One-year outcomes of catheter ablation for atrial fibrillation in young patients

BACKGROUND

Atrial fibrillation (AF) is relatively less frequent in younger patients (age < 50). Recently, studies have suggested that early restoration of sinus rhythm may lead to improved outcomes compared with rate control, however the efficacy of catheter ablation for AF in young is scarce.

METHODS

We included all hospitalized patients between 18 and 50 years with a diagnosis of AF from the Nationwide Readmission Database 2016-2017 from the Healthcare Cost and Utilization Project. Demographic and comorbidity data were collected and analyzed. Outcomes assessed included one-year AF readmission rates, all-cause readmission, ischemic stroke, and all-cause mortality. Subgroup analyses were performed for all demographic and comorbidity variables.

RESULTS

Overall, 52,598 patients (medium age 44, interquartile range 38-48, female 25.7%) were included in the study, including 2,146 (4.0%) who underwent catheter ablation for AF. Patients who underwent catheter ablation had a significantly lower rate of readmission for AF or any cause at one year (adjusted hazard ratios (HR) of 0.52 [95% confidence interval (CI): 0.43-0.63] and HR of 0.81 [95% CI: 0.72-0.89], respectively). There was no difference in 1-year readmission for stroke or all-cause mortality between the two groups. Subgroup analyses showed a consistent reduction in the risk of AF readmission among major demographic and comorbidity subgroups.

CONCLUSION

Catheter ablation in young patients with AF was associated with a reduction in 1-year AF related and all-cause readmissions. These data merit further prospective investigation for validation, through dedicated registries and multicenter collaborations to include young AF from diverse population.

Intraoperative defibrillation testing during replacements of implantable cardioverter-defibrillators: The Simpler trial

Funding Acknowledgements

Type of funding sources: Foundation. Main funding source(s): Maurice Kahn Foundation via the Mayo- Sheba Collaboration Fund.

Background

The need for intraoperative defibrillation testing (DFT) during implant and/or replacement of implantable cardioverter-defibrillators (ICDs) has been a matter of debate for many years. This debate was put to rest by the Simple and the Nordic ICD trials, and the practice of testing during new implantations has practically been nearly abandoned.

Nevertheless, induction of VF for testing purposes (VFT) may still have an important role in selective populations at risk for defibrillation failure, who were not included in the SIMPLE and Nordic trials. One such population includes those who undergo device replacements. Old registries demonstrated an increased incidence of significant findings in VFT during replacements. In the present study, we sought to test this observation.

Objectives

Evaluate frequency of significant findings and the safety of VFT in subjects undergoing device replacement.

Methods

A prospective observational multi-center study of VFT included consecutive patients undergoing ICD generator replacement in 5 centers in Israel, Europe, and the US. All centers followed the same VFT protocol. The primary outcome was defined as failure to terminate induced VF with a single shock at 10 Joules below the maximal capacity of the device. Secondary outcomes included complications of VFT. Patients were followed-up at 1 month and 6 months post-procedure. Data collection included documentation of any peri-operative complications and clinical endpoints (occurrence of appropriate shock, inappropriate shocks, lead failure, need for re-intervention, and infection).

Results

A total of 92 patients were eligible, and consented for the study, of which 84 underwent DFT during battery replacement. The median age was 68 years and 79.8% were male subjects. Induction of VF was successful in all 84 patients as well as VFT with a successful conversion on first attempt. During follow up one patient had two appropriate ICD shock events. In four patients, the ICD programming was changed. None suffered an inappropriate shock. There was no evidence of lead malfunction. A total of two deaths occurred, none of which were related to the device.

Conclusion

The present study found VFT was not associated with complications in patients undergoing ICD/CRTD generator replacement but produced no clinically important information.

Portable Device Improves the Detection of Atrial Fibrillation After Ablation

Asymptomatic recurrences of atrial fibrillation (AF) have been found to be common after ablation.A randomized controlled trial of AF screening using a handheld single-lead ECG monitor (BigThumb^®) or a traditional follow-up strategy was conducted in patients with non-valvular AF after catheter ablation. Consecutive patients were randomized to either BigThumb Group (BT Group) or Traditional Follow-up Group (TF Group). The ECGs collected via BigThumb were compared using the automated AF detection algorithm, artificial intelligence (AI) algorithm, and cardiologists' manual review. Subsequent changes in adherence to oral anticoagulation of patients were also recorded. In this study, we examined 218 patients (109 in each group). After a follow-up of 345.4 ± 60.2 days, AF-free survival rate was 64.2% in BT Group and 78.9% in TF Group (P = 0.0163), with more adherence to oral anticoagulation in BT Group (P = 0.0052). The participants in the BT Group recorded 26133 ECGs, among which 3299 (12.6%) were diagnosed as AF by cardiologists' manual review. The sensitivity and specificity of the AI algorithm were 94.4% and 98.5% respectively, which are significantly higher than the automated AF detection algorithm (90.7% and 96.2%).As per our findings, it was determined that follow-up after AF ablation using BigThumb leads to a more frequent detection of AF recurrence and more adherence to oral anticoagulation. AI algorithm improves the accuracy of ECG diagnosis and has the potential to reduce the manual review.

Impact of QRS morphology on response to conduction system pacing after atrioventricular junction ablation

Aims

His–Purkinje conduction system pacing (HPCSP) utilizing His (HBP) or left bundle branch pacing (LBBP) in patients with atrial fibrillation (AF) and wide QRS duration has not been well studied. We assessed the benefit of left bundle branch block (LBBB) correction during HPCSP in AF patients undergoing atrioventricular junction (AVJ) ablation with LBBB, compared with those with narrow QRS duration.

Methods and results

This is an observational study in consecutive patients with typical LBBB or narrow QRS duration in whom we attempted HPCSP after AVJ ablation for refractory AF with a left ventricular ejection fraction (LVEF) ≤ 50%. Echocardiographic responses and clinical outcomes were assessed at baseline and during 1 year of follow‐up. A total of 178 patients were enrolled, of which 170 achieved AVJ ablation + permanent HPCSP (age 69.3 ± 10.1 years; LVEF 34.3 ± 7.7%), 133 (78.2%) patients had a narrow QRS duration, and 37 (21.2%) had an LBBB. The QRS duration changed from a baseline of 159.7 ± 16.6 ms to a paced QRS duration of 110.4 ± 12.7 ms in the LBBB cohort and from 95.6 ± 10.4 to 100.8 ± 14.5 ms (both P < 0.001) in the narrow QRS cohort after AVJ ablation and pacing. Compared with the narrow QRS cohort, the LBBB cohort showed a greater absolute increase in LVEF (+22.3% vs. +14.2%, P < 0.001), higher super responder rate (71.4% vs. 49.2%, P = 0.011), and greater New York Heart Association (NYHA) class improvement (−1.9 vs. −1.4, P < 0.001) at 1 year.

Conclusion

Patients with LBBB have greater improvement in LVEF and NYHA class function than patients with narrow QRS from HPCSP after AVJ ablation.

P2832Outcomes of catheter ablation for atrial fibrillation based on sex: data from the cabana trial

Background

Sex-specific outcomes may differ amongst patients receiving catheter ablation for atrial fibrillation (AF).

Purpose

Assess sex-specific outcomes in the patients randomized to catheter ablation or drug therapy in CABANA.

Methods

CABANA randomized 2204 pts with AF to catheter ablation or drug therapy (rate/rhythm-control). The outcomes of combined death, disabling stroke, severe bleeding, or cardiac arrest (intention to treat-ITT) or all-cause death were not different. But, ablation significantly improved combined death or CV hospitalization. This analysis compares clinical characteristics by sex and determines sex-specific hazard ratios based on a comparison of ablation vs drug therapy.

Results

Females were 37.3% of ablation and 37.0% of drug therapy patients. Females were older, more often white race, had less CAD, or sleep apnea, but had higher NYHA Class, higher CHA2DS2VASc, and more often had paroxysmal (v. persistent) AF, and prior AF hospitalization. (Table) HTN, CVA and diabetes were the same (Table).

For the CABANA primary endpoint, an ITT comparison of ablation vs. drug therapy, showed a female hazard ratio (HR) of 1.14 (95% confidence interval (CI) 0.70–1.86), and a male HR of 0.74, (95% CI 0.52–1.06). For all-cause mortality, the female HR was 0.75 (95% CI 0.41–1.40) and male HR was 0.91 (95% CI 0.59–1.40) and for all-cause mortality or CV hospitalization, the female HR was 0.90 (95% CI 0.75–1.09) and male HR was 0.79 (95% CI 0.69–0.92). All interaction p values were non-significant.

Recurrent AF (post 90-day blanking) was significantly reduced for both females and males: female HR 0.64 (95% CI 0.51–0.82), male HR 0.46 95% CI 0.39–0.56), p=0.035

Clinical Characteristics and Outcomes Baseline Characteristics Female (N=818) Male (N=1385) p-value Age: Median (Q1, Q3) 69 (65, 74) 66 (60, 71) <0.001 White 766 (93.9%) 1259 (91.0%) 0.015 CAD 92 (11.2%) 332 (24.0%) <0.001 NYHA ≥II 345 (42.4%) 433 (31.5%) <0.001 Sleep apnea 136 (16.6%) 372 (26.9%) <0.001 CHA2DS2-VASc: Median (Q1, Q3) 3 (3, 4) 2 (1, 3) <0.001 Paroxysmal AF 406 (49.6%) 540 (39.0%) <0.001 Persistent AF 412 (50.4%) 845 (61.0%) AF Hospitalization 353 (43.2%) 521 (37.7%) 0.011

Conclusion

Significant sex-specific outcomes differences were not observed. Sex should not be used as a determining factor in selecting patients for AF therapy.

Acknowledgement/Funding

NIH, St Jude Medical Foundation and Corporation, Biosense Webster Inc., Medtronic Corporation, and Boston Scientific Corporation

The serum matrix metalloproteinase-9 level is an independent predictor of recurrence after ablation of persistent atrial fibrillation

OBJECTIVES

This study investigated whether the serum matrix metalloproteinase-9 level is an independent predictor of recurrence after catheter ablation for persistent atrial fibrillation.

METHODS

Fifty-eight consecutive patients with persistent atrial fibrillation were enrolled and underwent catheter ablation. The serum matrix metalloproteinase-9 level was detected before ablation and its relationship with recurrent arrhythmia was analyzed at the end of the follow-up.

RESULTS

After a mean follow-up of 12.1±7.2 months, 21 (36.2%) patients had a recurrence of their arrhythmia after catheter ablation. At baseline, the matrix metalloproteinase-9 level was higher in the patients with recurrence than in the non-recurrent group (305.77±88.90 vs 234.41±93.36 ng/ml, respectively, p=0.006). A multivariate analysis showed that the matrix metalloproteinase-9 level was an independent predictor of arrhythmia recurrence, as was a history of atrial fibrillation and the diameter of the left atrium.

CONCLUSION

The serum matrix metalloproteinase-9 level is an independent predictor of recurrent arrhythmia after catheter ablation in patients with persistent atrial fibrillation.

[Effects of ventricular demand and dual-chamber pacing models on the long-term clinical outcome and cardiac remodeling in patients with symptomatic bradycardia]

OBJECTIVE

To assess the effects of VVI (ventricular demand) and DDD (dual-chamber) pacing models on cardiac remodeling and the long-term clinical outcome of patients with symptomatic bradycardia.

METHODS

All patients with DDD and VVI pacing models at our hospital from January 1991 to January 2003 were retrospectively analyzed.

RESULTS

After a follow-up period of over 8 years in DDD and VVI groups (97 ± 27, 107 ± 44 months), left atrial diameter [(45 ± 12) mm vs (39 ± 12) mm, P < 0.01] and left ventricular end-diastolic diameter [(53 ± 11) mm vs (50 ± 9) mm, P = 0.01] in 57 patients with VVI pacing model were markedly enlarged than those at pre-implantation. And tricuspid regurgitation increased (42.4% vs 16.9%, P < 0.05). But in 59 patients with DDD pacing model, except for increased tricuspid regurgitation (42.1% vs 10.5%, P < 0.01), left atrial diameter [(37 ± 5) mm vs. (35 ± 5) mm, P = 0.07] and left ventricular end-diastolic diameter [(47 ± 7) mm vs (47 ± 5) mm, P = 0.32] were not significantly different. Mitral regurgitation significantly increased only in the VVI group (P < 0.01). The increases of left ventricular end-diastolic diameter (P = 0.04), mitral valve (P = 0.02) and tricuspid regurgitation (P < 0.01) were much more pronounced in the VVI group than those in the DDD group. Left ventricular ejection fraction (LVEF) showed no difference with that at pre-implantation (P = 0.11 in DDD group, P = 0.05 in VVI group). But the LVEF value was lower (P = 0.04) while the incidence of thrombosis was higher (P = 0.03) in the VVI group than those in the DDD group at post-implantation. However, the incidence of atrial fibrillation (P = 0.14), hospitalization (P = 0.08) and survival (P = 0.77) showed no significant difference between two groups.

CONCLUSION

DDD pacing offers more benefits over VVI pacing through improving cardiac functions and arresting left ventricular remodeling. However, neither groups showed any difference in decreasing mortality rate and hospitalization. Moreover, both pacing modes fail to reverse cardiac electrical and anatomical remodeling. It is imperative to explore more physiological pacing site and rational atrioventricular (AV) interval to improve the prognosis of patients.

Endoscopy-assisted radiofrequency ablation around the coronary sinus ostium in dogs: its effects on atrioventricular nodal properties and ventricular response during atrial fibrillation

INTRODUCTION

Radiofrequency ablation of the slow pathway can prolong atrioventricular (AV) nodal properties and RR intervals during atrial fibrillation (AF) in many patients with AV nodal reentrant tachycardia. However, it is not well elucidated whether these changes are related to the presence of dual AV nodal pathway physiology. The aim of this study was to evaluate changes of AV nodal properties and RR intervals during AF caused by ablation of two specific areas in dogs.

METHODS AND RESULTS

Assisted by fiberoptic endoscopy, linear lesions were created between the coronary sinus ostium and tricuspid valve annulus (area 1) or posterior to the ostium (area 2) in 15 dogs. Three additional dogs served as controls. The measurements were made under autonomic blockade. Catheter ablation could be assisted in all dogs by means of endoscopy. Linear lesions were confirmed at autopsy. AV nodal parameters and RR intervals showed no overall changes. Individual data showed that ablation of area 1 resulted in modification of AV nodal properties in 54.5% (facilitation in 36.3% and inhibition in 18.2%), whereas ablation of area 2 induced changes in 50% (facilitation in 10% and inhibition in 40%). The RR intervals were shortened in 33.3% and 20% and prolonged in 44.5% and 40% after ablation of areas 1 and 2, respectively. The RR intervals during AF correlated well with the Wenckebach cycle length and the AV node functional refractory period before and after ablation (r = 0.78 to 0.94, P < 0.01 for each).

CONCLUSIONS

Ablation of the two specific areas around the coronary sinus ostium was equally effective in modifying AV nodal properties and the ventricular response during AF without dual AV nodal pathway physiology. The ventricular rate to AF after ablation correlated well with the residual AV nodal properties.

Effects of chemical subendocardial ablation on activation rate gradient during ventricular fibrillation

The mechanism by which an endocardial-epicardial activation rate gradient develops after 1 or 2 min of sustained ventricular fibrillation is unknown. We recorded from electrodes on the epicardium and from hook electrodes in the endocardium in three open-chest control dogs during prolonged ventricular fibrillation. The same recordings were also made in seven dogs after right ventricular subendocardial ablation with Lugol solution and in three dogs after substitution of air for the cavitary blood. The effects of these interventions, i.e., Lugol ablation (n = 2) and the exposure to air (n = 2), on the subendocardial Purkinje fiber transmembrane action potential properties were also evaluated in vitro using microelectrode recording techniques. The in vivo studies showed a significant endocardial-epicardial rate gradient in the control dogs and in dogs that had air substituted for the cavitary blood. In comparison, in dogs that underwent chemical subendocardial ablation, the activation cycle lengths for the endocardium and epicardium were not significantly different. The in vitro studies showed that subendocardial Purkinje fiber action potentials could still be recorded for up to 10 min of exposure to air. In comparison, in the tissues subjected to chemical ablation, no transmembrane action potentials could be recorded from either the Purkinje fibers or superficial ventricular muscle cells. We conclude that the development of an endocardial-epicardial activation rate gradient during prolonged ventricular fibrillation depends on the presence of intact subendocardial Purkinje fibers and ventricular myocytes. The retained cavitary blood is not responsible for the development of the rate gradient.

The relation between atrial fibrillation wavefront characteristics and accessory pathway conduction

Although the source-sink relationship for impulse propagation in cardiac tissues has been demonstrated in vitro, there has been no verification of this hypothesis in humans. Accordingly, eight patients undergoing surgical division of their accessory pathways were studied. A 56-channel (7 x 8) bipolar plaque electrode array was placed over the atrioventricular groove on the accessory pathway and atrial fibrillation electrically induced. 10 episodes of QRS transition from consecutively preexcited to nonpreexcited complexes were analyzed. This showed that consecutively preexcited QRS complexes were always associated with uniform large atrial wavefronts. Immediately prior to QRS transition, four general types of changes were observed: (a) premature invasion by secondary wavefronts creating local conduction block (n = 5); (b) wavefront collision leading to wavefront curvature (n = 2); (c) transition from a uniform large atrial wavefront to multiple fractionated small wavefronts (n = 1); and (d) uniform atrial wavefronts "marching" into the accessory pathway refractory period (n = 2). We conclude that local atrial wavefront characteristics are important factors influencing impulse propagation through the accessory pathway. The findings that local wavefront collision, curvature, or fractionation often precede loss of accessory pathway conduction support the notion that source-sink relationship is an important determinant of the safety factor for impulse propagation in the human heart.

The effects of lidocaine on the vulnerable period during ventricular fibrillation

INTRODUCTION

It was postulated that a subthreshold defibrillation shock failed to halt ventricular fibrillation because the shock itself reinitiated ventricular fibrillation by falling into the vulnerable period of the wavefronts. Whether or not the timing of the vulnerable period is determined by the ventricular fibrillation cycle length is unknown.

METHODS AND RESULTS

We determined the patterns of epicardial activation in ten dogs by computerized mapping techniques during unsuccessful defibrillation. Lidocaine was then given to prolong the ventricular fibrillation cycle length, and the computerized mapping studies were repeated. The results showed that lidocaine increased the ventricular fibrillation cycle length from 110 +/- 13 msec to 156 +/- 5 msec (P < 0.001). Among 55 episodes of unsuccessful defibrillation, the site of the earliest postshock activation occurred in the center of the mapped tissue 12 times at baseline and 14 times during lidocaine infusion. At electrodes that registered as postshock early sites, the preshock intervals clustered within a narrow range both before (58 +/- 14 msec) and during (101 +/- 18 msec, P < 0.001) lidocaine infusion. The correlation between the preshock intervals and the ventricular fibrillation cycle length was significant for these 26 sites (r = 0.87, P < 0.001).

CONCLUSION

We conclude that a vulnerable period is present during ventricular fibrillation, and the timing of the vulnerable period is determined by the ventricular fibrillation cycle length.

Use-dependent effects of lidocaine on the upper limit of vulnerability in open chest dogs

OBJECTIVES

This study was performed to test the hypothesis that sodium channel activity is important in determining ventricular vulnerability to electric shocks.

BACKGROUND

It is unknown whether sodium channel activity determines the upper limit of vulnerability.

METHODS

The ventricles of 10 open chest dogs were paced at 300-, 500-, or 1,000-ms cycle lengths. The shock strength associated with a 50% probability of reaching the upper limit of vulnerability (ULV50) and the shock strength associated with a 50% probability of defibrillation (DFT50) were then determined by means of an up-down algorithm. Lidocaine (9.2-mg/kg body weight loading dose and 285-micrograms/kg per min maintenance dose) was then given, and the ULV50 and the DFT50 were redetermined after 1 h of stable infusion.

RESULTS

The mean (+/- SD) lidocaine concentration was 11.9 +/- 2.4 micrograms/ml. At baseline, the ULV50 tested with each S1 cycle length was not significantly different from the DFT50. During lidocaine infusion, the ULV50 determined with cycle lengths of 300 and 500 ms (18.9 +/- 11.3 and 16.1 +/- 8.9 J, respectively) were significantly (p < 0.05) higher than those simultaneously determined for the DFT50 (11.2 +/- 4.1 and 10.9 +/- 5.6 J, respectively). However, when determined with an S1 cycle length of 1,000 ms, the ULV50 (10.4 +/- 4.1 J) was not found to be significantly different from the DFT50 (10.3 +/- 5.3 J). Lidocaine infusion increased (p < 0.05) QRS duration and the effective refractory periods for cycle lengths of 300 and 500 ms but not 1,000 ms.

CONCLUSIONS

The effect of lidocaine on the upper limit of vulnerability is use dependent. These results are compatible with the hypothesis that sodium channel activity is important in determining ventricular vulnerability to electrical shocks.

The mechanism of termination of reentrant activity in ventricular fibrillation

The reentrant wave fronts in ventricular fibrillation (VF) have only a limited life span. The mechanisms by which these reentrant wave fronts terminate are unknown. We performed computerized mapping studies in six open-chest dogs before and after right ventricular subendocardial ablation with Lugol's solution. Recordings were made with 56 bipolar electrodes separated by 3 mm. Baseline pacing was performed on the right side of the tissue to create parallel activation wave fronts. A premature 50-V shock of either anodal or cathodal polarity was given to a bar electrode on the upper edge of the tissue. Counterclockwise reentrant wave fronts and VF were induced both before (60 episodes) and after (57 episodes) subendocardial ablation with either anodal or cathodal shocks. Among these reentrant wave fronts, 8 episodes before and 10 episodes after ablation had over 10 rotations (P = NS). The reentrant wave fronts in other episodes terminated with an average of 3.2 +/- 1.9 rotations before and 3.1 +/- 1.8 rotations after the ablation (P = NS). The reentrant wave-front cycle length was 118 +/- 19 milliseconds before and 124 +/- 20 milliseconds after ablation (P = .001). Conduction block occurred when the wave front was traveling across the myocardial fibers. When conduction was blocked in these episodes, the leading edge of the reentrant wave front encountered tissue that had been excited within the past 58 +/- 12 milliseconds (range, 28 to 77 milliseconds), which corresponded to 47 +/- 12% of the preceding VF cycle length. This period was significantly shorter than the recovery period in the same region that had allowed conduction (91 +/- 19 milliseconds; range, 48 to 137 milliseconds), which corresponded to 72 +/- 18% of the preceding VF cycle length (P < .001). In nine episodes, reentrant wave-front activity terminated when wave fronts that had originated from outside the mapped tissue interfered with the reentrant pathways. Conclusions are as follows: (1) The refractory period of fibrillating ventricular muscle ranges from 48 to 77 milliseconds. Because the refractory period is much shorter than the VF cycle length, a large excitable gap is present in the reentrant circuit. The presence of a large excitable gap contributes to reentrant wave-front termination. (2) Myocardial fiber orientation is an important determinant of the site of conduction block. (3) Although subendocardial ablation slowed the wave-front propagation, it did not prevent the generation and the maintenance of reentry and VF.

Effects of myocardial fiber orientation on the electrical induction of ventricular fibrillation

Although many studies have documented the importance of myocardial fiber orientation in the generation and maintenance of reentrant arrhythmia, its role in the induction of reentry by a single strong premature stimulus is held to be minimal. To study the importance of fiber orientation on the mechanisms of ventricular vulnerability to single strong premature electrical stimulation, computerized mapping studies using 56 closely (2. 5-5 mm) spaced epicardial bipolar electrodes were performed on six open-chest dogs to determine the patterns of activation after the application of a single strong premature stimulus. In an additional three dogs, both epicardial and endocardial mapping studies were performed with closely spaced electrodes. The baseline driving stimulus (S1) and the premature stimulus (S2) were given to the same or different sites on the right ventricular epicardium. When the line connecting the S1 and S2 (the electrical axis of stimulation) was roughly parallel to the fiber orientation, the vast majority of the early sites were found to lie between the S1 and S2 sites, with none of the early sites occurring on the side of the S2 site opposite to the S1 site. At the onset of ventricular fibrillation, figure-eight reentry was easily demonstrated. In comparison, when the S1 and S2 were given to the same site, or when the electrical axis of stimulation was roughly perpendicular to the fiber orientation, the early sites could occur on all sides of the S2 site. In these instances, a figure-eight reentry was not present at the onset of ventricular fibrillation, although this pattern could develop after the first few beats. These findings indicate that myocardial fiber orientation is an important factor in determining the patterns of activation during the electrical induction of ventricular fibrillation.

A reappraisal of ventricular fibrillation threshold testing

To test the hypothesis that ventricular fibrillation (VF) threshold testing is a probability function, 12 open-chest dogs were studied. The VF thresholds were tested by scanning the T wave with either the single premature stimulus method or with the train stimulus method. The dose-response curve method was used to determine the probability of inducing VF with different strengths of premature stimulation. Conventional methods and up-down methods were also used to test the VF threshold. The results showed that the VF threshold is a probability function. The conventional method VF threshold for the single premature stimulus and for the train stimulus methods corresponded to the current strength associated with a 23.4 +/- 13.2 and a 33.2 +/- 19.5% probability of VF (P < 0.05), respectively. In comparison, the triplicate VF threshold determined by the up-down algorithm method resulted in values that were not significantly different from a 50% probability of inducing VF, with a good correlation (r = 0.90, P < 0.001 for single and r = 0.89, P = 0.003 for the train stimulus method). We conclude that: 1) The VF threshold is a probability function, and 2) the triplicate VF threshold determined by the up-down algorithm method is the best alternative to the dose-response curve method in estimating 50% probability of inducing VF.

Effects of subendocardial ablation on anodal supernormal excitation and ventricular vulnerability in open-chest dogs

BACKGROUND

In Langendorff-perfused hearts and in hearts on cardiopulmonary bypass, chemical ablation of the subendocardium of both ventricles decreases ventricular vulnerability to fibrillation. It was hypothesized that the effects of ablation are a result of the elimination of the subendocardial Purkinje fiber network. This hypothesis has been supported by recent observations that the supernormal excitability that is demonstrable in the Purkinje fibers is associated with arrhythmogenesis.

METHODS AND RESULTS

We tested this hypothesis on 10 open-chest dogs by evaluating the strength-interval curves of anodal and cathodal stimulation with the assistance of computerized mapping techniques. The ventricular fibrillation threshold was also determined. The same test was then performed after chemical ablation of the subendocardium of either the right ventricle (six dogs) or both ventricles (four dogs). Anodal supernormality was consistently demonstrated in all the dogs studied both before and after subendocardial ablation. The ventricular fibrillation thresholds were 23 +/- 5 mA both before and after right ventricular subendocardial ablation (p = NS). The ventricular fibrillation thresholds before and after biventricular subendocardial ablation were 25 +/- 3 and 22 +/- 10 mA, respectively (p = NS).

CONCLUSIONS

We conclude that 1) subendocardial ablation does not decrease ventricular vulnerability when the heart is in situ and is not on cardiopulmonary bypass and 2) anodal supernormal excitability can be demonstrated in ventricles without a subendocardial Purkinje fiber network.

Effects of lidocaine on relation between defibrillation threshold and upper limit of vulnerability in open-chest dogs

BACKGROUND

The purpose of the present study was to test the effects of lidocaine on the relation between the defibrillation threshold and the upper limit of vulnerability.

METHODS AND RESULTS

The shock strength associated with a 50% probability of successful defibrillation (DFT50) and the shock strength associated with a 50% probability of reaching the upper limit of vulnerability (ULV50) were determined in 11 open-chest dogs by using the delayed up-down method before and during lidocaine (seven dogs) or normal saline (four dogs) infusion. The ventricles were paced at a cycle length of 300 msec. Shocks of various strengths were then given via a patch-patch electrode configuration on the anterior and posterior surfaces of the ventricle to determine the ULV50. Once ventricular fibrillation was induced, shocks were given 15-20 seconds later via the same electrode configuration to determine the DFT50. Lidocaine infusion resulted in a serum level of 15 +/- 4 micrograms/ml. This was associated with a lengthening of the QT interval but not with the widening of the QRS complex. In all dogs, both the ULV50 and the DFT50 increased significantly when tested during lidocaine infusion. Mean ULV50 during lidocaine infusion was 496 +/- 70 V or 13.1 +/- 4.3 J, which were significantly higher than the baseline values of 333 +/- 67 V or 5.3 +/- 2.2 J (p less than 0.001 for both voltage and energy). Mean DFT50 during lidocaine infusion was 407 +/- 41 V or 8.7 +/- 1.7 J, which were significantly higher than the baseline values of 300 +/- 38 V and 4.4 +/- 1.1 J (p = 0.004 for voltage and p = 0.013 for energy). The r values between the ULV50 and the DFT50 were 0.79 (p = 0.037) for voltage and 0.80 (p = 0.030) for energy at baseline and 0.85 (p = 0.016) for voltage and 0.88 (p = 0.009) for energy during the lidocaine infusion. However, the increments of the ULV50 (163 +/- 88 V or 7.8 +/- 4.6 J) were significantly greater than the increments of the DFT50 (107 +/- 51 V or 4.4 +/- 1.9 J, p = 0.035 for voltage and p = 0.023 for energy). Normal saline infusion did not alter DFT50 or ULV50.

CONCLUSIONS

Lidocaine infusion significantly increases both ULV50 and DFT50. These results are compatible with the upper limit of vulnerability hypothesis of defibrillation. However, the greater increase of the upper limit of vulnerability than the defibrillation threshold with lidocaine infusion indicates that other factors may also need to be considered to explain the results.