New Zealand's greatest athlete, Sir Peter Snell, has been stopped in his tracks by heart problems and will not be making a trip home for a celebration to mark the most memorable hour in New Zealand's Olympic history.

The 71-year-old said he could easily have died a year ago after blacking out for up to 15 seconds.

He has now been fitted with a pacemaker/defibrillator device on the recommendation of his cardiologist, after being diagnosed with a weakened heart that can no longer pump blood efficiently - because of a condition officially know as idiopathic dilated cardiomyopathy.

He is also on drugs to lower his blood pressure and reduce arrhythmia.

There had been plans to bring Sir Peter and fellow Arthur Lydiard-trained athlete Murray Halberg together to mark the day 50 years ago when New Zealand won gold in the 800m (Snell) and 5000m (Halberg) within 60 minutes at the Rome Olympics on September 2, 1960.

But in an email this week, he said he could not risk the trip.

About a year ago he crashed while riding his bike to work in Dallas, Texas, and a few weeks later he "passed out for 10-15 secs while playing racquetball".

He said he found himself "mindful of my mortality and not wanting to make a long tiring flight for a round of charity lunches and dinners".

Sir Peter said there was nothing he could have done to prevent the damage to the left ventricle of his heart.

At the time of his cycling accident, he said he could not remember what had happened because of his concussion.

The three-time Olympic champion, six-time world record-holder and Halberg Trust Sports Champion of the Century, said that over the previous couple of years he had noticed his fitness was declining and he was getting breathless while jogging.

"Cycling was okay so I decided that my lack of running due to knee osteoarthritis was responsible.

"As I later found out this, was not the case.

"At the time of passing out on the court I underwent a series of tests, including a coronary angiogram and a high resolution MRI scan."

He said it came as no surprise that his heart was found to be "big", because of the blood-oxygen demands required of an elite runner.

Though his coronary vessels were "large and clean", an MRI scan found a mass in his left ventricle.

"Only 33 per cent of the blood in the heart was being ejected with each beat, compared with 75 per cent normally."

Sir Peter collapsed for the second time early last month while playing racquetball and "was out for 15secs before my device delivered a shock and converted the heart into normal rhythm. This may have saved my life".

He was lucky his playing partner was a doctor but it was another warning for him.

"I have stopped engaging in intense exercise but am doing some cycling at a fairly light level."

Sir Peter had been keen to return to New Zealand after last year's "Three Knights" evening with Sir Murray Halberg and Sir John Walker.

He has reduced his workload by 50 per cent to spend more time in the garden, playing golf, doing his crafts and with his wife Miki.

He summed up his email simply: "Feel free to let people know. Life can be fragile."

Wins 800m Olympic gold at Rome

Wins 800m and 1500m at Tokyo Olympics

Inducted into Athlete Hall of Fame at University of Rhode Island

Recognised as NZ Athlete of the Century