Stuart P. Thomas

Impulse Propagation in Synthetic Strands of Neonatal Cardiac Myocytes With Genetically Reduced Levels of Connexin43

Abstract —Connexin43 (Cx43) is a major determinant of the electrical properties of the myocardium. Closure of gap junctions causes rapid slowing of propagation velocity (&thgr;), but the precise effect of a reduction in Cx43 levels due to genetic manipulation has only partially been clarified. In this study, morphological and electrical properties of synthetic strands of cultured neonatal ventricular myocytes from Cx43+/+ (wild type, WT) and Cx+/− (heterozygote, HZ) mice were compared. Quantitative immunofluorescence analysis of Cx43 demonstrated a 43% reduction of Cx43 expression in the HZ versus WT mice. Cell dimensions, connectivity, and alignment were independent of genotype. Measurement of electrical properties by microelectrodes and optical mapping showed no differences in action potential amplitude or minimum diastolic potential between WT and HZ. However, maximal upstroke velocity of the transmembrane action potential, dV/dtmax, was increased and action potential duration was reduced in HZ versus WT. &thgr; was similar in the two genotypes. Computer simulation of propagation and dV/dtmax showed a relatively small dependence of &thgr; on gap junction coupling, thus explaining the lack of observed differences in &thgr; between WT and HZ. Importantly, the simulations suggested that the difference in dV/dtmax is due to an upregulation of INa in HZ versus WT. Thus, heterozygote‐null mutation of Cx43 produces a complex electrical phenotype in synthetic strands that is characterized by both changes in ion channel function and cell‐to‐cell coupling. The lack of changes in &thgr; in this tissue is explained by the dominating role of myoplasmic resistance and the compensatory increase of dV/dtmax. (Circ Res. 2003;92:1209–1216.)

Comparison of epicardial and endocardial linear ablation using handheld probes.

BACKGROUND The optimal technique for producing linear radiofrequency thermal lesions in myocardial tissue is unclear. We compared epicardial ablation on the beating heart with endocardial ablation after cardioplegia. METHODS Radiofrequency lesions were produced using a multielectrode malleable handheld probe in ovine myocardium with three wall thicknesses. Detailed analysis of lesion dimensions was used to assess the effects of site of ablation, muscle thickness, and duration of ablation. RESULTS After epicardial atrial ablation, myocardial lesions were detected in all sections without macroscopically visible epicardial fat (n = 10), but only 43% (6/14) of sections with epicardial fat. Three of 24 atrial epicardial sections (13%) and 92% (23/25) of endocardial atrial lesion sections were clearly transmural. In thicker tissues lesion depth was independent of endocardial (right ventricle: 3.9 +/- 1.1 mm, left ventricle: 3.8 +/- 0.7 mm) or epicardial (right ventricle: 3.4 +/- 0.6 mm, left ventricle: 4.3 +/- 0.9 mm) ablation site. Epicardial lesions are less deep in thinner areas of myocardium (p = 0.003). Lesions were all wider than they were deep. There was no significant increase in lesion depth with the increase in ablation duration from 1 to 2 minutes. CONCLUSIONS Lesions were unlikely to be transmural with either technique when the wall thickness was greater than about 4 mm. Epicardial fat has an important negative effect on epicardial lesion formation. Where epicardial fat is absent epicardially produced lesions penetrate less deeply when the wall thickness is small, possibly due to endocardial cooling by circulating blood. Prolongation of the duration of ablation from 1 to 2 minutes does not significantly increase lesion depth.

Mechanism, localization and cure of atrial arrhythmias occurring after a new intraoperative endocardial radiofrequency ablation procedure for atrial fibrillation

OBJECTIVES The purpose of this study was to test a new pattern of radiofrequency ablation for atrial fibrillation (AFib) intended to optimize atrial activation, and to demonstrate the usefulness of catheter techniques for mapping and ablation of postoperative atrial arrhythmias. BACKGROUND Linear radiofrequency lesions have been used to cure AFib, but the optimal pattern of lesions is unknown and postoperative tachyarrhythmias are common. METHODS A radial pattern of linear radiofrequency lesions (Star) was made using an endocardial open surgical approach in 25 patients. Postoperative arrhythmias were induced and characterized during electrophysiological studies in 15 patients. RESULTS The AFib was abolished in most patients (91%), but atrial flutter (AFlut) occurred in 96% of patients postoperatively. At postoperative electrophysiological studies, 37 flutter morphologies were studied in 15 patients (46% spontaneous, cycle length [CL] 223 +/- 25 ms). Seven mechanisms (lesions discontinuity, n = 6; focal mechanism, n = 1) of AFlut were characterized in six patients. In these cases, flutter was abolished using further catheter radiofrequency ablation. In the remaining cases, flutter was usually localized to an area involving the interatrial septum, but no critical isthmus was identified for ablation. After 16 +/-10 months, 15 patients (65%) were asymptomatic with (n = 3) or without (n = 12) antiarrhythmic medications. Eight (35%) patients had persistent arrhythmias. Postoperative atrial electrical activation was near physiological. CONCLUSIONS The AFib maybe abolished using a radial pattern of linear endocardial radiofrequency lesions, but postoperative AFlut is common even when lesions are made under optimal conditions. Endocardial mapping techniques can be used to characterize the flutter mechanisms, thus enabling subsequent successful catheter ablation.

Organization of Myocardial Activation During Ventricular Fibrillation After Myocardial Infarction Evidence for Sustained High-Frequency Sources

Background—Studies of ventricular fibrillation (VF) in small mammals have revealed localized sustained stationary reentry. However, studies in large mammals with surface mapping techniques have demonstrated only relatively short-lived rotors. The purpose of this study was to identify whether sustained high-frequency activation with low beat-to-beat variability was present at intramural sites in a postinfarct ovine model of VF. Methods and Results—VF was induced in 12 sheep 77±40 days after anterior myocardial infarction. Electrical activation was recorded with 20 multielectrode transmural plunge needles. Unipolar electrogram frequency content and local cycle duration variability were studied in 30-second recordings beginning 5 seconds after the onset of VF. Higher mean beat frequency was associated with lower SD of the cycle duration intervals (r=−0.91, P<0.001). The mean beat frequency and the SD of cycle duration intervals of the highest-frequency electrode were 8.8±2.0 Hz and 17±11 ms. In 3 cases, a region with regular activation throughout the recording was identified (SD of the cycle duration interval, 6.0±0.7 ms). Two of these sites and 67% of all sites with low local cycle duration variability were intramural. They occurred within regions with a high dominant frequency as determined by fast Fourier transform of the unipolar electrogram. Conclusions—Regions with the highest frequency of activation during VF were always associated with a low local cycle duration variability and usually intramural in this chronic infarct model. In a minority of cases, a region of stable, rapid, and very regular activation could be identified. These findings support the hypothesis that relatively stable periodic sources form a component of the mechanism of VF in this model.

Pulmonary Vein Stenosis and Remodeling After Electrical Isolation for Treatment of atrial fibrillation: Short- and Medium-Term follow-up

Asymptomatic pulmonary vein (PV) stenosis after PV electrical isolation for atrial fibrillation has been reported in several studies and may be due to dynamic factors. The purpose of this study was to determine if PV stenosis progresses after the initial procedure. Consecutive patients (n = 26) requiring repeat procedures for atrial fibrillation recurrence were studied (mean age 55 ± 12 years). Segmental PV potential‐guided ostial ablation was performed with transvenous catheters. Biplane angiographic images were obtained before and after each procedure (52 procedures). Stenoses were found in 14 (16%) of 87 targeted veins immediately after the initial procedures. After 129 ± 94 days no new stenoses were found at the second procedure. PV stenoses were unchanged in 8 previously stenosed veins, slightly deteriorated in 1 vein, improved in 2 veins, and fully resolved in 3 veins. No patients had symptoms attributable to PV stenosis. PV stenosis occurred in 6 (9%) of 68 additional veins at the second procedure. No baseline or procedural characteristics predicted stenosis. Progression of PV stenosis is uncommon in the medium term. Complete or partial resolution of PV stenosis occurs in approximately one third of cases. Absence of PV stenosis after an initial procedure does not ensure PV stenosis will not occur with further ablation in the same vein.

Medium-Term Efficacy of Segmental Ostial Pulmonary Vein Isolation for the Treatment of Permanent and Persistent Atrial Fibrillation

Introduction: Previous studies suggest that segmental ostial isolation of the pulmonary veins for the treatment of patients with persistent and permanent atrial fibrillation is associated with a high rate of recurrence. Recurrence of atrial fibrillation is usually associated with electrical reconnection of the pulmonary veins to the left atrium.

Rapid loading of sotalol or amiodarone for management of recent onset symptomatic atrial fibrillation: a randomized, digoxin-controlled trial.

BACKGROUND Amiodarone and sotalol are commonly used for the maintenance of sinus rhythm, but the efficacy of these agents administered as high-dose infusions for rapid conversion of atrial fibrillation is unknown. Use in this context would facilitate drug initiation in patients in whom ongoing prophylactic therapy is indicated. METHODS We assessed the efficacy and safety of rapid high-dose intravenous infusions of amiodarone and sotalol for heart rate control and rapid reversion to sinus rhythm in patients who came to the emergency department with recent-onset symptomatic atrial fibrillation. Patients (n = 140) were randomized to receive 1.5mg/kg of sotalol infused in 10 minutes, 10mg/kg of amiodarone in 30 minutes, or 500 microg of digoxin in 20 minutes. Electrical cardioversion was attempted for patients not converting to sinus rhythm within 12 hours. RESULTS The rapid infusion of sotalol or amiodarone resulted in more rapid rate control than digoxin. Each of the 3 trial strategies resulted in similar rates of pharmacological conversion to sinus rhythm (amiodarone, 51%; sotalol, 44%; digoxin, 50%; P = not significant). The overall rates of cardioversion after trial drug infusion and defibrillation were high for all groups (amiodarone, 94%; sotalol, 95%,; digoxin, 98%; P = not significant), but there was a trend toward a higher incidence of serious adverse reactions in the amiodarone group. CONCLUSION The rapid infusion of sotalol or amiodarone in patients with symptomatic recent-onset atrial fibrillation results in rapid control of ventricular rate. Even with high-dose rapid infusions, all 3 agents are associated with a poor overall reversion rate within 12 hours. Almost all patients were returned to sinus rhythm with a combination of pharmacological therapy and electrical cardioversion.

The Effect of a Residual Isthmus of Surviving Tissue on Conduction after Linear Ablation in Atrial Myocardium

The aim of this study was to determine the relationship between the size of discontinuities in lines of ablation and wavefront propagation. Discontinuities in linear radiofrequency lesions used for the treatment of atrial fibrillation may be proarrhythmic and a major clinical problem. A better understanding of the electrophysiological properties of these discontinuities (isthmuses) may assist in their detection and treatment. Linear lesions were made in the right atrial free wall using a Nd:YAG laser in 12 dogs. Conduction properties across the discontinuities were studied by pacing from either side of the lesion. Two of the three isthmuses less than 0.8mm2 in cross section (smallest 0.2mm2) conducted at extrastimulus intervals of 300ms. All three failed to conduct at cycle lengths close to the atrial effective refractory period. Isthmuses above 0.8mm2 (n}=8) conducted at all cycle lengths. Conduction slowing (mean slowest conduction 0.5\plusmn; 0.3m/s) occurred in the region of the isthmus but the overall delay was only 6plusmn; 6ms where propagation through the isthmus occurred. The effect on conduction of small discontinuities in linear lesions is dependent on the size of the residual isthmus. All but the very smallest of discontinuities in linear lesions conduct and therefore have the potential to participate in reentrant arrhythmias. Efforts should be directed toward the development of ablation techniques that reliably produce continuous transmural linear lesions for cure of atrial fibrillation and flutter.The aim of this study was to determine the relationship between the size of discontinuities in lines of ablation and wavefront propagation. Discontinuities in linear radiofrequency lesions used for the treatment of atrial fibrillation may be proarrhythmic and a major clinical problem. A better understanding of the electrophysiological properties of these discontinuities (isthmuses) may assist in their detection and treatment. Linear lesions were made in the right atrial free wall using a Nd:YAG laser in 12 dogs. Conduction properties across the discontinuities were studied by pacing from either side of the lesion. Two of the three isthmuses less than 0.8mm2 in cross section (smallest 0.2mm2) conducted at extrastimulus intervals of 300ms. All three failed to conduct at cycle lengths close to the atrial effective refractory period. Isthmuses above 0.8mm2 (n}=8) conducted at all cycle lengths. Conduction slowing (mean slowest conduction 0.5\plusmn; 0.3m/s) occurred in the region of the isthmus but the overall delay was only 6plusmn; 6ms where propagation through the isthmus occurred. The effect on conduction of small discontinuities in linear lesions is dependent on the size of the residual isthmus. All but the very smallest of discontinuities in linear lesions conduct and therefore have the potential to participate in reentrant arrhythmias. Efforts should be directed toward the development of ablation techniques that reliably produce continuous transmural linear lesions for cure of atrial fibrillation and flutter.

Atrial arrhythmias after single-ring isolation of the posterior left atrium and pulmonary veins for atrial fibrillation: mechanisms and management.

Background—Single-ring isolation of the posterior left atrium is feasible, but the incidence and mechanisms of postprocedural arrhythmias have not been described in detail. Methods and Results—The first 100 consecutive patients (58.8±11.2 years old, 80 male) who underwent single-ring isolation for atrial fibrillation (66 intermittent, 18 persistent, 16 long-standing persistent) were followed up for 9.1±4.5 months. Recurrences were diagnosed by clinical symptoms and Holter monitoring. Patients with recurrences of sustained atrial arrhythmia >3 months after the procedure were offered a repeat procedure and were studied to determine the mechanisms of recurrence. Forty-six patients (46%) experienced sustained postprocedural atrial arrhythmias (35 had atrial fibrillation, and 34 had atrial flutter). Of these, 34 required a second procedure 7.0±3.1 months after their initial procedure. Reconnection of the posterior left atrium was seen in all patients with atrial fibrillation. Atrial flutter was most commonly due to mitral isthmus-dependent macroreentry (n=8, cycle length 368±116 ms) or macroreentry through 2 gaps in the ring of lesions (n=6, cycle length 328±115 ms). Posterior left atrium reisolation was achieved at the second procedure in all patients. Atrial flutter was successfully ablated and rendered noninducible in all patients. Six months after their last procedure, the Kaplan-Meier estimate of freedom from recurrence for all 100 patients was 81±5%. Conclusions—Atrial fibrillation and atrial flutter recurrence is common after single-ring isolation. Reconnection of the posterior left atrium and macroreentry are the common mechanisms. Repeat ablation results in satisfactory short-term outcomes.

Fibroblasts modulate cardiomyocyte excitability: implications for cardiac gene therapy

In an earlier study exploring the potential of gene transfer to repair myocardial conduction defects, we observed that myotubes, generated by forced expression of MyoD, exhibit reduced excitability when also modified to express connexin43 (Cx43). We hypothesized that this effect was caused by gap junction-mediated coupling between myotubes and the underlying fibroblast feeder layer. This intriguing possibility has important implications for ongoing efforts to develop strategies for repairing myocardial conduction defects by gene transfer, and also provides novel insights into the electrophysiological function of naturally occurring heterologous cell coupling within the heart. Although a conductive function for fibroblasts through heterologous coupling has previously been reported, the current study provides novel evidence that fibroblasts can modulate cardiomyocyte excitability in a Cx43-dependent manner. In a co-culture study system, neonatal rat cardiomyocytes were grown on monolayers of mouse fibroblasts with genetically altered Cx43 expression and the effect on intrinsic beat frequency examined. Cardiomyocytes grown on wild-type (WT) fibroblasts expressing native levels of Cx43 beat significantly slower than cells grown on fibroblasts devoid of this molecule (germline knockout) or with dominant-negative functional suppression. Expression of Cx43 in fibroblasts from Cx43 knockout mice restored cardiomyocyte beat frequency, to rates comparable with those observed in co-culture with WT fibroblasts.