Distinct Substrates of Idiopathic Ventricular Fibrillation Revealed by Arrhythmia Characteristics on Implantable Cardioverter-Defibrillator.

BACKGROUND: Idiopathic ventricular fibrillation (IVF) can be associated with undetected distinct conditions such as microstructural cardiomyopathic alterations (MiCM) or Purkinje (Purk) activities with structurally normal hearts. OBJECTIVE: This study sought to evaluate the characteristics of recurrent VF recorded on implantable defibrillator electrograms, associated with these substrates. METHODS: This was a multicenter collaboration study. At 32 centers, we selected patients with an initial diagnosis of IVF and recurrent arrhythmia at follow-up without antiarrhythmic drugs, in whom mapping demonstrated Purk or MiCM substrate. We analyzed variables related to previous ectopy, sinus rate preceding VF, trigger, and initial VF cycle lengths. Logistic regression with cross validation was used to evaluate the performance of criteria to discriminate Purk or MiCM substrates. RESULTS: Among 95 patients (35 women, age 35 ± 11 years) meeting the inclusion criteria, IVF was associated with MiCM in 41 and Purk in 54 patients. A total of 117 arrhythmia recurrences including 91% VF were recorded on defibrillator. Three variables were mostly discriminant. Sinus tachycardia (≤570 ms) was more frequent in MiCM (35.9% vs 13.4%, P = 0.014) whereas short-coupled (<350 ms) triggers were most frequent in Purk-related VF (95.5% vs 23.1%, P = 0.001), which also had shorter VFCLs (182 ± 15 ms vs 215 ± 24 ms, P < 0.001).The multivariable combination provided the highest prediction (accuracy = 0.93 ± 0.05, range 0.833-1.000), discriminating 81% of IVF substrates with a high probability (>80%). Ectopy were inconsistently present before VF. CONCLUSIONS: Characteristics of arrhythmia recurrences on implantable cardioverter- defibrillator provide phenotypic markers of the distinct and hidden substrates underlying IVF. These findings have significant clinical and genetic implications.

Efficacy and safety of the new TightRail™ mechanical sheath for transvenous lead extraction: Results of a French multicenter study.

INTRODUCTION: The aim of this study was to assess the safety and efficacy of the TightRail™ sheath for pacemaker/defibrillator transvenous lead extraction (TLE). METHODS: Multicenter observational study including patients who underwent a TLE with the TightRail™ sheath in five French university hospitals from September 2014 to January 2020. RESULTS: Two hundred and twenty-five patients (76% males, 71 ± 12 years) underwent a TLE procedure with the TightRail™. A total of 438 leads were extracted using the TightRail™, and the mean age of the extracted leads was 128 ± 85 months; of these, 344 (79%) were pacing leads and 94 (21%) were implantable cardioverter defibrillator leads. The overall clinical success of the extraction procedures was 93%. Overall, 410 of the 438 leads (95%) were extracted (complete or incomplete removal). After multilevel mixed-effects logistic regression model, we found that lead age (odds ratio [OR], 95% confidence interval [CI] for a 1 year increase: 1.11 [1.07-1.15], p < .001) and number of leads extracted (OR, 95% CI: 2.09 [1.50-2.96], p < .001) were the two independent factors associated with complete lead removal failure. Finally, there were 7 (3%) cases of major complications but no per-procedural death. CONCLUSION: This is the first large-scale survey assessing the efficacy and safety profile of the Tightrail™ mechanical sheath. The clinical success rate was 93%, and the lead removal failure was dependent on the age and number of leads. We show a satisfactory safety profile in this cohort of patients from primarily low-volume centers with older leads.

Device-Related Thrombosis After Percutaneous Left Atrial Appendage Occlusion for Atrial Fibrillation.

BACKGROUND: Transcatheter left atrial appendage (LAA) occlusion is an alternative strategy for stroke prevention in patients with atrial fibrillation (AF). OBJECTIVES: This study sought to determine the incidence, predictors, and prognosis of thrombus formation on devices in patients with AF who were treated with LAA closure. METHODS: The study retrospectively analyzed data from patients treated with 2 LAA closure devices seen in 8 centers in France from February 2012 to January 2017. RESULTS: A total of 469 consecutive patients with AF underwent LAA closure (272 Watchman devices [Atritech, Boston Scientific, Natick, Massachusetts] and 197 Amplatzer devices [St. Jude Medical, Minneapolis, Minnesota]). Mean follow-up was 13 ± 13 months, during which 339 (72.3%) patients underwent LAA imaging at least once. There were 98 major adverse events (26 thrombi on devices, 19 ischemic strokes, 2 transient ischemic attacks, 18 major hemorrhages, 33 deaths) recorded in 89 patients. The incidence of device-related thrombus in patients with LAA imaging was 7.2% per year. Older age (hazard ratio [HR]: 1.07 per 1-year increase; 95% confidence interval [CI]: 1.01 to 1.14; p = 0.02) and history of stroke (HR: 3.68; 95% CI: 1.17 to 11.62; p = 0.03) were predictors of thrombus formation on the devices, whereas dual antiplatelet therapy (HR: 0.10; 95% CI: 0.01 to 0.76; p = 0.03) and oral anticoagulation at discharge (HR: 0.26; 95% CI: 0.09 to 0.77; p = 0.02) were protective factors. Thrombus on the device (HR: 4.39; 95% CI: 1.05 to 18.43; p = 0.04) and vascular disease (HR: 5.03; 95% CI: 1.39 to 18.23; p = 0.01) were independent predictors of ischemic strokes and transient ischemic attacks during follow-up. CONCLUSIONS: Thrombus formation on the device is not uncommon in patients with AF who are treated by LAA closure. Such events are strongly associated with a higher risk of ischemic stroke during follow-up. (REgistry on Real-Life EXperience With Left Atrial Appendage Occlusion [RELEXAO]; NCT03279406).

Mitral and aortic valvular disease associated with benfluorex use.

Fenfluramine has been associated with an increased risk of pulmonary hypertension and valvular disease. Benfluorex is a fenfluramine derivative approved for the treatment of metabolic syndrome and type 2 diabetes mellitus. To date, only three isolated clinical cases of valvular disease and two recent case-control studies have been reported in patients exposed to benfluorex. Herein, the case is described of a patient with mitral and aortic valvular disease, with both echocardiographic and histopathological findings, who had been receiving benfluorex for several years, without any other etiology of valvular disease. The case is suggestive of toxic valvular lesions, similar to those observed previously during treatment with fenfluramine, pergolide, and cabergolide.

The use of epicardial electrogram as a simple guide to select the optimal site of left ventricular pacing in cardiac resynchronization therapy.

Cardiac resynchronization therapy (CRT) has been demonstrated to improve symptoms and survival in patients with left ventricular (LV) systolic dysfunction and dyssynchrony. To achieve this goal, the LV lead should be positioned in a region of delayed contraction. We hypothesized that pacing at the site of late electrical activation was also associated with long-term response to CRT. We conducted a retrospective study on 72 CRT patients. For each patient, we determined the electrical delay (ED) from the onset of QRS to the epicardial EGM and the ratio of ED to QRS duration (ED/QRS duration). After a followup of 30 ± 20 months, 47 patients responded to CRT. Responders had a significantly longer ED and greater ratio of ED/QRS duration than nonresponders. An ED/QRS duration ≥0.38 predicted a response to CRT with 89% specificity and 53% sensitivity.

Atrial flutter after non-congenital cardiac surgery: incidence, predictors and outcome.

BACKGROUND: Atrial arrhythmias (AAs) occur frequently after cardiac surgery. However, most authors do not differentiate postoperative atrial fibrillation (AF) from atrial flutter (AFL). Our study sought to investigate the incidence and predictors of AFL following non-congenital cardiac surgery, and the long-term outcome of this arrhythmia. METHODS AND RESULTS: We conducted an observational, retrospective, single-centre study. Among 821 patients in sinus rhythm (SR) before non-congenital cardiac surgery, AFL occurred in 42 patients (5.1%) and AF in 77 (9.4%). Independent predictors of AAs were older age (odds ratio (OR): 1.06, 95% confidence interval (CI): 1.01-1.11, p = 0.02), low postoperative hemoglobin level (OR: 0.43, CI: 0.29-0.63, p < 0.0001) and long cross-clamp time (OR: 1.02, CI: 1.002-1.037, p = 0.03). Compared to AF patients, those with AFL were younger (67 ± 10 versus 72 ± 8 years, p = 0.005) and less likely to have valve surgery (57% versus 76%, p = 0.03). After conversion to SR, of 41/42 patients with AFL, 5 (12%) developed AFL and 5 (12%) had AF within a mean follow-up of 9.6 ± 8.4 months: only 32% of patients were on long-term antiarrhythmic therapy and 5% had radiofrequency ablation of AFL in the early postoperative period. CONCLUSION: AFL following non-congenital cardiac surgery is not frequent. Compared to patients in AF, those with AFL are younger and less likely to have valve surgery. After conversion of AFL to SR, recurrence rate of AAs is relatively low. Therefore, AFL occurring in this setting does not require systematic catheter ablation in the postoperative period.

Is "hyper response" to cardiac resynchronization therapy in patients with nonischemic cardiomyopathy a recovery, a remission, or a control?

BACKGROUND AND OBJECTIVES: Some patients treated by cardiac resynchronization therapy (CRT) recover "normal" left ventricular (LV) function and functional status. However, whether this "normalization" persists or reverts over time remains unknown. The aim of the present study was to evaluate the long-term outcomes of LV function in patients hyper responder to CRT. METHODS: Eleven consecutive patients with nonischemic dilated cardiomyopathy, sinus rhythm, left bundle branch block (LBBB), New York Heart Association (NYHA) class III or IV, and optimal pharmacological treatment were hyper responder as they fulfilled concurrently the two following criteria: functional recovery (NYHA class I or II) and normalization of LV ejection fraction (LVEF). RESULTS: After a mean follow-up of 65 ± 30 months between CRT implantation and last evaluation LVEF improved from 26 ± 9 to 59 ± 6% (P < 0.0001). One patient died from pulmonary embolism 31 months after implantation. Three patients exhibited LVEF ≤ 50% at their last follow-up visit (two at 40% and one at 45%). In eight patients, brief cessation of pacing was feasible (three were pacemaker-dependent). Mean QRS duration decreased from 181 ± 23 ms to 143 ± 22 ms (P = 0.006). In one patient, pacing was interrupted for 2 years and LVEF decreased markedly (from 65% to 31%) but returned to normal after a few months when pacing was resumed. CONCLUSION: In hyper responder patients, "normalization" of LV function after CRT persists as long as pacing is maintained with an excellent survival.

Patients with non-ischaemic dilated cardiomyopathy and hyper-responders to cardiac resynchronization therapy: characteristics and long-term evolution.

AIMS: In some patients with non-ischaemic dilated cardiomyopathy (NIDCM), left bundle branch block (LBBB), and severe cardiac failure, cardiac resynchronization therapy (CRT) has been shown to reverse almost completely left ventricular (LV) function. These patients thus eligible to be qualified 'hyper-responders' are exclusively recruited in patients with NIDCM. Evaluate proportion of 'hyper-responders' among patients with NIDCM and try to determine their profile before implantation of CRT. METHODS AND RESULTS: Consecutive patients with DCM [LV ejection fraction (LVEF) <35%, LV end-diastolic diameter >60 mm], sinus rhythm, LBBB, and New York Heart Association (NYHA) class III and IV implanted with a CRT were prospectively followed. Patients were considered 'hyper-responders' if they fulfilled after a minimum period of 6 months the following criteria: functional recovery (NYHA class I or II) and LVEF >or=50%. Among the 51 NIDCM patients, 11 (21.5%) were 'hyper-responders' following CRT (LVEF increased from 26 +/- 9 to 59 +/- 6%, P < 0.01). In the 40 (78.5%) remaining patients, there was no or minimal functional improvement and only a modest increase in LVEF from 21 +/- 9 to 26 +/- 12% (P = 0.03). Comparison between 'hyper-responders' and remaining patients showed that baseline parameters of 'hyper-responders' were less severely deteriorated. CONCLUSION: In a significant subset of patients with CRT for NIDCM, a 'complete' functional recovery associated with normalization of LV function was observed. This observation suggests that LBBB could be the causative factor of DCM in this subgroup of patients. There was not a unique discriminating factor at baseline between 'hyper-responders' and remaining patients.

Primary failure of cardiac resynchronization therapy: what are the causes and is it worth considering a second attempt? A single-centre experience.

AIMS: Cardiac resynchronization therapy (CRT) has been validated as an effective treatment for patients with drug-refractory congestive heart failure and left bundle branch block. Failure of implantation of the left ventricular (LV) lead has been reported in 10-15% of patients. The goal of our study was to determine the causes of failure and the success rate following a repeat procedure by the same operators. METHODS AND RESULTS: We reviewed our last 100 consecutive cases of CRT before July 2007. The procedure was considered as unsuccessful if it had to be interrupted before the placement of the LV lead in an appropriate position, because of patient's haemodynamic status or if the procedure duration exceeded 3 h. Cardiac resynchronization therapy was unsuccessful in 10 patients (5 men, mean age: 72 +/- 11 years). The causes of failure of CRT were as follows: no target vein other than the great cardiac vein (n = 5), coronary sinus dissection (n = 1), and a lateral vein too small to provide a stable location for the LV lead (n = 4). A second procedure was attempted in four patients and was successful in all cases. CONCLUSION: In our study results, failure of CRT was observed in 10% of the patients. A second procedure can be attempted in a selected group of patients and is associated with a high success rate.

Cardiac resynchronization therapy: "nonresponders" and "hyperresponders".

BACKGROUND: Cardiac resynchronization therapy (CRT) is a recognized treatment modality for patients with dilated cardiomyopathy (DCM), left bundle branch block, and severe cardiac failure. However, 30% of patients are "nonresponders." Intriguingly, the opposite case has not been reported until recently: Do some patients treated with CRT have a "complete" recovery and thus can be considered "hyperresponders"? OBJECTIVE: The purpose of this study was to investigate patients treated with CRT who have a "complete" functional recovery, with normalization of left ventricular function after therapy. METHODS: Eighty-four consecutive patients with DCM, sinus rhythm, and left bundle branch block in New York Heart Association functional class III and IV who were implanted with a CRT device were prospectively followed. Patients were considered to be "hyperresponders" if they concurrently fulfilled two criteria: functional recovery and left ventricular ejection fraction > or = 50%. RESULTS: Among the 84 patients with DCM, 11 (13%) were "hyperresponders" within 6 to 24 months after CRT (left ventricular ejection fraction increased from 25% +/- 8% to 60% +/- 6.5%, P = .001). Comparison of baseline parameters between "hyperresponders" and the remaining patients showed that only etiology of the DCM was statistically discriminative. All "hyperresponders" belonged to the group of patients with nonischemic DCM (18% vs 0%, P = .05). CONCLUSION: In a subset of patients successfully implanted with a CRT device, "complete" functional recovery associated with normalization of LV function was observed, giving rise to the concept of "hyperresponders." This finding is observed exclusively in the subgroup of patients with nonischemic DCM and suggests that left bundle branch block may be the causal factor of DCM in this subgroup of patients.

Left ventricular pacing in patients with congestive heart failure.

Cardiac resynchronisation therapy (CRT) using biventricular (BIV) pacing has proved its effectiveness to correct myocardial asynchrony and improve clinical status of patients with severe congestive heart failure (CHF) and widened QRS. Despite a different effect on left ventricular electrical dispersion, left univentricular (LV) pacing is able to achieve the same mechanical synchronisation as BIV pacing in experimental studies and in humans. This results in clinical benefits of LV pacing at mid-term follow-up, with significant improvement in functional class, quality of life and exercise tolerance at the same extent as those observed with BIV stimulation in non randomised studies. Furthermore these benefits are obtained at lesser costs and with conventional dual-chamber devices. However, LV pacing has to be compared to BIV pacing in randomised trials before being definitely considered as a cost-effective alternative to BIV pacing.

Epidemiology, clinical features, and follow-up of patients with syncope and a positive adenosine triphosphate test result.

OBJECTIVES: We sought to evaluate epidemiology, clinical features, and outcomes of patients with syncope and an abnormal response to adenosine triphosphate (ATP). BACKGROUND: Syncope remains of unknown origin in almost 30% of the patients. Injection of ATP induces in some of these patients, but not in control patients, a ventricular pause > or =6 s. METHODS: Patients with syncope of unknown origin had an intravenous injection of 20 mg of ATP. All patients had a tilt test. RESULTS: Among 214 patients, 19 (8.9%) had a positive ATP test result. The proportion of positive test results was higher (p < 0.002) in women (14.3%) than in men (2.2%). Ten patients (4.7%) had positive ATP and tilt test results. These patients (exclusively women) were older (p < 0.05) at the time of their fist syncope than the 67 patients with a negative ATP test result but a positive tilt test result. There was a trend for these two test results to be correlated (p = 0.07). Side effects were of short duration and benign. The mean duration of pauses was longer in women (p = 0.009). During a mean period of 31 +/- 14 months, recurrences of syncope were reported in 25% of patients. CONCLUSIONS: The ATP test is a safe test with an "abnormal" result in <10% of patients with syncope of unknown origin. The profile of these patients is characteristic.

Evaluation of left bundle branch block as a reversible cause of non-ischaemic dilated cardiomyopathy with severe heart failure. A new concept of left ventricular dyssynchrony-induced cardiomyopathy.

OBJECTIVES: We sought to determine if amelioration of left bundle branch block (LBBB)-induced contraction disturbances achieved by left ventricular (LV)-based pacing could result in sustained reversal of severe LV dysfunction in certain patients with chronic heart failure due to non-ischaemic cardiomyopathy. BACKGROUND: It has been shown that LBBB induces asynchronous contraction of LV. However, whether such a functional contraction disturbance, if present for an extended period of time, could account for a dilated cardiomyopathy remains unknown. METHODS: The study population comprised 29 patients with dilated cardiomyopathy, sinus rhythm, LBBB and severe heart failure (14 patients in New York Heart Association (NYHA) class III and 15 in class IV). Patients were followed prospectively after resynchronization therapy. LV function was considered to be normalized when ejection fraction (EF) was >50% at 1 year. RESULTS: Five among the 29 patients (17%: group 1) demonstrated both complete normalization of LV function following resynchronization therapy (EF: from 19+/-6 to 55+/-3%, P = 0.001) and clinical improvement (mean NYHA class: 3.4+/-0.5 to 1.8+/-0.4, P = 0.02; 6-min walk distance: 300+/-136 to 444+/-75 m, P = 0.12; peak VO2: 11.9+/-4 to 15.8+/-2 ml/min/kg, p = 0.03). Among the remaining 24 patients (83%: group 2) EF improved but did not normalize (from 21+/-8 to 23+/-11%, ns). Baseline clinical features could not predict which patients would exhibit the reversal of LV dysfunction. CONCLUSIONS: Normalization of LV function 1 year after resynchronization therapy in a small but important number of patients suggests that long-standing LBBB may be a newly identified reversible cause of cardiomyopathy.

Value of entrainment mapping in determining the isthmus-dependent nature of atrial flutter in the presence of amiodarone.

INTRODUCTION: Entrainment mapping is a useful procedure for localizing macroreentrant tachycardia circuits. In patients with isthmus-dependent atrial flutter, entrainment mapping from the isthmus during tachycardia results in postpacing intervals (PPI) close to the tachycardia cycle length (TCL). However, the influence of antiarrhythmic drugs on the method's value is not clearly established. The aim of our study was to assess the value of entrainment mapping in the presence of amiodarone in patients undergoing radiofrequency ablation (RFA) of isthmus-dependent atrial flutter. METHODS AND RESULTS: The study consisted of 83 patients with isthmus-dependent atrial flutter: 52 were taking amiodarone at the time of RFA (group 1) and 31 were in a drug-free state (group 2). Entrainment mapping was performed from the cavotricuspid isthmus, and PPI minus TCL was determined. The two groups had similar baseline clinical characteristics. In all patients, RFA of the isthmus resulted in termination of tachycardia, confirming the isthmus-dependent nature of the flutter. TCL was significantly longer in group 1 than in group 2 (263 +/- 31 msec vs 238 +/- 27 msec, P < 0.0002). PPI minus TCL at the isthmus was significantly longer in group 1 than in group 2 (17 +/- 17 msec vs 8 +/- 4 msec, P < 0.01). More patients in group 1 had PPI-TCL>20 msec compared to group 2 (37% vs 10%, P = 0.01). CONCLUSION: Amiodarone significantly alters the entrainment mapping response from the isthmus. In this setting, long return cycles exceeding the TCL by >20 msec do not exclude isthmus-dependent atrial flutter.

Assessment of upgrading to biventricular pacing in patients with right ventricular pacing and congestive heart failure after atrioventricular junctional ablation for chronic atrial fibrillation.

AIMS: Effects of cardiac resynchronization therapy (CRT) in patients with right ventricular pacing and congestive heart failure (CHF) have only been reported in limited series. CRT in patients with atrial fibrillation remains controversial. Patients with AV junctional ablation offer a unique opportunity to study the effects of CRT in patients with right ventricular pacing combined with atrial fibrillation. The aims of the present study were to evaluate the effects of upgrading to biventricular pacing patients with CHF, permanent atrial fibrillation, and prior ablation of the atrioventricular (AV) junction followed by conventional right ventricular pacing. METHODS AND RESULTS: We studied 16 consecutive patients with permanent atrial fibrillation treated by AV junctional ablation. After a mean follow-up of 20+/-19 months (6 weeks to 5 years) they were successfully upgraded to biventricular pacing for severe CHF. Parameters were prospectively evaluated at baseline and at 6 months. The 14 surviving patients at 6 months demonstrated significant improvement (P<0.02) in New York Heart Association class but the exercise test parameters remained unchanged. Cardiothoracic ratio decreased by 5% (P=0.04), end-systolic diameter by 8% (P=0.001), end-diastolic diameter by 4% (P=0.08), systolic pulmonary artery pressure by 17% (P<0.0001) and mitral regurgitation area by 40% (P<0.05). Ejection fraction increased by 17% (P=0.11) and fractional shortening by 24% (P=0.01). CONCLUSION: CRT improves left ventricular performance and functional status in patients with permanent atrial fibrillation and prior remote right ventricular pacing.

Midterm benefits of left univentricular pacing in patients with congestive heart failure.

BACKGROUND: Resynchronization therapy by simultaneous pacing of the right and left ventricles has gained wide acceptance as a useful treatment for patients with severe congestive heart failure. Several short-term hemodynamic studies in humans and animals failed to demonstrate any benefit of biventricular pacing over left univentricular pacing, but long-term studies on this pacing mode are lacking. The objective of this study was to assess the outcome over a 1-year period of patients paced exclusively in the left ventricle. METHODS AND RESULTS: Clinical, angiographic, echocardiographic, and ergometric data were collected at baseline and after 12 months in 22 patients (age, 69.3+/-6.5 years) with NYHA class III or IV (10 patients), sinus rhythm, left bundle-branch block, and no bradycardia indication for pacing. After 12 months, compared with baseline values, NYHA class improved significantly by 40% (P<0.0001), 6-minute walk distance by 30% (P=0.01), peak VO2 by 26% (P=0.01), left ventricular end-diastolic diameter by 5% (P=0.02), ejection fraction by 22% (P=0.07), mitral regurgitation area by 40% (P=0.01), and norepinephrine level by 37% (P=0.04). CONCLUSIONS: In patients with severe congestive heart failure, sinus rhythm, and left bundle-branch block despite optimal pharmacological treatment, left univentricular pacing is feasible and results in significant midterm benefit in exercise tolerance and left ventricular function.

Contributions of high resolution electrograms memorized by DDDR pacemakers in the interpretation of arrhythmic events.

The accuracy of information retrievable from the memories of DDDR pacing systems has been limited by the absence of actual electrograms confirming the proper sensing of spontaneous cardiac activity versus that of extraneous signals. This study examined the diagnostic power of a new arrhythmia interpretation scheme, which includes the recording and storage of high resolution endocavitary atrial and ventricular electrograms (HREGM). HREGM stored in the memories of new generation pacemakers (PM) in response to nonsustained ventricular tachycardia (NSVT), sustained VT, and atrial arrhythmias were analyzed in a follow-up registry of 520 patients at 1 month, and 3 to 6 months after implantation of a PM for standard indications. For each sequence of stored HREGM, the accuracy of the PM response was examined, classified as accurate (true positive), versus inaccurate (false positive), versus undetermined, and the relative contribution of the HREGM in verifying the PM diagnosis was measured. During a follow-up of 4.9 +/- 2 months, 256 (49%) of the 520 patients had an event recorded, which was confirmed to be arrhythmic on the basis of HREGM. Overall, approximately 34% of atrialtachy response (ATR) episodes were confirmed to be appropriate. Similar percentages of episodes were prompted by oversensing of signals unrelated to cardiac arrhythmias, while nearly 12% of the episodes could not be clarified because of such brief duration as to preclude recording of their onset. Approximately one-third of NSVT, and one-half of VT detections were false positive. Ventricular oversensing, most often due to myopotential interference in presence of unipolar sensing, and atrial undersensing were both identified as sources of false-positive detections of ventricular events. The proportion of true-positive detections was significantly higher in the bipolar (83%) than unipolar configuration. Among 520 PM recipients, miscellaneous episodes of atrial arrhythmias were confirmed by HREGM in 37% of patients within 3 to 6 months of follow-up. Atrial fibrillation was confirmed in only 6% of patients, of whom over 50% already had histories of atrial fibrillation. The prevalence of unsuspected atrial arrhythmia in this unselected population was lower than previously reported.