None stand above me.
Normally a defibrillator is stored away, not in use.
Except when none stand below me.
A defibrillator is needed when someone is having a heart attack. They won't be standing, but paramedics will be standing over them.
Be careful with me...wish for me now, and love me now -- you may fear me seconds later.
The tool is a very dangerous last resort which has to be handled carefully.
I am small, but I have given so much.
Referring to saving lives.
I am small, but I have taken so much away.
Possibly referring to the dangers of using one.
Hearts in my world there are many -- larger than mine not any.
The most direct clue, followed by a bit of personification.