Background:  Continuous arrhythmia monitoring following ischemic stroke identifies atrial fibrillation (AF) in a high proportion of patients.  Due to the absence of pre-stroke arrhythmia monitoring in most patients, it is unclear whether post-stroke AF suggests a cause or effect relationship to the index event.  
Methods:  Data from the Optum© EHR de-identified database between 2007-2017 were linked to the Medtronic CareLink database of cardiovascular implantable electronic devices (CIED) capable of continuous AF monitoring.  Patients had ≥1 year of device data both pre- and post-ischemic stroke.  We quantified the percentage of ischemic stroke patients who had AF (≥6 minutes on any day) detected within the year following their stroke and calculated the percentage of these patients with AF present or absent in the year preceding their stroke.
Results:  Among 347 CIED patients (age 72.8±10.8, 64% male) with ischemic stroke, 150 (43.2%) had AF detected in the following year.  Among these, 115 (76.7%) also had AF detected in the year preceding the ischemic stroke, suggesting that AF may have caused the index event.  In contrast, 35 (23.3%) did not have AF detected in the preceding year, suggesting that the AF could be a consequence of the index event or newly developed and unrelated to the index event.  
Conclusion:  AF detected post-stroke is 3x more likely to indicate that AF was also present prior to the stroke.  Regardless of whether post-stroke AF suggests a cause or effect relationship to the index stroke, detection of post-stroke AF and treatment with oral anticoagulation is critical for recurrent stroke prevention.

IS POST-STROKE ATRIAL FIBRILLATION MORE LIKELY A CAUSE OR CONSEQUENCE OF ISCHEMIC STROKE? INSIGHTS FROM LONG-TERM CONTINUOUS ARRHYTHMIA MONITORING