Monday, April 9, 2012

Atrial Fibrillation & Stroke: Where's My Crystal Ball?

In a normal heart with a normal conduction system, heart rate varies according to need, speeding up during stress & exercise while slowing down when at rest.  On the other hand, atrial fibrillation is a very common dysrhythmia whereby the heart beats to various drummers, speeding up & slowing down at will w/o any rhyme or reason.  If'that was the only downside, we wouldn't worry too much.  However, atrial kick also accounts for approximately 10% of the ejection fraction which some patients w/congest heart failure depend upon.  More importantly, because of the relative akinesis (lack of movement) of the atrium, blood clots tend to form, only to be kicked out in response to a heart beat.  Should this errant clot go to the brain, a stroke then ensues.

That's why we tend to anticoagulate just about anyone w/atrial fibrillation, even the paroxysmal kind.  Our goal is to prevent a stroke.  Prior to the advent of the new direct thrombin inhibitor & even newer factor Xa inhibitor, neither of which require regular monitoring, we only had warfarin, a vitamin K antagonist, for outpatient use.  However, this required regular monitoring to keep the International Normalized Ratio (INR) in the correct zone.  Not enough warfarin, and your ischemic stroke risk increased.  But too much warfarin, and you risk having a hemorrhagic stroke rather than an ischemic/embolic one.

That's why we developed risk scores like CHADS2 and CHA2DS2-VASc to better predict stroke risk.  Of course, we also need to balance against the risk of hemorrhagic event using the HAS-BLED calculator.  The good news is that a study published early online last month in the American Journal of Cardiology concluded that both CHADS2 and CHA2DS2-VASc improved the ability to predict 1st hospitalization in patients w/atrial fibrillation and thus could be useful in guiding prophylactic therapy.  The authors evaluated claim data on 377,808 patients avg 74yo to arrive at their conclusion, noting that CHADS2 > 6 points more than doubled the risk of hospitalization while CHA2DS2-VASc > 9 points tripled one's risk, both compared to patients w/scores of 0 points.

What then caught my eye was a study published last week in Circulation in which the authors concluded that  troponin I and N-terminal pro-B-type natriuretic peptide (NT-proBNP) are independently associated with an increased risk of stroke & mortality beyond what was achieved by using CHADS2 and CHA2DS2-VASc alone.

The importance is this: CHADS2 and CHA2DS2-VASc have been validated and proven to be useful in determining who should receive anticoagulation prophylaxis.  And by adding two readily available test results, we can improve upon our prediction skills such that we can narrow down the group of patients who would benefit from anticoagulation while avoiding treatment in those who are more likely to be harmed.

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