Defibrillator System Research Articles

ICD-antiarrhythmic drug and ICD-pacemaker interactions.

Antiarrhythmic drugs and separate bradycardia pacing systems are prescribed commonly in patients with implantable cardioverter defibrillators (ICDs). Adverse effects of antiarrhythmic drugs on ICD function and adverse interactions between ICDs and pacemakers have been documented. The effect of antiarrhythmic drugs on the defibrillation threshold (DFT) in patients has not been well assessed. Most studies have been performed in animal models in which cardiac function was normal and drug doses were supraphysiologic. In addition, most studies have utilized monophasic defibrillation shock waveforms and epicardial lead systems. Despite the lack of clinical data applicable to current defibrillation systems, it appears that chronic amiodarone administration causes a significant DFT increase. In addition, antiarrhythmic drugs can influence antitachycardia pacing and tachycardia sensing. Defibrillation shocks can cause transient failure of pacemaker sensing and pacing, and cause spurious pacemaker reprogramming. Pacemaker function can result in ICD oversensing, leading to inappropriate therapy, or cause ICD undersensing, potentially resulting in failure to deliver therapy during ventricular fibrillation. The susceptibility of an ICD to undersensing appears related to the amplitude of the pacing stimulus artifact recorded by the ICD rate-sensing circuit and to the characteristics of the fibrillation electrogram. Preliminary data suggest that undersensing of ventricular fibrillation by current ICDs is an unlikely event.

Low‐Energy Transvenous Cardioversion of Atrial Fibrillation Using a Single Atrial Lead System

Clinical studies have shown that electrical conversion of atrial fibrillation (AF) is feasible with transvenous catheter electrodes at low energies. We developed a single atrial lead system that allows atrial pacing, sensing, and defibrillation to improve and facilitate this new therapeutic option. The lead consists of a tripolar sensing, pacing, and defibrillation system. Two defibrillation coil electrodes are positioned on a stylet-guided lead. A ring electrode located between the two coils serves as the cathode for atrial sensing and pacing. We used this lead to cardiovert patients with acute or chronic AF. The distal coil was positioned in the coronary sinus, and the proximal coil and the ring electrode in the right atrium. R wave synchronized biphasic shocks were delivered between the two coils. Atrial signal detection and pacing were performed using the proximal coil and the ring electrode. Eight patients with acute AF (38 +/- 9 min) and eight patients with chronic AF (6.6 +/- 5 months) were included. The fluoroscopy time for lead placement was 3.5 +/- 4.3 minutes. The atrial defibrillation threshold was 2.0 +/- 1.4 J for patients with acute AF and 9.2 +/- 5.9 J for patients with chronic AF (P < 0.01). The signal amplitude detected was 1.7 +/- 1.1 mV during AF and 4.0 +/- 2.9 mV after restoration of sinus rhythm (P < 0.001). Atrial pacing was feasible at a threshold of 4.4 +/- 3.3 V (0.5-msec pulse width). Atrial signal detection, atrial pacing, and low-energy atrial defibrillation using this single atrial lead system is feasible in various clinical settings. This system might lead to a simpler, less invasive approach for internal atrial cardioversion.

Electrical defibrillation optimization: an automated, iterative parallel finite-element approach.

To date, optimization of electrode systems for electrical defibrillation has been limited to hand-selected electrode configurations. In this paper we present an automated approach which combines detailed, three-dimensional (3-D) finite-element torso models with optimization techniques to provide a flexible analysis and design tool for electrical defibrillation optimization. Specifically, a parallel direct search (PDS) optimization technique is used with a representative objective function to find an electrode configuration which corresponds to the satisfaction of a postulated defibrillation criterion with a minimum amount of power and a low possibility of myocardium damage. For adequate representation of the thoracic inhomogeneities, 3-D finite-element torso models are used in the objective function computations. The CPU-intensive finite-element calculations required for the objective function evaluation have been implemented on a message-passing parallel computer in order to complete the optimization calculations in a timely manner. To illustrate the optimization procedure, it has been applied to a representative electrode configuration for transmyocardial defibrillation, namely the subcutaneous patch-right ventricular catheter (SP-RVC) system. Sensitivity of the optimal solutions to various tissue conductivities has been studied. Results for the optimization of defibrillation systems are presented which demonstrate the feasibility of the approach.

Undersensing of ventricular fibrillation in a noncommitted nonthoracotomy cardioverter defibrillator system.

Evaluation of the impact of undersensing on VF detection time and the relationship of undersensing to the programmed shock energy. Failure to reconfirm an ongoing arrhythmia due to undersensing by a noncommitted ICD might prolong the time to therapy. We measured initial detection times and redetection times at predischarge and at 2 and 6 months in 29 patients (22 men, mean age 60 years) with a noncommitted nonthoracotomy ICD. Telemetry data and output markers were used to analyze each induction. Undersensing leading to failure to reconfirm was present in 44 (11.1%) of 398 episodes of sustained VF and prolonged significantly the median initial detection time from 2.3 seconds (25th and 75th percentiles: 2 and 2.6 s, respectively) to 5.45 seconds (4.3 and 7.35 s, P < 0.0001). One episode required external defibrillation after reconfirmation failure occurred during charging; the total detection time prior to shock was 46 seconds. In a subset of 87 episodes with failed first shocks, the initial detection time was 2.3 seconds (2.1 and 2.8 s) and the redetection time 3 seconds (2.5 and 4.77 s, P < 0.0001). The presence of undersensing prolonged the redetection from 2.6 seconds (2.35 and 3.1 s) to 5.4 seconds (4.53 and 7.35 s, P < 0.0001). Undersensing was more prevalent during the redetection period (P = 0.004) and in episodes of sustained VF in which the first shock energy was higher than 15 J (19.7% vs 5.8%, P < 0.0001). In this automatic defibrillator system, undersensing occurs in 11% of the sustained VF inductions and prolongs detection time significantly. Redetection is longer than initial detection mostly due to the presence of undersensing, the frequency of which is proportional to the programmed energy. The clinical significance of this finding is unknown.

Rise in chronic defibrillation energy requirements necessitating implantable defibrillator lead system revision.

The chronic defibrillation energy requirement (DER) is believed to remain clinically stable in patients with defibrillators. Six patients (two with an epicardial and four with a nonthoracotomy system) were identified with a rise in their chronic DER, which eliminated a 10-J safety margin, thus necessitating a defibrillator lead system revision. The mean increase in DER was 14.7 +/- 4 J and was discovered at a mean of 16.0 +/- 18 months (range 2-41) following implantation. Management included placement of a defibrillator with a biphasic waveform, placement of an additional defibrillation electrode, or both. At 2 months following revision of the defibrillation system, a 10-J DER safety margin was present in each patient. In some patients, there is a progressive increase in the chronic DER with elimination of a 10-J safety margin necessitating revision of the defibrillation system. Routine reevaluation of the chronic DER, therefore, is necessary to identify these patients.

Automatic arrhythmia identification using analysis of the atrioventricular association. Application to a new generation of implantable defibrillators. Participating Centers of the Automatic Recognition of Arrhythmia Study Group.

Atrioventricular association is a key criterion for arrhythmia diagnosis. Its use in a defibrillator should significantly reduce the incidence of inappropriate shocks. Therefore, we evaluated the diagnostic accuracy of an algorithm that uses dual-chamber sensing and analysis of atrioventricular association to discriminate ventricular from supraventricular arrhythmias in a prototype of an implantable defibrillator. The algorithm performed a stepwise analysis of arrhythmias. The rhythm was first classified on the basis of cycle lengths. Each episode was then classified as supraventricular or ventricular in origin on the basis of the stability of cycle lengths and atrioventricular association. This algorithm was evaluated in 156 episodes of induced sustained tachycardias. Eighty-nine tachycardias were taken from the Ann Arbor electrogram library; the others were recorded in 50 patients during electrophysiological studies. The atrial and ventricular signals were stored on an external recorder and then injected into an external prototype of a defibrillator system. The algorithm correctly diagnosed 96% of ventricular tachycardia episodes, 100% of ventricular fibrillation episodes, and 92% of double-tachycardia episodes. The mean detection time for ventricular tachycardia was 2.6 +/- 0.8 seconds, and for ventricular fibrillation, it was 2.1 +/- 0.4 seconds. The positive predictive values for the diagnoses of atrial fibrillation and atrial flutter were 92% and 86%, respectively. For ventricular tachycardia and ventricular fibrillation, the values were 95% and 100%, respectively. Analysis of atrioventricular association promotes reliable differentiation between ventricular and supraventricular tachycardias and should enhance the diagnostic capabilities of implantable defibrillators.

Effect of shock timing on defibrillation success.

The goal of this study was to determine whether delivering transvenous defibrillation shocks, coordinated with the up/down-slope VF waveform patterns in the shocking lead, would improve the probability of successful defibrillation. Anesthetized swine (32-38 kg, n = 8) were implanted with an RV-->SVC + SQArray transvenous system to measure VF waveform patterns and to deliver shocks. The shocks were generated by a Cardiac Pacemakers Inc. biphasic waveform generator. Energy required for 50% success probability (E50) was determined using the multishock up-down protocol. VF was repeatedly induced and defibrillation shocks at E50 were given after 10 seconds. The defibrillation outcome, delivered energy (Ed), peak voltage (V), peak current (I), system impedance (Z) and VF waveform pattern at the time of shock were recorded and measured. Out of a total of 685 shocks, 324 (47%) succeeded and 361 (53%) failed. The Ed, V, I, and Z were similar for the two defibrillation outcome groups (success or failure). VF patterns were classified as high or low amplitude at the time of the shock based on the peak-to-peak amplitude of signals recorded between the shocking electrodes. Shocks that coincided with high amplitude VF patterns were further divided into shocks that occurred on the up-slope or on the down-slope. The probability of success when the E50 shocks were coincident with high or low amplitude fibrillation did not differ significantly (Student's t-test: 46% vs 48%. P = NS). However, during high amplitude fibrillation, shocks delivered on the up-slope were significantly more successful than those delivered on the down-slope (Chi-square: 67% vs 39%; P < 0.001). These results suggest that delivering defibrillation shocks during the up-slope of the high amplitude signal in the shocking lead may improve the probability of successful defibrillation of ICDs.

Ultrastructural Alterations in the Right and Left Ventricular Myocardium Following Multiple Low Energy Endocardial Countershocks in Anesthetized Dogs

Both high energy transthoracic and direct epicardial defibrillation can result in RV and LV myocardial damage, but little is known about the damage due to defibrillation using an endocardial RV electrode. Furthermore, disturbances in postdefibrillation oxidative metabolism have been reported and may be caused by primary injury of mitochondrial integrity and function, but information about ultrastructural mitochondrial alterations is rare. We therefore studied, in 13 fox hounds, RV and LV ultrastructural alterations following multiple low energy endocardial countershocks. Using an ICD and an endocardial defibrillation system a median of 54 (43-74) countershocks with a cumulative energy of 1,558 J (844-2,141 J) was delivered. After termination of countershocks, RV and LV myocardium was examined by electron microscopy. In both ventricles, severe myocardial alterations were found, including swollen mitochondria, disruption of mitochondrial crests, and loss of integrity of the mitochondrial inner and outer membranes. At the first time a semiquantitative score, originally developed for postischemic injury, was successfully used to grade the postcountershock mitochondrial alteration, which showed a more pronounced damage in the RV (2.69 +/- 0.22 points) compared to the LV (2.18 +/- 0.22 P = 0.021). We conclude that even the use of endocardial lead systems with low energy countershocks may lead to severe mitochondrial damage, especially in the RV.

Does an SVC electrode further reduce DFT in a hot-can ICD system?

Pectorally implanted ICDs that defibrillate with the RV electrode and the ICD housing have gained clinical acceptance. However, it is still debatable whether adding an SVC electrode connected to the housing will further reduce the threshold of defibrillation (DFT). This study utilized eight pigs. DFTs were measured with a 50 V step-down protocol starting at 650 V (20 J). Shock strength for 50% success (E50) was estimated with the average of three reversals. In addition to alpha dummy device, Lead I (Pacesetter Models 1558 and 1538) or Lead II (Endotak 72) were used. Leads I are active fixation, true bipolar sensing with 5-cm shocking coils. Lead II has an integrated bipolar sensing with a 4.7-cm RV and 6.9-cm SVC shocking coils. A 95 microF defibrillation system was used to deliver a 44% tilt tuned biphasic 1.6/2.5 ms waveform, and to measure lead impedance. The RV electrode was the anode during phase I. With Lead IRV-->CAN the DFT was 531 +/- 75 V (13.6 +/- 3.8J) and the E50 was 496 +/- 89 (12 +/- 4.3 J). These were not significantly (NS) different than the DFT for RV-->CAN and SVC which was 518 +/- 84 V (13 +/- 4.2 J) or the E50 which was 476 +/- 84 V (11 +/- 3.9 J). Similar results were obtained with Lead II. Despite a decrease in lead impedance there was no apparent benefit from the addition of the SVC electrode. Lead I provided equivalent DFT performance to Lead II.

Thoracotomy elevates the defibrillation threshold and modifies the defibrillation dose-response curve.

Despite innovations in nonthoracotomy defibrillation systems, thoracotomies are still required in some clinical settings and are utilized in many animal-based research protocols. The effect of a thoracotomy on defibrillation energy, however, has not been well characterized. Ten dogs in the immediate testing group underwent defibrillation testing immediately following a thoracotomy; another ten dogs in the delayed testing group were given 48 to 72 hours of recovery before defibrillation testing. A right ventricular endocardial coil to cutaneous thoracic patch biphasic system was used. At the time of defibrillation testing, the immediate testing group had a faster mean heart rate (144.7 +/- 30.2 vs 105.8 +/- 17.5 beats/min, P < 0.01), higher mean pulmonary artery pressures (systolic: 18.14 +/- 9.48 vs 11.28 +/- 6.46 mmHg, P = 0.1; diastolic: 6.59 +/- 2.88 vs 3.89 +/- 1.75 mmHg, P < 0.05), and higher mean defibrillation shock impedance (89.0 +/- 11.6 vs 70.9 +/- 7.3 omega, P < 0.002) than the delayed group. The mean ED50 (energy with a 50% success rate) was significantly higher in the immediate group than in the delayed group (26.9 +/- 14.9 vs 14.2 +/- 6.9 J, P < 0.05), and the slopes of the dose-response curves were significantly different (P = 0.03). In a right ventricular endocardial to cutaneous patch system, thoracotomy significantly and transiently increased the defibrillation threshold and modified the defibrillation dose-response curve.

Pacing threshold increase in nonthoracotomy implantable defibrillator leads: implications for battery longevity and margin of safety.

Just as a stable defibrillation threshold is required for implantable defibrillators to maintain efficacy and a margin of safety for the conversion of life-threatening ventricular arrhythmias, a stable pacing threshold is also required to provide bradycardia support and pacing to terminate ventricular tachycardias. This article reports the temporal course of pacing thresholds in patients treated with a tripolar, tined endocardial defibrillator lead capable of bipolar sensing and pacing, and defibrillation. Seventeen patients who underwent implantation of an implantable defibrillator system using an integrated bipolar pacing/sensing system were prospectively studied over 18 months. There were 16 males and one female, with a mean age of 69 +/- 5 years (range 61-75 years). At implantation, predischarge, and every 2 months thereafter, the pacing pulse-width threshold was tested at both 2.5 and 5.0 V stimulus amplitudes. After a mean follow-up of 363 +/- 173 days (range 34-597 days), the pacing threshold increased from 0.08 +/- 0.08 ms to 0.5 +/- 0.3 ms at the 2.5 V amplitude (p < or = 0.01, CI-0.57 to -0.27) and from 0.04 +/- 0.02 ms to 0.25 +/- 0.14 ms at the 5.0 V amplitude (p < or = 0.01, CI -0.28 to -0.14). Eight of the 17 patients (47%) received spontaneous implantable defibrillator shocks for clinically detected arrhythmias, and the total number of joules delivered via the leads did not correlate with the pacing threshold changes. We conclude that the pacing threshold for the nonthoracotomy implantable defibrillator lead system studied is not stable and increases with time. This finding has implications for defibrillator battery life in patients who use implantable defibrillators for bradycardia pacing.

Charge-burping theory correctly predicts optimal ratios of phase duration for biphasic defibrillation waveforms.

For biphasic waveforms, it is accepted that the ratio of the duration of phase 2 to the duration of phase 1 (phase-duration ratio) should be < or = 1. The charge-burping theory postulates that the beneficial effects of phase 2 are maximal when it completely removes the charge delivered by phase 1. It predicts that the phase-duration ratio should be < 1 when the time constant of the defibrillation system (tau s) exceeds the time constant of the cell membrane (tau m) but > 1 when tau s < tau m. This study tested the hypothesis that the optimal phase-duration ratio depends on tau s (the product of the defibrillator capacitance and pathway resistance). In a canine model of transvenous defibrillation (n = 8), we determined stored-energy defibrillation thresholds (DFTs) for biphasic waveforms from conventional capacitors (140 microF. tau s = 7.1 +/- 0.8 ms) and very small capacitors (40 microF. tau s = 2.0 +/- 0.2 ms). Each capacitance was tested with phase-duration ratios of 0.5, 1, 2, and 3. The duration of phase 1 approximated the optimal monophasic waveform, 6.3 +/- 0.7 ms for 140-microF waveforms and 2.8 +/- 0.2 ms for 40-microF waveforms. For 140-microF waveforms, the DFT was lower for phase-duration ratios < or = 1 than for phase-duration ratios > 1 (P = .0003). The reverse was true for 40-microF capacitors (P = .0008). There was a significant interaction between the effects of capacitance and phase-duration ratio on DFT (P = .0002). The lowest DFT for 40-microF waveforms was less than the lowest DFT for 140-microF waveforms (4.9 +/- 2.5 versus 6.4 +/- 2.4 J, P < .05). The optimal phase-duration ratio is < or = 1 for conventional capacitors and > 1 for small capacitors. This supports the predictions of the charge-burping theory.

A second defibrillator chest patch electrode will increase implantation rates for nonthoracotomy defibrillators.

Nonthoracotomy defibrillator systems can be implanted with a lower morbidity and mortality, compared to epicardial systems. However, implantation may be unsuccessful in up to 15% of patients, using a monophasic waveform. It was the purpose of this study to prospectively examine the efficacy of a second chest patch electrode in a nonthoracotomy defibrillator system. Fourteen patients (mean age 62 +/- 11 years, ejection fraction = 0.29 +/- 0.12) with elevated defibrillation thresholds, defined as > or = 24 J, were studied. The initial lead system consisted of a right ventricular electrode (cathode), a left innominate vein, and subscapular chest patch electrode (anodes). If the initial defibrillation threshold was > or = 24 J, a second chest patch electrode was added. This was placed subcutaneously in the anterior chest (8 cases), or submuscularly in the subscapular space (6 cases). This resulted in a decrease in the system impedance at the defibrillation threshold, from 72.3 +/- 13.3 omega to 52.2 +/- 8.6 omega. Additionally, the defibrillation threshold decreased from > or = 24 J, with a single patch, to 16.6 +/- 2.8 J with two patches. These changes were associated with successful implantation of a nonthoracotomy defibrillator system in all cases. In conclusion, the addition of a second chest patch electrode (using a subscapular approach) will result in lower defibrillation thresholds in patients with high defibrillation thresholds, and will subsequently increase implantation rates for nonthoracotomy defibrillators.

Incidence of lead system malfunction detected during implantable defibrillator generator replacement.

Implantable cardioverter-defibrillator (ICD) generator replacement due to a depleted battery is a frequently performed procedure. The frequency with which sensing and defibrillation system failures are identified during device replacement procedures has not been previously described. Therefore, the purpose of this study was to prospectively determine the frequency of lead system malfunction detected at the time of device replacement in 55 consecutive patients undergoing ICD generator replacement. The mean age of the patients was 63 +/- 10 years and 40 of them were men. Forty-nine patients had an epicardial lead system, and six patients had a nonthoracotomy lead system. Four [7%] of these 55 patients were noted to have previously undetected lead system failure, either sensing (n = 3) or defibrillation (n = 1), necessitating system revision. The lead systems that failed were 40 +/- 6 months old (33-49 months). In summary, during ICD generator replacement, previously undetected problems with sensing or defibrillation may be identified in approximately 10% of patients. Therefore, a comprehensive evaluation of the sensing and the defibrillation functions should be an essential component of the ICD generator replacement procedure.

Defibrillation thresholds with monophasic versus biphasic shocks delivered through a single-lead endocardial defibrillation system

Defibrillation thresholds with monophasic versus biphasic shocks delivered through a single-lead endocardial defibrillation system

Defibrillator Twiddler's Syndrome Causing Device Failure in a Subpectoral Transvenous System

Twiddler's syndrome is well described as a complication of cardiac pacing. Defibrillator twiddler's syndrome has been recently reported with abdominal implantations of epicardial and transvenous defibrillator systems. We report a case of a patient with a transvenous defibrillator system implanted with the pulse generator placed in the subpectoral plane. The patient developed twiddler's syndrome, which resulted in retraction of both leads. This caused inappropriate shocks due to sensing both the atrial and ventricular electrograms. While the subpectoral position leaves the generator deeper and more difficult for the patient to access, it may not lessen the chance of twiddler's syndrome. It is possible that the subpectoral position may actually predispose the patient to this malady.

Atrial fibrillation detection in humans using the METRIX™ atrial defibrillation system

Atrial fibrillation detection in humans using the METRIX™ atrial defibrillation system

Feasibility of METRIX™ atrial defibrillation system in humans with atrial fibrillation

Feasibility of METRIX™ atrial defibrillation system in humans with atrial fibrillation

Long-term stability of the defibrillation threshold with a pectoral unipolar defibrillation system

Long-term stability of the defibrillation threshold with a pectoral unipolar defibrillation system

Clinical predictors of defibrillation energy requirements in patients treated with a nonthoracotomy defibrillator system

Many factors can influence defibrillation energy requirements (DER) in patients with a nonthoracotomy defibrillator. No large studies, however, have correlated clinical characteristics with the DER. In this study, 124 patients underwent the same DER protocol with the identical biphasic waveform, nonthoracotomy lead system, and lead configuration. These patients were 63 ± 12 years old (mean ± SD); 99 were men; the ejection fraction was 0.32 ± 0.13 and 36 were taking an antiarrhythmic medication. New York Heart Association congestive heart failure class I was present in 28, class II in 70, and class III in 26 patients. Male sex (454 ± 94 V vs 406 ± 91 V for female sex) was associated with a significantly higher DER ( p = 0.02) and an increased risk of a DER >550 V ( p = 0.047). No other clinical variable was associated with the DER or a DEr >550 V. In conclusion, women tend to have lower DERs than men.