The point raised about defibrillating on a metal stretcher seems, at least a priori, an interesting one. It is (theoretically, at least) possible that the metal back-plane could well shunt away a proportion of the shock. It will also change the overall impedance, so that will create a mismatch in older machines that don't correct for source / load impedance problems "on the fly". Again, this could lead to a reduction in the actual energy actually dumped into the myocardium.
In fact, the situation could be more complicated, since the field lines will be distorted somewhat, and the "direction" of the shock with respect to the orientation of the heart could be changed. Whether clothing provides adequate insulation depends on the field distribution.
I have no idea how big these effects are- the symmetry of the problem and the electrical non-linearity and inhomogeneity of structures in the chest make a "back of the envelope" calculation a little difficult, or even a finite element simulation (I've been pondering it all day!!). It may well all be negligible, but I suspect that experiments are the only way to proceed if you want to be sure, as I think that there isn't going to be a good analytical way out. I wonder if anyone has ever looked at this properly- anyone know?
I don't think that things should be any worse with a manual defibrillator, though, since a poor contact is still a contact to the skin, and not directly to a stretcher underneath the patient.
I'm sure I've heard anecdotes of people being successfully shocked on metal ship decks...
Ari
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