Jon Niese #49 of the New York Mets throws against...

Jon Niese #49 of the New York Mets throws against the Texas Rangers at Rangers Ballpark in Arlington. (June 25, 2011) Credit: Getty Images

DETROIT — The Mets still are unsure what caused Jonathon Niese’s rapid heartbeat, or tachycardia, during Saturday’s game against the Rangers. But they were relieved yesterday after Niese had a battery of tests and was examined by a cardiologist at Henry Ford Hospital.

“So far, all the tests have come back normal,” assistant general manager John Ricco said.

Niese had a treadmill stress test, an EKG and an echocardiogram, then rejoined the team wearing a Holter monitor, which will keep track of his heartbeat the next 24 hours.

Once those readings are checked today, the Mets hope to have a better grasp of what’s going on. Of particular concern is Niese, 24, saying he’s experienced the rapid heartbeat previously, but it’s unknown if that is a dangerous condition.

“All they’ve reported so far is what they’ve found so far,” Ricco said. “I’ll have an answer for that after they’re all done.”

On Saturday, Niese was removed with two outs in the sixth inning after having trouble catching his breath and feeling his heart race. The symptoms passed quickly, and Niese waited until yesterday to have the tests done in Detroit.

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