The purpose of the study is to evaluate the performance of ECG classification algorithms and their ability to classify heart rhythms into multiple categories of rhythms and heart rates.

Atrial fibrillation (Afib) is the most common cardiac arrhythmia in the United States, affecting up to one in four individuals across the lifespan, and is associated with substantial morbidity and mortality. Fifteen percent of strokes in the United States are attributable to Afib, and nearly 20% of these occur in individuals with no prior Afib history. Because Afib is often paroxysmal, one-time screening is unlikely to capture those at risk. Thus, there has been an increasing interest in leveraging monitoring for Afib via wearable devices, which provide a novel method to monitor Afib and determine the burden of Afib in the general population.

The primary objective of this study is to assess the sensitivity and specificity of the investigation device (WHOOP ECG Feature) for the detection of Afib as compared to a gold-standard reference (12-Lead ECG Holter Device).

Inclusion Criteria:

• Aged 22 years or older.
• Ability to provide informed consent.
• Willing to participate and to follow the procedures per the Principal Investigator's instructions.
• Resided in the United States.
• Wrist circumference: 130 mm to 245 mm at band wear position.
• Previous medical history of persistent, paroxysmal, or permanent or chronic AF and being in AF at time of enrollment (AF cohort only).
• No known history of AF and being in normal sinus rhythm at the time of enrollment (NSR cohort only).

Exclusion Criteria:

-Subjects with an implantable pacemaker device or implantable cardioverter- defibrillator device.

• Medical history of a life-threatening cardiac arrhythmia, e.g., ventricular tachycardia or fibrillation.
• Any known allergies to medical adhesives, isopropyl alcohol, or ECG patch.
• Any known allergy or sensitivity to thermoplastics, metals with physical vapor deposition (PVD) coatings or Elastane used in the wrist fitness devices.
• Clinically significant body tremors that compromise study measurements.
• Pregnant at the time of enrollment.
• Any physical disability that prevents safe and adequate testing.
• Physical or medical impairments that preclude exercise testing, including, but not limited to, back pain and leg claudication.
• Mental impairment as determined by the Investigator, or designee.
• Subjects with any medical history, physical examination finding, vital sign or other finding or assessment that could compromise subject safety, study integrity or accurate assessment of study objectives. This includes, but is not limited to, known untreated medical conditions that may be considered clinically significant, such as significant anemia, electrolyte imbalance, untreated or uncontrolled thyroid disease, and open wound(s) in the area of test band positioning.
• Vital sign measurements, medical history or physical examination finding that makes the subject inappropriate for participation according to the Investigator.
• Scarring, tattoos, large, pigmented moles, or other skin pathology in the area of sensor location.
• Severe skin conditions on either wrist, that would preclude wearing the sensor. Severe symptomatic skin injury, disorder, allergy, or disease such as eczema, rosacea, impetigo, dermatomyositis or contact dermatitis on wrist or other areas where sensors or electrodes would be placed.
• Clinically significant hand tremors as judged by the Investigator.
• Participated in phase 1 of the study (Only for phase 2 cohort).

The study population will include adults seen in primary care and cardiology clinics associated with a single health system or subjects recruited through study advertisement. The prospective cohort will comprise two distinct populations - individuals with a known history of persistent atrial fibrillation and individuals without a history of cardiac arrhythmia of any kind.