Continuous AF monitoring does not reduce risk for stroke, systemic embolism: LOOP
Continuous implantable loop recorder monitoring resulted in a threefold increase in atrial fibrillation detection and subsequent anticoagulation initiation, but failed to reduce risk for stroke or systemic embolism in high-risk patients.
Jesper H. Svendsen, MD, DMSc, clinical professor in the department of clinical medicine at the University of Copenhagen, Denmark, and colleagues designed the LOOP study to assess whether continuous monitoring via an implantable loop recorder for unknown AF could prevent stroke in high-risk adults.
“Although AF was diagnosed in more than 30% of participants in the implantable loop recorder group, and 91% of these [participants] initiated oral anticoagulation, the overall risk reduction [of stroke and systemic embolism] was 20% and nonsignificant,” Svendsen said. “If we assume that the 20% risk reduction is a correct estimate, then the number needed to screen to avoid one stroke would be 62 individuals.”
Increased AF detection; anticoagulation initiation
The trial involved 6,004 adults aged 70 to 90 years without AF at baseline but with at least one additional stroke risk factor (mean age, 75 years; 47.3% women). Researchers randomly assigned participants in a 1:3 ratio to implantable loop recorder monitoring (n = 1,501) or usual care (n = 4,503). In the implantable loop recorder group, anticoagulation was initiated if an AF episode lasted 6 min or longer.
During a median follow-up of 64.5 months, 477 participants in the implantable loop recorder group (31.8%) and 550 participants in the usual care group (12.2%) were diagnosed with AF (HR = 3.17; 95% CI, 2.81-3.59; P < .0001). Oral anticoagulation was initiated in 445 participants in the implantable loop recorder group (29.7%) and 591 participants in the usual care group (13.1%; HR = 2.72; 95% CI, 2.41-3.08; P < .0001).
The primary outcome — time to first stroke or systemic embolism —occurred in 4.5% and 5.6% of participants in the implantable loop recorder and usual care groups, respectively (HR = 0.8; 95% CI, 0.61-1.05; P = .11).
Major bleeding occurred in 4.3% and 3.5% of participants in the implantable loop recorder and usual care groups, respectively (HR = 1.26; 95% CI, 0.95-1.69; P = .11). There were no between-group differences in mortality; nine participants (0.6%) in the implantable loop recorder group experienced complications that led to device explantation.
In subgroup analyses, researchers observed that participants in the tertile with the highest baseline systolic BP appeared to benefit from screening, Svendsen said.
“The majority of our patients, more than 90%, had hypertension as their inclusion disease, so there is a tight association between our primary endpoint and hypertension,” Svendsen said. “But it is important to say that this subgroup analysis is only hypothesis-generating.”
The results were simultaneously published in the Lancet.
‘We can learn a lot’
In a related editorial, Bed Freedman, OAM, deputy director of cardiovascular research strategy at the Heart Research Institute, Charles Perkins Centre, and professor of cardiology at the University of Sydney, Australia, and Nicole Lowres, PhD, a postdoctoral research fellow with the Heart Research Institute at the University of Sydney, wrote that the LOOP study results would conventionally be considered negative; however, “we can learn a lot from this large trial by examining why it did not succeed.
“The most important explanation is that shorter atrial fibrillation episodes found by long-term implantable loop records might not have the same stroke risk as atrial fibrillation detected through single-timepoint or less-intense monitoring. In studies of atrial fibrillation detected incidentally by implanted cardiac devices, stroke risk was increased in patients with atrial fibrillation episodes of longer than 5-6 min, but most of the prognostic information was carried by episodes lasting 24 hours or longer,” Freedman and Lowres wrote.
In the LOOP study, only 16% of participants had AF episodes lasting 24 hour or longer.
Svendsen said the findings demonstrate that all AF episodes may not be worth screening for.
“Our study concludes that short-lasting episodes are probably different from the AF that is detected by routine screening,” Svendsen said. “[Clinicians] are primarily identifying longer-lasting AF episodes. Short-lasting episodes are probably not as serious.”
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This is an important study. The major issue, in my opinion, is that we don't know how much AF one needs to have a stroke, and therefore don't know how much AF we should be looking for to treat. The implantable loop recorder may be too sensitive, and many patients may have infrequent, short episodes of AF that do not cause stroke and do not require anticoagulation. Other issues that may have influenced the results include the high rate of AF detection in the control group — much higher than seen in other studies. Lastly, the high adoption rate of anticoagulation in both groups is greater than that seen even in clinical practice.
We now need to match the AF-detection tool with the duration of AF that is actionable. The LOOP study shows us that a highly-sensitive device that can pick up minutes of AF may not be necessary and opens the door to a plethora of mHealth devices that may be more suitable for the job.
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Svendsen JH, et al. Hot Line: LOOP. Presented at: European Society of Cardiology Congress; Aug. 27-30, 2021 (virtual meeting).