In what may be a world first, doctors in the US state of New Jersey say a fitness tracker has saved a man's life.

English teacher Jeff Bravo was at school when he had a seizure.

His heart rate sparked wildly and he was rushed to hospital where doctors used the information from his Fitbit to treat him.

"They shocked my heart, it was able to go back to a normal rhythm and I was sent home later that day," he said.