Stroke risk vs. afib episode duration
COMO, ITALY. The risk of an ischemic stroke or transient ischemic attack (TIA) increases with age, hypertension, diabetes, congestive heart failure, and a history of a previous stroke or TIA. The National Registry of Atrial Fibrillation Scheme has developed a rating system (CHADS2 index) which classifies the overall risk of stroke. Thus, being over the age of 75 years, having hypertension, diabetes, or congestive heart failure each increases the risk by 1 point, while having suffered a previous stroke or TIA increases it by 2 points. The annual stroke risk for afibbers estimated by the CHAD-score developers is as follows:
This estimation is based on data from 1733 patients. A later, much larger study (5089 patients) found the following stroke risk:
Combining the data from the two sources produces the following stroke risk:
There is also some evidence that the risk of stroke among atrial fibrillation patients with underlying heart disease increases with the duration of episodes. A group of Italian researchers have now combined CHAD score and episode duration into a new scheme aimed at further improving stroke risk prediction.

Their one-year study included 568 AF patients (aged between 60 and 80 years with 49% being male) who had had a pacemaker (Medtronic AT500) implanted to deal with sinus node disease (bradycardia-tachycardia syndrome). About half the patients had hypertension, a third were over the age of 75 years, 8% had diabetes, 1.5% had congestive heart failure, and 1.4% had suffered a previous stroke or TIA. Nineteen percent were taking a daily aspirin for stroke prevention, while 25% were on warfarin. Thus, just over half the group did not receive anti-platelet therapy (aspirin) or anticoagulation (warfarin). Beta-blockers were prescribed for 10% of patients and antiarrhythmics for 46%.

Study participants who had been afib-free or experienced no more than one episode (lasting less than 5 minutes) over the one-year monitoring period were considered afib-free (group A). Patients with one episode lasting more that 5 minutes, but less than 24 hours were stratified as group B, while those with one or more episodes lasting more than 24 hours were classified as group C. (NOTE: This group would presumably include permanent afibbers).

The overall annual incidence of stroke in the group with a CHAD score of 0 was 0%, although 1.1% did suffer a TIA. This finding confirms our long held belief that lone afibbers with no risk factors for stroke (as defined by the CHAD score) have an extremely low risk of ischemic stroke. The annual incidence of stroke in the groups with CHAD scores of 1, 2, and 3 or greater was 0.7%, 2.7%, and 18% respectively. Considering afib status, the annual stroke rate was 1.2% in the afib-free (group A), 0% in group B, but a rather high 2.6% in group C. By combining CHAD score and afib duration, the researchers uncovered two distinctly different "populations". The first consisted of afib-free individuals (group A) with a CHAD score of 2 or less, group B members with a CHAD score of 1 or less, and group C with a CHAD score of 0. in this group the average annual stroke risk was 0.8% - actually somewhat lower than what is considered normal for an age-matched population. Study participants not meeting the above criteria had an average annual stroke risk of 5%.

As a separate part of the study, the Italian group simulated the results of 24-hour, 7-day, and 30-day Holter monitoring and found that this type of monitoring, in general, only picks up about 50% of the afib events actually occurring. The researchers conclude that using both CHAD score and afib duration to determine the need for anti-coagulation would result in a much better management of stroke prevention. NOTE: Two of the authors of this study are employees of Medtronic, but I see no reason to believe that this would, in any way, have influenced the outcome or reporting of the study.

Botto, GL, et al. Presence and duration of atrial fibrillation detected by continuous monitoring: crucial implications for the risk of thromboembolic events. Journal of Cardiovascular Electrophysiology, Vol. 20, March 2009, pp. 241-48

Editor's comments: This study very clearly confirms that afibbers with no risk factors for stroke have a very low risk of ischemic stroke, irrespective of the duration of their episodes. As a matter of fact, this particular study found an annual stroke risk of 0% among participants with a CHAD score of 0. Even those with a CHAD score of 1, only had an average annual stroke risk of 0.7%.