Sigh, like most things this has all been done before.
If you don't know how a defibrillator works don't make it up - I do know how they work, because I'm a paramedic. So lets get rid of this mindless babble once and for all with some electrophysiology.
If you're too stupid to read this and continue to post babble so be it. Alternately accept the final paragraph as gospel and skip everything in between
Ok lets look at how the heart works.
The heart muscle cells are specialised muscle cells that have all the normal features of muscle cells - that is Conductivity, contractility and excitability. And the special one Automaticity.
Conductivity refers to the ability of the muscle cell to conduct an impulse to depolarise to the next muscle cell.
Contractility refers to the shortening of the muscle cell in response to an external stimulus.
Excitability refers to the ability of the muscle cell to electrically depolarise in response to an external stimulus.
Automaticity is the ability of the muscle cell to spontaneously generate an impulse to contract.
You need to know all that crap to comprehend this:
The heart has a specific electrical conduction system , which has a higher rate of automaticity than any other cell in the heart.
This conduction system is heirarchical , which is to say the higher up the heart, the higher its rate of automaticity.
The SA node (in the atria or top chamber of the heart) fires 60-80 times a minute at rest
The AV node (on the junction of the atria and venticals) fires 40-60 times a minute
the Bundle of HIS (in the interventricular septum) fires 30-50 times a minute.
the ventricular wall up to 30 times a minute.
Now when everything works right the SA node fires off and through excitability and conduction, fires off the rest of the heart before its inate automaticity fires it off.
This firing of the cells, involves electrical depolarisation of the cell, which leads to a calcium eflux from the mitochondria, and electromechanical bonding, which allows the actin and myelin filaments to actually cause contraction.
After the contraction the individual muscle cells have a refractory period during which they are unable to contract regardless of what you do to them, and again the top half of the heart should repolarise quickest starting the cycle all over again.
After the absolute refractory period comes the relative refractory period, during which the cell may fire if a current or stimulus greater than threshold is applied to it. Threshold, is the current level below which the cell will just absorb the current and do nothing. During the relative refractory period threshold starts at >100ma and reduces to the resting threshold of around 65ma.
If you apply enough current in the relative refractory period, you can fire off some of the heart, but not the rest (as it is just plain not ready to go yet). When you do this you create an arrhythmia.
Now in a healthy heart, the most likely thing you will do is cause a single aberant beat, next most likely is a couple of aberant beats then back to normal, next most likely is a run of ventricular tachycardia then back to normal, next most likely is VT deteriorating into ventricular fibrillation, and least likely is to spark VF at the outset.
Which leads us to ventricular fibrillation, which is a chaotic wave of electrical activity which washes over the heart, only depolarising a small bit of it at any given time, as the rest is refractory, so this wave wanders around the heart in a psuedorandom pattern - and the heart just quivers. So *NO* blood is pumped and you die.
So lets have some fun talking about a defibrillator.
Defibrillators use large surface area paddles - and resistance decreases as a function of the amount of conducting surface. Which is either done with a paddle (ala ER) or a large conductive pad. Both of which have specialised silver/silver nitrate gel on them to reduce impedance. or resistance. A defib uses a large shock to totally stop the heart and hope that the normal SA node or other coordinated electrical activity will reassert itself. You can't shock Flatline ala TV. Defibs cause flatline for 15-20 seconds.
Transthoracic impedance measured through defib pads (referenced from the Lifepak 12 (ala ER) user manual) ranges between 25-100 Ohms.
Peak current flow at 25Ohms, is 80A for 2milliseconds. or at 50Ohms its 57A for, or at 100Ohms os 35A.
25Ohms is based on internal paddles - large surface area applied directly to the heart, 100Ohms is normal skin resistance through defib pads.
So by applying the concept of V=IR you can see that defibs use nowhere near 50kV.
Anyone wants to check this, its page A-6 of the International version of the Lifepak 12 instructions
Page F1 of the Zoll 1600 defibrillator instructions will also show you peak voltage delivery of 2000volts.
Lets consider the farcical suggestion that sticking needles into your skin would reduce that resistance below 100Ohms - and you betcha if it was possible to do so, and address the issues of COST and SKIN BURNING, and PAIN caused using these things it would have been done.
Cost of an IV cannula = $2.90, Cost of a set of Defib Pads for a Lifepak 12 = $36.92 or Zoll 1600 =$35.17. If the theory worked to use a needle in each arm, we would be doing that and saving hundreds of thousands of dollars each year. Not to mention that dumping 2000V through 100ohm resistance creates signifigant HEAT and therefore skin burning and therefore pain. Therefore this suggestion is the worst sort of superstitious drivel.
Can a 9v or 12v battery kill you - I guess theoretically yes. 9v on an internal paddle would give you 90ma, which would cause an ectopic beat, and if you were incredibly unlucky when you first applied that maybe VF.
But then chances are the fact that your chest was open would do you a whole lot of harm before the battery did. And the fact that we're talking DC current what would most likely happen is that as soon as you took the current back off the heart would beat normally again.
Is 240v dangerous? Yes, 240v can kill you. With spectacular contact like Defib pads you could get up to 2.4AMPS of current which would be a bad thing. The reality though as many people have references is that dry skill alone, normally has high enough resistance , so that current flow is too small to kill. Which is why it was chosen as a standard. So that in normal conditions it was harmless. Otherwise we'd all use high voltage power and the nasty electric company wouldn't bother buying all those transformers and substations to step down from hundreds of thousands of volts to 240.
Arm to Arm *WILL NOT* kill you - the apex of the heart is below shoulder height. Right arm to left leg is the dangerous one. Again if it was arm to arm, and needles work, why do you think we bother to apply the pads to the right side of chest and down on the left side.
So the promised gospel last paragraph:
240v can kill but extremely rarely even causes a signifigant injury. The stats about this are skewed, since if you just get a little boot from 240v , most people don't report it, so they are not part of the data set. Defibs use around 2000v not 50kv. AC is more dangerous than DC because it is more likely to provoke Ventricular fibrillation. 9v Batteries are HARMLESS. Sticking probes in your skin will just hurt a lot. And finally - theres a lot of people around here who think they know more than they actually do. These people should learn the famous quote - it is better to keep your mouth closed (hands off the keyboard) and be thought a fool, than open it (or type) and confirm the opinion. YOU DO NOT HAVE TO HAVE THE ANSWERS and/or an opinion about everything. |
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