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Re: defibrillators
On 18/11/2018 06:39, RichD wrote:
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Assuming here you mean the smart AED defibrillators for the public to  
use rather than the proper medical ones where you can change the shock  
pulse energy, length and even see the actual EGC waveforms.

Basically it consists of an EGC with dual purpose sensor pads and if  
appropriate it administers an electric shock of the right sort to stop  
the heart and allow it to restart with a stable pumping rhythm again.

It is smart so it won't shock a healthy patient and it warns when it is  
about to adminster the shock. Some require you to press the button. It  
talks you through what to do and when and has a slightly annoying  
metronome for doing CPR by which for some reason persists even if the  
patients heart has recovered a good rhythm. If there is no heartbeat  
then it will offer to shock the patient every couple of minutes.

When/if the real paramedics arrive then they will use their own kit.

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If the heart is no longer functioning as a pump you lose oxygen  
circulation to the brain and active cooling so damage occurs pretty  
rapidly if you don't do good CPR whilst the kit is being fetched and  
attached. Survival rate for the patient falls by about 10% for every  
minute that the heart is in a failed state so *time* really matters.

Ventricular fibrillation is the serious killer. It was someone suffering  
this sort of heart attack that led to our community buying one.

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Have you forgotten how to use google? eg

Martin Brown

Re: defibrillators
On Sat, 17 Nov 2018 22:39:53 -0800 (PST), RichD

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A high voltage generator, capacitor, and some control stuff. Basically
a big-ass flash unit with different controls.

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It stops the heart - the idea being that the heart will restart,
normally, on its own.  It does nothing for a heart attack (MI), unless
the heart goes into fibrillation, tachycardia, or some other
arrhythmia (stop, restart).  I does nothing for cardiac arrest, even.
The heart already stopped and it didn't restart itself so whacking it
again does nothing. It's only purpose is to correct an arrhythmia
(which is big enough).

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Just had them used on me (multiple times over past decade, or so).

Re: defibrillators
On Sat, 17 Nov 2018 22:39:53 -0800, RichD wrote:

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It's been awhile since I've studied this, but here's what I remember...

In VFib, the normal cardiac rhythm is taken over by a fast cyclic
loop - with an excitatory wave circling the heart (often called the
"circus movement").  As with any excitable tissue, there's a refractory  
period -- i.e. it can only fire just so many times/second.  The classic  
defibrillation method essentially excites the entire heart (nerve and
muscle) at the same time, which gets the whole heart into this refractory  
period.  This (temporarily) stops the excitatory wave.

The important thing is that there's some way to "restart" the heart.
If the normal source (the sino-atrial [SA] node) fails, other parts of  
the heart can take over, hopefully helped along by CNS control.  If the  
VFib was due to some pathological tissue it can return (the patient is  
likely toast in that case).  Also the heart can be so fatigued from VFib
(remember it's doesn't work as a pump, so oxygen perfusion is gone)  
that restarting the heart may be difficult.  CPR can be a life- saver as  
it can keep some blood flowing even while in fibrillation.  (Good for the  
brain too!)

The technology of defibrillators has greatly improved over the years, as  
has our understanding of some of the physiology of VFib, and drugs to  
address some of our biochemical problems, but the flaws present in our  
bodies are mostly unchanged.  

   -F (Bioengineer)

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