If you're using a trigger to do that, it might as well be better to make the trigger do the healing. (At least if the healing is instant anyway)
I don't know what you did but setting the No Target Required to true should allow the hero to instantly use the item.
Here's a small test-map of Healing Salve that can be instantly used on the user.
To be honest, it looks kind of odd since the healing has to be done 1 second later after using it. (Blame WC3 for doing things in periods of 1 second)
This could be easily solved if you just trigger the healing.
Edit: Oh, you should just base the item ability off of Healing Salve. Just make sure that you set the fields to your preference.