Every morning Alison Box is filled with dread as she creeps into her teenage son's bedroom to check whether he has survived the night.

Christopher, 13, has been diagnosed with an irregular heart rhythm leading it to stop beating while he sleeps.

The life-threatening condition was detected by doctors after he became one of the first children in the country to be fitted with a state-of-the-art heart monitor.

But next week, Christopher, of Little Horton, Bradford, will be fitted with a pacemaker in a bid to return his life to normal.

Called a reveal device, the monitor implanted in Christopher's chest automatically detects and records changes in his heart's rhythm. Data is then downloaded on to a computer system at Leeds General Infirmary every three months.

Alison, 35, a teaching assistant, said she first noticed something was wrong last year when Christopher, who had never shown any signs of ill health, suddenly started to experience dizzy spells on the football pitch and collapsed at home.

But she never imagined anything so serious to be wrong with him.

She said: "During the night his heart rhythm is stopping for up to three seconds.

"I'm coming to the point now when I'm waking him up at 4am as that seems to be the time when the heart is slowest. It's so frightening.

"Without the monitor, we wouldn't have realised what was happening as he was asleep. If they hadn't have implanted it, he might not have been here now."

The monitor, which Christopher can also activate when he is dizzy or has chest pains, was fitted last June.

And doctors are now investigating the family's medical history after Alison's 15-year-old nephew Nicholas was diagnosed with a similar condition. Her niece Lauren has also been fitted with a reveal device at the age of five although her daughter Briar, eight, has shown no signs of the condition.

"It seems to be hereditary and the doctors are looking into it. The older they get, the worse it is," said Alison, whose partner Christopher Williams, 35, owns Biscuit clothes shop in North Parade, Bradford.

"When Christopher gets up in the morning, he's very lethargic as his heart's going so slowly during the night and almost stops. I don't think the doctors were expecting to find anything so serious."

Christopher, a pupil at Thornton Grammar School, will undergo the operation to fit the pacemaker on Monday.

LGI consultant paediatric cardiologist Dr Mike Blackburn said that reveal devices, introduced two years ago, were now fitted in patients as young as three, but that only one to two per cent of pacemakers were implanted in children.

He said: "The reveal device has been a real boon for the paediatric side. We are now putting one in every couple of weeks.

"Often the patients have been having problems for a time and we have been trying to capture it on tape. But with the reveal device, we have had a result every time.

"The self-activating model we use now is very useful for children who are not able to tell their parents about the symptoms."

The pacemaker will use a sensor to detect the heartbeat and electrodes to stimulate the heart muscle will be triggered if the rhythm becomes too slow - usually below 68 to 72 beats per minute.

But despite his heart rate of only 20 beats per minute at night, Christopher will also be prescribed medication during the day when it pumps too fast.

Alison said: "He'll still be able to do everything he likes such as swimming. The only things he won't be able to do are contact sports like rugby or kick boxing.

"He's a real livewire. He seems to be dealing with it quite well, but as the weeks go by, he is getting more worried."