Atrial Fibrillation Burden as a Digital Biomarker

Atrial fibrillation burden is the proportion of a monitored period during which a person is in atrial fibrillation, usually expressed as a percentage of monitored time, sometimes alongside the number and duration of episodes. It replaces a binary question, has this person ever had atrial fibrillation, with a quantitative one that can change.

That shift is the reason the measure exists. A detection is permanent once it happens and cannot serve as a response endpoint. Burden can fall after ablation, rise before a clinical event, and respond to rate or rhythm control, which makes it usable in trials. An American Heart Association scientific statement made the argument explicitly, framing burden as a move beyond treating atrial fibrillation as a binary entity, and burden has since been associated with ischaemic stroke risk in people with paroxysmal atrial fibrillation.

Burden is computed as time in atrial fibrillation divided by time monitored, which makes the denominator as important as the numerator. Implanted devices and insertable monitors record continuously, so their denominator is close to complete. Consumer wearables sample intermittently, only while worn, and often only when the wearer is still, so their denominator is partial and variable.

Detection itself uses two sensor types. Photoplethysmography screens continuously for irregular pulse patterns and is what drives background notification. Single lead electrocardiography, taken by the wearer on request, provides a recording a clinician can review. Neither is equivalent to continuous implanted monitoring, and burden figures from the two sources are different quantities even when both are written as a percentage.

The main research uses are recurrence quantification after ablation or cardioversion, where burden is markedly more sensitive than intermittent recording, and anticoagulation strategy trials that test whether treatment can be tied to the amount of arrhythmia rather than given continuously. Burden is also used to characterise disease progression, since paroxysmal atrial fibrillation tends to become more persistent over time.

The regulatory position strengthened when an atrial fibrillation history feature on a consumer wearable was qualified by the United States Food and Drug Administration as a medical device development tool, a route intended specifically to support use in regulated studies. Burden is reported alongside a symptom and quality of life instrument, because the relationship between detected arrhythmia and how a person feels is weak in both directions.

Burden depends on monitoring coverage, and comparing a smartwatch figure with an implanted monitor figure will misstate the result. Wear time and adherence become part of the measurement rather than a nuisance variable, and studies should report them alongside burden itself.

Many episodes are asymptomatic and many symptoms occur without arrhythmia, so burden and symptom burden must both be reported. The largest open question is clinical rather than technical: for brief, asymptomatic, device detected episodes, the evidence on whether anticoagulation improves outcomes without adding unacceptable bleeding risk is still developing, so a burden figure does not by itself indicate what to do.

• Chen LY, et al. Atrial fibrillation burden: moving beyond atrial fibrillation as a binary entity. A scientific statement from the American Heart Association. Circulation. 2018. pubmed.ncbi.nlm.nih.gov
• Go AS, et al. Association of burden of atrial fibrillation with risk of ischemic stroke in adults with paroxysmal atrial fibrillation. JAMA Cardiol. 2018. pubmed.ncbi.nlm.nih.gov
• Perino AC, et al. Practice variation in anticoagulation prescription and outcomes after device-detected atrial fibrillation. Circulation. 2019. pubmed.ncbi.nlm.nih.gov
• Perez MV, et al. Large-scale assessment of a smartwatch to identify atrial fibrillation. N Engl J Med. 2019. pubmed.ncbi.nlm.nih.gov