Hello everyone, I'm new to the forum and have need for help on an odd project so I don't end up killing myself with it.

I'm a medical student doing a research project in cardiac arrest using isolated small animal hearts. We have to induce ventricular fibrillation to do this and I will have to build the device that does this. I've read several similar studies that have done this so I know what the current is like they are outputting. It seems rather small to me but they are slowly increasing 60hz AC current to 3-5ma for 30 seconds, holding it there for 30 seconds, then reducing it to half that current for one minute.

It would seem to me that this could be done with a potentiometer plugged into wall current but that just seems extremely dangerous. I know I am building a device designed to put a heart into Vfib after all, I just don't want to put myself into Vfib using it!

I don't think I would need a transformer to do this as the current would remain the same, just the voltage would change right? Sorry, I've tinkered with electronics all my life but I would barely call myself a hobbyist with my level of success at making things that worked. Thats why I'm reaching out for help. Thanks!

 

The transformer is to keep you (or the animal) from completing the circuit with just 1 paddle and earth. You can still get it if you touch both paddles. So yes a good idea!
The current sounds about right but I would expect the voltage might be higher than what you get from the wall outlet. Do you have a number for the voltage?
Is it an external shock or open heart?

 

Your understanding of a transformer isn't quite right. The primary to secondary turns ratio determines the voltage relationship between the input and output voltages. A 10:1 turns ratio means that with 120 Vac in, you get 12 Vac out.

It is the physical size of the transformer that limits the amount of current available from the secondary. Transformers the size of a ping pong ball and a soccer ball both can be designed for a 12 Vac output, but one will deliver only 0.1 A before the output voltage begins to decrease, while the other can deliver 100 A with no output voltage sag.

3-5 mA sounds like open heart / direct contact rather than through the skin. What does your literature say about the output voltage required while delivering this current? If it is less than 9 Vac RMS, then we can whip up a simple opamp circuit that will deliver a sinewave current with nice regulation. A full-blown design would have 2 or 3 opamps, adjustable output current, voltage, and frequency, and be battery powered.

ak

 

If you want, you can regulate the current waveform so that the current is independent of the resistance of the heart -the voltage applied will be continuously adjusted to maintain the desired current waveform. A Howland current pump is one way to do that.

Tutorial:
http://www.youtube.com/watch?v=_x4aqCCz_Iw

 

thanks for the responses everybody, glad I asked for help, from what ronv said it looks like I would've killed myself at step one! As for voltage, that's been the problem, nobody I've found listed their voltages! Just the current and frequency. And, yes this is in an isolated heart outside the body. The setup will be something called a langendorff heart setup, you can see videos of it on youtube to see what I'm doing. The previous studies have been done in the full rat by applying the current through a wire (or wires, not sure) placed into the superior vena cava (main vein coming into the heart), the numbers should be the same though for what I'm doing as the current will still be applied directly to the heart.
So on to the next question about this, wall current can be practically any amperage up to the point that I trigger the breaker right? So I will need to build an AC power supply to keep from tripping the breaker/burning the lab down correct? Or am I building the power supply when I make this device?
Thanks again for the help, its greatly appreciated.

 

and since the amperage is variable, how do I know what voltage to bring it to to achieve the desired amperage? I know this is a simple a straight forward electronics question, but I've never been able to figure out how that was done.

 

It is, but we don't know the resistance of the heart. But having said that I suppose you can measure it. I suspect it is around 100 ohms or even less as it is the skin that offers the high resistance to electricity. Voltage = current X resistance.
I would think something as simple as a 115 to 24 volt step down transformer and a 5,000 ohm potentiometer might be sufficient. The low voltage would be safe for you but not for the exposed heart.
Correct me if I am wrong but this device is just to stop the heart not to make any measurement??

 

I built a muscle stimulator for my final year biomedical project that had essentially had the same goal as this proposed device: to deliver current of a desired level and frequency through a load.

As previously mentioned, the load (animal heart) that you are driving current through will have an impedance that will vary quite a lot. Since it is the level of current density passing through the tissue that will ultimately cause the desired effect, controlling current is the best way to ensure consistency and repeatability.

There are two ways to skin the cat, you can control a voltage waveform and pass it across the load, tweaking it to get the desired current or you can control a current waveform and pass it through a load (independent of load impedance, in the fashion DickCappels has mentioned).

The current level requirements that you mentioned are pretty low and since the impedance of internal organs is pretty low (high conductivity under the skin) the voltages to supply your circuitry would not need to be very high.

Some questions:

Does the current waveform need to be sinusoidal or can it be square pulses?

What kind of electrodes will you be using?

 

Thanks ronv, I will try and measure the resistance of one of these hearts in the lab next week if I get the chance. And yes, the device is only to stop the heart (induce ventricular fibrillation) and will not be taking measurements. After a preset amount of time I will then defibrillate the heart with a medical defibrillator at its lowest setting, I think thats like 5 joules or something. So this device should be pretty simple to build I'd think.
I think you're right blah2222, this device is basically just a muscle stimulator. I'm afraid I don't know enough about electronics to understand the difference between a voltage waveform and a current waveform, I will have to do some research to see what that is until I can comment on it. I would think the voltage should be quite low. I know another researcher that built a similar device using a 115 to 40v transformer but the current was still to high and it did not work, it kept charring the electrodes and the heart! Not what I want! haha.
I haven't decided yet on the electrodes but probably some very small wires embedded into the heart tissue itself? I don't know that it matters if the waveform is square or sinusoidal, nobody has stated in the literature that I've seen. I'm not sure how much it matters in the induction of fibrillation but they all say to use biphasic waveform DEfibrillation, as monophasic can ionize the solution were using to keep the heart alive causing problems further down the line.

 

I did some innergoogle digging, and it's surprising 1. that defibrillation is over 100 years old, and 2. that there is so little tech data about it. 5 J is 5 watt-seconds, or 500 watts for 10 milliseconds. If you need anything like that kind of energy for 30 continuous seconds, then a simple opamp circuit won't do it. But what might work, if you would rather buy than build, is a low cost function generator to make the sine wave and an audio amplifier to pump it up.

ak

 

The defib part is 5j and I have a defibrillator already to do that, It only runs for a few hundred milliseconds or so. Normally if we were applying the electrodes to the skin it would be around 360 joules! The circuit I'm building is to cause fibrillation. That has to be applied for a few minutes to reliably cause it from what I've read.

 

A current source drives the desired amount of current through the heart, regardless of how much voltage it takes to do that, within the limits of the machine to deliver voltage.

4 instance: A 10 ma driver connected to a 30 volt supply will deliver 10 ma into 100 ohms by adjusting itself to 1 volt. That same driver will drive 10 ma through 1000 ohms by adjusting itself to 10 volts. That same driver will fail to drive 10 ma through 10,000 ohms because it runs out of voltage at 3 ma.

How to do a constant current sine wave escapes me because a sine wave has a continuously changing voltage. If it always delivered 10 ma into a fixed resistance, that would be a square wave. I suppose you could declare the "constant" current to be the current delivered at the peak of the sine wave voltage, but again, not as easy to auto-regulate as DC or a square wave.

And, here's how I would do it, and the winning amplifier is on this page:

http://www.mouser.com/Semiconductor...Z1yyhnnvZ1z0wbflZ1z0wbyxZ1z0wd73&Ns=Pricing|0

 

ps, You could use a cheaper amplifier and fit a push-pull output stage on it.

I do think it's a bit of overkill, but better to weaken it from too strong than to try to patch up a weak design.

Common tales of the current it takes to kill a person describe abut a third of what I designed for, and a mouse isn't as big as a person.

If you want a different frequency, detach the 3.9k resistor and connect a signal generator to it. Don't forget to attach the ground wire of the signal generator to the circuit common.