A carpenter friend of mine was just in the hospital for a little while yesterday with fibrillation issues. They put some electrode patches on his chest and put some current through to stimulate or regulate or whatever. But out of curiosity he asked how many volts they were using and the tech or doc responded 200 joules.
That is interesting (1 Joule = 1 Watt-second, btw) So, 200 Joules would be 20kW for 1/100 of a second, perhaps. Since chest impedance varies, it would be impossible to set a particular voltage and "shutter speed" as the current would be unpredictible- I'd imagine the defribillator has the "Joule" setting, since the defribillator will automatically adjust the voltage, current and cycle length to administer the proper shock to constrict the heart and get it moving. And thus the voltage and current would never be consistant, and would be meaningless means of measurement. But that's just a guess...
[This message has been edited by SteveFehr (edited 02-21-2007).]
#130816 - 02/21/0705:08 PMRe: Medical electric units?
The body is basicaly operated on and controlled by electricity. Food and Oxygen are metabolized into energy, which is transmitted down the nerves by a change in the disposition of electrolytes, resulting in an incredibly weak electrical current. That current is sufficient to cause the muscles to contract, which is why 5 milliamperes will override your body's own electricity and make you hold onto a live wire. That's also why the bad guys on TV go down 'right now' when hit with a Taser. The amount of energy in any medical device is tiny when compared to the units that we are used to for measuring power, so they use a Joule, which is a very small unit for energy. You can pick up the tiny impulses and run a sensitive chart recorder...called an EKG; or you can whack the body big-time with enough power to seize or control muscles...that's what a defibrillator does. Generally when using the machine, the Doctor starts with a setting that is just high enough to get the desired result, and increases it in small increments if needed.
#130817 - 02/21/0707:32 PMRe: Medical electric units?
In the same way that the Watt is the unit of power, the joule is the unit of energy. As Steve pointed out, one joule is the amount of energy delivered at a 1-watt rate for one second.
The cardiologist doesn't really care about voltage or current; he just wants to know how much energy is being delivered to the patient. Hence, the use of the joule as the measurement.
In its simplest form, a defibrillator is a capacitor with a charging circuit, a discharge switch, and a network that produces an optimal discharge waveform. The capacitor is charged to the desired energy level, then discharged through the electrodes.
The energy level is related to the voltage to which the capacitor is charged by this equation:
E = C * V^2 / 2
where E is the energy in joules, C is the capacitance in farads, and V is the voltage.
The capacitor in a 200-joule defibrillator is around 100 uF, so it needs to be charged to 2 kV (!) to reach 200 joules:
E = (100e-6 * 2000^2 / 2) = 200 J
By the way, if you ever grab a 100 uF capacitor with 2 kV across it, it will be a memorable experience.
Notice that the energy increases as the square of the voltage; charging the same capacitor to twice the voltage (4 kV) yields four times the energy (800 J, not 400 J). For the same reason, a car moving at 60 MPH has four times the kinetic energy of one moving at 30 MPH.
...put some current through to stimulate or regulate or whatever.
Contrary to what many people think from watching TV, the main purpose of the shock is not to start the heart, but to stop a life-threatening irregular heartbeat. It's a sort of great big reset pushbutton.
I hope everything turns out well for your friend.
Edit: I'll get these UBL codes right the first time some day...
[This message has been edited by John Crighton (edited 02-21-2007).]
#130818 - 02/21/0707:55 PMRe: Medical electric units?
Ya know, I would like to think that nothing I have or will have ever designed and built, has ever broken or will ever break. But we know that's just not how it goes. So what if it does break and happens to be a piece of medical equipment, designed to shock people, but in a nice way? So how do I make sure that it can help more people than in vaporizes? Well as long as we are on GOD's green Earth, or any of His other properties, we know that we can't get more energy out of a system than we put in. You will most likely find that a specified amount of energy is stored in a device and then isolated reliably from the input circuitry. Only then will a firing circuit discharge it into the human body. As long as we can verify that we can't discharge before isolating, we know that the worst that can happen is the uncontrolled discharge of less energy than we put in. When you start talking about stored energy,"joules" is a natural word to use.
Now if I happened to be MacGyver with chest pains, I'd cough and run for the aspirin like everyone else would. But then I would fetch my camera strobe, headphones, and aluminum foil. I would use the headphone/foil paddles attached to where the xenon tube was, to shock myself back to life for another season of working for the fat guy with no hair. Priceless!
But seriously, a camera strobe probably has more in common with a defibrillator than one might think. Joe
I've heard stories of the interns getting knocked on their butt by not "clear!" 'ing fast enough. But I dont believe any of them were hurt badly, at least not where we were. There was one story of someone's nitro-glycerine patch exploding, or at least releasing a puff of smoke scaring the docs a bit...
The most dangerous thing about those new automatic ones you see all over the malls now are the sub-standard LiIon battery packs! There have been several cases of those catching fire now, and they store a LOT of energy too...
Talking about MacGyver does anybody remember the hilarious scene in one of the "In like Flint" movies where Flint restarts a fellow's heart by unscrewing a light bulb, sticking one hand in the fixture and massaging the guys chest with the other? now that is classic film making
Joe, This could be why electro-medical equipment has such a strict testing regime, it's not the sort of thing you EVER want to have fail at any time. I know a guy that works at our local hospital as a test technician, we got to talking one night at the Supply house Xmas party, he tried to explain to me how an ECG machine worked in very great detail, my eyes glazed over, he was telling me of things I'd never even heard of before. This guy is at the top of his profession, mind you, you wouldn't have just anybody fixing your defibrillators and what-not.
So happens that my girlfriend is a spinal cord monitor tech. She hooks up a machine to a patient during back surgeries, using probes at certain spots on the head and body, that monitors electrical pulses that the body uses to communicate down the spinal cord. If the doctor over-tightens some hardware, or makes some other motion that might cause damage to the spinal cord, she sees it on her monitor and alerts the doctor. She basically saves people from being crippled.
I tell her that we basically are in the same business, except that her income makes my income look "cute".