EBME Forums Forums Maintenance Technical Assistance duel diathermy operating

Richard, again I do not disagree with what you are saying. I am offering an alternative explanatiom of a phenomenon that is based on circuit theory rather than observational fact, and may allow others to build their own picture of what is happening.
I may be talking through my backside but it is the way I look at complex things.

Does anybody have the time to actually read and digest this stuff? How do you guys find the time to research and write such dissertations? Maybe when I get my pipe and slippers, I'll be able to join in the fun!

Hi guys

I have my pipe and slippers on today so for the benefit of people who are new to the profession and may be wondering what the dingo's kidneys this is all about or who find their eyes glazing over I'll try to summarise, starting with the bits where we all agree (I have to say I haven't read every line, my eyes wouldn't let me).

I have given each point a number so we can agree where we disagree as it were.

(throughout I am using the the term dispersive electrode to differentiate it from the actual return point which is probably one end of a transformer winding somewhere within the generator)

in all cases

1) the ouput and return current of a generator must be equal (Kirchoff's laws)

2) the output curent need not return through the dispersive electrode (leakage)

3) the stray capacitance is designed to be small so the leakage current is small (as a proportion)

4) the Ohmic heating is IR^2

5) the resistance of bulk tissue is fairly low

6) the heating is effect is fairly low

in the case of multiple generators

7) the leakage currents of the generators share a common reference (the patient/earth)

8) the output current of one generator may pass through an alternate dispersive electrode, the mains earth and the stray capacitance back to the first return.

9) the current which follows this convoluted path is small (in proportion)

10) the current in each dispersive electrode is very much as would be expected if there were only one generator

11) the danger to the patient presented by this small change in dispersive currents is small

12) the alarm circuits may pick up on this change

13) the Ohmic heating where the currents pass though the same area of tissue is the sum of the generators individual contributions

14) if the dispersive electrodes are sensibly placed this should present no problem

Anybody disagree with any of this so far?

Interferential currents....I am on a lot shakier ground here

15) absolute frequency stability is not the prime design criterion for electrosurgeries

16) the difference frequency will not be at all stable.

17) for us to be concerned any difference frequency must be very small compared to the primaries, say less than 500Hz cf. 500KHz.

OK so far?

18) sinA cosB = 1/2[Sin(A+B)/2 + sin(A-B)/2]
this says there are difference frequencies, but it's a multiplication! In a resistance?

19) sinA + sinB = 2 [sin(A+B)/2.cos(A-B)/2]
Well at least it's simple addition on the LHS. But what is that thing on the RHS? I guess it could be graphed but I certainly can't visualise it.

20) the trig identity "might" say that SinA + SinB leads to components at the difference frequency

Do the difference frequency currents actually exist?

Anyone up for an experiment?

If you try connecting two a.f generators to a single resistor I guess you will be able to see the difference frequency on a scope. Does the envelope get bigger and smaller (otherwise known as amplitude modulation) or does the envelope have a constant amplitude but moves up and down? The first contains energy components at low frequncy, but does the second? I dunno, but I think it what will be seen and is what happens in tissue.

Of course this could be a load of b****x. It has been known! And it's a long time since I thought about ONC maths.

Excellent, but I would take you up on point 8 though. Did you mean that the current from generator A could return through the return of generator B via the earth? If so how could it jump the isolated winding of B and then back across the isolated winding of A. If this is not what you meant, could you offer a little more explanation.

An experiment would clear up all the disagreements, unfortunately which of us is going to get the opportunity? But if someone does I'd suggest using a isolated mains transformer for the Scope supply.

Adrian.

Point 8.

Kirchoff's law says the ouput and return currents must be equal. Any current which apparently gets away (leaks) must get back. The only way to prevent it getting back would be perfect isolation, but then there woudn't be any leakage in the fist place!

Any current which originates in the active end of a generator's output transfomer and leaks to ground must get back to the return end of its own transformer through the return end's capacitance to ground. This includes any current which may have made its way to ground through the alternate dispersive electrode (and down the wire and then through the circuit capitance to ground). OK?

This alternate current must exist but will be small, in fact negligable.

I AM NOT SUGGESTING TRYING THE EXPERIMENT WITH DIATHERMY. 2 low volatge a.f sources is all.

Apparently it's possible for a current density of as little as 100uA/mm^2 to 1mA/mm^2 HF current over 10s to cause a burn. Sounds negligible compared to a HF current of 300mA generated by an ESU into a typical load.

The problem is it's hard to find a.f. generators that are floating. Plus the power needs to be fairly high else the possible mix frequency will be lost in the noise and won't be detected by the use of a scope. Also by using diathermy that is intended to be used in practice the experiment be seen as more valid, else someone could always argue about circuit differences.

It's probably all getting a bit academic, in practical terms maybe asking the manufactures would be a better option. But it's good to consider these things as an exercise.

Adrian.

I think things are being taken out of context with this 'mixing argument' that keeps being 'pushed'. Actually I think 'mix-frequencies' are a red-herring that keeps dragging the discussion away from the 'real-world' where it's safety that's the consideration.

My opinion is also that product and difference frequencies are likely to be negligible, for all practical purposes, and probably just contribute to ESU radiated noise if they're not significant enough to cause burns or relatively lower-frequency neuromuscular stimulation.

Inadvertent burns via alternative paths via dual ESU operation, considering aspects of safety are what the users, the manufacturers and we should be interested in preventing - the more significant problems that could potentially occur.

I also believe the mixing argument isn't a problem, but I haven't heard any conclusive evidence that says this is the case. So I still think it needs to be considered.

The problem with inadvertent burns for the patient and staff is a genuine problem, but I think no different than when one diathermy is been used, apart from the overall increase in output powers. The reason been I'm still not convinced that the alternate current path via the other generator is anything more than negligible.

Adrian.

There is a possibility that the frequencies of the 2 ESUs may differ by a small amout (50 Hz say) and may interfere with each other and produce a beat frequency of 50Hz that may be very dangerous to the heart (Physiotherapists call this Interferential Frequency therapy)