Case Study

Secondary-Prevention Patient with High DFTs: A Case Study for a High-Output ICD

Jo Ann LeQuang

Jo Ann LeQuang


With numerous implantable cardioverter-defibrillators (ICDs) available to the clinician, selecting the appropriate device for a given patient requires specific and individualized consideration. For example, if the patient requires a defibrillator but has no pacing indications, physicians might be rightfully hesitant about introducing an “extra” lead into the vasculature. This can be particularly important if the patient receives the initial implant at a relatively young age and will likely have an implanted device for the rest of his or her life. Selecting a device that allows patients access to magnetic resonance imaging (MRI) technology can also be an important option, as MRI has evolved increasingly into “the gold standard” for unparalleled soft tissue imaging with no X-ray exposure. It has been estimated that more than half of ICD patients will require an MRI at some point over the life of their implanted device.1 Responsiveness to therapy is another individual variant to consider. Younger patients are more likely to have higher defibrillation thresholds (DFTs) than older patients, requiring more energy to shock the heart back to normal sinus rhythm.2 In addition, it is crucial to consider whether the patient’s indication is primary or secondary prevention. Secondary prevention patients have a history of potentially lethal ventricular tachyarrhythmias at the outset of device therapy, and thus may approach device therapy with a different psychological mindset than primary prevention patients.

Case Report

A 40-year-old male with a history of sudden cardiac arrest was recommended for ICD therapy. The implanting physician decided to implant a subcutaneous ICD to avoid unnecessary hardware in the vasculature. The subcutaneous ICD offers defibrillation by relying on specific pulse generator placement in the chest and a subcutaneous lead placed in such a way as to create a shocking vector through the heart. The subcutaneous ICD lead is placed on the exterior of the heart, unlike a conventional ICD, which utilizes a defibrillation lead fixated inside the right ventricle. The subcutaneous lead has an electrode which analyzes the heart rhythm, in contrast to a conventional ICD that relies on intracardiac signals to detect individual beats. 

The implant proceeded uneventfully and the patient was to undergo DFT testing. The subcutaneous device provides rescue shocks at 10 to 80 Joules (J), in steps of 5 J. Rescue shocks are not programmable, but the manual shocks (e.g., for use in DFT testing) can be programmed to a specific energy value. The patient was induced to ventricular fibrillation (VF) and a manual shock was delivered, which failed to convert the patient. The clinical team then programmed the subcutaneous ICD to the maximum output of 80 J, but the device was still unable to defibrillate the patient’s heart. The patient was rescued using external defibrillation equipment. 

The implanting physician recognized that the device placement may have played a role, and found that by repositioning the ICD to a more posterior than lateral position, the shocking vector could then more efficiently direct energy to the left side of the heart. With the subcutaneous device in this position, defibrillation could be achieved with adequate safety margins, and the implantation procedure concluded. The patient returned home and had an uneventful recovery.

Over the course of the next year, the patient experienced four inappropriate shocks. On two occasions, he was shocked inappropriately while brushing his teeth; another inappropriate shock took place when he was mowing the lawn. The fourth inappropriate shock occurred while he was at rest. Although the patient understood the gravity of his condition, he found these inappropriate therapy deliveries to be very distressing, and he returned to the clinic to request that the device be explanted. The clinical team advised him that rather than explant the system, he should consider replacing the existing device with a new system. The patient learned about the new device and consented.

In selecting the revised device, the clinical team had to consider the patient’s high DFTs, his young age, and the ability of the device to accurately detect ventricular tachyarrhythmias so that he did not receive inappropriate shocks. The new device was a conventional implanted system with the highest available energy on the market, 45 J stored and 42 J delivered (Inventra DX ProMRI, BIOTRONIK). This ICD also offers MRI compatibility, and has a single lead to be placed inside the right ventricle with floating atrial dipoles to gather atrial diagnostic data along with ventricular pacing, sensing, and defibrillation. Moreover, this ICD allows the physician to program rescue shocks to maximum output for the first attempt — an important consideration for a patient with high DFTs.

With any ICD system, it is important to balance sensitivity with specificity. In this case, that meant assuring that the device detect every potentially dangerous episode of ventricular tachyarrhythmia without the possibility that the device deliver inappropriate shocks by identifying a benign rhythm as a dangerous one.

Inappropriate shocks may be delivered when the device detects a supraventricular tachycardia (SVT) and inappropriately interprets it as an episode of VT or VF. The Inventra DX ProMRI device is a single-lead system that can provide dual-chamber diagnostics. The system uses a special defibrillation lead that is affixed to the right ventricle with floating dipoles in the atrial blood pool that allow for atrial sensing. Atrial sensing allows for more reliable SVT detection, which can help prevent inappropriate shocks. Atrial diagnostics from these atrial dipoles can be downloaded in the form of dual-chamber intracardiac electrograms (IEGMs), making more accurate arrhythmia diagnosis possible for clinicians. Moreover, atrial diagnostic information can provide early detection of atrial fibrillation and other atrial arrhythmias. Thus, this relatively young patient could be spared the presence of a second lead in his vasculature without giving up the important benefits of dual-chamber diagnostics. Since this patient was extremely upset by his prior inappropriate shocks, the benefits of atrial diagnostics were of particular importance.

Other important features of this device were BIOTRONIK Home Monitoring® and MRI compatibility. Home Monitoring® allows for continuous monitoring of the implanted device and the ability for the device to self-report unusual or remarkable conditions directly to the patient’s clinical team, as well as automatic daily report transmission. The ProMRI feature made this system compatible with MRI scanning technology; MRI scans are contraindicated for some ICD patients.

The patient returned to the clinic, and the subcutaneous device was successfully explanted and the new device implanted. During DFT testing, the patient was induced into VF and then successfully defibrillated at 35 J. Although this is a high DFT, when the device was programmed to the maximum setting, it allowed the patient a 7 J safety margin. The device was programmed so that the first shock delivery would be at maximum output. The surgery concluded uneventfully, and the patient returned home and made a rapid recovery. Since the device revision, the patient has experienced no inappropriate shocks, and has overcome his malaise and wariness of device therapy.


High DFTs occur in a subset of all ICD patients. While risk factors for high DFTs have been described, it is impossible to use risk stratification to determine exactly which patients will or will not have high DFTs. In a retrospective analysis (n=1642 ICD patients), high DFTs have been correlated to younger age (<60 years), male gender, compromised left ventricular function, secondary prevention, and the use of amiodarone.3 This patient had three of those risk factors (younger age, male, secondary prevention), and it is thought that an accumulation of risk factors may elevate an individual’s risk. High DFTs are associated with complications and failed defibrillation. A survey of 12,397 ICD patients reported that 9.4% had inadequate safety margins, which in turn were associated with greater odds for complications (1.22 odds ratio, 95% confidence interval [CI], 1.09 to 1.37, P=.0006), hospitalization of more than three days (1.24 odds ratio, 95% CI, 1.19 to 1.30, P<.0001), and increased in-hospital mortality (1.96 odds ratio, 95% CI 1.63 to 2.36, P<.0001).4 

Device selection for an individual patient can be challenging and requires the consideration of multiple factors. The possibility of high DFTs is clearly an overriding concern, as successful defibrillation is the essential function of the device. The choice of an Inventra DX ProMRI system offered this patient several other benefits, including programmable first rescue shocks, MRI compatibility, and atrial diagnostics, which corrected the patient’s initial problem with inappropriate therapy. The latter was available in a single-lead system, minimizing the use of leads in the veins (one ventricular lead instead of a ventricular plus an atrial lead). 


This case study discussed a younger patient with high DFTs who required an ICD for secondary prevention and then elected for device revision when his initial subcutaneous system delivered four inappropriate shocks. Device selection must take into account the patient’s individual needs and requirements. Based on this patient’s high DFTs and young age, a device with high-energy outputs, atrial diagnostics in a single-lead system, and MR compatibility was a good choice for the device revision.

Disclosure: The author is with LeQ Medical, and discloses receiving a fee from BIOTRONIK for medical writing services for this article.


  1. Kalin R, Stanton M. Current clinical issues for MRI scanning of pacemaker and defibrillator patients. Pacing Clin Electrophysiol. 2005;28:326-328.
  2. 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.
  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. Hsu JC, Marcus GM, Al-Khatib SM, et al. Predictors of an inadequate defibrillation safety margin at ICD implantation: insights from the National Cardiovascular Data Registry. J  Am Coll Cardiol. 2014;64(3):256-264.

This article is published with support from BIOTRONIK, Inc.