Traditionally, atrial fibrillation (AF) has been diagnosed with ECGs, telemetry, or holter monitors that provide days of tracings. In the past 20 years, new implantable devices (e.g., loop recorders, implantable defibrillators, pacemakers) have been introduced that give continuous readings over months to years. Longer tracings are able to detect asymptomatic episodes of atrial tachyarrhythmias that have undetermined clinical significance. Two events that have been loosely defined are atrial high-rate episodes (AHRE) and subclinical AF. While criteria aren't standardized across professional organizations, the following definitions are often seen:
- AHRE - atrial tachyarrhythmia episodes with a rate > 170 beats/min detected by cardiac implantable electronic devices
- Subclinical AF - an asymptomatic AHRE lasting greater than 6 minutes and less than 24 hours
Observational studies have found these events, which often precede the development of clinical AF, are associated with a higher-than-normal but
less-than-AF stroke risk, spurring two recent studies that evaluated the effects of antithrombotics in affected patients. The first study (N=2536) compared edoxaban to placebo in patients with AHREs. It was stopped early due to futility, as CV events were not significantly lower with edoxaban, and major bleeding was worse. The other study (N=4012) randomized patients with subclinical AF to apixaban or aspirin. After 3.5 years of follow-up, apixaban reduced stroke risk (0.78%/patient-year vs 1.21%/patient-year) but increased major bleeding (1.53%/patient-year vs 1.12%/patient-year).
These studies do not support anticoagulation in AHRE or subclinical AF, while recent AHA guidelines state that it is "reasonable" in certain high-risk patients. Over the course of both trials, a significant number of subjects developed clinical AF (18% and 24%, respectively), underpinning the importance of continued close monitoring.