Early changes in defibrillation threshold following implantation of a nonthoracotomy system in dogs

Effects of respiration phase on ventricular defibrillation threshold in a hot can electrode system

The impedance of defibrillation pathways is an important determinant of ventricular defibrillation efficacy. The hypothesis in this study was that the respiration phase (end-inspiration versus end-expiration) may alter impedance and/or defibrillation efficacy in a "hot can" electrode system. Defibrillation threshold (DFT) parameters were evaluated at end-expiration and at end-inspiration phases in random order by a biphasic waveform in ten anesthetized pigs (body weight: 19.1 +/- 2.4 kg; heart weight: 97 +/- 10 g). Pigs were intubated with a cuffed endotracheal tube and ventilated through a Drager SAV respirator with tidal volume of 400-500 mL. A transvenous defibrillation lead (6 cm long, 6.5 Fr) was inserted into the right ventricular apex. A titanium can electrode (92-cm2 surface area) was placed in the left pectoral area. The right ventricular lead was the anode for the first phase and the cathode for the second phase. The DFT was determined by a "down-up down-up" protocol. Statistical analysis was performed with a Wilcoxon matched pair test. The median impedance at DFT for expiration and inspiration phases were 37.8 +/- 3.1 omega, and 39.3 +/- 3.6 omega, respectively (P = 0.02). The stored energy at DFT for expiration and inspiration phases were 5.7 +/- 1.9 J and 6.0 +/- 1.0 J, respectively (P = 0.594). Shocks delivered at end-inspiration exhibited a statistically significant increase in electrode impedance in a " hot can" electrode system. The finding that DFT energy was not significantly different at both respiration phases indicates that respiration phase does not significantly affect defibrillation energy requirements.

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.

The effect of electrode size on transvenous defibrillation energy requirements: a prospective evaluation

Recent technological advances have resulted in high success rates for implantation of nonthoracotomy defibrillation lead systems. Further decreases in defibrillator size, facilitating pectoral placement, will depend in part on lowering defibrillation energy requirements. The purpose of this study was to determine if endocardial defibrillation energy requirements are influenced by electrode size. Thirteen adult mongrel dogs were studied under general anesthesia. A 9 Fr integrated bipolar pace/sense/defibrillation electrode (cathode) was positioned transvenously at the RV apex. The second defibrillation electrode (anode) was positioned at the junction of the RA and SVC. Two diameters of the proximal electrode, 7 Fr and 11 Fr, were sequentially tested in random order in each animal. The DFT for each electrode was determined using a 50-V up-down method. Energy, leading edge voltage, and current, current distribution, and total resistance were measured. The mean defibrillation voltage threshold with the 11 Fr proximal electrode was significantly less than with the 7 Fr proximal electrode (551.1 +/- 76.5 V vs 588.5 +/- 54.6 V, P < 0.01). Similarly, the mean DFT with the 11 Fr electrode was less than with the 7 Fr electrode (20.7 +/- 5.7 J vs 23.3 +/- 4.4 J, P < 0.01). Lower DFTs were found using the larger electrode in 11 of the 13 animals studied. However, there was no difference in defibrillation lead impedance between the two electrode systems. Endocardial defibrillation energy requirements may be lowered with a larger diameter proximal electrode. The mechanism by which this occurs may be due to a more even distribution of current gradients with the larger electrode. Determination of the optimal electrode size requires evaluation in humans, as this may allow further reduction in defibrillation energy requirements and defibrillator size.

Utility of predischarge and one-month transvenous implantable defibrillator tests

We evaluated the utility of performing predischarge implantable cardioverter-defibrillator (ICD) testing and ICD testing at 1 month in detecting ICD-related problems, identifying 4 patients with an unacceptable increase in defibrillation threshold. Given that ICD testing is noninvasive and has minimal (if any) associated risk, we recommend that predischarge and 1-month follow-up ICD testing should be performed after implantation.

[Influence of waveform and configuration of electrodes on the defibrillation threshold of implantable cardioverter-defibrillators]

The defibrillation threshold (DFT) is no threshold in the true sense. Between energy levels which defibrillate in all cases and energy levels which never defibrillate, a broad range of energies exists which might or might not defibrillate. Thus, the value of the DFT is dependant on the protocol used for its determination. Usually the DFT presents an energy at which the implantable cardioverter-defibrillator (ICD) will defibrillate successfully at a rate of approximately 75%. To achieve a 100% success rate the energy has to be programmed 15 J above the DFT or twice the DFT.Using DFT measurements the energy needed for internal defibrillation could be gradually reduced in the last years. Major break throughs have been the introduction of the biphasic defibrillation waveform and the use of pectorally implanted ICD shells as defibrillation electrodes. The shortening of the defibrillation impulse by the use of lower capacitances could not improve DFTs but allowed to construct ICDs of smaller volume. Addition of a superior vena cava electrode or a subcutaneous array electrode at the left lateral chest to the standard bipolar electrode system (right ventricle, pectoral ICD can) allowed for tri- and quadripolar lead configurations which reduced DFTs on average only slightly but reduced the standard deviation of DFTs significantly and thus helped to avoid high DFTs. Besides building smaller ICDs, reduction of DFTs and thus programming of lower defibrillation ICD energies allows for improved battery longevities and reduced capacitor charging times and thus a lower incidence of syncopes.

Circadian variation in defibrillation energy requirements

BACKGROUND

Reports have demonstrated a circadian variation in the incidence of acute myocardial infarction, ventricular arrhythmias, and sudden cardiac death. We tested the hypothesis that a similar circadian variation exists for defibrillation energy requirements in humans.

METHODS AND RESULTS

We reviewed the time of defibrillation threshold (DFT) measurements in 134 patients with implantable cardioverter-defibrillators (ICDs) who underwent 345 DFT measurements. The DFT was determined in 130 patients at implantation, in 121 at a 2 months, and in 94 at 6 months. All patients had nonthoracotomy systems. The morning DFT (8 AM to 12 noon) was 15.1 +/- 1.2 J compared with 13.1 +/- 0.9 J in the midafternoon (12 noon to 4 PM) and 13.0 +/- 0.7 J in the late afternoon (4 to 8 PM), P < .02. In a separate group of 930 patients implanted with an ICD system with date and time stamps for each therapy, we reviewed 1238 episodes of ventricular tachyarrhythmias treated with shock therapy. To corroborate the hypothesis that energy requirements for arrhythmia termination vary during the course of the day, we plotted the failed first shock frequency for all episodes per hour. There was a significant peak in failed first shocks in the morning compared with other time intervals (P = .02).

CONCLUSIONS

There is a morning peak in DFT and a corresponding morning peak in failed first shock frequency. This morning peak resembles the peaks seen in other cardiac events, specifically sudden cardiac death. These findings have important implications for appropriate ICD function, particularly in patients with marginal DFTs.

Early postoperative rise in defibrillation threshold in patients with nonthoracotomy defibrillation lead systems: attenuation with biphasic shock waveforms

INTRODUCTION

In patients with non-thoracotomy defibrillation lead (NTL) systems coupled with monophasic shock waveforms, the defibrillation threshold (DFT) rises early after implantation. There is little information regarding features predictive of the DFT rise, or DFT changes early after implantation of NTL systems coupled with biphasic shock waveforms.

METHODS AND RESULTS

DFT measurements were performed serially at implantation, prior to hospital discharge (mean 4 +/- 3 days), and at follow-up (mean 49 +/- 22 days) in 146 patients with an NTL system. Factors were assessed for association with a "clinically important" early postimplantation DFT rise, defined as a rise of > or = 2 energy steps (2 to 4 J per step; > or = 5 J total). A clinically important early postimplantation DFT rise occurred in 48 patients (33%). Univariate predictors of the rise included the monophasic shock waveform, the Medtronic Transvene lead system, the presence of a subcutaneous defibrillation patch, and the number of shocks delivered during the implantation procedure. However, the only independent predictor of a clinically important DFT rise was the monophasic shock waveform (F = 18, P < 0.001). For the monophasic patient group (n = 79), the incidence of a DFT rise was 53% (n = 42). For the biphasic patient group (n = 67), the incidence of a DFT rise was 9% (n = 6). The clinical characteristics of the monophasic and biphasic groups were not significantly different, nor were their DFTs at implantation. Among a subgroup of 18 consecutive patients who underwent serial DFT testing utilizing both monophasic and biphasic waveforms, the incidence of a clinically important DFT rise with monophasic (n = 9,50%) was higher than with biphasic shocks (n = 3,17%; P = 0.05).

CONCLUSION

NTL systems coupled with biphasic shock waveforms have an attenuated incidence of a clinically important DFT rise early after implantation, relative to patients with NTL systems coupled to monophasic waveforms.

Increase in defibrillation threshold in non-thoracotomy implantable defibrillators using a biphasic waveform

We have previously reported a chronic increase in defibrillation threshold in a non-thoracotomy implantable cardioverter-defibrillator (ICD) system using monophasic waveforms. To determine if this phenomenon is related to the lead system or the waveform used, we studied the chronic defibrillation threshold in consecutive patients receiving an ICD capable of delivering biphasic waveforms with the same lead system previously evaluated. Twenty-five patients received an ICD with biphasic shock waveform and have been followed for 4 to 15 months. All have undergone defibrillation threshold measurements using the identical testing protocol with biphasic waveforms at implant and at 2 months. Coronary artery disease was present in 15, idiopathic dilated cardiomyopathy in 9, and valvular heart disease in 1. The presenting arrhythmia was ventricular fibrillation in 11, ventricular tachycardia in 10, and syncope with inducible ventricular tachycardia in 4. The configuration of the shocking electrodes was randomized; the lead-only configuration was used in 14 patients (56%), and a subcutaneous patch was used in the remaining patients. Mean defibrillation threshold using a step-down technique was 9.8 +/- 1.0 J at implant, 13.2 +/- 1.6 J at 2 months, and 12.4 +/- 1.5 J at 6 months (p = 0.01 by analysis of variance). There was no change in clinical status, cardiac size, radiographic lead position, or impedance between implant and 2 months. These findings suggest the increase in defibrillation threshold in this ICD system is not related to the type of waveform used, but rather is a feature of non-thoracotomy as opposed to epicardial electrodes.