Fabrice Muamba survived his cardiac arrest in March for two reasons: his fitness and the speed with which he received excellent medical attention. His heart stopped in the middle of a football match because of an abnormal cardiac arrhythmia, or irregular heartbeat, caused by an undiagnosed heart condition, which he may have had since birth. His heart stopped for more than an hour, but he was kept alive and "healthy" by CPR – life support.
Muamba was fitted with an implantable cardioverter defibrillator (ICD) before being discharged from hospital – a small device to detect any future irregular heartbeats. The ICD works in two ways: it first tries to correct the arrhythmia. If that doesn't work, the device then switches to defibrillator mode and shocks the heart back into time.
Since then the player has talked about some adverse effects such as short-term memory problems. It appeared that he was recovering slowly, and he intended to make a comeback with Bolton.
However, the medical advice to retire from football would be based on a balance of probabilities. His doctors clearly feel that the risk of another serious arrhythmia and cardiac arrest, which could be fatal or cause serious complications such as pneumonia, is too high.