The .22 was designed for target shooting and varmint control: rats, squirrels, rabbits, crows, and other small animals. A well-placed head shot from a .22 MIGHT stop an attacker, but even this isn't very reliable. A former patient of mine tried to commit suicide by shooting himself in the head with a .22 at contact range. It put a nice divot in his forehead and penetrated into his brain, but didn't even knock him out. He had a mild brain syndrome for a number of months but was able to make a full recovery and return to college. Interestingly, the brain trauma made him completely forget why he wanted to kill himself!
Anyway, DO NOT think of the .22 as a man-stopper. It simply is not! A shot in the heart, lung, spleen, gut, or major artery might kill an attacker minutes, hours or days later, but that does you no good if he kinfes or shoots you a second or two after you fire. The point of defensive pistol use is to STOP or incapacitate, not kill an attacker. For that, the .380 or 9mm is about the practical minimum. I think most cops carry .40's these days.
Do yourself a favor and retire your .22 for uses it was designed for, not self defense.