Defibrillation

[Kaeroll]

This came up in a conversation I had with a medic student I know... in a lot of TV shows, defibrillation is used to revive flatliners. According to Wikipedia, this is not done in real life, and we understand why that is.

 

My friend suggested that it might actually work, as a similar method is used to restart hearts during transplants (albeit by directly shocking the heart rather than the chest). Can anyone settle this for us?

[GDG]

My understanding is that a defibrillator works by delivering a large enough charge that all of the heart muscle cells are depolarized, and thus unable to contract until they have restored their ion imbalances. Basically, it takes all the overactive muscle cells and gives them a "time out." This interrupts the fibrillation (if successful).

 

I'm not sure how transplanted hearts are restarted, but I suspect they use a temporary pacemaker. This also shocks the heart, but at a much smaller voltage, and in an appropriate rhythm. The idea here is not to depolarize all the cardiac tissue, but rather to stimulate the heart's natural pacemaker to begin initiating contractions.

[Glider]

Defribrillators work as stated by GDG, and are used in cases of ventricular fibrillation (VF), and is why it's called a de-fibrillator. Fibrillation is where groups of myocardial cells begin to contract independently and out of phase with other groups around them, but at least there is activity. VF is a life-threatening rhythm.

 

Ventricular fibrillation is significantly different from cardiac arrest (ventricular asystole or VA), which is a lethal rhythm. There is no activity and no fibrillation so, nothing to defibrilate.

 

A direct charge to the myocardium (propagating to the sinoatrial and atrioventriculoar nodes) can reinitiate cardiac activity in an otherwise healthy heart. However, in people who are sick enough to go into cardiac arrest in the first place, the system is usually not sufficiently healthy to re-start by that method. If it was, it wold not have stopped in the first place.

Edited April 11, 2009 by Glider
To correct acronym

[Kaeroll]

Thanks for the replies. I'll pass them on.