Younger patients with implantable cardioverter-defibrillators (ICDs), a history of atrial fibrillation (AF) and not on statins received a high number of inappropriate shocks, which was associated with a greater risk for mortality, according to a study in the Feb. 1 issue of the Journal of the American College of Cardiology.
Johannes B. van Rees, MD, of the Leiden University Medical Center in Leiden, the Netherlands, and colleagues evaluated 1,544 patients who had ICDs implanted between 1996 and 2006. Seventy-nine percent of patients were male, all had a median age of 61 and the mean follow-up period was 41 months. The occurrence of inappropriate ICD shocks and all-cause mortality were the primary endpoints.
“We discovered that in a large group of patients there was a relatively high number of inappropriate shocks, much higher than we expected,” senior study author Martin J. Schalij, MD, PhD, told Cardiovascular Business News.
In fact, during the five-year follow-up period, the incidence of inappropriate shocks rose to 18 percent. Thirteen percent of patients experienced a total of 665 ICD shocks during the mean 41-month follow-up period. The cumulative event rate for a first inappropriate shock during the first year was 7 percent, while the rate at three years increased to 13 percent.
A second inappropriate shock took place in 36 percent of the 204 patients who received a first inappropriate shock with an average time from first to second shock of 11 months.
Independent predictors of inappropriate shocks were a history of AF and a younger age (less than 70).
Schalij noted that younger patients may receive more inappropriate shocks due to the fact that they are more active and normal heart rates are higher than expected. “If they exercise they will have a higher heart rate and could receive an inappropriate shock because most of these devices look at heart rate,” he said.
Schalij noted that doctors must focus on programming the devices and tailoring them to specific patients, while industry should do more to develop algorithms that could help prevent these inappropriate shocks.
Experiencing a first inappropriate shock increased the risk of mortality by 60 percent.
The researchers also found that 76 percent of the inappropriate shocks were due to the misdiagnosis of supraventricular tachycardia. According to Schalij, this may be due to some devices not able to successfully discriminate between a supraventricular tachycardia and a ventricular arrhythmia.
Surprisingly, newer generations of ICDs were not better at reducing inappropriate shocks when compared to older devices, despite their more complex algorithms and features, Schalij said. He said that this complexity sometimes makes algorithms to prevent inappropriate shocks go unused.
“In the past we thought that these inappropriate shocks weren’t harmful; however, now we have found that they may increase mortality.”