More strategy, fewer shocks

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A study with nearly 89,000 patients from more than 2,500 centres reveals that strategic programming of implantable cardioverter defibrillators can reduce the number of inappropriate shocks and improve survival rate and quality of life.

Results of a new study presented in May at the Heart Rhythm Society 2010 Scientific Sessions, in Denver, USA, showed that more strategic programming of implantable cardioverter defibrillators (ICDs) and cardiac resynchronisation therapy devices with defibrillation (CRT-Ds) would significantly reduce morbidity from shocks. By programming variables, such as faster ventricular tachycardia/ventricular fibrillation (VT/VF) thresholds and longer detection durations, overall shock incidence was reduced to between 17% and 28%, according to the registry’s investigators.


The study also identified programming and patient characteristics that increased the risk of shocks. The observational study involved 88,804 patients from more than 2,500 centres. “Improving programming by using evidence-based shock reduction strategies can significantly reduce shocks to patients,” said lead author Bruce Wilkoff, Cleveland Clinic, Cleveland, USA, who presented the data in a late-breaking clinical trials session. “Most importantly, strategies to minimise shocks may further improve survival and quality of life in ICD patients.”

An observational cohort analysis was performed on a de-identified database from the Medtronic CareLink Network. Average follow-up was over two years. Patients were included in the study if they had a CRT-D (42%) or dual-chamber ICD (58%). Four shock reduction strategies as well as clinical characteristics were reviewed in the study: Slowest VT/VF detection threshold, VF number of intervals to detect, supraventricular tachycardia discriminators ON, antitachycardia pacing ON for fast VTs and atrial fibrillation (AF) with rapid ventricular response, gender, age, ICD type, ICD replacement. The primary endpoint was number of spontaneous all-cause shocked episodes per 100 patient-years.

Wilkoff told delegates that strategic programming of faster VT/VF detection thresholds, longer detection durations, supraventricular tachycardia discriminators, and antitachycardia pacing for fast ventricular tachycardia reduced shocks. “Clinical actions to reduce morbidity from shocks should include ensuring adequate rate control for patients with AF, as well as programming to increase the VT/VF detection rate and duration thresholds,” he said.

The 88,804 patients had 2.5 ± 1.3 years of follow-up, 75% were male, 67 ± 12 years old, 33% had a replacement device, and 12% had AF with rapid ventricular response. At baseline, slowest VT/VF detection was equal to or greater than 188bpm in 33% of the patients, 168–187bpm in 22%, 151–167bpm in 21%, and less than or equal to 150bpm in 23%; VF number of intervals to detect was 12/16 in 37% of the patients, 18/24 in 59%, and 24/32 or 30/40 in 3%; 84% of patients had supraventricular tachycardia discriminators ON; and 86% had antitachycardia pacing ON.


There were 19,458 patients (22%) who had a total of 72,239 shock episodes. After adjusting for all variables, the investigators showed that a slower VT/VF rate detection threshold was associated with a 21–148% increase in shocks, depending on the rate of detection. There was also an increase in shocks, with VF number of intervals to detect 12/16 (55%), patients aged less than 70 years (23–57%), AF (38–244% with rapid ventricular response), and patients with replacement devices.

VF with number of intervals to detect 24/32 or 30/40 was associated with 17% fewer shocks. Also associated with fewer shocks were supraventricular tachycardia discriminators ON (-22%), antitachycardia pacing ON (-28%), females (-19%), and patients with CRT-D devices (-23%).

AF has a dramatic impact on the incidence of shocks and is dependent on the rapid ventricular rate (RVR) observed during AF, Wilkoff said. AF with RVR increases shock risk by 244% compared to patients without AF. However, with remote interrogation alerts, it is possible to identify patients at risk for future shocks, since there often is a delay between the onset of the AF with RVR and the shock.


Based on the findings of this trial, clinical actions to reduce morbidity from shocks should include ensuring adequate rate control for patients with AF as well as programming to increase the VT/VF detection rate and duration threshold.

Wilkoff told delegates that physicians need to realise that it is necessary to think beyond implantation and spend more time programming devices. In a press conference, he said “Defibrillators are not medications. There is a categorical difference between the two. Medications, you take a pill, what you get is what you took. Defibrillators are not that way. They are malleable, and they are malleable on the basis of a number of things, including clinical parameters, but in particular they are under the control of the physician, and the programming choices that he makes.

Study discussant Michael Gold, Medical University of South Carolina, Charleston, USA, said, “We have become much more hyperaware of the potential risks of shocks. Shocks are not benign whether appropriate or inappropriate. From the perspective of quality, which is a theme for this conference, there is a great opportunity to reduce shocks further with better adoption of these evidence-based programming strategies.

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