Sub-Clinical Atrial Fibrillation Biomarker Study

Inclusion Criteria: * Permanent pacemaker or defibrillator (with or without resynchronization) or insertable cardiac monitor * At least one episode of device-detected SCAF ≥ 6 minutes in duration but no single episode \> 24 hours in duration at any time prior to enrollment. Any atrial high rate episode with average \> 175 beats/min will be considered as SCAF. No distinction will be made between atrial fibrillation and atrial flutter. SCAF requires electrogram confirmation (at least one episode) unless ≥ 6 hours in duration * Age ≥ 55 years * Risk Factor(s) for Stroke: Previous stroke, TIA or systemic arterial embolism OR Age at least 75 OR Age 65-74 with at least 2 other risk factors OR Age 55-64 with at least 3 other risk factors Other risk factors are: * Hypertension * CHF * Diabetes * Vascular disease (i.e. CAD, PAD or Aortic Plaque) * Female * Must be from a participating Canadian recruitment centre * Consent to participate in the ARTESiA parent study Exclusion Criteria: * Clinical atrial fibrillation documented by surface ECG (12 lead ECG, Telemetry, Holter) lasting ≥ 6 minutes, with or without clinical symptoms * Mechanical valve prosthesis, recent (within past 6 months) deep vein thrombosis or pulmonary embolism or other condition requiring treatment with an anticoagulant * Allergy to aspirin or apixaban * Severe renal insufficiency (serum creatinine \> 2.5 mg/dL \[221 μmol/L\] or a calculated creatinine clearance \< 25 ml/min) * Serious bleeding in the last 6 months or at high risk of bleeding (this includes, but is not limited to: prior intracranial hemorrhage, active peptic ulcer disease, clinically significant thrombocytopenia or anemia, recent stroke within past 10 days, documented hemorrhagic tendencies or blood dyscrasias) * Moderate to severe hepatic impairment * Ongoing need for combination therapy with aspirin and clopidogrel (or other combination of two platelet inhibitors) * Meets criteria for requiring lower dose of apixaban AND also has ongoing need for strong inhibitors of both CYP3A4 and P-glycoprotein (e.g., ketoconazole, itraconazole, ritonavir or clarithromycin) * Ongoing need for strong inducers of both CYP3A4 and P-glycoprotein (e.g., rifampin, carbamazepine, phenytoin, St. John's wort) * Received an investigational drug in the past 30 days * Participants considered by the investigator to be unsuitable for the study for any of the following reasons: * Not agreeable for treatment with either aspirin or apixaban or anticipated to have poor compliance on study drug treatment * Unwilling to attend study follow-up visits * Life expectancy less than 2 years due to concomitant disease * Women who are pregnant, breast-feeding or of child-bearing potential without an acceptable form of contraception in place (sterilization, abstinence or other method with less than 1% failure rate)