Wednesday, November 10, 2010

Atrial Fibrillation

Today we talked about atrial fibrillation in a patient with a history of rheumatic fever. On GIM, atrial fibrillation is a diagnosis we see very commonly.

One of the best atrial fibrillation resources is the 2006 ACC/AHA Guidelines which you can find here. As a side note, the ACC/AHA have guidelines on a huge variety of cardiovascular topics that provide an evidence base for cardiac decision making on the wards. Here's the list.

Briefly about atrial fibrillation:

1. Afib is a very common arrhythmia. Seen in ~10% of patients greater than 85 years old.

2. While afib can cause distressing symptoms (palpitations, SOB, etc.), the biggest concern is thromboembolism and the impact of afib on cardiac function.

3. Generally we classify afib as:
a) paroxysmal (less than 7 days duration)
b) persistent (lasting greater than 7 days and terminating spontaneously or with cardioversion)
c) permanent (afib lasting greater than 1year and failed cardioversion/not attempted).

Regardless of the classification,
they all portend the same yearly risk of stroke.

A fourth classification is "lone atrial fibrillation" which applies to atrial fibrillation in a patient with no structural heart disease (and often less than 60 years old). These patients are generally felt to be at a low risk of stroke.

4. Most common causes of afib: HTN, valvular disease, hyperthyroidism, heart failure and alcohol.

Usually afib is not associated with stable coronary artery disease unless it results in an MI or heart failure. Afib can also be seen in the context of surgery or a pulmonary embolism.

5. Minimum set of investigations for new onset atrial fibrillation: ECG (confirm diagnosis, assess for other complicating disease), CBC, lytes, TSH and 2D Echo (to assess for structural heart disease).

6. Management consists of rate vs. rhythm control and thromboembolism risk reduction.

a) Rate vs. rhythm control:

Often a trial of cardioversion is given for the first episode of atrial fibrillation. In the long term, most patients with recurrent atrial fibrillation are managed with rate control rather than rhythm control (unless they have significant symptoms or cardiovascular compromise when in afib).

The AFFIRM Trial demonstrated that there was no survival or stroke risk advantage to a rhythm control strategy and that rhythm control more frequently resulted in medication side effects and hospitalization.

Rate control is usually accomplished with B-blockers, non-dihydropyridine calcium channel blockers (Dilt, Verapamil) or Digoxin. The guidelines suggest a HR target of less than 80bpm at rest and less than 110bpm with exercise. However, the recently published RACE-II Trial suggests that strict rate control may confer no additional benefit over lenient rate control.
b) Thromboembolism risk reduction:

The yearly risk of stroke depends on a variety of factors. Several risk scores exist, but one of the easiest to use is the CHADS2 Score. This only applies for patients with non-valvular atrial fibrillation:

Chronic Heart Failure - 1 point
HTN - 1 point
Age greater than 75 - 1 point
Diabetes - 1 point
Stroke/TIA - 2 points

If the CHADS2 score is 0 then no therapy or ASA is reasonable. With a CHADS2 score of 1 then ASA or Warfarin (INR 2-3) is acceptable and with a CHADS2 score greater than 1 warfarin is recommended.

The reason for this is that the benefit you obtain from ASA or warfarin is determined by your yearly risk of stroke. ASA provides a RRR of ~25% while warfarin gives a RRR of ~66%. The risk of a significant bleeding complication from ASA is small, while with warfarin is ~1%/year (depending on the patient).

Remember, for patients with valvular atrial fibrillation, their stroke risk is much higher (as in our patient this morning) so they should be managed with warfarin.

Interestingly, the recently published RE-LY trial showed that Dabigatran, a direct thrombin inhibitor that does not require monitoring may be superior to warfarin in terms of stroke prevention or bleeding risk (depending on the dose). This could revolutionize the way atrial fibrillation in managed.

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