A new study, published in the New England Journal of Medicine, has added weight to the view that subclinical atrial fibrillation is a possible cause of cryptogenic stroke after finding that 13% of strokes and systemic embolisms in the study population were associated with subclinical atrial tachyarrhythmias.
Subclinical atrial fibrillation is often suspected as being the cause in the (about) 25% of patients who present with ischaemic stroke and in whom no aetiologic cause be identified (cryptogenic stroke). Although assessing the prevalence and the prognostic value of subclinical atrial fibrillation can be difficult, there is some evidence that pacemakers can be programmed to detect episodes of atrial tachycardia (and possibly the presence of subclinical atrial fibrillation).
Therefore, an objective of the ASSERT (Asymptomatic atrial fibrillation and stroke evaluation in pacemaker patients and the atrial fibrillation reduction atrial pacing trial) study was to assess whether subclinical episodes of high atrial rate detected by implanted devices were associated with an increased risk of stroke in patients who had no other evidence of atrial fibrillation.
Investigators Jeff Healey (Population Health Research Institute, McMaster University, Hamilton, Canada) and colleagues assessed the incidence of stroke and systemic embolism in 2451 patients with a newly implanted pacemaker and 129 patients with a newly implanted implantable cardioverter defibrillator (ICD). After enrolment, patients were monitored for the presence of subclinical atrial tachyarrhythmias (episodes of high atrial rate of >190 beats per minute for more than six minutes) for three months and were then followed for a mean of two and a half years.
During the first three months, at least one tachyarrhythmia was detected in 261 patients (10.1%) and the median number of atrial tachyarrhythmias in this group of patients was two. Of these patients, 11 had a stroke or systemic embolism during the follow-up period compared with 40 of the 2319 patients who did not have an episode of subclinical atrial tachyarrhythmia (a rate of 1.69 per year vs. a rate of 0.69% per year, respectively, p=0.007). The risk of stroke and systemic embolism was not altered after adjustment for baseline factors.
Overall, 51 patients in the study had a stroke or systemic embolism and 11 of them had had a subclinical tachyarrhythmia detected within three months. However, no patients had had clinical atrial fibrillation in this timeframe. Healey et al reported: “The population attributable risk of ischaemic stroke or systemic embolism associated with subclinical atrial tachyarrhythmia was 13%.” According to the investigators, this risk is similar to that reported for clinical atrial fibrillation by the Framingham investigators. Additionally, the absolute rate of stroke increased with increasing CHADS2 score. In patients with a CHADS2 score greater than two and episodes of subclinical atrial tachyarrhythmias, the absolute rate of stroke was 3.78% per year.
Healey et al wrote that a “major finding” in their study was the “substantial incidence” of atrial tachyarrhythmias in the study group (10.1% in the first three months and 34.7% in the follow-up period). Another important finding, they said, was that subclinical tachyarrhythmia was shown to be an independent risk factor for stroke. They added that although underpowered for such an analysis, the study indicated that the risk of stroke was higher when the episodes of atrial tachyarrhythmia were of longer duration. They concluded: “The data from the present study support the concept that there is a link between subclinical atrial fibrillation and cryptogenic stroke.”
Stuart Connolly, one of the investigators and director of the Division of Cardiology, Salim Yusuf Chair in Cardiology, McMaster University, Hamilton Health Sciences, Ontario, Canada, told Cardiac Rhythm News: “The findings of ASSERT are of potentially great importance to our efforts to prevent stroke. It is likely that sub-clinical atrial fibrillation occurs not just in patients with pacemakers but in other elderly patients with risk factors. I think that future research needs to figure out who is at high risk for silent atrial fibrillation and to find ways to monitor them to detect it. Finally we need to know if anticoagulant therapy is effective in patients with silent short episodes of atrial fibrillation.”