRA, or RAA sites.

Electrocardiographic and electrophysiologic characterization of atypical atrial flutter in man: use of activation and entrainment mapping and implications for catheter ablation

INTRODUCTION

Although the circuit in typical counterclockwise atrial flutter has been clearly delineated, the mechanisms of "atypical atrial flutters" have been less well characterized. The purpose of this study was to investigate the ECG and electrophysiologic (EP) characteristics of atypical atrial flutter.

METHODS AND RESULTS

Thirty-three patients with at least one form of atypical atrial flutter underwent EP evaluation with multipolar atrial activation and entrainment mapping. Nineteen patients with clockwise flutter had: (1) stereotypic ECG morphology; (2) same cycle length as counterclockwise flutter; (3) clockwise activation around the tricuspid annulus; (4) recording of discrete split potentials along the length of the crista terminalis, suggesting the presence of conduction block; (5) concealed entrainment from the low right atrial isthmus; (6) successful ablation in this isthmus. Twenty patients with atypical flutter not consistent with a clockwise mechanism ("true atypical flutter") showed: (1) heterogeneous ECG morphology; (2) cycle length shorter than that of clockwise flutter; (3) frequent transitions from and to atrial fibrillation; (4) could be entrained in only six patients and, when accomplished, demonstrated surface fusion when entraining from the low right atrial isthmus.

CONCLUSIONS

Atypical flutter falls into two broad categories. Clockwise flutter uses the same circuit with the same endocardial barriers as its counterclockwise counterpart and is best considered a form of typical flutter. True atypical flutter induced in the EP laboratory is a heterogeneous group of arrhythmias that are transitional to atrial fibrillation. Although it may superficially resemble clockwise or counterclockwise flutter based on the 12-lead ECG alone, the distinction can be readily made from a combined evaluation including activation and entrainment mapping.

Primary ablation of atrial flutter and atrial fibrillation

Direct treatment of atrial flutter and atrial fibrillation--that is, attempting to prevent arrhythmia recurrences by ablating atrial tissue--has been a challenge because of uncertainty about the location of optimal target tissues as well as the amount of atrial tissue requiring destruction to effect cure. Advances have yielded success rates for ablation of the common form of atrial flutter comparable to those for other types of supraventricular tachycardia and provide reason for optimism about the use of catheter techniques, to treat atrial fibrillation definitively. This article discusses some of these advances as well as the current status of catheter ablation for atrial flutter and atrial fibrillation and, finally, what the future may bring.

Unique properties of cardiac action potentials recorded with voltage-sensitive dyes

INTRODUCTION

Optical mapping with voltage-sensitive dyes has made it possible to record cardiac action potentials with high spatial resolution that is unattainable by conventional techniques. Optically recorded signals possess distinct properties that differ importantly from electrograms recorded with extracellular electrodes or action potentials recorded with microelectrode techniques. Despite the growing application of optical mapping to cardiac electrophysiology, relatively little quantitative information is available regarding the characteristics of optical action potentials recorded from cardiac tissue.

METHODS AND RESULTS

A high-resolution optical mapping system and microelectrode techniques were used to determine the characteristics of guinea pig ventricular action potentials recorded with the voltage-sensitive dye di-4-ANEPPS. The effects of optical magnification, tissue-light interaction, sampling rate, voltage resolution, spatial resolution, and cardiac motion on action potential signal characteristics were determined. The optical action potential signal represents the relative change in transmembrane potential arising from a volume of cells, where the area of a recording site is determined by optical magnification and detector area, and the depth of recording is determined by system optics and the visible light transmission characteristics of cardiac muscle. Using photographic lenses, the depth of tissue contributing to the signal is < 250 microns. The action potential plateau and final repolarization can be accurately reconstructed from data digitized at modest sampling rates (450 to 750 Hz), since the frequency content of optical action potentials is band-limited to approximately 150 Hz. However, faster sampling rates are needed to depict the subtle details of the action potential upstroke. In addition to temporal resolution, it is essential to achieve sufficient dynamic range and voltage resolution to accurately represent the time course of membrane potential change. Voltage resolution is inversely related to the square of spatial resolution, hence, there exists an inherent trade-off between increased spatial resolution and diminished voltage resolution. Cardiac motion, which can otherwise limit spatial resolution as well as signal fidelity, can be effectively reduced using mechanical stabilization of the heart without distorting action potential characteristics.

CONCLUSIONS

Optical mapping with voltage-sensitive dyes provides high-fidelity multisite action potential recording with flexible spatial resolution. When recording cardiac action potentials with voltage-sensitive dyes, the interdependence of temporal, spatial, and voltage resolutions must be carefully considered.

Pharmacologic alterations in human type I atrial flutter cycle length and monophasic action potential duration. Evidence of a fully excitable gap in the reentrant circuit

OBJECTIVES

This study compared the effect of changes in action potential duration versus conduction velocity on atrial flutter cycle length to determine whether there is a fully or partially excitable gap in atrial flutter.

BACKGROUND

In an excitable gap reentrant circuit, cycle length is proportional to conduction velocity. Action potential duration is not a direct determinant of cycle length when the gap is fully excitable.

METHODS

Right atrial monophasic action potentials were recorded from 41 patients during type I atrial flutter before and during pharmacologic interventions.

RESULTS

Adenosine (17 +/- 3 mg [mean +/- SD]) shortened (p < 0.001) action potential duration but did not change cycle length. Edrophonium (10 mg) had no significant effect on action potential duration or cycle length. Isoproterenol (0.03 microgram/kg body weight per min) shortened (p < 0.05) and procainamide (15 mg/kg, then 2 mg/min) prolonged (p < 0.001) action potential duration and cycle length. Alterations in cycle length were not correlated with changes in action potential duration. Procainamide's prolongation of action potential duration was reversed by adenosine without affecting cycle length. Procainamide's prolongation of action potential duration and cycle length was partially reversed by isoproterenol. Adenosine's and isoproterenol's shortening of action potential duration and isoproterenol's shortening of cycle length were enhanced by procainamide.

CONCLUSIONS

Atrial flutter cycle length is determined primarily by conduction velocity and does not depend directly on action potential duration. Atrial flutter has a fully excitable gap, and procainamide does not convert the gap from full to partial excitability. Adenosine and isoproterenol interact with procainamide such that their effects are enhanced and procainamide's effects are diminished.

Implantable atrial defibrillators

Due to the limited efficacy of antiarrhythmic drugs for atrial fibrillation, several nonpharmacologic therapeutic options have evolved. One of these is an implantable atrial defibrillator. Recent studies have shown that internal atrial defibrillation is feasible with relatively low energies. To date, the optimal electrode configuration involves large surface area catheters in the right atrium and coronary sinus. In humans, atrial defibrillation can generally be achieved with < 2 J using this electrode configuration and a biphasic shock waveform. For shocks < 5 J, there is no significant pathological damage to the atria or coronary sinus. Further investigation is needed to guarantee that atrial defibrillation shocks do not provoke ventricular arrhythmias. Preliminary data suggest that atrial defibrillation shocks synchronized to R waves that are not closely coupled are safe. In addition, the shocks are well tolerated if the shock energy is < 1.5 J. With additional studies to confirm the safety of implantable atrial defibrillators, further reduce shock energy, and improve patient tolerance, an implantable atrial defibrillator can become an acceptable therapy for patients with symptomatic, paroxysmal atrial fibrillation.