From the Journals

Is AFib a stroke cause or innocent bystander? The debate continues


 

Discovery of substantial atrial fibrillation (AFib) is usually an indication to start oral anticoagulation (OAC) for stroke prevention, but it’s far from settled whether such AFib is actually a direct cause of thromboembolic stroke. And that has implications for whether patients with occasional bouts of the arrhythmia need to be on continuous OAC.

It’s possible that some with infrequent paroxysmal AFib can get away with OAC maintained only about as long as the arrhythmia persists, and then go off the drugs, say researchers based on their study, which, they caution, would need the support of prospective trials before such a strategy could be considered.

But importantly, in their patients who had been continuously monitored by their cardiac implantable electronic devices (CIEDs) prior to experiencing a stroke, the 30-day risk of that stroke more than tripled if their AFib burden on 1 day reached at least 5-6 hours. The risk jumped especially high within the first few days after accumulating that amount of AFib in a day, but then fell off sharply over the next few days.

Based on the study, “Your risk of stroke goes up acutely when you have an episode of AFib, and it decreases rapidly, back to baseline – certainly by 30 days and it looked like in our data by 5 days,” Daniel E. Singer, MD, of Massachusetts General Hospital, Boston, said in an interview.

Increasingly, he noted, “there’s a widespread belief that AFib is a risk marker, not a causal risk factor.” In that scenario, most embolic strokes are caused by thrombi formed as a result of an atrial myopathy, characterized by fibrosis and inflammation, that also happens to trigger AFib.

But the current findings are, “from a mechanistic point of view, very much in favor of AFib being a causal risk factor, acutely raising the risk of stroke,” said Dr. Singer, who is lead author on the analysis published online Sept. 29 in JAMA Cardiology.

Some studies have “shown that anticoagulants seem to lower stroke risk even in patients without atrial fib, and even from sources not likely to be coming from the atrium,” Mintu P. Turakhia, MD, of Stanford (Calif.) University, Palo Alto, said in an interview. Collectively they point to “atrial fibrillation as a cause of and a noncausal marker for stroke.”

For example, Dr. Turakhia pointed out in an editorial accompanying the current report that stroke in patients with CIEDs “may occur during prolonged periods of sinus rhythm.”

The current study, he said in an interview, doesn’t preclude atrial myopathy as one direct cause of stroke-associated thrombus, because probably both the myopathy and AFib can be culprits. Still, AFib itself it may bear more responsibility for strokes in patients with fewer competing risks for stroke.

In such patients at lower vascular risk, who may have a CHA2DS2-VASc score of only 1 or 2, for example, “AFib can become a more important cause” of ischemic stroke, Dr. Turakhia said. That’s when AFib is more likely to be temporally related to stroke as the likely culprit, the mechanism addressed by Dr. Singer and associates.

“I think we’re all trying to grapple with what the truth is,” Dr. Singer observed. Still, the current study was unusual for primarily looking at the temporal relationship between AFib and stroke, rather than stroke risk. “And once again, as we found in our earlier study, but now a much larger study, it’s a tight relationship.”

Based on the current results, he said, the risk is “high when you have AFib, and it decreases very rapidly after the AFib is over.” And, “it takes multiple hours of AFib to raise stroke risk.” Inclusion in the analysis required accumulation of at least 5.5 hours of AFib on at least 1 day in a month, the cut point at which stroke risk started to climb significantly in an earlier trial.

In the current analysis, however, the 30-day odds ratio for stroke was a nonsignificant 2.75 for an AFib burden of 6-23 hours in a day and jumped to a significant 5.0 for a burden in excess of 23 hours in a day. “That’s a lot of AFib” before the risk actually goes up, and supports AFib as causative, Dr. Singer said. If it were the myopathy itself triggering stroke in these particular patients, the risk would be ongoing and not subject to a threshold of AFib burden.

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