While the thought of running 26.2 miles may be heart-stopping to some, Chris Taylor is fully prepared to cope with such an event.
The runner from Lancashire will be the first ever person to run the marathon while fitted with the medical device – similar to that recently fitted to footballer Fabrice Muamba.
Taylor suffered from a genetic disorder that means the walls of his heart are different to normal and means he could suffer a heart attack when involved in physical actitives such as playing sport or running.
He told the Daily Mail that he hopes his effort this Sunday will help to highlight his disorder as well as raise funds for the British Heart Foundation.
He added: "It's an extremely rare condition and the medical understanding of it is changing all the time.
"I wanted to do something that would help raise of awareness of my condition and others like it."
The subcutaneous implantable cardioverter-defibrillator is fitted under the skin of his arm and monitors his heart. If for any reason the heart stops it immediately delivers a shock to get it re-started.
Taylor is hoping to complete the race in around six hours and it has been a long-term aim to take part in the London Marathon since he watched his grandfather run the 26.2 miles many years ago.
For those planning London hotel deals in 2013, organisers have confirmed that the ballot for next year's London Marathon will open on April 30th.