Kyle Larson has been pulled out of today's STP 500 NASCAR Sprint Cup Series race at Martinsville Speedway, after the 22-year-old fainted at an autograph session on Saturday afternoon following the end of practice.

His place in the #42 Ganassi Chevrolet will be filled by Regan Smith, the team announced on Sunday.

Larson had been taken to hospital when he fainted after signing autographs for fans at his merchandise hauler. Reports at the time said that he was awake and alert and was exhibiting no problem in recalling the events of the two practice sessions earlier in the day.

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Initially the Chip Ganassi Racing team said that Larson's condition did not appear serious and that they expected him to race in this week's Cup race on Sunday afternoon, but that doctors at Martinsville Memorial Hospital had referred him to specialists for further tests.

Larson was sent on to Carolinas Medical Center in Charlotte to be evaluated late Saturday night as there were no available consultant neurologists in Martinsville itself.

Even though the doctors found nothing wrong with the results of the tests, there was concern about Larson having fainted for no immediately clear reason and as a result decided to recommend that Larson sit out today's race rather than risk any recurrence while behind the wheel until the underlying cause was better understood.

"After fainting yesterday at an autograph session in Martinsville, VA, Kyle Larson was first evaluated at a local hospital in Martinsville and ultimately evaluated at a Charlotte hospital," the team said in a statement released Sunday morning.

"Although all tests came back negative and Larson feels completely fine, the doctors felt he should be held for more testing today," the statement continued.

"Subsequently, Larson will be unable to race today in the STP 500 in Martinsville. Regan Smith has agreed to fill-in for Larson today in the #42 Target Chevrolet SS."

It's been an unusually busy year for Smith, who as well as running a full time campaign with JR Motorsports in the NASCAR Xfinity Series has also already had three Cup race outings on top after deputising in the Stewart-Haas Racing #41 car during Kurt Busch's suspension over domestic abuse allegations.

Smith had a consistent run while filling in, finishing in 16th in the Daytona 500, 17th at Atlanta and 16th again at Las Vegas before Busch returned to active duty in time for Phoenix. Smith has nine previous Sprint Cup Series starts at Martinsville with a best finish of 13th in 2011, and 175 Cup start in total.

"I pretty much just got wind that something was going on last night," explained Smith of how today's ride had come about. "We had a plan in affect that if for some reason Kyle couldn't race today. I didn't know anything until this morning when I woke up. I had a text message when I woke up this morning and headed this way.

"We were fortunate to have a little time last night to work with the seat and try and get me as comfortable as we could with what we had to work with from that stand point, last minute and stuff. Outside of that getting the seat in the car and getting that stuff ready that has been about the extent of it.

"I talked to Jamie this morning here for a little bit just now in the hauler," he continued, referring to his Ganassi team mate for today Jamie McMurray. "And have gone over with both teams where their cars are at and what they thought of their cars. And what they think they are going to do today, what adjustments they have made for today. I talked to Jamie about the tyre, about the track and how it is reacting compared to the last time I was here, which as we mentioned before was a while ago. I got a pretty good feel for all that stuff."

Larson had been due to start from seventh place on the grid for today's race, but the change of drivers will mean Smith will have to drop to the back of the grid before the green flag.

With the the 0.526-mile oval - nicknamed the Paperclip - already the shortest and most cramped circuit on the Sprint Cup calendar, making his way forward through the traffic without being caught and lapped by the leaders will be a tough task for Smith.

"There are going to be a lot of challenges today," he admitted. "I haven't been here in a while for one. It's a place where when you have to start 43rd like we do; the leader is going to be right there quick. I think one of our struggles is going to be trying to stay on the lead lap early on. If we can do that and maybe find a position where we are able to get some track position throughout the course of the event then great, we will take advantage of those when we can."

The team was busy working first thing on Sunday morning replacing Larson's race seat with one fitted to Smith's measurements. Larson meanwhile remains in hospital and under observation. Fortunately next week sees all the NASCAR championships take a week off for Easter, giving Larson more time to rest and recuperate which should ease some of the pressure on him.

Larson will still be eligible to make it into the Chase despite missing today's race because he had had already qualified for the event, satisfying NASCAR's requirement that a driver must compete each weekend.

As for Smith, he had had mixed emotions about being the current in-vogue 'super sub' for Chevrolet-powered teams to turn to this season.

"I would rather be racing than watching it on TV!" he said. "I'm okay with that. I think I have made it clear I want to get back to doing this on a more permanent basis on the Cup side. If being the guy that everybody calls on can help lead to that down the road, then that is great.

"These situations they aren't easy. You are hopping in somebody else's car and then somebody else's set-up, somebody else's team, somebody else's guys and trying to plug yourself in as best you can. And do as good as you can for that team and for those guys. With that said, you try to make the most of it.

"I'm getting a lot of experience with different people and different teams," he added. "And learning how every place works. Every place is different and every place has different operations, every crew chief talks different, every spotter talks different, so it is difficult. Anytime you can get in a rhythm and have your own deal it is obviously easier. But you it is still a race car, it still has a steering wheel and it still has gas pedals. We still do the same stuff with it, so you can work around a lot of that."