A 29-year-old woman presented with nonischemic dilated cardiomyopathy (diagnosed 5 years prior), atrial flutter, and persistent atrial fibrillation (AF). While using a wearable defibrillator (LifeVest, ZOLL Corporation), she had two episodes of rapidly conducted AF that required her to manually abort therapy. She was placed on amiodarone and underwent cardioversion of AF. A week later, she sustained an episode of ventricular fibrillation (VF) and had a successful defibrillation by the LifeVest. She was admitted to the hospital following the LifeVest shock, and a transvenous defibrillator implant was planned for secondary prevention of cardiac arrest. Due to her history of atrial arrhythmias and her younger age, she received an Inventra 7 VR-T DX (BIOTRONIK, Inc.). This device was selected because it is a single ventricular lead system that offers atrial diagnostics in addition to normal arrhythmia sensing and discrimination offered by other single-chamber ICD models. During the ICD implant, she failed defibrillation threshold (DFT) testing at 24 Joules (J), but was successfully defibrillated at 28 J. The maximum energy delivered by the BIOTRONIK ICD is 42 J, leaving an acceptable safety margin. She was discharged home after an otherwise uneventful recovery.
Five months after the device implantation, she presented to the Emergency Department after having received a series of ICD shocks. Device interrogation was performed; her therapy zone programming and shock history are shown in Figure 1. Her baseline rhythm had been predominantly AF (Figure 2) since implant, as she was unable to undergo elective cardioversion due to the presence of left atrial thrombus that was newly diagnosed after a transesophageal echocardiogram. The intracardiac electrograms (IEGMs) show the arrests were due to VF, and she received five appropriate shocks. Four of the shocks failed to convert her out of VF, but the fifth shock was successful (Figure 3).
Her electrolytes were within normal ranges, and she had no signs of cardiac ischemia. Her amiodarone was discontinued in favor of sotalol. Due to the failed defibrillator shocks, a defibrillator coil was placed in the azygos vein and attached to the SVC port on the existing ICD generator (Figure 4). In the EP lab, VF was induced and successfully defibrillated with a 25 J shock (Figure 5). However, AF persisted despite multiple shocks. She was discharged on anticoagulation and sotalol in addition to therapy for chronic heart failure.
This case highlights several points relevant to device selection for this patient. Minimizing leads in the vasculature while maintaining atrial diagnostic information were important due to the patient’s young age and concomitant atrial arrhythmias. Additionally, she has several risk factors for having high defibrillation threshold, and this particular case highlights the importance of high-energy shocks.
DFT is defined as the minimum amount of energy required to reliably defibrillate the heart; it is a probabilistic value. High DFTs have been described as >25 J or ≤10 J of the maximum output of the implanted device. High DFT incidence varies, but occurs in 5% to 10% of ICD implants.1-4 High DFTs may be observed without overt risk factors or indicators, but are seen more frequently with younger age, severely reduced EF (<20%), male gender, and secondary prevention implant.5 Furthermore, high DFTs can be dynamic and change with disease progression, concomitant arrhythmias, pharmacological therapy, and other factors. High DFTs are more frequent in initial implants versus revisions and in younger rather than older patients.2,5 While these risk factors can identify patients who are more likely to have a high DFT, they have not been used to stratify individual patient risk.
Nonischemic dilated cardiomyopathy (DCM) is associated with high DFTs. In a study of 313 patients with an initial ICD implant, high DFTs occurred in 44% of the patients with nonischemic DCM compared to 30% of controls (P=0.03).1 Large left ventricular mass is an independent predictor of high DFTs, but only accounts for 5% of the variability in DFT testing.4
This patient has persistent AF, which is not shown to have a statistically significant association with high DFTs, but is a frequent comorbid condition and is seen in patients with enlarged left ventricular mass and left ventricular dysfunction. While AF in and of itself is not a known predictive factor for high DFTs, congestive heart failure (CHF) does have a significant association. In the study by Verma et al, 75% of those with high DFTs had a prior hospitalization for CHF versus just 21% of control patients (P=0.001). In that same study, it was found that 62% of patients with high DFTs were New York Heart Association (NYHA) Class III or IV (associated with more advanced forms of heart failure) versus 30% of controls (P=0.01).1
Managing patients with high DFTs can be challenging. Routine DFT testing at implant is controversial, as the majority of patients will not have high DFTs and the testing procedure may cause morbidity.7-9 When the DFT is close to the implanted device’s maximum output, the odds of unsuccessful defibrillation are higher regardless of proper detection. Stepwise approaches to the management of high DFT have been proposed, and once treatable factors (such as medication changes, treatment of pneumothorax) have been corrected, device interventions can be considered. These include reprogramming the SVC coil out of the shock vector in dual-coil systems, changing to a high-energy can, RV lead revision, repositioning of an SVC coil in dual-coil systems, or addition of a second coil in single-coil systems. Due to this patient’s cardiac characteristics, a high-energy device was selected for the initial implant, and due to the device configuration, the addition of a coil could be accomplished in a fairly straightforward manner. About 30% of ICD patients die of sudden cardiac arrest,10 and in many cases, this results from the device’s failure to defibrillate the heart.11 This makes a strong case for implantable devices with high-energy outputs, such as the device selected for use in this patient.
This case study reports on the management of high DFT in a young patient who required five maximum-output shocks at 42 J to terminate her clinical arrhythmia. In younger patients with concurrent atrial and ventricular arrhythmias, a single-chamber system with an atrial sensing bipole is advantageous to minimize hardware in the vasculature. The Inventra combines that diagnostic ability with high-energy defibrillation and is valuable for treating patients with high DFTs. The addition of an azygos coil was needed in this patient, highlighting management options in patients with clinically significant high DFTs.
Disclosures: The authors have no conflicts of interest to report regarding the content herein. The authors acknowledge medical editing support from Jo Ann LeQuang of LeQ Medical.
- Verma A, Kaplan AJ, Sarak B, et al. Incidence of very high defibrillation thresholds (DFT) and efficacy of subcutaneous (SQ) array insertion during implantable cardioverter defibrillator (ICD) implantation. J Interv Card Electrophysiol. 2010;29(2):127-133.
- Russo A, Sauer W, Gerstenfeld E. Defibrillation threshold testing: is it really necessary at the time of implantable cardioverter-defibrillator insertion? Heart Rhythm. 2005;2:456-461.
- Osswald BR, De Simone R, Most S, Tochtermann U, Tanzeem A, Karck M. High defibrillation threshold in patients with implantable defibrillator: how effective is the subcutaneous finger lead? Eur J Cardiothorac Surg. 2009;35(3):489-492.
- Hodgson DM, Olsovsky MR, Shorofsky SR, Daly B, Gold MR. Clinical predictors of defibrillation thresholds with an active pectoral pulse generator lead system. Pacing Clin Electrophysiol. 2002;25(4 Pt 1):408-413.
- Shih MJ, Kakodkar SA, Kaid Y, et al. Reassessing Risk Factors for High Defibrillation Threshold: The EF-SAGA Risk Score and Implications for Device Testing. Pacing Clin Electrophysiol. 2016;39(5):483-489.
- Quin EM, Cuoco FA, Forcina MS, et al. Defibrillation thresholds in hypertrophic cardiomyopathy. J Cardiovasc Electrophysiol. 2011;22(5):569-572.
- Gold M, Kroll M, Ellenbogen K. Defibrillation testing at ICD implantation: are we asking the wrong question? Pacing Clin Electrophysiol. 2009;35:567-569.
- Viskin S, Rosso R. The top 10 reasons to avoid defibrillation testing during ICD implantation. Heart Rhythm. 2008;5:391-393.
- Birnie D, Tung S, Simpson C. Complications associated with defibrillation threshold testing: the Canadian experience. Heart Rhythm. 2008;5:387-390.
- Anderson K. Sudden cardiac death unresponsive to implantable defibrillator therapy: an urgent target for clinicians, industry and government. J Interv Card Electrophysiol. 2005;14:71-78.
- Mitchell L, Pineda E, Titus J, Bartosch P, Benditt D. Sudden death in patients with implantable cardioverter defibrillators: the importance of post-shock electromechanical dissociation. J Am Coll Cardiol. 2002;39:1323-1328.