Stroke Prevention and Rhythm Interventions in Atrial Fibrillation

Atrial fibrillation affects approximately 1% of the general population, and the prevalence of AF increases with increasing age, such that 10-15% of individuals aged 80 years or greater suffer from AF. Various projections estimate at least a doubling of the prevalence of AF over the next 30-40 years. Stroke is a serious, often disabling complication of AF. The Framingham Heart Study demonstrates a roughly five-fold increase in the risk of stroke in patients with AF. The proportion of strokes attributable to AF also increases with age, such that one quarter to one third of all strokes in octogenarians are related to atrial fibrillation.

AF-related strokes tend to be more severe than ischemic strokes of other etiologies, resulting in a higher rate of stroke-related disability. Vitamin K antagonists have been shown to be highly effective in reducing the risk of stroke in patients with AF. A recent meta-analysis of 6 trials of VKA versus placebo, in over 2900 patients with AF, demonstrated a roughly 65% reduction in thromboembolic stroke. Thus, vitamin K antagonists (VKA) have been universally recommended for most AF patients felt to be at moderate to high risk for stroke. The CHADS2 risk score fairly accurately distinguishes patients at low, moderate and high risk for stroke using clinical variables in a bedside risk score. In recent years, VKA have been recommended for AF patients with a CHADS2 risk score of at least 2 or greater, with the optional use of either ASA or VKA for those with a score of 1. Most recently, the Canadian Cardiovascular Society's 2010 AF Guidelines now recommend anticoagulation for patients with a CHADS2 risk score of 1 or greater, in the absence of increased bleeding risk. The 2012 update to the Canadian Cardiovascular Society Guidelines additionally recommends the use of the CHA2DS2-Vasc score in patients with a CHADS2 risk score of 0, and subsequent use of OAC even in a proportion of these patients.

The benefits of VKA are largely dependent upon achieving adequate anticoagulation, ideally an INR of between 2 and 3, for the majority of treatment time. However, achieving and maintaining therapeutic anticoagulation in clinical practice can be quite challenging. Warfarin and other VKA have a variety of limitations in clinical practice. The individual patient's response to warfarin is often unpredictable, with a narrow therapeutic window. The drug has a slow onset and offset of action, with the need for regular INR monitoring and frequent dose adjustments. There are often numerous food and drug interactions with warfarin. Patients may be non-compliant with routine INR monitoring. Physicians may be wary of the risk of bleeding complications, particularly the risk of intracranial haemorrhage, which rises substantially once INR approaches 4 or beyond. Such fear of bleeding may lead physicians to underprescribe warfarin to patients at increased stroke risk. Adverse events with warfarin are common, leading to a high rate of discontinuation. As a result of these limitations, it is estimated that only half of eligible AF patients worldwide receive any VKA at all. In those patients receiving VKA, only 40-60% actually achieve therapeutic anticoagulation. The adequacy of anticoagulation with VKA is often termed "Time in Therapeutic Range (TTR)", which estimates the overall period of time that an individual patient or group of patients spend with an INR between 2 and 3. Numerous practice audits and observational registries suggest that the average TTR globally is roughly 45-55%, whereas it is estimated that a TTR of 65% is required to optimally reduce the risk of AF-related stroke.

In addition, a variety of other issues must be considered in the management of patients with AF, such as rate vs. rhythm control, drug selection, device therapy, and management of associated co-morbidities, in particular, hypertension.

Recently, two new classes of oral anticoagulants have entered the Canadian marketplace. These include dabigatran, a direct thrombin inhibitor, and rivaroxaban, a factor Xa inhibitor. Both drugs offer distinct advantages over warfarin, provide predictable anticoagulation without the need for monitoring, and have different safety and tolerability profiles. It is unknown how Canadian physicians currently assess and manage stroke risk in patients with AF, and it is also unknown how physicians will adopt new oral anticoagulants into their practices. Recent guidelines also highlight the need for physicians to assess bleeding risk in AF patients, something that most physicians do not perform in a systematic manner. Thus, in this new era of stroke prevention strategies and guidelines, a prospective AF registry is clearly warranted to better understand physician approaches to the use of OAC in clinical practice, and to characterize patient variables determining the use of specific agents.