PORT ST. LUCIE, Fla. -- Shaun Marcum may have served up a two-run homer Saturday in his spring debut, but more vexing for the right-hander was the lack of feel for his changeup.

"It's a good thing it's Spring Training, and we can get out to the bullpen on Monday and keep working on it, and improve it and get better each time out," Marcum said after giving up three runs in two innings against the Nationals. "The changeup is my main pitch. That's definitely a feel pitch for me, so I throw it more than I throw anything else during the spring."

Marcum said he typically does not even use any of his breaking balls until his third or fourth start, relying on a fastball-changeup combination in the interim. Once he has established command of his signature pitch, Marcum begins working in his entire repertoire.

"If I feel like I can throw my fastball and changeup where I want to," Marcum said. "My breaking balls come pretty easy for me."

Top prospect Wheeler impressive in debut

PORT ST. LUCIE, Fla. -- Zack Wheeler took the mound in the third inning Saturday for his first big league appearance, walked the first Nationals batter he faced and threw one of his next pitches to the backstop.

That was enough for catcher John Buck and third baseman David Wright to jog to the mound, seeking to calm their ace of the future.

Buck tried humor: "You a little amped up?" he asked. "Is your heart beating?"

"He kind of chuckled and was like, 'Yeah, just a little,'" Buck said. "It was kind of funny. He was a little shocked that I asked right in the middle of an intense moment."

The chat worked. Wheeler finished the third inning with consecutive strikeouts of Tyler Moore and Chad Tracy, both on fastballs clocked in the mid-90s, then generated three ground balls in a scoreless fourth. Relying heavily on his fastball and high-80s slider, Wheeler also dropped in two overhand curves in his outing.

"There wasn't really nerves," Wheeler said. "I was just a little pumped up going out there. After I got settled down, after the first couple of batters, I think everything went well."

"Yeah, he was amped up," Buck agreed. "He should be amped up."

Amped or not, Wheeler showcased the kind of pure stuff that has made him one of the top pitching prospects in all of baseball. Manager Terry Collins reiterated Saturday that Wheeler will almost certainly begin the regular season back at Triple-A, though continued success can only accelerate his timetable.

"Obviously a great arm," was Collins' evaluation. "The first inning I'm sure he was all geeked up and he settled down. The second inning he really pounded the strike zone, which was pretty impressive to see."

Feliciano has non-threatening hole in exterior of heart

PORT ST. LUCIE, Fla. -- Mets reliever Pedro Feliciano has been diagnosed with a non-threatening hole in the exterior of his heart, a condition that he said has caused an irregular heartbeat.

"They said everything was fine, just there was a little hole outside my heart," Feliciano said upon returning to Mets camp Saturday. "I've never felt [anything] with my heart, so it's weird."

Feliciano flew to New York for an examination by team doctors Friday, after his routine pre-Spring Training physical revealed a non-orthopedic irregularity. The left-hander returned a day later and will wear a heart monitor this week as part of an additional battery of tests.

"It was kind of a surprise for me, after I worked out with rehab and Puerto Rican Winter Ball and everything, and now have this?" Feliciano said. "It's hard, but I'll be here."

The Mets would not reveal the specifics of Feliciano's condition.

"My understanding is he has a heart condition that could possibly be very serious," manager Terry Collins said. "That's why we're being very, very careful with him. Until we get further results on some of the tests they took in New York, he's not to be on the field."

A member of the Mets from 2002-04 and 2006-10, Feliciano returned to the club this winter on a Minor League deal worth $1 million, plus incentives, if he makes the team. He is one of a half-dozen lefties battling for jobs in the Mets' Opening Day bullpen.

Outfielder Jamie Hoffman, whose physical was also flagged due to a previous heart condition, has been cleared to play.

Mets' single-game tickets on sale Friday

PORT ST. LUCIE, Fla. -- Mets single-game tickets will go on sale Fri. at 10:00 a.m. ET at Mets.com, the Mets announced Saturday. An exclusive presale for Citi credit and debit cardholders will start on Monday at noon and run through Wednesday.

New York-area cardholders will also have the opportunity to purchase tickets on Monday at 1 p.m at Bryant Park, where they will have the chance to meet former Mets Darryl Strawberry and John Franco. Fans will be able to visit Mets.com/Presale for more information.