In the United States ECT is used when all known anti-depressents have failed to help a person and they have treatment refractory depression or if the person is suicidal and anti-depressents don't help. In other countries it may be used beyond that point but basically should be considered an option after a person cannot be treated with anti-depressents. For some more information on anti-depressents google "Depression Central". In the United States, trans cranial magnetic stimulation has now been FDA approved and is proving to be safer long term than ECT and effective as well. I am not sure of its status in other countries but you could ask about it.
Do NOT let anyone perform ECT on you! You might as well ask for a lobotomy. That is MY humble opinion.