ICDs are highly effective. When the device detects a dangerous heart rhythm, it delivers a shock in the vast majority of cases. Clinical evidence consistently shows appropriate shock delivery rates above 95%, though the exact figure varies depending on the device, the condition being treated, and how the device is programmed.
ICDs do not fail often, but they are not infallible. Lead fractures, sensing errors, and inappropriate shocks (where the device fires when it does not need to) are all recognised but relatively uncommon complications. Your device clinic monitors for signs of these at every check-up, and remote monitoring can flag concerns between appointments.
If your ICD delivers a shock, contact your device clinic or go to A&E. Whether the shock was appropriate or not, it should always be reviewed. Our page on living with an ICD covers what to do if your device activates.