Question: What kind of heart rhythm problems will cause my defibrillator to fire?
Answer: Your defibrillator is designed to respond to a high heart rate that's usually associated with a life-threatening arrhythmia. So most of the time, it will respond to either ventricular tachycardia or ventricular fibrillation which are lethal rhythms from the bottom chamber of the heart that would be associated with loss of consciousness and even sudden death.
On the other hand, the device will respond to high heart rates in general such as high levels of exercise can sometimes reach the heart rate cutoff for a defibrillator shock and likewise other arrhythmias such as atrial fibrillation can cause the device to see a high heart rate and deliver a shock.
Finally, on rare occasions, the device will be fooled by a problem with the lead or some problem in the circuitry such that it thinks there's an abnormally high heart rate and deliver a shock when indeed there is not an abnormal rhythm. You can get an idea as to whether the shock was appropriate in part by what kind of symptoms you were feeling at the time of the shock. If you feel like your heart was racing and you were starting to pass out, that would suggest an appropriate shock. On the other hand, if it shocked during a high level of exercise when you felt great, you'd have to wonder whether your heart rate had reached the cutoff for that device and it had delivered a shock for the normal high heart rate that's seen with exercise.