Approximately 25% of all strokes are of unknown cause, and it has long been hypothesised that short subclinical episodes of atrial fibrillation (AF) may be an important common etiologic factor.  While several studies have attempted to detect episodes of subclinical AF, most have been hampered by the infrequency of such episodes and the unlikelihood of picking them up even with monitoring over a period of days or weeks.  In the US around 400,000 pacemakers are inserted each year, many of which are able to detect and record episodes of rapid atrial rate, which correlate with electrocardiographically documented atrial fibrillation. In this study Healy et al. evaluated whether subclinical episodes of AF detected by these devices were associated with an increased risk of ischemic stroke.

Recruiting 2580 individuals, 65 years of age or older, with hypertension and no history of atrial fibrillation, in whom a pacemaker or defibrillator had recently been implanted, patients were monitored for 3 months to detect subclinical AF (episodes of atrial rate >190 bpm for more than 6 minutes) and were followed for a mean of 2.5 years for the primary outcome of ischemic stroke or systemic embolism. By 3 months, subclinical AF had been detected in 261 patients (10.1%). Subclinical AF was associated with an increased risk of clinical AF (HR, 5.56; 95% CI, 3.78 to 8.17; P<0.001) and of ischemic stroke or systemic embolism (HR, 2.49; 95% CI, 1.28 to 4.85; P=0.007). Of 51 patients who had a primary outcome event, 11 had had subclinical AF detected by 3 months, and none had had clinical atrial fibrillation by 3 months.  Overall, the population attributable risk of stroke or systemic embolism associated with subclinical AF was 13% with subclinical AF remaining predictive even after adjustment for other predictors of stroke (HR, 2.50; 95% CI, 1.28 to 4.89; P=0.008).

Conclusions

Subclinical AF occurred frequently in patients in this study and was associated with a significantly increased risk of ischemic stroke or systemic embolism. .

• Healey JS, Connolly SJ, Gold MR, Israel CW, Van Gelder IC, Capucci A, Lau CP, Fain E, Yang S, Bailleul C, Morillo CA, Carlson M, Themeles E, Kaufman ES, Hohnloser SH; ASSERT Investigators. Subclinical atrial fibrillation and the risk of stroke. N Engl J Med. 2012 Jan 12;366(2):120-9.