"I was breathing heavy and the heart was racing a little bit," Niese said. "But it wasn't really a big issue."
He was examined by a Rangers team doctor, who was satisfied with his immediate condition, Niese said. Manager Terry Collins suggested Mets doctors may want to look at Niese in New York, but Niese hoped to proceed with the team to Detroit after Sunday's series finale in Texas and have the examination on the road.
The temperature was 94 degrees at the first pitch and Collins said that the heat might have contributed to Niese's condition.
"I really don't know what it's from," Niese said. "The heat could have been a part of it. But it's not a big deal. It's more precautionary than anything."
Niese said he has experienced similar rapid heartbeats on the mound "on occasion" in the past -- and not just in hot weather.
"Just random times," Niese said. "Sometimes not in the heat. It's one of those things where the adrenaline goes and the heart starts racing and you take a deep breath and calm down."
Niese said he has never been specifically examined for the condition, but added that he annually passes a rigorous spring training physical.
"They always do the tests before the season with our hearts and all this stuff and I've had no issues before," he said. "Just like after the game today, they checked everything out and it was good."
Collins noticed Niese was laboring even before the sixth inning started.
"He got the ball between innings and he didn't go right on the mound," the manager said. "He got the first guy out, and then I saw him walking around the back of the mound after the first guy. I thought, 'Ahh, what's going on here?' I asked [pitching coach] Dan [Warthen], 'Hey, is he all right?' [Warthen] said he might be cramping up.
"And then he got the second guy out, and that's when he stood behind the mound and I could see him taking deep breaths. So that's when I went out. I said, 'Are you OK?' And he said, 'My heart is really racing. I can't get it stopped.' I said, 'Let's just stand here for a second and get it calmed down.' And we did, and he said, 'I think I'm OK now.' I said, 'Well, this is going to be your last hitter, because I'm not taking any chances.'"
Niese did face the additional batter, and was removed after Nelson Cruz doubled.
Niese limited the Rangers to two runs (both solo homers) on six hits through 5 2/3 innings. He struck out seven and walked two, earning the win in the Mets' 14-5 victory to improve to 7-6 on the season.
"It's something that I've always managed," Niese insisted. "It's not a big deal."
Adam Rubin covers the Mets for ESPNNewYork.com.