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Implantable cardiac monitoring (ICM) turned up a substantial burden of serious arrhythmic events in high-risk patients after a heart attack, despite not meeting conventional ejection fraction criteria, the SMART-MI trial showed.

The trial homed in on a high-risk population of recent MI survivors with a left ventricular ejection fraction (LVEF) in the 36% to 50% range and core lab-detected cardiac autonomic dysfunction on a 20-minute high-resolution resting electrocardiogram.

About 30% of such patients had a serious arrhythmic event during the median 21 months of follow-up when monitored by a Reveal LINQ subcutaneous device, reported Axel Bauer, MD, of the Medical University of Innsbruck in Austria, during the virtual European Society of Cardiology congress.

By comparison, usual care picked up such events -- atrial fibrillation (Afib) for ≥6 minutes, higher-degree atrioventricular block, fast non-sustained ventricular tachycardia (VT), or sustained VT or ventricular fibrillation -- in only 6.0% of the control group in the randomized trial (HR 6.3, 95% CI 3.4-11.8).

Over 3 years, the cumulative detection rate had widened even further to 41.2% with ICM versus 10.7% among controls.

The amount of arrhythmias impressed ESC press conference panelist François Schiele, MD, PhD, of University Hospital Jean Minjoz in Besançon, France.

Without loop recorder data, some clinical events may be getting chalked up to the procedure, stent thrombosis, or other thrombotic events, added press conference moderator Carlos Aguiar, MD, of the Hospital Santa Cruz in Lisbon.

Types and frequency of arrhythmias in the study population were comparable to what has been seen in reduced LVEF MI survivors, Bauer said in an ESC press release.

While guidelines recommend prophylactic implantable cardioverter-defibrillators (ICDs) in patients with LVEF <35%, most arrhythmic deaths after MI actually occur in patients with higher ejection fraction.

Loop recorder-identified subclinical events may serve as a warning signal to address impending complications for those patients, Bauer told reporters at a press conference for the Hot Line trial session.

"Telemedical monitoring with ICMs allows for continuous cardiac risk assessment, and I think this could be a new indication for ICMs in general," he said. "This might open a window of opportunity for preventive interventions."

Aguiar noted that it's a common clinical conundrum.

For patients for an ICD but worrisome features on imaging or clinical features, "they leave you concerned," he said. "They sometimes make you think it would be great if this patient's LVEF were really below 35%, because I am not comfortable leaving this patient without an ICD."

Bauer agreed, noting that there could be implications for oral anticoagulation as well.

But future studies will be needed to test whether the added detection indeed leads to improved clinical outcome. With 400 patients, the trial was underpowered to detect an efficacy impact on outcomes.

However, irrespective of detection method, serious arrhythmic events predicted major adverse cardiac and cerebrovascular events (CV death, stroke, systemic arterial thromboembolism, and unplanned admission for decompensated heart failure). Hazard ratios were 6.8 with ICM and 7.3 in the control group (both P<0.001), with similar positive predictive accuracy (61% and 62%, respectively).

Sensitivity, though, was 61% with ICM versus 20% with usual care (P=0.007).

Risk stratification would be important for the large patient population of MI survivors with preserved LVEF, rather than implanting all with loop recorders, said Bauer, noting that only 20% to 30% of screened patients entered the trial.

The allowed enrollment of patients with either abnormal periodic repolarization dynamics or abnormal deceleration capacity as a signal of cardiac autonomic dysfunction. But, it might be even better to combine those with other imaging markers to further narrow down to a group for whom more invasive diagnostic procedures are justified, he added.

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