Take-home Messages:

• Standard risk stratification models for risk of stroke in patients with atrial fibrillation fail to adequately assess individual patient risk.
• Too many patients are categorized as “moderate risk,” leaving the best approach for patient management unclear.
• A more refined schema, known as CHA2DS2-VASc, appears to do a better job of categorizing risk and guiding therapy.

How do you define a “low-risk” atrial fibrillation (AF) patient? Stroke risk is a continuum and the artificial division of individuals into low, moderate, and high risk is poorly predictive. It evolved to help target high-risk patients, but often the result was a small number of low-risk patients – some of whom still went on to stroke – and a huge pool of intermediate-risk patients. What do you do then for these patients in the middle? Treat them as high risk? Treat them as low risk? Find some middle ground and cross your fingers?

This is particularly important in younger patients where a decision may lead to many years of unnecessary anticoagulation therapy – or unnecessary risk of a stroke. Gregory Y. H. Lip, MD, argues that the presence of a stroke risk factor increases risk, and should enter into calculations of what approach to take if present in association with AF.

What about schema that help assess risk and stratify patients? The most widely used is CHADS2, but Lip et al have shown in a 12-year follow-up study of patients with lone AF, for example, that neither baseline CHADS2 scores nor CHADS2 score at the time of thromboembolic event was predictive for thromboembolism (TE).1

CHA2DS2-VASc

Lip and colleagues built a better risk stratification schema for predicting stroke and TE in AF patients. Usually, models are derived from trial cohorts, which is useful, but often many potential risk factors are not included. So, Dr. Lip and others refined the 2006 Birmingham/National Institute for Health and Clinical Excellence (NICE) stroke risk stratification schema into a risk factor-based approach by reclassifying and/or incorporating additional new risk factors where relevant.

This schema was then compared with existing models in a real-world cohort of patients with AF (n = 1,084) from the Euro Heart Survey.2 Risk categorization differed widely between the different schemes evaluated: individuals classified as high risk ranged from 10.2% with the Framingham schema to 75.7% with the Birmingham 2009 schema. The classic CHADS2 model categorized the largest proportion (61.9%) into the intermediate-risk strata, whereas the Birmingham 2009 schema classified 15.1% into this category. The Birmingham 2009 schema classified only 9.2% as low risk, whereas the Framingham scheme categorized 48.3% as low risk.

Those classified as low risk by the Birmingham 2009 and NICE schema were truly low risk with no TE events recorded, whereas TE events occurred in 1.4% of low-risk CHADS2 subjects. When expressed as a scoring system, the Birmingham 2009 schema – given the acronym CHA2DS2-VASc -- showed an increase in TE rate with increasing scores (p value for trend = 0.003).

A Little More Analysis

The original CHADS2 model is very simple and certainly a good risk stratifier. However, a little further analysis provides more refined risk assessment, which can be particularly helpful in those patients who comprise the large pool of “intermediate” risk.

Specifically, how does it differ from earlier models?

• Scoring has expanded from CHADS2 (total 6) to a maximum of 9 points.
• The two definitive risk factors – age >75 years and prior stroke/TIA – each count 2 points; the rest are all 1 point each.
• Age can actually account for 0 (<65 years), 1 (65-74 years), or 2 points (>75 years).
• Vascular disease, such as peripheral artery disease or MI, is now part of the scheme.

According to Dr. Lip, this novel, simple stroke risk stratification schema improves upon the predictive power for TE over CHADS2, with low event rates in low-risk subjects and the classification of only a small proportion of subjects into the intermediate-risk category. This schema could improve stroke risk stratification in patients with AF and is now recommended in the ESC guidelines for managing AF.3

In terms of managing young patients, in particular, Dr. Lip suggests identifying the low-risk patient adequately and correctly. Points to help:

• Look hard for risk factors.
• Use stroke risk schema that more reliably identify patients at low risk by being more inclusive of stroke risk factors.
• Keep in mind that stroke risk is not static.
• CHA2DS2-VASc simplifies (dichotomizes) selection of patients for anticoagulation.
• In young patients, AF ablation may offer a “cure” and, if truly low risk, then no antithrombotic therapy may be appropriate.

References:

1. Potpara TS, Stankovic GR, Beleslin BD, et al. A 12-Year Follow-up Study of Patients With Newly Diagnosed Lone Atrial Fibrillation: Implications of Arrhythmia Progression on Prognosis: The Belgrade Atrial Fibrillation Study. Chest 2012;141:339-47.
2. Lip GY, Nieuwlaat R, Pisters R, et al. Refining clinical risk stratification for predicting stroke and thromboembolism in atrial fibrillation using a novel risk factor-based approach: the Euro Heart Survey on atrial fibrillation. Chest. 2010;137:263-72.
3. Camm AJ Kirchhof P, Lip GY, et al. Guidelines for the management of atrial fibrillation: the Task Force for the Management of Atrial Fibrillation of the European Society of Cardiology (ESC). Eur Heart J 2010;31:2369-429.

Keywords: Thromboembolism, Stroke, Follow-Up Studies, Peripheral Arterial Disease, Risk Factors, Risk Assessment

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