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| Can patients at elevated risk of stroke treated with anticoagulants be further risk stratified? |
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September 2007
Patients with atrial fibrillation have a varied risk of stroke, depending on age and comorbid conditions. The objective of a recently published study was to assess the predictive value of stroke risk classification schemes and to identify patients with atrial fibrillation who are at a substantial risk of stroke despite optimised anticoagulant therapy.
In the August issue of Stroke, Lawrence Baruch MD from the Department of Medicine, Bronx Veterans Affairs Medical Center, Bronx, and the Mount Sinai School of Medicine, New York, NY, USA, and colleagues retrospectively compared 7 risk stratification schemes in a large clinical trial-based programme of patients with atrial fibrillation (the Stroke Prevention using an ORal Thrombin Inhibitor in atrial Fibrillation [SPORTIF] III and IV studies).
This trial compared the efficacy of adjusted-dose warfarin and the direct thrombin inhibitor ximelagatran (36 mg twice daily) in preventing thromboembolic events in 7,329 patients with chronic or paroxysmal nonvalvular atrial fibrillation who were at moderate or high risk of ischaemic stroke. The main outcome measure was ischaemic stroke, as determined by a central event adjudication committee.
The investigators found that during 11,245 patient-years of follow-up, 159 patients had an ischaemic stroke (1.4%/year). As indicated by c statistics and hazard ratios, 3 of the classification schemes predicted stroke significantly better than chance:
- Framingham (c=0.64),
- CHADS2 (an acronym for congestive heart failure, hypertension, age >75, diabetes mellitus, and prior stroke or transient ischemic attack) (c=0.65), and
- Stroke Prevention in Atrial Fibrillation (SPAF) (c=0.61).
The authors pointed out that this predictive ability may allow clinicians to target high-risk patients for more aggressive intervention.
Daniel G Hackam MD, PhD, FRCPC from the Divisions of Clinical Pharmacology and Clinical Neurological Sciences, and Department of Epidemiology and Biostatistics, University of Western Ontario, London, Ontario, Canada, commented on Lawrence Baruch’s study in the same issue of stroke.
He noted that predicting which patients with atrial fibrillation will have a stroke or systemic embolic event is not easy because of the inherent complexities of the analyses and comparisons concerned. "At first glance, the data Baruch and colleagues present might suggest caution regarding the use of formal risk stratification schemes to predict stroke in patients with atrial fibrillation. However, many potential caveats apply. In much of the original validation work for such schemes, patients were not selected by the presence of additional risk factors such as hypertension or heart failure, whereas in the SPORTIF programme, only patients with atrial fibrillation judged to be at high risk because of the presence of concomitant stroke risk factors were included. Therefore, as recognized by the authors, the present study included few patients at a low risk for stroke, thereby hampering the ability of the stratification schemes to separate patients on the basis of predicted risk.Clinical trials are often criticized because they include patients at the extreme lower end of the risk spectrum, but the converse is actually true in this case, because of the deliberate selection criteria of SPORTIF".
Furthermore, he underlined that: "these limitations should be balanced against the strengths of this study. Clinical trials provide a high degree of quality control, including the formalized adjudication of individual outcomes such as stroke, haemorrhage, and systemic embolus. Such adjudication is almost certainly more accurate than reliance on administrative healthcare databases although a combination of both techniques is probably better than either technique used alone. Also, side-by-side comparisons of the predictive capabilities of multiple risk stratification schemes in the world of atrial fibrillation are uncommon; the investigators should, therefore, be congratulated for attempting such an analysis".
"Despite the above caveats, the study by Baruch and colleagues does provide evidence that in patients with atrial fibrillation who are already well-treated with anticoagulation, existing risk stratification schemes may not work as well as we might think. Confirming this finding would require prospective validation in a community-derived sample of patients with atrial fibrillation who have a broader risk spectrum, preferably both treated and not treated with anticoagulants. Until such time, physicians need to pay attention to the entire panoply of treatable risk factors in patients with atrial fibrillation; it is likely that vasculoprotective therapies such as statins and antihypertensive agents play an important additive role to coumarin anticoagulants in this setting", he said.
Reference:
Stroke 2007; 38 (9): 2459–2463.
Stroke 2007; 38 (9): 2409.
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