Wearable Defibrillator: Early Post-MI Experience

July 31, 2013

WASHINGTON, DC — Some patients deemed high risk for sudden cardiac death in the first weeks following an acute MI, when guidelines are strict about avoiding use of implantable cardioverter-defibrillators (ICDs), can be successfully treated by shocks from a wearable cardioverter-defibrillator (WCD), according to an analysis of manufacturer-supplied data published today in the Journal of theAmerican College of Cardiology [1].

The LifeVest (Zoll Medical, Pittsburgh, PA) WCD has been available for about a decade but, lacking any randomized trials, is still seeking indications for which it might be routinely used. In the current study, the device successfully treated 1.4% of >8000 post-MI patients with low LVEF wearing the WCD for at least some time in the period ICDs are to be avoided (40 days after MI; 90 days for patients who underwent acute primary revascularization).

Three-fourths of shocks from the vest occurred in the first month after MI, 96% in the first three months. Of the patients receiving appropriate shocks, 91% were successfully resuscitated.

Appropriateness Questions Remain

"Although some may argue that prescription of a defibrillator, whether an ICD or WCD, is not appropriate during the waiting period post-MI as LV function improves, our study suggests that a small but not necessarily unimportant group of patients may derive benefit from defibrillation early after MI," write the authors, led by Dr Andrew E Epstein (University of Pennsylvania, Philadelphia).

"Given the limitations inherent in using a manufacturer's database," writes Dr Paul C Zei (Stanford University, CA) in an accompanying editorial[2], "it seems we can still draw some modest conclusions from this current study. First, there is a small but definite subpopulation of post-MI patients who are at risk of [sudden cardiac death] SCD during the mandated waiting period for ICD implantation." Also in the study, he notes, "patients who received appropriate WCD therapy appear to have increased mortality."

As Epstein et al note, for all patients in the analysis, actuarial survival was 96% at three months, 94% at six months, and 93% at 12 months. For those who received appropriate shocks, it was 73%, 70%, and 65% at three, six, and 12 months, respectively.

Zei points out that their clinical trajectory after appropriate shocks remained worse than for those who did not need appropriate shocks. Whether their greater mortality "is due to the negative impact of ventricular arrhythmias on clinical status, or that ventricular arrhythmias [are] a marker for poor clinical outcomes, or that the shocks themselves contribute to clinical destabilization is unclear," he writes. "It is also not clear whether those who received inappropriate WCD therapies also had poorer outcomes. That being said, this population appears effectively treated by the WCD for ventricular arrhythmias."

Challenging Subject, More Enlightenment

Epstein et al looked at 8453 patients with a WCD prescribed within 90 days of an acute MI with an LVEF <35% who wore the vest for at least 15 minutes (mean actual time 69 days). The device features three defibrillation electrode pads and four sensing electrodes, held close to the body by an elastic belt; it can deliver up to five 150-J shocks (a mean of two appropriate shocks were delivered per event).

A total of 309 appropriate shocks occurred in 133 patients during 146 ventricular-arrhythmia events; 91% were successfully resuscitated. A total of 114 inappropriate shocks were delivered to 99 patients (during 102 events), none of which induced arrhythmias or caused burns or other sequelae, according to Epstein et al.

"In the end, this study still does not quite answer the elusive question of how to best risk-stratify post-MI low-EF patients in the early days postinfarction," according to the editorial. "Some portion will go on to recover LV function, while others will fail to do so and will eventually meet criteria for ICD therapy." A few of the patients will have SCD during the waiting period, it notes.

"Does the relatively low incidence of risk within this time frame warrant prescribing a WCD for every patient with low EF post-MI? Should the WCD be prescribed for a fixed duration? If not, what are the criteria for determining when it is safe to stop wearing the WCD? Despite these unanswered questions, this study provides a bit more enlightenment on this challenging subject."

Epstein discloses honoraria and research funding from Biotronik, Boston Scientific, Medtronic, and St Jude Medical and consulting for Zoll. Disclosures for the coauthors are listed in the paper. Zei's editorial contained no disclosures.

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