Stroke risk in paroxysmal AF

STOCKHOLM, SWEDEN. Recent headlines proclaimed that stroke (ischemic) is just as common in paroxysmal atrial fibrillation patients as it is among permanent afibbers. Not surprisingly, the conclusion of the Swedish study was, “It is therefore important to increase the use of anticoagulants (warfarin) among patients with paroxysmal atrial fibrillation”. Let us look beyond the headlines and abbreviated summary (abstract) of the study and see what it really means for otherwise healthy lone afibbers.

The study involved 855 paroxysmal and 1126 permanent afibbers who were treated for atrial fibrillation (AF) in two Stockholm hospitals during 2002. At the end of the 3.6-year follow-up period, 110 patients had suffered a first ischemic stroke. This corresponds to a stroke incidence of 2.1%/year in paroxysmal and 2.5%/year in permanent afibbers.

Before proceeding any further, it should be pointed out that the study group did not consist of very healthy people. As a matter of fact, more than a third of them died during the follow-up. Almost half (48%) had hypertension, 50% had heart failure (64% in the permanent group), 10% had some sort of valvular defect, 12% had thyroid problems, and 14% had chronic pulmonary disease. Only 4% of the group had lone atrial fibrillation and no strokes, of any kind, occurred in this group during the 3.6 years of follow-up. In contrast, the rate of stroke among patients with heart failure was 9%/year, 14%/year among those who had already had one stroke, 10%/year among those who had experienced a heart attack, and 9%/year among patients diagnosed with hypertension. In other words, 96% of the participants upon which the conclusion of the study are based bear no resemblance whatsoever to a group of otherwise healthy lone afibbers.

The Swedish researchers also evaluated stroke risk based on CHADS2 score where a score of 0 corresponds to no increased risk factors for ischemic stroke and a score of 1 indicates the presence of one risk factor (hypertension, diabetes, heart failure or age over 75 years). Study participants with a CHADS2 score of 0 to 1 had a stroke risk of 1.0%/year if they were paroxysmal and 1.0%/year if they were permanent. A rate of 1.0%/year is exactly what would be expected from an age- and sex-matched sample of the general population. In other words, afibbers with a CHADS2 score of 1 or 0 do not have an increased risk of stroke. The researchers also investigated the relationship between age and stroke risk and found that afibbers below the age of 70 years had an annual stroke risk of less than 0.5% (less than half of that found in the general population).

The only conclusion that can be reached from this data is that any increased stroke risk that may be associated with AF applies only to very sick, old people. Similarly, it is clear that warfarin therapy only benefits that same group of people. When considering paroxysmal afibbers only, the stroke rate was 1.3%/year among patients on warfarin as compared to 5.3% among those not on warfarin. However, when patients with a prior stroke or TIA (transient ischemic attack) were excluded from the analysis, there was no statistically significant difference in stroke risk between warfarin-treated patients and those not on warfarin. Considering the flip side of this finding, one could certainly be pardoned for concluding that warfarin, like aspirin, really only benefits patients who have already suffered an ischemic stroke or TIA. NOTE: Two of the authors of this study have financial ties to AstraZeneca, a major Swedish pharmaceutical company.

Friberg, L, et al. Stroke in paroxysmal atrial fibrillation: report from the Stockholm Cohort of Atrial Fibrillation. European Heart Journal, January 27, 2009 [Epub ahead of print]

Editor’s comment: The results of this study unequivocally confirm that lone, otherwise healthy afibbers have a very low risk of ischemic stroke; as a matter of fact, substantially lower than that of the general population. A more detailed examination of the data presented in the report would also lead one to the inescapable conclusion that afib, as such, is not a risk factor for stroke – rather it is the conditions that often accompany it (heart disease, hypertension, etc) that increase the stroke risk.