Implanted defibrillator does not prevent an arrhythmia. It only treats it. If you have frequent recurrences of arrhythmia and shocks, your doctor will put you on appropriate medications to suppress the arrhythmia if permitted by your cardiac condition. The device may also need programming to optimise its detection and treatment algorithms as well. Certain arrhythmias like ventricular tachycardia may be treated with overdrive pacing before attempting a shock. Oversensing of T waves or other signals can cause false arrhythmia detection and inappropriate shocks. Sometimes you may feel a “shock” when actually none has been delivered. This is known as “phantom shock”. All these possibilities can be checked by device programming / interrogation as the device stores electrograms of the episodes of arrhythmia detected by it. Device programming is a non-invasive procedure in which a programmer head with radio communication facility is placed over the device and the information collected in the programmer device for display / print out.