Calculate Defibrillation current from Measured Voltage Waveform

[ChrisReed]

We are planning to use a NI-9215 (4Ch, 100kS/s 16-bit Analog Input Module) coupled with a cDAQ-9191 (WiFi Chassis) to measure various differential voltage points across a patient during Defibrillation.

The reason for this is we want to determine if the Defib shock signal finds any alternative paths other than directly across the Defib paddles.  ie. to determine if a staff member has been accidentally shocked while working on the patient.

Does anyone have an idea how I can calculate what the current (peak/average) value is on any particular channel based on the measured voltage waveform alone?

Chris

[Liqian]

hi, ChriesReed

It seems that it is difficult to get the current value with the measured voltage waveform alone, unless you know the equivalent impedance among the differential voltage points.

Maybe you can choose a fixed value as the equivalent impedence by your experience (for example 50Ohm), or you can try to measure the impedence first.

[ChrisReed]

Thanks for the reply Liqian,

Yes that is the problem I can't figure out either.  We have several differential probes across the patient and we don't know what each impedance is.

We could place a known resistance across the probes, and measure the voltage drop across it, but then this parallel resistance would be affecting the circuit.

Or I could place a low value sense resistor in series with one of the Probe leads and measure the voltage drop across that to determine the current, but the Probe leads go into a very high impedance 100 MOhm High Voltage Divider so virtually no current is going to flow into here (ie. our measuring circuit does not interfere in anyway with the patient) - I expect we are looking at sub microvolt levels being generated across the sense resistor.

Chris

[Liqian]

Hi, Chris

Now the problem is how to determine the impedance between different voltage points.

The first method you mentioned is to place a known resistance in parallel with the probes. I wonder whether you can get the total value of current flowing through the probes by an ammeter. If you can, the problem would be easy. Because the total value of current flowing through the probes is the sum of the current flowing through the resistance (can be calculated) and the current flowing through the patient.

The second method is to place a known resistance in series with one of the probe leads, which requires meters with high resolution.

[ChrisReed]

Yeah I think we're going to try the second method and use a fairly high resistance (say 100Ohms) for a sense resistor, which will have no effect on the input impedance to the Isolation Amplifier, but will at least give us a more reasonable voltage drop that hopefully wont disappear in the noise floor of the Isolation Amplifier.  It will still be in the microvolts range though.

[gregc]

A good article on testing:

Search google:

Testing Implantable Medical Devices

Max Cortner

[johnsold]

Chris,

There is a body of literature on the distribution of currents through various tissues of the torso during external defibrillation.  Of course most of it is aimed at determining the effectiveness of the defibrillation process itself.  Some of the articles extensively discuss modeling the tissues through which the current passes.  Those may be of assistance with what you want.

I doubt you will be able to measure or calculate currents the way you described.  There are too many unknowns.

I would suggest an alternative approach. Map the distribution of voltages at various points on the body. Based on estimates of contact impedance and published safety limits, you may be able to determine the risk of shock to other persons.

Lynn