EBME Forums Forums Maintenance Technical Assistance duel diathermy operating

Nice assessment RoJo, we all bring different ideas to the forum and we can't be right all the time. In fact some of the points made can be considered to be right under different circumstances. And opinions can change during a debate as we take on each others ideas.

I do still stand by my assessment of the technical use of two diathermies, but I don't consider other peoples input to the discussion not to be worthy of respect.

Adrian.

I was only using the touching plates as a starting point for a general analysis of what might be happening when you have two isolated, floating, generators linked via a variable impedence. It is often useful in cct analysis to consider what happens under various limit situations. I still use this as one limit, when the potential of the plates would be common and the generator voltage levels relative to the plates, and each other, would be variable. The other limit is an infinite impedence, also not achievable, when both generators would operate independant of each other. We are considering an in between situation where currents must flow between areas of different potential, but these currents are not part of the normal operational current, as this must flow back through its own plate. I suggest these currents are capacative and cycle back and forth between the two circuits.
I am not an expert, I am only offering a way of thinking about a complex electrical problem, in the same way that you might use the water analogy (pressure, flow, and restriction) to think about Ohms Law (voltage, current and resistance)

Hi RICK,

Yeah, I know that and my response must have come across as snotty and offhand, as it usually does when I'm preoccupied with getting my point across - that's something I need to work on so I apologise for that if it's caused offence. What you write sounds reasonable - my view is that under certain circumstances it is actually possible to obtain current paths or loops that are unexpected due to the arrangements of two neutral plates during the simultaneous operation of two electrosurgery units, even if they float independently. Effectively leakage between units, if you like.

Unexpected currents flowing between the neutral electrodes seems plausible to me since we accept that capacitive paths and inductive effects cause leakage currents around the patient all the time i.e. HF leakage current can flow through unexpected paths and is a potential source of burns (which is why I'm pushing this argument so much). Via an ECG electrode for example, if it's placed near the surgical field in the path of HF current between active and neutral (it shouldn't be as far as I'm aware, even if the monitor has electrosurgery filtering, protection and is isolated appropriately). This is because both electrosurgery unit and ECG monitor may share a common reference at HF due to capacitive leakage and inductive coupling effects.

So I guess what I'm saying is that you can have leakage currents flowing between two dual electrosurgery units, which may also be referenced to an alternative leakage path (this complicates the issue again - earth perhaps?) even though they're individually 'floating', and that differences in potential between the neutral plates could cause currents to flow between them. Call it a leakage path, whatever, it's still a possible source of current that could heat tissue under the wrong circumstances. These currents are due to two generators being referenced together in some way (via patient tissue) and possibly to some other leakage path(s) probably via capacitive coupling. Considering that two plates in contact with the patient and so are the actives when operated simultaneously and in contact with tissue. The frequency, phase and magnitude of the current is another issue and will be dependent upon the complex interaction between generators outputs if they're not in-phase with each other.

Because it's practically impossible to totally eradicate HF leakage paths (or prevent radiated EM fields associated with electrosurgery) - even with floating units - then users still must consider the ideal placement of the neutral plates and leads, relative to each other, the patient's bare skin (to help prevent damaged leads or capacitive effects causing burns), prevent them from draping over any earthed items or the patient for example, to limit capacitive leakage currents that could cause burns at a localised contact point where there's a possibility of a high current density path. With return current monitoring and split plate technologies the problems are not as great as they used to be; as far as I'm aware it's the older units out there that can cause problems since they may not have the necessary safety features built-in.

I think we are talking about the same thing. Ignoring leakage due to imperfect insulators, the rest is made up of inductive and capacitive interaction, with the majority, in normal circumstances, being due to the leakage capacitance between different parts of the equipment.

Yes I can see the possibility for an unwanted current to flow as two floating generators could be at different potentials. But I would suspect that it would be very small as a similar effect occurs with static electricity. Large differences in voltage can be achieved but the power behind such floating is usually quite small.

But with that said, this would only be my own opinion expressed for the benefit of this site. If I was asked for my opinion by a Trust what advice should be given to medical staff my answer would be different. I would suggest that only equipment that has been recommended to be used in this manner by the manufacturer should be use in these procedures. If the models in question did not mention this capability in their manuals I may try approaching the companies directly. Try giving a list of other diathermies that can be used like this and ask if their model could do the same, if not, why not? I may even drop the hint that if they were unwilling to provide a answer in writing that they were likely to loose the Trusts business with future purchases. The reason being the Trust would wish to have this capability as procedures of this kind are quite common in electro surgery. In my opinion if a manufacturer was unwilling to give a statement either way it would look very poor. After all if they don't understand the workings of their own equipment how can they claim safe operation!

Adrian.

The point is that HF current can take a more preferential path irrespective of whether equipment is floating or not. It's just less likely if the electrosurgery equipment is isolated that's all. It's difficult to predict the path of all of the components of the RF current, in a practical scenario, precisely, unless you take all the precautions that are recognised in preventing adverse scenarios from occurring. The factors that will contribute to patient burns are the time that the current flows, magnitude, frequency, impedance of the tissue volume directly under the contact area and the contact area that the current is applied over/through the skin. A pin-point application of a relatively small current can have greater heating effects, over time, than a significantly higher current applied momentarily to a larger area - that's why no burns occur at the neutral return plate (also called the dispersive - energy is dispersed - heating is the movement and distribution of energy) and why it's important to ensure a good electrical contact with the patient over the entire area of the neutral return plate or dispersive electrode, call it what you want.

My thinking in the dual-plate scenario is that if you have two plates side by side and current does flow between them because they're not placed appropriately for dual operation (as I've described before) then perhaps this current, returned via the edges of the plate either capacitively or via tissue (a relatively small surface area), that's in contact with the patient, could cause heating (of the plate/tissue in between the plates), maybe burns or at least unecessary leakage that causes alarms if the ESU has monitoring for this. This is just a suggestion I'm making but I think the currents that predominantly cause burns are usually associated with inadvertent current paths from leads and between leads, accessories that are draped over bare skin, components of the system that couple to the patient capacitively or are damaged - creating a parallel path with the active or even preferential path for RF current, whatever. What we've been discussing is touching on the Physics of electrosurgery, so it's not really of much interest to most people. Perhaps we should leave it to Physicists and those 'in the know'.

Thinking about what you said - if we don't know the principles of the device and think objectively about all of the potential issues concerning safety then how can we advise the operators or understand the advice of manufacturers?

I believe we are in a better position to understand the principles involved with electrical safety. And if we don't know the answer, we have better background knowledge to read up and find out. Maybe we have more of an idea where we should look, what gaps are in our knowledge and a better understanding of what issues may arise. I stress that we only advise and I have aired on the side of caution with my last comments.

ValleyLab have equipment that can operate in this manner. And as far as I'm aware it's two floating generators, without any linking between the two I can't see how any clever circuitry could possible overcome different potentials between two floating generators.

I say put the ball in the court of the manufactures, not I don't have the ability to comprehend the issues involved!

Adrian.

Well to my knowledge the majority of modern ESUs monitor return current to ensure that excessive leakage is not a problem. Some floating ESUs also monitor plate voltage, if it starts to float towards excessively high voltages, to avoid large potentials on the patient. There's split plate monitoring to ensure good contact with the patient and return lead continuity. On some ESUs there's also means of monitoring any imbalances in return current across split plates so that the even distribution of return current is maintained. So I guess the issues that I've been raising are of enough concern to the manufacturers at least. What you're saying makes sense of course - the safest and simplest option is to get the manufacturer involved - that's the advice I suggested way back in the postings.

I think I am either not understanding your problem or I am not explaining myself adequateley. Imagine each generator is represented by a capacitor plate (not the patient connection plate). The generators are at different potentials and so a potential for an equalisation current between the generators capacitor plates) exists. A significant current cannot flow between the plates of a capacitor, ie across the dielectric, and so the equalisation must take place through the cicuitry connected to the capacitor. In our case the path of least resistance is between the patient plates. So the difference in potential between the generators causes a current to flow through the patient. This is the capacitive current that I talk about. If the generators were completeley isolated it would not happen, but because of leakage capacitance which allows one generator to react (think about reactive impedence) with another, a leakage current, capacitive current, heating current, choose a name, will flow through the patient.
If we were talking about isolated d.c. generators this would not happen; the generators would remain at their different potentials, with a common potential at the patient. Any capacitance between them would charge up and remain in a charged state as d.c.current cannot flow between the plates (not the patient plates) of a capacitor. Think of two battery's with one terminal of each patient linked via the patient plate and the patient. When you connect the other side of the battery to the patient current flows directly to its own plate. However we are using r.f. which introduces the possibility of circulating currents between (not through)otherwise unconnected parts of the circuitry, wether they are connected to a patient or not. When you connect them to a patient these circulating currents, another name, take the easiest path, through the patient.
I think I have said all I can think of now. I still think I am not expressing myself adequateley, and that we are talking about the same thing; r.f. currents can flow between isolated generators, via the patient, and cause local anomalies.

Apologies beforehand - this posting is a silly length and I promise it's the last and my best shot at explaining what I'm getting at for RICK's benefit. If you want to get a pen and paper and while away a few minutes......

RICK,

Lets's think about it another way. If ESU1 return electrode1, surgery site1 becomes referenced to a potential, say the potential on another ESU - ESU2, return electrode2, surgery site2, also connected to the patient, for example, they will be connected together via the patient or referenced to each other by a complex load - the patient tissue.

What I've been consistent with is that I think that under certain circumstances it is possible for some ESU current1 (what I've described as leakage current) to return via ESU2 return electrode2 to ESU1 return electrode1 instead of between the ESU active1 and the ESU1 return electrode1 through surgery site1. Probably best to draw this.

Say for example if the impedance between ESU active1 and ESU2 return electrode2 added to the impedance between ESU2 return electrode2 and ESU1 return electrode1 is actually lower than the impedance between ESU active1 ESU return elctrode1 through the site of surgery1.

In this case current may flow through this route, preferentially, via ESU2, return electrode2 rather than through the surgical site (that's a higher impedance path) because of where the return elctrodes have been positioned relative to each other and the surgical sites.

Thus some current could flow from ESU1 active1 to ESU2 return electrode2 and then on to ESU1 return electrode1. Current paths then depend upon the relative impedances and coupling between the surgical sites, the actives (when they're applied) and their respective return electrodes.

You could say return currents are 'crossing' but they're actually returning to their respective generators by a convoluted route. The return path the currents take is unexpected and probably unwanted and could heat tissue, that's located inbetween the surgical sites, unecessarily.

Hence there seems to be a justifiable reason when Dual ESUs are used, in my mind, to keep respective impedances between each ESU active and the respective return as low as possible i.e. place the ESU return electrode in a suitable position on the patient that's likely to present the lowest impedance across the ESU generator used at that surgical site.

If this doesn't seem like a credible argument then here's somthing that is well understood by most engineers and Surgeons who are at least familiar with ESU operation. It's a similar argument as to what can happen above with regard to unexpected curent paths leading to heating or burns in areas you don't expect them:

Let's imagine that an ESU floating return electrode attached to the patient's lower back or upper thigh capacitively couples to EARTH, possibly via the return lead that's draped over the earthed operating table pedestal i.e. the ESU return then becomes EARTH-REFERENCED to some extent at HF. If the patient then becomes capacitively coupled to earth at a location away from the surgical site (say at the elbow) then a parallel path i.e. a leakage path may occur.

This means that some of the current from the active (say in the shoulder) could possibly flow from it back to the return electrode on the patient via the elbow (by coupling), EARTH (operating table) and the ESU return lead (by coupling) rather than between the shoulder and the thigh or lower back where the return electrode is placed.

If the elbow to return path presents a relatively low impedance to HF then the current flow could be significant. Whatever the magnitude of current a path coupling to EARTH via the elbow returning current back to the return electrode via the 'floating' return lead,coupled to EARTH, would be considered an inadvertent or unexpected path under normal conditions.

Given the wrong conditions (producing a high-enough current density) then a burn could be generated at the elbow where the patient is capacitively coupled to EARTH. By this well known phenomenon it's possible that some active current (leakage in this context) can return to the generator via an alternate path where you don't expect it.

Similar to the arguemnt above, except that ESU2 return elctrode2 is analogous to EARTH, despite it floating - that's irrelevent - it's the path that can be created between the coupling of return electrodes that's important. That's why I think electrodes should not touch or be too close relative to their respective distances to the surgical sites.

For the two ESUs then it's not about current 'crossing' from one to the other it's about the potential for current flowing preferentially between the plates setting up unexpected return paths possibly flowing through areas of tissue where you don't want it to.

Thankfully the HF leakage and EARTHING problem is monitored by modern ESUs - patient voltage, patient earth, plate continuity, return current monitoring, etc, etc, and other similar forms of monitoring are used to help reduce these and associated problems, practically speaking. But their not always infallible.

I'm just using the second expalnation as an analogy with the first to demonstrate the idea that coupling between circuits at different potentials may cause inadvertent return current paths for HF back to the generator.