Despite proven survival benefits of implantable cardioverter defibrillators in patients with ventricular tachycardia and ventricular fibrillation, device-related shocks remain a problem and are associated with an increased risk of mortality, according to study findings in the Feb. 1 Journal of the American College of Cardiology.
Dr. Johannes B. van Reese and his colleagues at Leiden (the Netherlands) University Medical Center sought to evaluate the occurrence of inappropriate shocks, identify parameters that may predict their occurrence, and assess their long-term impact. The investigators analyzed medical records from 1,658 patients at the center who received an implantable cardioverter defibrillator (ICD) system equipped with intracardiac electrogram storage between 1996 and 2006.
A total of 114 patients were lost to follow-up; 56% of the remaining 1,544 patients received an ICD for primary prevention and 64% had ischemic heart disease.
During 41 months of follow-up (±18 months), 204 patients (13%) had a total of 665 inappropriate shocks. The cumulative incidence of inappropriate shocks "steadily increased" across the extended follow-up period, reaching 7% at 1 year, 13% at 3 years and 18% at 5 years (J. Am. Coll. Cardiol. 2011;57:556-62).
Several factors independently predicted the occurrence of inappropriate shocks, including age less than 70 years, a history of atrial fibrillation or nonischemic heart disease, or nonuse of statins. The main cause of inappropriate shocks was misdiagnosis of supraventricular tachycardia, which occurred in 155 (76%) of the 204 patients.
There was no significant difference in the occurrence of inappropriate shocks among different types of devices. However, patients with a single-chamber ICD received significantly more shocks because of misdiagnosis of sinus tachycardia than did patients with a dual-chamber ICD (24% vs. 8%). Also, patients who had a defibrillator for cardiac resynchronization therapy tended to experience more inappropriate shocks as a result of abnormal sensing than did ICD recipients with a single-chamber ICD (15% vs. 8%), the investigators found.
Despite improved technology in ICDs, the researchers found that patients who underwent implantation between May 2004 and 2006 were at a greater risk of experiencing inappropriate shocks than were those who received their ICD between 1996 and May 2004. The authors said that evolving guidelines on who could receive ICDs may have caused more critical patients to receive the device in later years, ultimately increasing the number of inappropriate shocks experienced.
In what the authors considered "the most important finding," patients who experienced inappropriate shocks had a higher risk of all-cause mortality. Specifically, 298 (19%) patients died during follow-up, and after adjusting for potential confounders, the researchers found a 60% increased risk of death after a first inappropriate shock. The risk of mortality increased with each subsequent inappropriate shock, from a hazard ratio of 1.6 after the first shock to 3.7 after five inappropriate shocks.
Dr. Martin J. Schalij, one of the study’s authors, described these findings as a "serious issue" necessitating "that greater efforts be made to lower the number of these shocks," he said in a statement.
Although two analyses from ICD clinical trials also found an association between inappropriate shocks and increased mortality, Dr. Schalij noted that this current study is the first trial to do so in a large, general patient population.
The researchers could not determine the cause of the increased mortality associated with the shocks. However, Dr. Schalij, said that more should be done to reverse this trend. "It is not acceptable that so many patients suffer from inappropriate shocks. ICD therapy must be improved, through both patient-tailored programming of the devices and the development of superior algorithms to allow ICDs to better determine false alarms, such as supraventricular arrhythmias."
The authors reported receiving grants from Biotronik, Boston Scientific, Bristol-Myers Squibb Medical Imaging, Edwards Lifesciences, GE Healthcare, Medtronic, and St. Jude.