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Jonathan Lyne, Consultant Cardiologist, Mater Private Hospital, Dublin

Clinical Articles, Lead Article

Inappropriate ICD shocks – challenges and advances

Over 30 years ago the first implantable cardioverter defibrillator (ICD) was deployed in man. Since, millions of devices have been implanted worldwide and “innumerable lives have been saved”.1 There have been many challenges on the way and a number of significant limitations remain.

One disturbing complication of ICD treatment has been the problem of inappropriate shocks. Such inappropriate electrical discharges result in multiple adverse effects including impaired quality of life, psychiatric disturbances, and even provocation of nonfatal and fatal ventricular arrhythmias.2 In the MADIT II study2 83 (11.5%) of the 719 MADIT II ICD patients experienced inappropriate shocks, representing some 31.2% of the overall shock episodes. Atrial fibrillation was the most common trigger of inappropriate shocks (44%) followed by supraventricular tachycardia (36%) and abnormal sensing (20%). Inappropriate shock occurrence was associated with increased probability of mortality in follow-up.

Inappropriate shocks increase death risk

Another recent study4 (See Arrhythmia Watch, February 2011 for full report) by a Dutch research team from Leiden University Medical Centre asserts that despite the benefits of ICD therapy inappropriate shocks from these devices are “common” and place patients at a greater risk of arrhythmia and associated mortality. The study examined the incidence, predictors, and patient outcomes of inappropriate ICD shocks in a large, real-world patient population.


Dr Jonathan Lyne

Seeking to evaluate ICD implantation outside the setting of clinical trials, the follow-up period was 41 months (±18 months), but they also recorded data at one year and five years. Of the 1,554 patients who had complete records, 13% (204) experienced an inappropriate shock during the main follow-up period, resulting in a total of 665 inappropriate shocks. The cumulative incidence of inappropriate shocks “steadily increased” across the extended follow-up period, reaching 18% of patients at 5 years. In what the authors considered “the most important finding,” patients who experienced inappropriate shocks were at a greater risk of all-cause mortality. A total of 298 (19%) patients died during  follow-up, and after adjusting for potential confounders, the research team found a 60% increased risk of death after experiencing a first inappropriate shock (hazard ratio [HR]: 1.6, 95% CI: 1.1 to 2.3; p = 0.01). The risk of mortality increased with each subsequent inappropriate shock, up to an HR of 3.7 after 5 inappropriate shocks.

Several factors independently predicted the occurrence of an inappropriate shock, including age <70 years; history of atrial fibrillation, underlying nonischaemic heart disease, or nonuse of statins; and the occurrence of appropriate shocks. The main cause of inappropriate shocks was a failure to discriminate supraventricular tachycardia, which occurred in 155 (76%) patients. According to study author Dr Martin J. Schalij the study’s finding that inappropriate ICD shocks impact mortality is a “serious issue” which necessitates “that greater efforts be made to lower the number of these shocks.” He noted that while two other analyses—both conducted as part of ICD clinical trials—have shown an association between inappropriate shocks and increased mortality, this study is the first to do so in a large, general-patient population.

Reducing the number of inappropriate shocks may prove challenging. The researchers found that despite improved technology in ICDs, patients who underwent implantation between May 2004 and 2006 were at a greater risk of experiencing inappropriate shocks than those who received their ICD between 1996 and May 2004. Acknowledging this “paradox,” the authors wrote that evolving guidelines on who could receive ICDs may have caused more critical patients to receive the device in later years, potentially increasing the number of inappropriate shocks experienced. However numerous other factors may also have had a significant role in this finding.

Although the researchers could not determine the exact cause of the increase, still more must be done to reverse this trend, noted Dr Schalij. “It is not acceptable that so many patients suffer from inappropriate shocks,” he stated. “ICD therapy must be improved, through both patient-tailored programming of the devices and the development of superior algorithms to allow ICDs to better recognise false alarms, such as supraventricular arrhythmias”.

New ‘smart’ devices

Data on the new Protecta™ portfolio of ICDs and cardiac resynchronisation therapy-defibrillators (CRT-Ds) which aim to reduce the incidence of inappropriate shocks, were presented at Cardiostim 2010.4 These implantable defibrillators feature SmartShock™ Technology, comprising six advanced shock-reduction algorithms that can apparently reduce inappropriate shocks significantly. Findings from the Virtual ICD study utilising a statistical model predicted 98% of studied patients with SmartShock Technology would be free of inappropriate shocks one year after implant and 92% at five years post-implant.5

Further data released at Cardiostim from the Managed Ventricular Pacing (MVP) trial also show that 24% of patients who received anti-tachycardia pacing (ATP) to terminate a potentially life-threatening arrhythmia visited the hospital, clinic or emergency room at least once within three days of receiving therapy, versus 56% of patients who were treated with appropriate shock therapy, and 61% of patients who received an inappropriate shock. This shows shock therapy may correlate with greater health care utilisation and associated costs compared to ATP, the manufacturer suggests.

Additionally, new data released on SmartShock Technology showed lead fracture and T-wave oversensing algorithms can be safely applied without delaying the detection of life-threatening arrhythmias. Major shock reduction clinical trials (Shock-Less, PREPARE, PainFree Rx I and II, WAVE, ADVANCE III and EMPIRIC) will provide further data in more than 5,500 patients worldwide. This data is keenly awaited as are any further advances in the reduction of inappropriate shocks which may spare patients unnecessary anxiety and may prevent the increased mortality demonstrated by this device complication.


Jonathan Lyne, Consultant Cardiologist

Mater Private Hospital, Dublin


1 Maisel WH, Kramer DB. Implantable cardioverter-defibrillator lead performance. Circulation 2008;117:2721-3.

2 Daubert JP, Zareba W, Cannom DS et al. Inappropriate implantable cardioverter-defibrillator shocks in MADIT II. J Am Coll Cardiol 2008;51:357-65.

3 Inappropriate Implantable Cardioverter-Defibrillator Shocks: Incidence, Predictors, and Impact on Mortality, van Rees, JB, et al, J Am Coll Cardiol 2011;57:556–62.

4 Medtronic press release

5 Volosin et al. Virtual ICD: A Model to Evaluate Shock Reduction Strategies. Heart Rhythm. Vol. 7, N. 5, May supplement 2010. (PO3-125).

Published on: September 28, 2011

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