Atrial therapies reduce atrial arrhythmia burden in defibrillator patients.

BACKGROUND: Approximately 25% of patients who receive an implantable cardioverter-defibrillator (ICD) to treat ventricular tachyarrhythmias have documented atrial tachyarrhythmias before implantation. This study assessed the ability of device-based prevention and termination therapies to reduce the burden of spontaneous atrial tachyarrhythmias. METHODS AND RESULTS: Patients with a standard indication for the implantation of an ICD and 2 episodes of atrial tachyarrhythmias in the preceding year received a dual-chamber ICD (Medtronic 7250 Jewel AF) that uses pacing and shock therapies for prevention and/or termination of atrial tachyarrhythmias. In a multicenter trial, patients were randomized to 3-month periods with atrial therapies "on" or "off" and subsequently crossed over. Analysis was performed on the 52 of 269 patients who had episodes of atrial tachyarrhythmia and had >/=30 days of follow-up with atrial therapies on and off. The atrial therapies resulted in a reduction of atrial tachyarrhythmia burden from a mean of 58.5 to 7.8 h/mo. A paired analysis (Wilcoxon signed-rank test) showed that the median difference in burden (1.1 h/mo) was highly significant (P=0.007). When the subgroup of 41 patients treated only with atrial pacing therapies was analyzed, the reduction in burden persisted (P=0.01). CONCLUSIONS: In this study, patients with a standard ICD indication and atrial tachyarrhythmias had a significant reduction in atrial tachyarrhythmia burden with use of atrial pacing and shock therapies.

Is sinus node modification appropriate for inappropriate sinus tachycardia with features of postural orthostatic tachycardia syndrome?

Inappropriate sinus tachycardia and postural orthostatic tachycardia are ill-defined syndromes with overlapping features. Although sinus node modification has been reported to effectively slow the sinus rate, long-term clinical response has not been adequately assessed. Furthermore, whether patients with postural orthostatic tachycardia would benefit from sinus node modification is unknown. The study prospectively assessed the short- and long-term clinical outcomes of seven consecutive female patients with postural orthostatic tachycardia syndrome and inappropriate sinus tachycardia who were treated with sinus node modification. The study was conducted in a tertiary care center. The electrophysiological and clinical responses were prospectively assessed as defined by autonomic function testing, including Valsalva maneuver, deep breathing, tilt table testing, and quantitative sudomotor axonal reflex testing. Among the study population (mean age was 41+/-6 years), 5 (71%) patients had successful sinus node modification. At baseline, heart rates were 101+/-12 beats/min before modification and 77+/-9 beats/min after modification (P = 0.001). With isoproterenol, heart rates were 136+/-9 and 105+/-12 beats/min (P = 0.002) before and after modification, respectively. The mean heart rate during 24-hour Holter monitoring was also significantly reduced: 96+/-9 and 72+/-6 beats/min (P = 0.005) before and after modification, respectively. Despite the significant reduction in heart rate, autonomic symptom score index (based on ten categories of clinical symptoms) was unchanged before (15.6+/-4.1) and after (14.6+/-3.6) sinus node modification (P = 0.38). Sinus rate can be effectively slowed by sinus node modification. Clinical symptoms are not significantly improved after sinus node modification in patients with inappropriate sinus tachycardia and postural orthostatic tachycardia. A primary subtle autonomic disregulation is frequently present in this population. Sinus node modification is not recommended in this patient population.

Prospective postmarket device studies versus returned product analysis as a predictor of system survival.

Monitoring and reporting mechanisms are vital tools for clinicians to assess ICD system performance over time for optimal patient care. This article explores the various reporting mechanisms available to the clinician, both historical and current, and compares and contrasts two such methods. The lead survival rates obtained by return product analysis (RPA) are compared with those from an ongoing prospective chronic study that actively follows patients for clinical ICD system failures (Tachyarrhythmia Chronic Systems Study [TCSS]). Examination of available data shows that a prospective study such as the TCSS is capable of detecting clinically significant adverse events in 2.2% of the 3,958 leads followed. By comparison, RPA-based monitoring of the same leads detects "out of specification" events in 0.5% of the 78,571 leads followed. Statistical analyses of two separate families of leads (RV leads and SQ Patch leads) show that survival rates obtained by the two methods begin to differ at approximately 2 years of implant experience, with 95% confidence intervals no longer overlapping at 3 years. The authors conclude that prospective chronic device studies are a superior tool for the ongoing monitoring of implanted device performance compared to RPA-based reports.

Factors affecting the frequency of subcutaneous lead usage in implantable defibrillators.

Subcutaneous leads (SQ) add complexity to the defibrillation system and the implant procedure. New low output devices might increase the requirement for SQ arrays, although this might be offset by the effects of active can and biphasic technology. This study sought to assess the impact of these technologies on SQ lead usage, and to determine if clinical variables could predict the need for an SQ lead. Patients receiving nonthoracotomy systems (n = 554) at our institution underwent step-down-to-failure DFT testing with implant criteria of a 10-J safety margin. SQ leads were used only after several endovascular configurations failed. Use of biphasic waveforms significantly lowered the frequency of use of SQ leads from 48% to 3.7% (P < 0.000001). SQ leads were required in 4.4% of patients with cold can devices and 2.6% of patients with active can devices (P = NS). There was no increase in SQ lead usage with low energy (< 30-J delivered energy) devices. Clinical variables (including EF, heart disease, arrhythmia, and prior bypass) did not predict the need for an SQ lead. The implant DFT using SQ arrays (14.5 +/- 6.5 J) was not significantly lower than that for SQ patches (16.6 + 6.0 J). We conclude that biphasic waveforms significantly reduce the need for SQ leads. Despite this reduction, 3.7% of implants still use an SQ lead to achieve adequate safety margins. The introduction of lower output devices has not increased the need for SQ leads, and when an SQ lead is required, there is not a significant difference in the implant DFT of patches versus arrays. Clinical variables cannot predict which patients require SQ leads.

Defibrillator challenges for the new millennium: the marriage of device and patient-making and maintaining a good match.

Although it has become clear that implantable cardioverter defibrillators (ICDs) are effective, important challenges remain for the physician. Due to the limitations of available risk stratification tools, patient selection for primary sudden death prevention remains controversial in many populations. Additionally, the proliferation of device choices has led to challenges in matching the appropriate device to the individual patient: device size is balanced against longevity; the advantages of dual chamber systems is weighed against their increased complexity; physician and patient preferences in device implant site are constrained by site-dependent effects on defibrillation effectiveness and lead failure rates; and special consideration must be given to the patient with a preexisting pacemaker. After ICD placement, determination of appropriate follow-up frequency and methodology to assess device function must be considered. This article will review patient selection, device implant site selection, device-device interactions, single versus dual chamber ICD selection, and follow-up.

Global right atrial mapping of human atrial flutter: the presence of posteromedial (sinus venosa region) functional block and double potentials : a study in biplane fluoroscopy and intracardiac echocardiography.

BACKGROUND: Previous studies of atrial flutter have found linear block at the crista terminalis; this was thought to predispose the patient to the arrhythmia. More recent observations, however, have demonstrated crista conduction. We sought to characterize the posterior boundary of atrial flutter. METHODS AND RESULTS: Patients with counterclockwise flutter (n=20), clockwise flutter (n=3), or both (n=5) were studied using two 20-pole catheters. Biplane fluoroscopy determined catheter positions. During counterclockwise flutter, craniocaudal activation occurred along the entire lateral and posterior right atrial walls. Septal activation proceeded caudocranially. In all patients, a line of block was seen in the posteromedial (sinus venosa) right atrium; this was manifested by the presence of double potentials where the upward and downward activations collided. Anatomic location was confirmed by intracardiac echocardiography in 9 patients. In patients with clockwise flutter, the line of block and double potentials were seen in the same location during counterclockwise flutter, but the activation sequence around the line of block was reversed. Pacing near the site of double potentials during sinus rhythm excluded a fixed line of block, and premature atrial complexes demonstrated functional block with manifest double potentials. In 2 patients, posterior ectopy organized to subsequently initiate isthmus-dependent atrial flutter. CONCLUSIONS: (1) A functional line of block is seen at the posteromedial (sinus venosa region) right atrium during counterclockwise and clockwise atrial flutter. (2) All lateral wall right atrial activation can be uniform during flutter, without linear block or double potentials in the region of the crista terminalis. (3) Activation at the site of posteromedial right atrial functional block can organize to subsequently initiate isthmus-dependent atrial flutter.

Economic outcomes of implantable cardioverter-defibrillators.

BACKGROUND: We reviewed the literature pertaining to the cost-effectiveness of implantable cardioverter-defibrillator (ICD) therapy in the management of ventricular fibrillation and tachycardia. Discussed are the methodology, advantages, and limitations of economic-outcomes analyses as related to ICD therapy; the impact of new technology and physician practice patterns; and methodological recommendations for future studies. METHODS AND RESULTS: Articles published between 1990 and 1997 were screened for cost-effectiveness analyses of ICD versus antiarrhythmic drug therapy. Randomized clinical trials, prospective and retrospective studies, and economic models were included. These studies report incremental cost-effectiveness ratios ranging from cost savings of $13 975 per life-year saved (LYS) to an incremental cost of $114 917 per LYS for ICD therapy. Differences were due to study type, cost-reporting methodology, ICD technology used, and length of follow-up. Assuming current technology and physician practice patterns, we find that ICD total therapy costs may break even in 1 to 3 years. CONCLUSIONS: Recent literature suggests that ICDs are a cost-effective therapy for management of life-threatening ventricular tachyarrhythmias. The advent of new technology and patient management practices should further improve the cost-effectiveness of ICD therapy. Future studies of ICD cost-effectiveness should address the implications of truncated follow-up periods and quality of life.

Are routine arrhythmia inductions necessary in patients with pectoral implantable cardioverter defibrillators?

INTRODUCTION: The value of ventricular arrhythmia inductions as part of routine implantable cardioverter defibrillator (ICD) follow-up in new-generation pectoral ICDs is unknown. METHODS AND RESULTS: We performed a retrospective analysis of a prospectively collected database analyzing data from 153 patients with pectoral ICDs who had routine arrhythmia inductions at predismissal, and 3 months and 1 year after implantation. Routine predismissal ventricular fibrillation (VF) induction yielded important findings in 8.8% of patients, all in patients with implantation defibrillation threshold (DFT) > or = 15 J or with concomitant pacemaker systems. At 3 months and 1 year, routine VF induction yielded important findings in 5.9% and 3.8% of tested patients, respectively, all in patients who had high DFT on prior testing. Ventricular tachycardia (VT) induction at predismissal, and 3 months and 1 year after implantation resulted in programming change in 37.4%, 28.1%, and 13.8% of tested patients, almost all in patients with inducible VT on baseline electrophysiologic study and clinical episodes since implantation. CONCLUSION: Although helpful in identifying potentially important ICD malfunctions, routine arrhythmia inductions during the first year after ICD implantation may not be necessary in all cases. VF inductions have a low yield in patients with previously low DFTs who lack concomitant pacemakers. VT inductions have a low yield in patients without baseline inducible VT and in the absence of clinical events. Definite recommendations regarding patient selection must await larger prospective studies as well as consensus in the medical community about what comprises an acceptable risk justifying avoidance of the costs and inconveniences of routine arrhythmia inductions.

Defibrillation thresholds are increased by right-sided implantation of totally transvenous implantable cardioverter defibrillators.

Whether an ICD is placed via a left- or right-sided approach depends on venous access, the presence of a preexisting pacemaker, and other factors. Since the DFT is affected by lead position, which in turn is determined in part by the side of access, right-sided venous access could adversely affect DFTs. Furthermore, right-sided active can placement directs electric current toward the right hemithorax, which could further increase DFTs. This study sought to determine whether DFTs were increased by right-sided vascular access, and whether active can technology was beneficial or detrimental with right-sided ICD placement. Stepdown to failure DFTs were found in 290 patients receiving transvenous systems at the time of initial ICD implantation. Of these, 271 (93%) received left-sided systems and 19 (7%) received right-sided systems. The mean DFT in systems placed via left-sided vascular access was 11.3 +/- 5.3 J versus 17.0 +/- 4.9 J for right-sided implantation (P < 0.0001); right-sided DFTs were elevated for both active can and cold can systems. Right-sided active can devices had a lower DFT than right-sided cold can systems (15 +/- 4.1 J vs 19 +/- 4.8 J, P = 0.05). The right-sided implantation of implantable defibrillators results in significantly higher DFTs than the left-sided approach. This may be due to the less favorable distribution of the defibrillating field relative to the myocardium with the devices on the right. When right-sided implantation is clinically mandated, active can devices result in lower thresholds and should be used.

Preliminary clinical results of a biphasic waveform and an RV lead system.

Biphasic defibrillation waveforms have provided a reduction in defibrillation thresholds in transvenous ICD systems. Although a variety of biphasic waveforms have been tested, the optimal pulse durations and tilts have yet to be identified. A multicenter clinical study was conducted to evaluate the performance of a new ICD biphasic waveform and new RV active fixation steroid eluting lead system. Fifty-three patients were entered into the study. Mean age was 63 years with a mean ejection fraction of 36.8%. Primary indication for implantation was monomorphic ventricular tachycardia alone (54.7%). Forty-eight patients (90.6%) were implanted with an RV shocking lead and active can alone as the anodal contact. The ICD can was the cathode. In four cases (7.5%), an additional SVC or CS lead was used due to a high DFT with the RV lead alone. In an additional case, a chronic SVC lead was used although the RV-Can DFT was acceptable. DFT for all cases at implant was 9.8 +/- 3.7 J. Repeat testing at 3 months for a subset of patients showed a reduction in DFT (7.4 +/- 3.0 J), P value = 0.03. Sensing and pacing characteristics of the RV lead system remained excellent during the study period (acute 0.047 +/- 0.005 ms at 5.4 V and 9.9 +/- 6.2 mV R wave; chronic 0.067 +/- 0.11 ms at 5.4 V and 9.3 +/- 5.4 mV R wave). It is concluded that this lead system provides good acute and chronic sensing and pacing characteristics with good DFT values in combination with this waveform.

A stepwise testing protocol for modern implantable cardioverter-defibrillator systems to prevent pacemaker-implantable cardioverter-defibrillator interactions.

Current use of newer implantable cardioverter-defibrillators (ICDs) has changed the spectrum of pacemaker-ICD interactions and provided new tools for testing and understanding those interactions. Testing for pacemaker-ICD interactions was performed in 31 procedures involving 22 patients. The protocol included: (1) evaluation of pacemaker stimulus artifact amplitude and its ratio to that of the evoked ventricular electrogram, (2) testing for inhibition of ventricular fibrillation (VF) detection by the ICD during asynchronous pacing at maximum output, (3) evaluation by pacemaker event marker recordings of pacemaker sensing behavior while programmed to nonasynchronous mode during ventricular tachycardia (VT) or VF, and (4) evaluation of postshock interactions. Inhibition of detection of VT/VF was found in 6 of 22 patients (27.2%). Large stimulus artifact amplitude (>2 mV) or stimulus artifact:evoked QRS ratio > 1/3 had a positive predictive accuracy of 18% and 14.4%, respectively, and a negative predictive accuracy of 100% and 92.3%, respectively, for clinically significant interaction. Asynchronous pacing occurred in 16 of 31 procedures (51.6%), and was due to underdetection by the pacemaker in 4 of 16 (25%) and noise reversion in 12 of 16 (75%). Postshock phenomena occurred in 6 cases, 3 of which were clinically significant. Overall, 11 of 22 patients (50%) had clinically significant interactions discovered by this protocol, which led to system revisions in 6 and to pacemaker output reprogramming in 5. Thus, pacemaker-ICD interactions are frequently detected using a thorough and systematic protocol. Most cases can be managed by system revision or pacemaker reprogramming.

The potential usage of dual chamber pacing in patients with implantable cardioverter defibrillators.

UNLABELLED: Bradycardia support by ICDs has been limited to fixed rate, ventricular pacing. Concomitant placement of a pacemaker and an ICD exposes a patient to potentially life-threatening device interactions. ICDs capable of dual chamber pacing have recently become available. The number of ICD recipients who stand to benefit from the addition of dual chamber pacing is debated, but no data have addressed this question. This retrospective study analyzed all patients who received nonthoractomy ICD system placement at the Mayo Clinic in Rochester, MN between March 1991 and October 1996 in order to determine the proportion of patients in whom a dual chamber pacing ICD may be indicated. DEFINITIONS: (1) Definitely indicated = pacemaker present at ICD implant or NASPE Class I pacing indication; (2) Probably indicated = NASPE Class II pacing indication, NYHA Functional Class III or IV, or history of systolic congestive heart failure; (3) Possibly indicated = history of paroxysmal atrial fibrillation or an ejection fraction < or = 20%. The results were that nonthoracotomy ICDs were placed in 253 patients. A dual chamber ICD would have been definitely indicated in 11% of the study group, probably indicated in 28%, and possibly indicated in 14%. Chronic atrial fibrillation was present at ICD implant in 6.7% of patients and developed in 0.9%/yr during follow-up. The addition of dual chamber pacing to ICDs stands to potentially benefit approximately half (53%) of ICD recipients. These data do not address all patients who may benefit from dual chamber sensing.

Cardiac parasympathetic stimulation via QRS-synchronous low-energy shocks in humans.

INTRODUCTION: In patients receiving test shocks to verify lead connections at implantation, we anecdotally have observed postshock delay. The purpose of this study was to determine whether QRS-synchronous low-energy shocks delivered by implantable defibrillators result in postshock cycle length prolongation, and to determine the mechanism of this phenomenon. METHODS AND RESULTS: Twenty-five patients undergoing defibrillator testing were studied, three with epicardial patches and 22 with transvenous leads. Each patient received QRS-synchronous shocks of 0.2, 0.4, 0.6, and 2.0 J in random order. Patients were further randomized to receive either saline or 2.0 mg atropine intravenously, and then given a second sequence of shocks. At baseline, the postshock cycle length (1,035+/-245 msec) was significantly longer than the preshock cycle length (968+/-177 msec, P = 0.01). In patients with a coronary sinus (CS) or superior vena cava (SVC) lead, the mean prolongation was 91+/-160 msec, compared with 12+/-106 msec for patients without such a lead (P < 0.0001). All energy levels resulted in significant postshock prolongation compared with preshock cycle lengths (P < 0.05). Postshock prolongation before atropine was 76+/-162 msec, compared with -13+/-52 msec afterward (P < 0.00001). Biphasic shocks resulted in greater postshock prolongation than monophasic shocks of equal energy. CONCLUSION: Low-energy shocks delivered during the QRS complex cause postshock cycle length prolongation in man. This effect required the presence of a CS or SVC lead. Atropine inhibited this effect, suggesting the phenomenon was mediated by direct cardiac parasympathetic nerve stimulation by the intracardiac shock.

Comparison of results in two implantable defibrillators. Jewel 7219D Investigators.

The Jewel 7219D was the first non-thoracotomy implantable cardioverter-defibrillator (ICD) with biphasic shock capability small enough to be placed in the prepectoral subcutaneous position. Size reduction of ICDs is desirable, but safety and efficacy of smaller devices must be demonstrated. Outcomes of patients treated with the Jewel 7219D defibrillator (n = 1,781) and with its precursor model PCD 7217B (n = 2,637) were compared. To use PCD patients (n = 2,637) as historical (n = 2,574) and concurrent controls (n = 63), statistical adjustments using the Cox proportional-hazards regression model were made. Jewel recipients (n = 1,781) treated in 106 US and 32 non-US centers exhibited similar characteristics including a mean age of 59 years, 78% men, ejection fraction of 34%, history of aborted sudden cardiac death in 41%, and coronary artery disease in 70%. Implantation was completed in 1,777 of 1,781 (99.9%) attempts and success with the first electrode configuration and polarity was 89.5%. Kaplan-Meier cumulative first-year survivals for cardiac and all-cause mortality were 98.5% and 93.3%. Complication-free first-year survival for Jewel implants in prepectoral subcutaneous (n = 582), subpectoral submuscular (n = 366), and abdominal (n = 449) positions did not differ (p > 0.05). First-year survival free of pocket-related complications exceeded 98% in all locations. Adjusted cardiac and all-cause first-year mortality, and efficacy in terminating spontaneous tachyarrhythmias did not differ between the 2 device groups. In conclusion, the safety and efficacy of Jewel model 7219D in the prepectoral subcutaneous position are at least equal to either those of Jewel models implanted in different positions or to those of the previously extensively characterized PCD 7217B.

Randomized prospective pilot study of long-term dual-site atrial pacing for prevention of atrial fibrillation.

OBJECTIVE: To determine whether dual-site atrial pacing is feasible, safe, and effective. DESIGN: We undertook a randomized prospective single-blind crossover study. MATERIAL AND METHODS: Nine patients with at least two episodes per month of symptomatic paroxysmal atrial fibrillation participated in a randomized crossover study involving three separate 3-month blocks of single-site atrial pacing, dual-site atrial pacing, and control (support-only) pacing. RESULTS: Dual-site atrial pacing resulted in shorter P wave duration (81 +/- 14 ms) than did single-site pacing (111 +/- 12 ms) or control sinus rhythm (123 +/- 9 ms) (P<0.0001) and in fewer premature atrial complexes on Holter monitoring (P = 0.06). The arrhythmia-free interval was longer with dual-site pacing (67 +/- 17 days) than with single-site (62 +/- 30 days) or support-only (49 +/- 34 days) pacing (P = 0.10). This pilot study was not statistically powered to detect a difference between pacing modes. CONCLUSION: (1) Dual-site atrial pacing is feasible and safe; (2) it shortens the P wave duration and tends to decrease premature atrial complexes on Holter monitoring; (3) any atrial pacing tends to prolong the arrhythmia-free interval; and (4) this pilot study enrolled too few patients to determine whether a significant difference in pacing modes exists and supports the need for a larger study.

Importance of pacemaker noise reversion as a potential mechanism of pacemaker-ICD interactions.

Numerous types of interactions between pacemakers and implantable cardioverter defibrillators (ICDs) have been described. Pacemaker outputs preventing appropriate detection of ventricular tachycardia or ventricular fibrillation by the ICD is one of the more serious. Asynchronous pacemaker activity during ventricular arrhythmias may be caused by either nonsensing of the arrhythmia or by noise reversion, which is an algorithm that causes the pacemaker to switch to asynchronous pacing when repetitive sensing at a high rate occurs. We analyzed the mechanisms underlying asynchronous pacemaker activity in ventricular arrhythmias using pacemaker telemetry during the arrhythmia. Thirty-nine induced arrhythmias from 26 different procedures in 19 patients with both pacemakers and ICDs were analyzed. Of the 39 arrhythmias, asynchronous pacemaker activity occurred in 16. The underlying mechanism was nonsensing in 4 episodes and noise reversion in 12 episodes. Clinically significant interference with detection arose on three occasions. Conditions favoring the occurrence of noise reversion include specific pacemaker models, arrhythmia cycle lengths in the range causing noise reversion of the individual pacemaker model, long noise sampling periods, and VVI pacing mode. Noise reversion can be diagnosed by telemetering the pacemaker marker channel during ventricular arrhythmias as a part of routine pacemaker-ICD interaction evaluation. It can be prevented or minimized by programming short ventricular refractory periods or using pacemakers with short retriggerable refractory periods.

High failure rate for an epicardial implantable cardioverter-defibrillator lead: implications for long-term follow-up of patients with an implantable cardioverter-defibrillator.

OBJECTIVES: The purpose of this study was to determine the risk of epicardial lead failure during long-term follow-up and its mode of presentation. BACKGROUND: Despite the high prevalence of epicardial lead-based implantable cardioverter-defibrillators, their long-term performance is unknown, and appropriate follow-up has not been established. METHODS: The study group comprised all patients in whom an epicardial lead system was implanted at the Mayo Clinic between October 31, 1984 and November 3, 1994. The number of lead fractures and leads with fluid within the insulation and the mode of presentation were determined retrospectively by review of patient visits, radiographs of lead systems and data derived from formal lead testing. RESULTS: At 4 years, the survival rate free of lead malfunction, using formal lead testing, for 160 Medtronic epicardial patches (models 6897 and 6921) was 72% compared with 92.5% for the 179 Cardiac Pacemaker, Inc. (CPI) patches (models 0040 and 0041) (p = 0.01). In addition, five Medtronic patches in three patients had fluid within the lead insulation but no obvious fracture. No CPI patches had fluid identified within the leads. Of 330 Medtronic epicardial pace/sense leads (model 6917), the 4-year survival rate free of lead malfunction as assessed by lead testing was 96%. In all, 19 presentations of lead malfunction were found in 17 patients (2 patients had more than one lead fracture at different times). In 11 (58%) of these presentations, the patients were asymptomatic despite the presence of obvious lead fracture. CONCLUSIONS: Epicardial lead malfunction is common on long-term follow-up, and some leads have a failure rate of 28% at 4 years. Many patients with fractured leads remain asymptomatic, despite involvement of multiple leads in some cases. Therefore, consideration should be given to regular periodic lead testing in addition to routine X-ray examination, as asymptomatic lead malfunction can present with normal chest X-ray findings.

Stability of the defibrillation probability curve with the development of ventricular dysfunction in the canine rapid paced model.

Most patients with implantable defibrillators have diminished cardiac function. Progressive heart failure might impair defibrillation efficacy, leading to interpreted device failure. This study sought to determine the effect of ventricular dysfunction on defibrillation energy using a biphasic endocardial system. Eleven dogs were ventricularly paced at 225 pulses/min for 2 weeks to induce ventricular dysfunction, and five control dogs remained unpaced. Dose response defibrillation probability curves were generated for each animal at baseline, after 2 weeks (at which time the pacemakers were turned off in the paced group), and then 1 week later. The defibrillation thresholds, ED20, ED50, and ED80 (the 20%, 50%, and 80% effective defibrillation energies, respectively) were determined for each dog at each study. In the paced dogs, the mean ejection fraction fell from 55% to 25% after pacing (P < 0.0001), and rose to 46% after its discontinuation (P = 0.0002). The defibrillation threshold, ED20, ED50, and ED80 remained unchanged in both the control and paced groups for all three studies, even after adjustment for dog weight or left ventricular mass. Rapid pacing produced no change in left ventricular mass. It induced ventricular cavity dilatation and wall thinning, which had opposing effects on defibrillation energy requirements, resulting in no net change of the ED50 in heart failure. In conclusion, the defibrillation efficacy of a biphasic transvenous system is not changed by the development of heart failure using the rapid paced canine model.

Pneumothorax: an unusual cause of ICD defibrillation failure.

We describe two patients with defibrillation failure of implantable cardioverter defibrillators (ICDs) resulting from large left pneumothoraxes following subclavian vein puncture during the implantation. Following pneumothorax drainage, low defibrillation thresholds (DFTs) were attained without further manipulations. The absence of other signs and symptoms of pneumothorax and the presence of satisfactory pacing function during the procedure, resulted in a significant delay in diagnosis. Pneumothorax should be included in the differential diagnosis when unexpected high DFTs are found during ICD implantation or predischarge testing. This complication is avoidable by a different surgical approach, cephalic vein cutdown.