(January 25, 2008 - Insidermedicine) A new blood thinning drug called idraparinux increases the risk for undesirable bleeding in patients with atrial fibrillation more than vitamin K antagonists, although it is just as effective at preventing strokes caused by blood clots, according to research published in The Lancet.

Here are some facts about atrial fibrillation:

•    Atrial fibrillation is a condition in which the heart beats irregularly and, usually, more quickly than normal.

•    Atrial fibrillation can cause blood to stagnate in the heart, which can lead to the development of blood clots. These clots can travel throughout the bloodstream and become lodged in a vital organ, such as the brain, where it causes a stroke.

•    While vitamin K antagonists are an established and effective means of preventing the development of blood clots in patients with atrial fibrillation, they are inconvenient to use. Patients taking these drugs must have regular blood tests in order to keep the dose of the drug properly adjusted.

Researchers from the Academic Medical Centre in Amsterdam and the AMADEUS Investigators Group randomly treated patients with atrial fibrillation with either the new blood thinning drug idraparinux or with a vitamin K antagonist. The investigators stopped the study earlier than initially planned, however, because patients receiving idraparinux were experiencing the potentially serious side effect of undesirable bleeding more often than those taking vitamin K antagonists.

Patients taking idraparinux were 74% more likely to have undesirable bleeding than those taking vitamin K antagonists. Most alarmingly, 21 individuals taking idraparinux experienced bleeding within the brain, compared with only 9 taking vitamin K antagonists. Those who were elderly or whose kidneys were not functioning at full capacity were at higher risk of bleeding.

Today's research demonstrates that vitamin K antagonists remain the best prevention against blood clots to the brain in patients with atrial fibrillation, even though they are inconvenient to use. The new blood thinning drug idraparinux requires only a once weekly injection, but increases patients' risk of having undesirable bleeding.

For Insidermedicine in Depth, I'm Dr. Susan Sharma.