I also have not encountered the theory about chest compression
controlling fibrillation. Since defibrillation is also controlled during
open heart surgery by paddles directly applied to the heart and through
the use of wire catheters from implantable defibrillators, it is not the
only means.
I thought it might be interesting to describe the heart function in
electrical terms since that is the basis of understanding for most of us
in this group.
Heart muscle can be viewed as a simple free running flip-flop
oscillator. When cut into pieces, each piece of heart muscle will
automatically fire (contract) at its own natural rate. It can be
triggered earlier by input from neighboring tissue. Once it fires, it
goes through a relaxation stage when it is quite resistant to triggering
by neighboring tissue. This firing at a cellular level is call
depolarization. It is caused by the cell wall suddenly becoming
permeable to sodium and potassium ions and loosing its charge (which of
course is a current flow). The cell then takes time to recharge by
pumping ions back across the cell wall.
The master clock for the heart is the sinoatrial (SA) node (the sinus
node mentioned by Gary). This node sets the pace for the heart in
response to assorted hormone and brain inputs. It is located in the
right atrium. Remember the heart has four chambers. The right atrium is
a collecting spot for the returning blood and when it contracts, moves
the blood next door to the right ventricle. The right ventricle pumps
into the lungs. The left atrium holds returning oxygenated blood and
pushes it into the left ventricle which pumps into the rest of the body.
Obviously the ventricles are the workhorses.
When the sinoatrial node fires, a wave of depolarization spreads over
both atria (1/10 of a second), but is protected from reaching the
ventricles by a layer of insulation. At the base of the right atrium it
reaches the atrioventricular (AV) node. This is a delay line (another
1/10 of a second) and passes the signal to the left and right bundle
branches which are special conductors to get the signal quickly to all
parts of the ventricles. The AV delay provides the time for the atria to
finish filling the ventricles before the much more significant
contraction of the ventricles.
Since all these conductors are live tissues, injury or irritation,
depending on where it occurs, can cause all sorts of problems like fast
or slow rhythms, lack of coordination of atria and ventricles, etc. One
solution is implantable pacemakers which in their simplest forms
electrically trigger the ventricles (the atria are left to themselves
since they are not as important).
Fibrillation occurs when something (like electric shock or irritation)
triggers a piece of heart muscle. This in turn triggers neighboring
cells. Unfortunately when the coordinated signal arrives from elsewhere,
the cells which have just fired can't respond since they have not gone
through their refractory period. These misfired cells then wait (while
other parts are recovering) and having waited too long, fire on their
own again. When several locations of the heart are doing this, the heart
just quivers instead of making a coordinated pumping effort. The fix is
to provide an electrical jolt which doesn't bother with triggering, it
just hits all the cells with enough energy to force depolarization
anyway. Then all cells together go through their refractory period and
are ready for a coordinated trigger (if it still exists). This is why
very high shock levels can avoid causing fibrillation.
Defibrillators have come a long way from the old days when they just
applied a severe 60 cycle AC shock. These days they try to provide a
minimal level impulse coordinated with any residual heart beat to force
the heart into unified action. The impulse can be applied with external
paddles, and now is available as a built in part of implanted pacemakers
so the impulse can be applied directly to the heart using the pacing
electrodes.
Bob Johnson
-----Original Message-----
From: owner-emc-p...@majordomo.ieee.org
[mailto:owner-emc-p...@majordomo.ieee.org] On Behalf Of Bill Owsley
Sent: Monday, November 19, 2001 1:22 PM
To: Gary McInturff; 'Gregg Kervill'; 'Rich Nute'
Cc: jrbar...@lexmark.com; emc-p...@majordomo.ieee.org
Subject: RE: Define Continuous DC Voltage
I've never seen cardiac function or resuscitation explained this way...
and I'm an EMT-D. The D is for defibrillator and the EMT is for
emergency medical technician. And with very few exceptions, the rest of
the medical aspects of this discussion have been suspect.
As my kids say - don't go there...
- Bill
At 12:02 PM 11/19/2001 , Gary McInturff wrote:
From a few courses several years back.
The heart has something called the Sinus node (spelling could be
wrong) The responsibility of that node is to control the timing of the
electric wave "front" if you will. The heart actually has about three
pulses. Looking at a heart waveform on a monitor you will see a small
blip,
big blip, and another smaller blip (those all being medical terms
naturally). Those are the QRS waves, and sweep across the heart, from
the
input side to the output, although the "big blip' is the main blood
moving
event. I no longer remember exactly what each of the pulses does, but
all
three are needed for the pumping of blood through the heart chambers,
and
the sinus node does all the time for these events.
When "low" level current disrupts this timing sequence the heart
starts to fibrillate - it beats unrhymtically and "quivers" not only
does it
not pump blood but it works itself into exhaustion. The node needs to be
allowed to resynchronize. That is done with a "high" current applied to
a
defibrillator paddle from one side of the chest to the other.
When this is done it is not the current through the heart that
is
effective but the current through the muscles of the chest that are
effective. The current causes the muscles to constrict hard enough to
squeeze the heart muscle and prevent it from the uncontrolled and
uncoordinated pulsing. The current is release, the muscles relax, and it
is
hoped that the sinus node re-takes control of the heart. This is the
reason
that the old method of reviving someone by slamming them in the chest
worked. It forced the heart to stop long enough for the sinus node to
reestablish itself.
People that work around high voltage are more prone to death by
falling from the high voltage lines, or internal burns that actual heart
failure do to heart fibrillation.
Gary
-----Original Message-----
From: Gregg Kervill [mailto:gkerv...@eu-link.com]
Sent: Tuesday, November 13, 2001 1:12 PM
To: 'Rich Nute'
Cc: jrbar...@lexmark.com; emc-p...@majordomo.ieee.org
Subject: RE: Define Continuous DC Voltage
Hi Rich
> There was also a very good (but short) article by Tektronix in the
70's
> called The Lethal Current.
>
> It concluded that currents between 100 mA and 3 Amps were more
lethal
that
> currents of more than 3 Amps because those high currents tended to
'restart'
> the heart.
Hmm. Having been the manager of product safety at Tektronix in
the '70's, I don't recall such an article. At least not by that
name.
- I'll try and find it - it may have called the fatal current circa 72-5
published in the UK
B-I-G SNIP
INFORMATION OVERLOAD!!!!!! I'm squeamish
So, Gregg's statement that there is both a lower and upper limit
for fibrillation is correct (although I do not agree with Gregg's
values).
Hang on - I'm trying to quote from an article I read in the early 70's -
and
the figures were from the article.
I'm sure it was from Tex - we had a number of the big valve 'scopes
(plugins
and more than 100 valves) - wonderful things and the only ones that
allowed
a delay longer than the TB sweep.
I'll try to dig the article out - I found it very useful - particularly
since the safety standards at that time were pretty Spartan.
Thank goodness they have - and continue to help and provide guidance.
Best regards
Gregg
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British Prime Minister Tony Blair pointed to the victims of the Sept. 11
attacks on the World Trade Center and the Pentagon, and said the Taliban
regime had no "moral inhibition" on slaughtering innocent people.
"There is no compromise possible with such people, no meeting of minds,
no point of understanding with
such terror," he said. "There is just a choice: Defeat it or be defeated
by it and defeat it we must."
"Whatever the dangers of the action we take, the dangers of inaction are
far, far greater," he said.
----------------------------
Bill Owsley, ows...@cisco.com
919) 392-8341
Compliance Engineer
Cisco Systems
7025 Kit Creek Road
POB 14987
RTP. NC. 27709
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