Prevention of thromboembolism in atrial fibrillation

Atrial fibrillation is associated with an increased risk of peripheral thromboembolism (1).

• the main risk factors for thromboembolic episodes are:
  • age
    • prevalence of AF also increases with age, from <0.5% in 40-50 years to 5% in those >65 years and 10% in those >75 (2)
  • hypertension
  • diabetes
  • previous history of thromboembolism
• three quarters of strokes in patients with atrial fibrillation are thought to be caused by embolism from left atrial thrombus.
  • thrombus is found predominantly in the left atrial appendage.
  • transoesophageal echocardiography can detect left atrial thrombus in 15-30% of patients with chronic atrial fibrillation.

Antithrombotic therapy to prevent thrombo-embolism is recommended for all patients with AF, except in those at low risk (lone AF, aged <65 years, or with contraindications) (1).

The selection of antithrombotic therapy should be considered using the same criteria irrespective of the pattern of AF (i.e. paroxysmal, persistent, or permanent) (2).

The overall risk of stroke in non-rheumatic atrial fibrillation is 4.5% per year. The risk is higher in atrial fibrillation caused by rheumatic fever.

Assessment of the risk of bleeding should be considered when prescribing antithrombotic therapy (whether with vitamin K antagonist or aspirin)

• the bleeding risk with aspirin should be considered as being similar to vitamin K antagonist, especially in the elderly
• the HAS-BLED score should be considered as a calculation to assess bleeding risk, (2)

DOACs have a rapid onset of action and short half-life and attain more predictable blood concentrations than vitamin K antagonists (such as warfarin), allowing standard fixed dosing regimens and obviating the need for laboratory monitoring (3)

• these factors, along with lower risk of major bleeding including intracranial hemorrhage, provide considerable advantages of DOACs over warfarin for
  thromboembolic prophylaxis in patients with atrial fibrillation

Although DOACs have a safer bleeding profile than warfarin, major bleeding still occurs in about 3-4% of patients taking DOACs every year (3)

• despite a lower incidence than warfarin, intracranial hemorrhage associated with DOAC usage remains a concern
• intracranial hemorrhage is responsible for up to 45% of all bleeding related deaths in DOAC treated patients and carries a fourfold increased risk of mortality compared with major extracranial bleeds

DOACs are contraindicated in patients with mechanical valve prostheses owing to an increased thrombosis risk (3)

Absolute contraindications to the use of anticoagulation therapy may include (3):

• severe thrombocytopenia,
• recent trauma or surgery,
• recent hemorrhagic stroke,
• recent intracranial hemorrhage,
• intracranial masses,
• or decompensated liver disease

Clinicians need to carefully assess each of these risks and weigh the risk of a life threatening bleed against the risk of disabling stroke when withholding anticoagulation.

Reference:

• (1) Treatment of AF - British Medical Bulletin 2008; 88: 75–94
• (2) European Heart Rhythm Association 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 (19):2369-429.
• (3) Ponamgi SP et al. Screening and management of atrial fibrillation in primary care. BMJ 2021;372:mn379 http://dx.doi.org/10.1136/bmj.mn379