This was very frustrating when playing assault. I could hand out med kits all day long, but I couldn't revive anyone without fumbling through the mouse scroll wheel to whip out the defibrillators. By the time I was ready to yell "Clear!" it was too late. So...many...dead. If only I could have got the paddles out in time!
I spent a while googleing and was coming up empty so I headed to the official forums at which point I ran across a post by wazups2x
The fix is to bind "Select Gadget 1" and "Toggle Weapon Underslung" to the same key.