Effects of Atrial Fibrillation Screening According to Echocardiographic Measures of Cardiac Structure and Function: A Post Hoc Analysis of the LOOP Randomized Clinical Trial

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Abstract
Background: Subclinical atrial fibrillation (SCAF) poses an increased stroke risk, but whether oral anticoagulation for SCAF prevents stroke is unclear. We sought to investigate the treatment effect of SCAF screening according to measures of cardiac structure and function. Methods: This was an echocardiographic substudy of the LOOP (Atrial Fibrillation Detected by Continuous ECG Monitoring) study, which randomized older people at risk of stroke to usual care or an implantable loop recorder (ILR) with monitoring for SCAF and subsequent oral anticoagulation. A subset (24% of trial population) underwent echocardiography to measure left ventricular size and function, left atrial volume and strain, and valvular pathology. The primary outcome was a composite of stroke or systemic embolism. Results: The study included 1422 participants (ILR: n=1001; control: n=421; mean age: 74 years; men: 54%). During follow‐up, 354 (25%) were diagnosed with AF (ILR versus control: 30% versus 12%). During a median follow‐up of 5.5 years (interquartile range, 4.9–5.9 years), 55 (4%) developed the primary outcome (ILR versus control: 3.9% versus 3.8%). No conventional measure of cardiac structure and function modified the treatment effect from randomization. However, left atrial contraction strain significantly modified the treatment effect ( P interaction=0.003), such that a lower risk of the primary outcome was noted from ILR with lower left atrial contraction strain values (hazard ratio [HR], 0.38 [95% CI, 0.16–0.87], for participants with contraction strain<16.5%). Conclusion: In a post hoc analysis of the LOOP study, conventional echocardiographic measures did not modify the effect of SCAF screening for stroke prevention. However, a significant stroke risk reduction was observed from ILR randomization among participants with reduced left atrial contraction strain.

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