July 1, 2003
A: FROM MENTOR JOAN LUSK IN RI
Electric shock won't cure everything that can be wrong with a heart -
it won't cure a heart attack, in which the blood supply to the heart
itself is cut off.. What it's good for is defibrillation: getting
the heart to beat with a normal rhythm when its beating has become
disorganized. All the individual muscle fibers have to contract
together with the others in their part of the heart, so that the
blood is pushed out of the atria or ventricles when it ought to be.
If the individual fibers are contracting at random times, in a
disorganized fashion, the heart won't be pumping. The shock sort of
resets the beat. I don't know exactly how it does that - possibly
the electric field is strong enough to affect all the cells at once,
and they take up their usual regular rhythm from that starting point.
There is a natural pacemaker that sets the beat - possible the
external shock might work through that, sort of the way an implanted
pacemaker does.

If the heart is actually beating on its own and the patient has a
pulse, then applying the shock can be dangerous - it might stop the
heart rather than start a non-beating one. Here are a few sites that
will tell you more.

http://www.concordhospital.org/cardiacservices/cardiac_AED_story.html

http://www.med.umich.edu/1libr/aha/aha_electric_car.htm

http://www.healthatoz.com/healthatoz/Atoz/ency/defibrillation.html