An 18-year-old boy in the US has credited a smartwatch for saving his life after the device alerted him that his heart rate was much higher than normal, leading to the diagnosis of a life-threatening condition.

Paul Houle, a senior at Tabor Academy in Marion, Massachusetts, felt back pain after participating in two football practices in sweltering heat.

"I thought I was just sore and a little out of shape because the practice was hard. I didn't think much of it," Houle said.

Back at his dorm room after practice, Houle checked his Apple Watch's heart rate monitor. The watch indicated his heart rate was 145 beats per minute, a full 60 to 80 beats higher than an average resting heart rate.