EBME Forums Forums Maintenance Technical Assistance duel diathermy operating

Adrian,

You are absolutely correct - I was twisting your words and misrepresenting what you and DD were getting at - I sincerely apologise. It is all my misunderstanding. I realise that as an expert giving advice to the clinical staff you are performing an essential role that is absolutely necessary for the safety of the patient since the operation of Electrosurgery is not within the field of expertise of Surgeons.

My advice and previous postings was absolute rubbish regarding the technical aspects - of course I made it all up just to try to win an argument. Again my apologies and I hope to see you contributing to the forum, on other issues, in the near future.

Best Wishes,

DD

Why not try the old codger Seathy. He still does diathermy training for the college.

By the way, how is your car !!!!!!!

Before you go to all that trouble DD:

During dual operation, using floating output devices, I believe you can get currents flowing between the neutral electrodes - currents effectively 'crossing' between plates - since the active outputs are not synchronised.

The floating nature of the non-synchronised outputs means that the instantaneous voltage on each return plate can be different causing currents to flow between the plates (and I assume between actives and 'all over the place' - depending upon impedances and the tissue beteen both plates and actives).

The likelihood of this depends upon how close the neutral plates are located relative to the surgery site and their respective active electrodes and to each other, the tissue and the impedances of the different tissues between the electrodes and neutral plates.

Interferential currents (interfering currents to laymen) can cause low frequency complex components (harmonics) to be produced and is also due to the effects of the arc struck between active and the surgery site to rectify current and produce low frequency components.

These low frequecy currents have the potential to cause stimulation to the patient (usually muscular) but don't usually because of the LF blocking filters built into the output stages of the electrosurgery units.

Actually I'm not so sure there would be current returning to a different plate if both generators were floating. The reason comes down to current circuits, how can the return current get back to it's origin via another generator. If current leaves a generator, the same current has to return else the circuit is broken. If no other route can be found, the only option is via the correct return plate.

However if there was an interaction between the high frequencies causing a beat frequency the LF blocking circuitry would be useless. This is because only high frequency would be leaving the generators; the low frequency would be generated at the site due to the mixing.

But I'm not convinced there would be interaction because of the two separate return paths explained above. I am open to the possibility if a valid explanation for interaction can be made.

Adrian.

The voltages from each active, assuming identical electrosurgery units, become referenced to each other by virtue of the fact that the neutral plates and actives are connected a common circuit. Electrodes on either unit are floating but commoned via the patient. If each active floats relative to the other and the patient completes the common circuit between neutral plates then current will flow between plates when each plate is at a different voltage relative to each other and when the non-synchronised actives are operated simultaneously as they might be under dual operation.

Simplifying things; I think Kirchoff's law still holds for HF current within a bulk conducting volume (patient) - this law still has to be satified whatever the circuit - most of the current required to satisfy this must return to the respective neutral, it's just the route that is unexpected. The implication being that there could, under certain circumstances, be current between actives at different potentials. I'm not certain, as I'm definitely not an expert, but it wouldn't surprise me if some electrosurgery units can detect this and alarm under these circumstances (as I've posted before).

If there is a difference in voltages between the floating neutral electrodes, they are referenced via the 'commoned' actives, that are producing output, and the likelihood of this is dependent upon the impedances in the circuit (between each actives and each respective neutral electrode and between the neutral electrodes). That's why it's important that the plate contact area is maximised and a good contact is made with the patient to minimise current density and what I posted earlier is important:

Your comments:

Actually I think the predominant contribution is from the arc (or arcing of the active) rectifying HF current. Yes the generator is outputting HF but what if the filtering blocks the return of dc? My point is that if a rectifier (the effect due to the arc rectifying HF current) is placed at the floating output of a HF generator then the HF current output will be produced though not necessarily referenced to the return because of filtering or circuitry dedicated to blocking it - The dc or low frequency current components will have no return path through the generator if blocked.

Could an electrosurgery expert or someone who has some literature or a text post something perhaps?

I like to think about what happens when the two plates are touching, you cannot have a potential difference between them, therefore it is the absolute potential of the floating generators that moves up and down with any phase differences etc. If you then start to seperate the plates with a low impedence between them a difference in potential may then exist and this is created by the flow of capacative currents between the two systems.

I find that sometimes it's good to draw a simple circuit diagram of problems like this to help work out what is going on. If anyone would like to send there fax number I could fax you what I would draw. If you do draw your own diagram I sure you will see a link created by the patient, however this link can not be used as an alternative return path. i.e. current leaving Generator A can not return to Generator A via Generator B.

An example of intermoduation or beat frequency been put to good use is the Physiotherapy Interferential equipment. TENS is a good alterative to drugs for pain relief, but as it is low frequency it only simulates at the surface. Interferentials use four pads with two high frequencies, where these frequencies mix a lower beat frequency of the difference is created. So it could be said the TENS effect can be felt deep in a knee or shoulder joint. The patient does not experience the tingling where the pads are applied because the low frequency beat frequency does not exist at these points. Neither leaving or returning to the generators.

Adrian.

RICK,

I agree your argument on the face of it it sounds reasonable. But to my knowledge the plates in a dual electrosurgery operating system using dual floating outputs must ABSOLUTELY NOT touch - that is one of the first things to consider.

I do not claim to be an expert I am just reporting what I have seen.
At my previous hospital, which was a cardio-respiratory specialist centre, they did lots of by-pass grafts with two teams of surgeons operating at once. One opening the chest and the other harvesting the graft from the leg or arm.
They had two Tyco Valleylab ForceFX machines and before that two Escmann TD411 machines operating at once.
By placing the return electrodes near the operating site there were no problems.
I hope this is plain enough, simple enough and non-controvesial. It is an observation not a prescription or an instruction.
We also had some old GU Solstars around, these had earth referenced outputs. If these were used in the above situations in place of one of the machines, the newer one, of either make, alarmed as it thought that its plate or the patient was touching earth. Which effectively was what was happening.

Now to the controversial bits.
I believe as technicians we can only offer advice and that the clinicians make the decisions based on that advice. So the buck stops with them as they are legally responsible for the care of the patient. Professionally we should give the best advice possible.
Under the Health and Safety at Work Act we are obliged to bring to the appropriate person's attention any hazardous or potentially hazardous situation. If we do not then we are liable under the law. If we do, then it up to the responsible person to sort out the hazard.
So I believe we can tell the clinicians we believe something is incorrect (in writing to prove you did it), advise them on a suitable action and then leave them to do what they think appropriate and to take the consequences of their actions.
I also believe that they do not have enough specialist knowledge in certain technical areas to know the actual correct action so this means that our advice should be sound but it is still up to them whether they act on it or not.
I used to be part of a ward based clinical support team and our phrase to new doctors was "usually in this situation the team did....." We carefully phrased it so we were not telling them what to do, but were in reallity. But they were responsible for their action and could not say "But the technician told me to do...."
Lets face it, through experience and speciallist knowledge we probably do know more than the clinicians in certain areas so they are relying on us to help them out but if something is wrong they take the can.
There have been postings worrying about technicians problems we can always say we advised, the clinicans cannot hide away....as much as they might try.

Robert

Issued in good faith as a personal opinion.
There, does that get me out of the brown stuff?

Adrian,

It's only my opinions I'm expressing - I don't purport to be an expert but me not being an expert doesn't make your understanding correct or any more valid than mine. I've not said that the currents don't return to the generator what I've been consistent with is that the path the current takes to return is not necessarily as one would expect - that's why I use quotation marks when describing 'crossed' currents. The implication being that they're not really 'crossing' and not necessarily returning to the other electrosurgery generator and that it's a description used by others i.e Rojo in this case. Saying that I'm not discounting where the HF current flows because it's a complex issue. I have said:

Commenting on Rojo's suggestions about the route of the current (via meaning 'by way of') not that I believe the current won't be returned to it's respective generator; I continue:

Current flowing between the neutral plates ('crossing'). Why not if you have generator outputs interacting even if the outputs have different phase and amplitude? And to try to explain what I'm getting at further:

In any case when you have two generators giving 'compatible' outputs with a common reference and tied outputs, even if they are out of phase and of differing amplitude, as you've commented yourself, then there is bound to be some level of interaction with currents flowing between areas of differing potentials. We cannot predict with certainty where the currents are going so we ensure that the plates are positioned in areas that should avoid the potential problems I've mentioned.

Adrian in my opinion it is not possible to draw a 'representative circuit' to describe what's happening to currents and voltages that are out of phase and of different amplitudes when connected to the patient and interacting. For a start you can't relate this to electrosurgery output settings and the relative values of impedances between all of the electrodes. The patient is not a uniform, homogenous, conducting volume. The circuit topology is not predictable: the patient varies considerably in shape. You don't know where the plates and active electrodes are going to be applied, etc, etc. But you do accept there is interaction (mixing as you put it, between the active outputs) so that's something.

I think that it's just accepted that the safest option to prevent unpredictable current paths is to assess the best location each neutral plate with respect to the active it's associated with and not try to prove these unpredictable currents don't exist when in practice manufacturers comment on them and are aware of potential problems.

And RICK I think plates must not be allowed to connect for at least four reasons in my opinion:

1). Touching plates might overlap and not contact the patient over it's surface this could present a relatively poor contact area and higher current density at the overlapping return plate.

2). You could get capacitive coupling, at HF, between the plates (see 4).

3). Currents flowing between plate edges, at different potentials, may cause heating and burns over this small area.

4). That if plates touched then the two electrosurgery units would remain at the same potential i.e. they would not float independently of each other (see1).