Chronic Rise of Defibrillation Thresholds in Transvenous ICD Lead Systems
ABSTRACT & COMMENTARY
Synopsis: Biphasic shocks prevent chronic rises in defibrillation thresholds with transvenous ICD lead systems.
Source: Gold MR, et al. J Am Coll Cardiol 1997;30: 233-236.
Gold and colleagues describe the patterns of defibrillation threshold (DFT) change over time with monophasic vs. biphasic shocks in a group of patients with transvenous implantable cardioverter-defibrillator (ICD) systems. Twenty-four patients received a single model transvenous defibrillation lead (CPI Endotak C). This lead has two defibrillation coils that are placed in the right ventricular apex and in the right atrium or superior vena cava. Intraoperative DFT testing was performed using reversed polarity (i.e., right atrial coil as cathode, using both monophasic and biphasic waveforms). A step-down technique for DFT measurement was used. DFT measurement was then repeated 205 ± 105 days later using a similar protocol. There was a significant rise in DFT with monophasic shocks from 13.7 ± 6.0 J to 16.8 ± 6.7 J. Monophasic DFT increased in 15 of the 24 patients. In five patients, an increase to either 25 or 30 J was seen. Values this high might compromise defibrillation efficacy during clinical use even if maximum device output was programmed. There was no increase over time in mean DFT when biphasic shocks were used. The initial value, 10.2 ± 4.3 J was better than that seen with monophasic shocks and the chronic mean value remained stable at 10.2 ± 4.8 J. Although transvenous threshold increased in 8 of the 24 patients, no patient had a measured late DFT of more than 20 J. Shock impedance increased slightly over time with both waveforms. Gold et al conclude that biphasic shocks prevent chronic rises in DFT with transvenous ICD lead systems.
COMMENT BY JOHN P. DiMARCO, MD, PhD
This paper illustrates one of the reasons why transvenous ICDs that use biphasic waveforms are the current standard in virtually all electrophysiology laboratories. When an ICD is implanted, a DFT is measured and the device is then programmed to have at least a 10 J (or 100 volt) safety margin during clinical follow-up. Early ICDs employed a monophasic shock waveform, and the acute DFT was often marginal. Frequently subcutaneous patches or arrays were added to decrease impedance and lower the DFT. It was also noted that DFT would rise over time in a significant fraction of patients. In some cases, this DFT rise could eliminate the desired 10 J safety margin and revision of the device or lead would be necessary. Biphasic waveforms usually yield a lower acute DFT and, as shown in this paper, have a lesser tendency to have a DFT change over time. This has enhanced the safety and efficacy of ICD use and has minimized the need for later lead replacements or revisions.
One of the problems with papers on chronic features of ICD therapy is that they are usually outdated by the time they are published. The most popular ICD systems now on the market incorporate technology that allows the generator can to be part of the defibrillation pathway, either as a single anode paired with a right ventricular cathode or as a dual anode when linked to a right atrial or superior vena cava coil. Biphasic shock waveforms are virtually always selected. These newer systems have been shown to offer defibrillation characteristics superior to those seen with two transvenous coils. It is hoped that these systems will also have stable DFTs over long periods of follow-up. The observation by Gold et al that a biphasic waveform is the key to long-term DFT stability supports this hope.
Several minor questions are not addressed in this paper. Shock waveforms differ slightly between manufacturers, and it is hoped that the trend shown here can be applied to all devices. Gold et al don’t say why they routinely used reverse polarity. The newer systems that incorporate the generator in the shocking pathway typically will not allow this. The observation that at least a few patients had a rise in DFT of more than 5 J even with a biphasic waveform is somewhat worrisome and argues against aggressive programming to the lowest shock energy with a 10 J safety margin as routine practice. Finally, Gold et al excluded patients from the study if they experienced new cardiac events or required changes in antiarrhythmic drug therapy after the initial implant. Cardiologists following ICD recipients must remain aware that many factors can affect DFT, and periodic assessment of ICD function in the laboratory will still be required in many patients.
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