Question: If my defibrillator fires does that mean it just saved my life from a life threatening heart rhythm problem?
Answer: If your defibrillator has gone off, whether you felt it or not, whether you've lost consciousness and were told that a shock occurred or whether you feel the shock, there's a good chance that it operated appropriately and shocked in response to a lethal rhythm. That's more likely the case if indeed you were losing consciousness or had lost consciousness.
Occasionally, a defibrillator will shock for an inappropriate reason. That can be a high heart rate such as sinus tachycardia or the rhythm that happens when you exercise or another non-lethal arrhythmia may reach the rate cutoff such that the device -- which can't really tell the difference between a bad fast rhythm and a not so bad fast rhythm -- the device would go ahead and apply a shock.
Another possible cause for a shock is if there's a malfunction in the device. While this is uncommon in terms of the device itself, or occasionally will occur with a lead problem such that it over senses and then the device thinks there's an abnormal fast heart rhythm and a shock is delivered.
Whenever a shock does occur, it's a good idea to go ahead and call your doctor and let your doctor's office know what happened. If it's the first time it's ever gone off, then it might make sense to call anytime day or night. But certainly if there's been multiple shocks, two shocks in a row, that's considered really an emergency and should prompt you to get urgent medical attention.