Case Study

Managing a Young Patient with High Defibrillation Thresholds

Rajeev Joshi, MD
North Texas Heart Center
Dallas, Texas

Rajeev Joshi, MD
North Texas Heart Center
Dallas, Texas

Selecting the optimal device for primary prevention patients has to be a carefully thought out process, particularly when the patient is relatively young with a larger body habitus. Selecting a state-of-the-art ICD with high-output capability and a complete range of diagnostics becomes a crucial factor. 

In this case study, we describe a 51-year-old male patient brought in for ICD implantation with a primary prevention indication. Device selection had to take into account advanced features, high output, complete diagnostics, and remote monitoring capabilities. Since the patient was relatively young, a device that did not require multiple leads was preferable. We chose the Inventra 7 VR-T DX® (BIOTRONIK, Inc.).

The Inventra 7 VR-T DX offers high-output defibrillation therapy along with single-chamber physiologic rate-adaptive pacing and atrial tracking. The device is implanted with a single-pass lead that is fixated in the right ventricle, but two atrial sensing electrodes allow for atrial diagnostic reporting. Like the other devices in the Inventra family, the Inventra VR-T DX offers BIOTRONIK Home Monitoring®. Inventra devices also offer programmable shock therapy, which can be set up into three zones (VT-1, VT-2, and VF). The first two shocks in any zone can be programmed, and the remaining shocks in that zone (up to eight) are delivered at the maximum programmable energy of 45 Joules (41.5 J delivered). 

Since our patient had no pacing indication, this high-output system was preferred because of the patient’s larger body habitus and other factors that could influence the patient’s DFTs over time, including that certain antiarrhythmic agents may elevate thresholds.1,2 A retrospective analysis of 1,642 consecutive ICD patients who underwent DFT testing found five significant predictors for high DFT: younger age (defined as under 60 years), male gender, compromised left ventricular (LV) function, amiodarone use, and secondary prevention. Interestingly, older patients were less at risk for high DFTs; in fact, every 10-year increment in age reduced the chances for high DFT by 0.35 times.3 A study of 168 ICD patients undergoing DFT testing found on univariate analysis that high DFTs could also be correlated with LV end-diastolic diameter, LV ejection fraction, and the impedance of the defibrillation lead.4 While risk factors can be statistically identified, it is not always possible to state which particular patient will experience high thresholds, which can occur for unknown reasons as well.

In our patient, the device was implanted successfully with no complications or difficulties. DFT testing was conducted, and the device was programmed to deliver an initial shock at 28 J with the second follow-up shock at a maximum output of 45 J. Ventricular fibrillation (VF) was induced using shock-on-T induction. VF was appropriately detected by the device, and the device charged and delivered an initial 28-Joule shock, which failed to terminate VF. A second high-energy shock restored the patient to normal sinus rhythm. 

Having a high-output system proved useful for this round of DFT testing, as the first shock would not have defibrillated the patient. It was then decided to adjust the biphasic waveform to determine if the patient might successfully be defibrillated at a shock of around 30 J. Adjustment of the biphasic waveform may help improve defibrillation efficacy by reducing thresholds.5 As noted previously, Inventra devices offer programmable biphasic waveforms. Defibrillation energy is delivered in a waveform consisting of two phases. In a conventional system, the first shock starts with 100% of the initial charging voltage, then the switching voltage (which commences phase 2 of the waveform) starts at 40% of the initial voltage, and terminates with the cutoff voltage (20% of the initial voltage). Phase 1 (sometimes called “tilt”) receives about 60% of the total shock energy, and phase 2 gets about 50% (also called tilt), resulting in a waveform called a “60/50 fixed-tilt, voltage-controlled biphasic shock.”  This can be programmed to biphasic II, in which the initial charging voltage is 100%, the switching voltage is 40%, and the cutoff voltage is time controlled rather than voltage controlled and set to 2 ms. Many factors can result in an elevated DFT, including the use of certain class III antiarrhythmic agents. In a study of 62 defibrillation patients (mean age 54±13 years, ejection fraction 43%±17%), the use of biphasic 2 shocks significantly reduced the defibrillation threshold of patients treated with amiodarone (10.7±4.9 J vs 13.4±5.6 J; P<0.0001) or sotalol (6.1±3.3 J vs 9.1±4.6 J; P<0.05).2

After waiting about five minutes, the patient was again induced into VF and the device detected, charged, and delivered a 30-Joule shock that successfully converted the patient. This success allowed for a good safety margin for the patient. Testing was concluded and the pocket was closed in three layers. 

At this point, it could be seen on the atrial electrogram that the patient had developed atrial fibrillation (AF). The atrial diagnostics of the Inventra 7 VR-T DX allowed for the episode of AF to be promptly and accurately detected. External electrical cardioversion was performed, which restored normal sinus rhythm.

Upon conclusion of the procedure and subsequent recovery from anesthesia, the patient and their family member were instructed about CardioMessenger Smart (BIOTRONIK, Inc.). They were informed that the device was set up to transmit daily and provide reports on atrial arrhythmias, defibrillation shocks, and relevant device information (such as lead impedance values falling out of range). Because of this system, the required post-discharge device check before patient discharge from the hospital is fully automated. The device can be programmed to run through the appropriate tests based on a clock-time setting, eliminating the need to have a representative or technical support person be present to use a programmer for testing before the patient is released. Therefore, discharge is faster, easier, and more streamlined.

Finally, the patient and his family were instructed in the use of BIOTRONIK Home Monitoring. The patient was shown how to set up the device by plugging it in near his bed and how to use it on his first night at home. This ensured that the first transmission would occur promptly, and that the physician and sales representative could confirm that the BIOTRONIK Home Monitoring system was set up properly and that the device was functioning correctly. The patient was discharged the same day of the procedure. The next day, the BIOTRONIK Home Monitoring system successfully transmitted all testing numbers, including threshold values, and confirmed appropriate device function.

Disclosures: Dr. Joshi has no conflicts of interest to report regarding the content herein. The author acknowledges medical editing support from Jo Ann LeQuang of LeQ Medical. 


  1. Apps A, Miller CP, Fellows S, Jones M. Cardiac devices with class 1C antiarrhythmics: a potentially toxic combination. BMJ Case Rep. 2015 Aug 18;2015.
  2. Merkely B, Lubinski A, Kiss O, et al. Shortening the second phase duration of biphasic shocks: effects of class III antiarrhythmic drugs on defibrillation efficacy in humans. J Cardiovasc Electrophysiol. 2001;12(7):824-827.
  3. 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.
  4. Lubinski A, Lewicka-Nowak E, Zienciuk A, et al. Clinical predictors of defibrillation threshold in patients with implantable cardioverter-defibrillators. Kardiologia polska. 2005;62(4):317-328; discussion 329-331.
  5. Gabriels J, Budzikowski AS, Kassotis JT. Defibrillation waveform duration adjustment increases the proportion of acceptable defibrillation thresholds in patients implanted with single-coil defibrillation leads. Cardiology. 2013;124(2):71-75.