Often during the insertion of an implantable cardioverter-defibrillator (ICD), testing is performed to confirm that the devices can in fact detect and terminate ventricular fibrillation (VF). While true defibrillation threshold testing calls for repeated VF induction while decrementing shock strength for defibrillation until failure occurs, many operators opt to demonstrate the termination of VF at least once by a shock energy greater than ten Joules below the device’s maximum programmable output. Those that advocate testing termination of VF by the ICD argue that even with the technological advances that provide higher output devices, an inadequate traditional ten Joule safety margin may still occur.

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Given that physicians who implant ICDs have an obligation to determine whether a selected lead configuration can terminate VF, a study published July 14 in the Journal of the American College of Cardiology developed a simple risk score to help predict an inadequate defibrillation safety margin (DSM) for ICDs, and found an inadequate DSM to be “associated with an increased risk for in-hospital adverse events.”

The study, led by Jonathan Hsu, MD, MAS, Division of Cardiology, University of California, San Diego, analyzed ICD Registry implants between 2010 and 2012. Making use of logistic regression models, Hsu and his co-authors identified factors most predictive of an inadequate DSM (defined as the lowest successful energy tested less than ten Joules from maximum device output) and evaluated the association of an inadequate DSM with adverse events. Among the 132,477 implants analyzed, an inadequate DSM occurred in 12,397 patients (9.4 percent). Identifying several patient characteristics that predicted an inadequate DSM, Hsu and his colleagues developed a simple risk score utilizing eight pre-procedural variables to help distinguish those ICD recipients at higher and lower risk. These characteristics included age, gender, race, New York Heart Association class, no ischemic heart disease, current renal dialysis, indication for secondary prevention, and whether the ICD was a single chamber device or cardiac resynchronization therapy with a defibrillator. After multivariable adjustment for potential confounders, an inadequate DSM was associated with more than 20 percent greater odds of procedural complications and prolonged hospital stay and almost two-fold greater odds of in-hospital mortality.

The authors note that “this simple risk score may identify ICD recipients in whom defibrillation threshold testing is most likely to change clinical management, therefore distinguishing those who most warrant this testing.” They add that moving forward, “future studies are necessary to evaluate the mechanisms underlying the association between an inadequate DSM and adverse events and whether defibrillation threshold testing itself is responsible for the adverse outcomes observed.”

In a commenting editorial Bruce Lindsay, MD, FACC, Cleveland Clinic Foundation, notes, “While a scoring system like this is imperfect, it would allow some risk estimation that could guide physicians on the need for testing, appropriate precautions, and the possibility that a subcutaneous coil might be required … Based on these findings, some low volume centers might choose to refer patients wither higher risk scores to a more experienced center for the implant procedure.”

Keywords: National Cardiovascular Data Registries, ICD Registry

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