Brian VickersThe Red Bull Racing team has revealed that blood clots are the cause of Brian Vickers' absence from this weekend's NASCAR Sprint Cup Series race at Dover.

The team announced yesterday that Casey Mears would substitute for Vickers due to a recent diagnosis of a medical condition, after he was hospitalized on Wednesday evening during a visit to Washington.

At Dover, the team's vice president and general manager Jay Frye revealed the seriousness of the diagnosis, and said it is uncertain when Vickers will be back in the No. 83 car.

"This past Wednesday, Brian was in Washington, D.C., preparing for his Thursday visit with Congressman Doc Hastings at the Walter Reed Army Medical Centers," said Frye. "He was having some discomfort in his chest and, with the encouragement of Dr. Jerry Petty, Brian went to an emergency room in the D.C. area to get checked out.

"Those tests were conducted including a CT scan. The scan discovered several blood clots in his veins, not his arteries. He was admitted Wednesday evening to the hospital, currently he's being given medication to dissolve the clots. He's been thoroughly tested and given a good clean bill of health. The doctors are not sure what caused the clots or how it happened.

"We've been in constant contact with Brian, he's in great spirits and doing very well. At this point there's really no timetable for his return.

"It is a minor setback in a young man's life and his career. It's a major setback for our race team for this weekend. We're really not sure what the next steps are. We've got great reason for optimism that everything is going to be fine."

Frye said one of the clots in Vickers' chest was found "around his lungs" and while he insisted that they do not know exactly what caused them, he said there are contributing factors. Inhalation of carbon monoxide is said to be possibly one of them.

"Some of the contributing factors are things that we do every week – and we do it a lot," said Frye. "So is that the reason? I don't know. I'm not a doctor, but I'm sure it all adds up somewhere in there."

Vickers is currently in his seventh full season in the NASCAR Sprint Cup series and his fourth with Red Bull Racing Team, having joined the outfit for its debut season in 2007 after three years with Hendrick Motorsports. Last year the 26-year-old claimed Red Bull's first victory at Michigan after starting from pole position, and he then went on to make the Chase, finishing 12th in the final standings, his best ever at NASCAR's top level.

It is still unclear whether Mears will continue to fill in for Vickers in coming weeks if needed.