Mike said:
When the heart goes into ventricular fibrillation, you're right, it is
very
erratic and does not pump blood very efficiently. The purpose of the
defibrillator is to attempt to get the "electrical system" of the heart
back
into a normal sinus rhythm.
Right. The heart contracts in traveling waves which keep it in sync.
There's a moment after a contraction called the "refractory period" when
the muscle ignores electrical stimulation, and that's what allows the
contraction wave to flow without doubling back and getting chaotic. Once
in the chaotic state (fibrillation) it's difficult to get back to order.
A defibrillator forces all susceptible tissue to contract hard. Then ALL
heat muscle tissue goes into a refractory state. The place in the heart
where the contraction wave is supposed to begin (sorry, I forgot the
term) typically leaves the refractory state most quickly. Then a nerve
prompt, or the tissue's own propensity to periodically contract, can
start an orderly contraction wave.
If the heart is too badly damaged, some parts may fire prematurely
causing an improper pattern and possibly fibrillation. Or a section may
not contract at all, forcing the wave to propagate around the dead spot.
So early defibrillation is critical to successfully rebooting the heart.
Back to fibrillation current. Medical research has not been able to find
a lower limit for fibrillation current in *damaged* hearts since ANY
stimulus can push it over the edge. And we don't know who's heart is
damaged, so the best we can do is base safety levels on statistical
parameters. IEC 471 typically uses the 5 percentile fibrillation level
(the level which causes fibrillation in 5% of the target population) as
a guide. Equipment standards add a somewhat arbitrary safety factor to
that level.
So it's clear as mud, nothing's perfectly safe, and being born is
ultimately fatal. We make products as safe as we can consistent with
being able to use and sell them, and write standards in part to force
the competition to increase their safety levels, leveling the playing
field, so we can sell our safer products. Bit by bit we make it a safer
world. Now if we can just pass safety standards for guns and bombs...
Heber Farnsworth, P.E.
Agency Compliance Engineer, Physio-Control Corp.
(MY mutterings, no one elses)
