Path: utzoo!utgpu!watmath!iuvax!purdue!ames!coherent!dplatt
From: dplatt@coherent.com (Dave Platt)
Newsgroups: sci.electronics
Subject: Re: Re: Touching a "hot" connector
Message-ID: <31069@coherent.com>
Date: 8 Aug 89 20:35:31 GMT
References: <427@edai.ed.ac.uk> <880007@hpmtlx.HP.COM>
Reply-To: dplatt@coherent.com (Dave Platt)
Organization: Coherent Thought Inc., Palo Alto CA
Lines: 72

In article <880007@hpmtlx.HP.COM> heumann@hpmtlx.HP.COM (John Heumann) writes:

> I believe your presumption that all currents above a threshold are lethal 
> is incorrect.  What I recall is that currents within a fairly narrow
> range induce ventricular fibrillation and are potentially lethal.  Once you
> exceed this range, higher currents are actually LESS dangerous since they
> cause simple tetany rather than fibrillation.  (Of course if you go to really
> high currents you can literally fry, but now were talking about lots of
> kilovolts).

There's another factor involved: timing.  The heart's response to a
pulse of current will depend to some degree on the point in the
heartbeat-cycle at which the pulse occurs.

There's a very interesting writeup on this phenomenon in a book on
biological rhythms published by The Scientific American Library.
Briefly: the heart's internal "pacemaker" can be modelled as an
oscillator, with electrical charges travelling around the heart-muscle
in a recirculating pattern.

A strong electrical shock (strength >> a threshold) will typically reset
the "phase" of this oscillator to a specific value, regardless of the
phase of the oscillator prior to the shock.  That is... if you apply a
strong shock to the heart, it will beat immediately, with the next beat
following at the usual interval.

A weak electrical shock (strength << threshold) will not reset the phase
of the clock to its starting point... instead, it will simply shift the
phase of the clock by a lesser or greater amount (it'll shorten or
lengthen the interval to the next heartbeat, without triggering a beat
immediately).

If you draw up a three-dimensional graph (X = starting phase, Y =
current applied, Z = resulting phase), you'll find that there is an
inescapable singularity in the Z-axis values... this is a topological
necessity.  In effect, for one certain X (starting phase) and one
certain Y (current), the resulting phase is indeterminate.

This phenomenon exists in many sort of perturbable phase oscillators
(circadian rhythms, etc.).  Different systems display the singularity in
different ways.  In some systems, the singularity is a repelling
point... the system "slides" back into stable behavior at some
unpredictable point in its cycle.  In other systems, the singularity is
an attracting point... the oscillator "hangs up" and begins exhibiting
nonregular behavior.  The heartbeat-cycle is of the latter sort; if the
oscillator is disrupted, the heart fibrillates.

Tests on animals have shown that this singularity isn't simply a
mathematical artifact... it really exists.  At one specific point in the
heartbeat cycle, a properly-measured pulse of current will very probably
throw the heart into fibrillation.  The _same_ pulse, at a different
time in the heartbeat cycle, does not cause fibrillation; a stronger or
weaker pulse does not cause fibrillation.  Once the heart starts
fibrillating, it will not tend to regularize itself; someone must apply
a pulse of current (>> threshold) in order to kick the oscillator out of
its unstable mode and back into a regular phased operation.

So, it seems that your chances of surviving a shock depend not only on
the strength of the shock, but also on the timing of the shock itself.
The fact that you've been "bitten" by a wild voltage, and survived, does
NOT mean that the same voltage/current could not kill you the next time.

Don't do this at home, kids.

Disclaimer:  I've probably gotten some of the details wrong, as it has
been a couple of years since I read about the phenomenon.

-- 
Dave Platt    FIDONET:  Dave Platt on 1:204/444        VOICE: (415) 493-8805
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