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Capacitive Coupling / Parasitic Capacitance Questions

Capacitive Coupling / Parasitic Capacitance Questions

(OP)
Hello All,

I have (2) questions regarding Capacitive Coupling. I am pretty out of my league here as I am ME, and been a long time since I've had a circuits class. Any help is appreciated

I am designing a handle for monopolar surgical instrument. Now the handle itself has quite the irregular shape; additionally, there is an electrode running through parts of it to charge the tool for the desired electrical cutting and cauterization. The handle is also metal, as I mentioned it is well insulated against direct coupling, or arcing, but as I understand it there is no way to avoid some level parasitic capacitance or induction charging on that metal handle. Additionally, the longer the device is used the more charge it builds potentially discharging in unintended places.

1st question: Since I have the CAD model and geometry of the components, the material properties of the handle, including the dielectric properties of the insulator components and I know the power settings of the electrode, is it possible to estimate the capacitance of the handle? We don't want surgeons accidentally zapping themselves.

2nd question: Is there anyway to accurately measure that? I mean without discharging? So if I put an electro-generator on a prototype is there some sort of probe or dmm I could use to accurately and precisely measure the capacitance, and I could record it over time?

Thank you!

RE: Capacitive Coupling / Parasitic Capacitance Questions

It's been many years since I worked with medical electronics, but how about earthing?

Benta.

RE: Capacitive Coupling / Parasitic Capacitance Questions

Q1. Yes, that should be doable, since it's essentially the thickness and value of the dielectric
Q2. Yes, you get a capacitance meter

Nevertheless, if you are worried about capacitance, then you have insufficient shielding, i.e., as benta stated, grounding. However, you've not even stated whether the voltages are AC or DC, or what frequency, if the former.

TTFN
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RE: Capacitive Coupling / Parasitic Capacitance Questions

Disclaimer: This is not my area so all advice given should be carefully checked and double-checked.

Depending on the signal / power you're sending down the cable to the tool, perhaps the tool handle could be effectively an extension of the transmission line in terms of electrical properties. If so, parasitic capacitance is simply part of the transmission line, along with series inductance.

Note: there can be complications depending on how the currents flow.

The handle should be firmly grounded, perhaps via the coaxial feed cable, so there should be no chance of shock. This might even call for redundancy, so perhaps an additional grounding wire. Or triaxial cable.

RE: Capacitive Coupling / Parasitic Capacitance Questions

(OP)
I've attached a picture regarding the load of these devices and how they're used. The issue with conductive capacitance is that something could build up enough charge to burn the patient or surgeon unintentionally.

So for surgeries using a monopolar device: The device is attached to an electrosurgical generator. The settings can vary depending on what the surgeon is trying to do cut or coagulate etc. However, their general response is to crank up the voltage(AC). The generators can vary quite a bit but have 3 ranges low power 50-100watts mid power100-200 and high power 300-400 watts. The frequency also changes depending on how the surgeon wants to cut, but on the high end It's a high frequency ~1MHz.

So when things are working the way they're supposed to the electrodes cut taking the path of least resistance. However, with all that induced charge patients can get burned in other areas. So the device is split up into zones so as to classify zone 1 is the intended cut area, zone 2 and 3 are parts of the instrument that are most likely to discharge in conductive capacitance, but our design has proven valid to oppose this on earlier model. However, now we have a new dilemma we're introducing more metal where typically the handle would be a composite. AKA Zone 4, the zone that the surgeon manipulates. The concern is that during prolonged surgery.

That's how the monopolar instrument works, the return pad (aka grounding pad although not a true ground) is the electrode placed underneath the patient in this type of surgery. So adding a ground line from the handle would take care of it, but it generally is not accepted in hospitals.

The worry is that the composite (Radel 5000 grade) will wear (legitimate concern) and the handle will induce a large enough charge to arc to the electrode or through the insulation, and burn the surgeon.

RE: Capacitive Coupling / Parasitic Capacitance Questions

(OP)
Capacitive coupling: The condition that
occurs when alternating (AC) electrical current is
transferred from one conductor (an electrode), across
intact insulation, into adjacent conductive materials
(tissue or skin) or another metal surgical instrument.
Capacitance is stored electrical charge.

RE: Capacitive Coupling / Parasitic Capacitance Questions

400W? You're not going to have any problem. You can wrap the handle in tin foil and then put a scope between the foil and local ground to see just how paltry any capacitive charge is going to be.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Capacitive Coupling / Parasitic Capacitance Questions

But according to the load profile this device can manage 200W into 1000ohms. That's several hundred volts oscillating around 1MHz. You wouldn't need much capacitance to couple a significant voltage to the handle. Probably not enough to kill anyone (provided the power supply continues to limit current as it should) but possibly enough for a jolt or a burn.

To address your questions:
1) Yes, in theory. For a first pass estimate, can you approximate the handle to a cylinder with a concentric conductor? If you can, you can get a ballpark estimate using the capacitance of concentric conductors forumla. For example, plug your numbers into the first one on this page.

2) Accurately measuring parasitic capacitance is tricky. Stray fields and the effect of the measuring device can be significant. Devices do exist and might be worth a shot. For a complex shape like the tool you describe it might be easier to take operational voltage measurements instead (be sure to load your measurement to quantify the source impedance of the coupled voltage).

But to echo earlier suggestions, it's a Good Idea to ground the enclosure of a device with a metal enclosure, containing live circuits to 100's of volts. Off the top of my head it solves three problems at once - it reduces the capacitively coupled voltage you're describing; it bleeds off any static build-up; and more fundamentally, any breakdown of the insulation will be detected by a ground trip. You wouldn't need a grounding line, just a shield in the cable that runs between the power supply and the tool.

RE: Capacitive Coupling / Parasitic Capacitance Questions

The handle is firmly grounded for safety? Right?

If not, then why not?

If it is, then how could anyone get a shock from it? *


* There is "magic" with RF (not really), but at 1 MHz the wavelength is 300m (or a bit less in a transmission line), so you won't stumble into any unexpected one-quarter wavelength lines in a ten foot cable. One-quarter wavelength is where a chunk of metal can be grounded at one end and high voltage at the other. At least until the FCC shows up and requests that you turn it off.

RE: Capacitive Coupling / Parasitic Capacitance Questions

The cable from the "ESU" to the handle is not simply a single conductor? It's at least a coaxial cable, right?

RE: Capacitive Coupling / Parasitic Capacitance Questions

In the latest picture, which way is that arc going?

If the handle is grounded, perhaps it's ground seeking current headed back? Taking a short cut back to ground?

How can we know?

RE: Capacitive Coupling / Parasitic Capacitance Questions

(OP)
@ everyone
I don't know if it has a shielding wire to the handle electrode but I guess so, it still doesn't solve the issue. Otherwise there wouldn't be the need for companies to come up with stuff like active electrode monitoring for these devices. You guys know better than me about that stuff. I put a hyperlink below so you can see the electrodes and cords. The idea of the surgery is to purposely create a controlled arc for the purpose of cutting or coagulating or desiccating etc. The electrode is hot, it works through the insert and creates an arc trying to complete a circuit to the grounding pad. The issue at hand is when these are charged they can sometimes discharge in unexpected places, like through the insulation. So the insulation handles direct coupling, but as far as I can tell short of actually adding another ground wire to the handle there is no way to avoid it building charge. Adding an additional grounding wire is something for Bovie or Coviden or someone to work on, I'm just trying to determine if there is anyway to legitimately test for this in a prototype (incidentally it might as well be a big block metal) The electrode is insulated by a few mm of radel, and a few mm of air.

@liteyear
inside the body it has caused death, but generally it's cuts or burns in unexpected places. perforated bowels are seriously painful and have lots of complications. But you nailed it, a jolt or burn to the hand of a surgeon or nurse and game over.

So as to answer everyone's question: The surgical instrument has to be take apart for cleaning. This generally means that the handle and the piece that is actually grasping tissue or cutting are separate items. In order to take advantage of the electrosurgical properties the insert has to be energized from the electrode the electrode has to stay embedded in the handle. However, the handle still has to actuate the "hot" insert. The way this usually addressed is to have a non-conductive handle. The make or break of this new design is if you can have a conductive handle but insulate it enough or adjust it somehow to not discharge into the doctor's hand.

So if I was to design a test, I would test worst case scenario, like the handle's been through an autoclave a bunch of times and the surgeon has the settings cranked to the max and has been laying on the foot pedal for a ton of time with an open circuit. If can determine that it won't discharge into someone's hand I'm super happy. I just was thinking if I could quantitatively capture the increase in capacitance I could demonstrate such a test would be valid and that even if the handle is building a charge, it's not enough burn someone

RE: Capacitive Coupling / Parasitic Capacitance Questions

(OP)
@VE1BLL

I think so. So imagine you're on your back on a surgical table. Underneath you is a ground pad electrode. So it might be taking a shortcut back to the ground, or it could be with the charge it's built up that the difference in the potentials has narrowed to the point that it's taking multiple paths back to the ground? I really wish I knew the electrical behavior better. I mean I know the old adage that it will always take the path of least resistance only holds true if there is big difference in the electrical potential difference right?

The part that's circled is the part displaying the undesired behavior but if you look at the rest of the picture you can see that the instrument is cutting in the desired location as well. I put a link to where the cables are, I think they are shielded because the plugs into the generator are multi-pronged, but I really don't know.

RE: Capacitive Coupling / Parasitic Capacitance Questions

Words like "charged", etc. imply DC and usually apply to static electrity problems. This system is apparently using AC / RF, so that vocabulary isn't quite right.

In general, metal things (especially handles) around high voltage are supposed to be grounded. I don't understand how any other design approach could ever be contemplated, except all plastic double insulated. Is this system Approved?

Actuating the arc could be by an integrated low voltage control embedded in the handle. Trigger squeeze to control power.

You may wish to review the design concepts of modern MIG/TIG welding and Plasma Cutting handles. They're way ahead of where this design has reached.

The ESU and handle designs should be integrated. I think that you hinted that they're separate. If so, that's a problem.

The whole system could be improved if the return path were better controlled. An isolated/balanced system with a return path paddle and plenty of saline splashed around. As it is, the return path and its current density is a matter of luck.


RE: Capacitive Coupling / Parasitic Capacitance Questions

If the handle is grounded, as it should be, those arcs may be ground seeking from the patient to the grounded handle. You should rule this possibility out before worrying about parasitic capacitance.

The whole design concept should address all these safety factors from the start.

"Path of least resistance" is untrue. Current and voltage follows Kirchoff's laws and distributes themselves accordingly. A complication is arcing where the ionized path provides a lower voltage drop and hogs more of the current than expected. Kirchoff's laws apply at any voltage, noting any nonlinearities.

RE: Capacitive Coupling / Parasitic Capacitance Questions

" So adding a ground line from the handle would take care of it, but it generally is not accepted in hospitals."

There is absolutely no reason the probe couldn't have a shield that runs back to the box in the same 2-conductor cable that is currently being used.

TTFN
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RE: Capacitive Coupling / Parasitic Capacitance Questions

(OP)
@VE1BLL

I get that, it's the first thing that popped into my mind when I was informed of this potential failure mode. Unfortunately, there is nothing I can do about it. The handle is separate and has to use a standard post electrode that works with standard cables and generator units. This system isn't anything new. It has been in frequent use since the early 90's. I'm not redesigning the system. The electrical components are all standard equipment. The only thing new here is that instead of a composite handle body or dip coating a metal handle for insulation; a metal handle is shelled out and insulated with composite pieces. What I'm looking for is to better understand how to realistically measure and induce the capacitive coupling so as to test for it and not have it happen to a surgeon or nurse.

@IRstuff I really don't know. I didn't design the cable, it probably is. I posted a link to some of the most popular cables and the electrodes. They're standard uniform accessories, and my hands are bound to using standard connectors/fasteners. What is there is there, so the question is will the handle work in those conditions or won't it. I'm trying to figure that out theoretically first, and then try to quantitatively measure those issues in prototype testing.

RE: Capacitive Coupling / Parasitic Capacitance Questions

(OP)
@VE1BLL

Also, Thank you. What are the right terms for an AC system? All I can think to call them otherwise is conductors...

I mean this was my introduction to this phenomenon, I attached the picture earlier showing the arc through the halar tube.

Capacitive coupling: Capacitive coupling occurs in the presence of a capacitator and is created when two conductors are separated by an insulator. It can occur many times during a laparoscopic procedure, as tissue, trocar and instrument (itself consisting of active electrode and primary insulation) are in close proximity. Capacitive coupling can transfer current to non-target tissue through intact insulation, thereby causing stray electrosurgical burns. There’s nothing a surgeon can do to prevent the latter two causes of electrosurgical burns. What’s more, in an American College of Surgeons survey, 49 percent of surgeons said they had not heard of capacitive coupling. That’s why it’s especially important that monitored monopolar instruments are in place.

- See more at: http://encision.com/news/electrosurgery-in-the-new...

RE: Capacitive Coupling / Parasitic Capacitance Questions

Well I stand corrected! A picture is worth a bazillion words. That's not at all the style handle I was picturing. Thanks for the cool pictures. All of them! I see what you're up against now.

Seems silly the originators didn't make the probe a small gap with the ground immediately associated with the tip. Like tweezers, so no current has to flow through the patient.

Back to your issue. I was not kidding when I said wrap it in foil. That would be like the most intimate hand grip a user could manage. That foil would be the maximum a hand could ever couple with. Then, measure the potential, preferably with a fast scope, to the associated grounding mat. This setup will show you the maximum that could be capacitively coupled into a gripping hand. You can also add a 10K high-voltage resistor to stand in for the human's skin resistance if desired. That won't change the potential but it would change the current seen exiting thru the simulated user.

If you don't have a high voltage scope probe definitely use a resistor, as mentioned, but stack it up as a resistor divider to reduce the voltage the probe would see. The voltage divider equation will allow you to calculate exactly what was applied to the top of the divider by the 'foil hand'.

If you have a scope some foil and a handful of resistors you could be testing in minutes.

One point. Don't use wire-wound resistors as they will spoil things with their inherent inductance.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Capacitive Coupling / Parasitic Capacitance Questions

Going back to the top of the thread: "Additionally, the longer the device is used the more charge it builds potentially discharging in unintended places."

That description does not apply to AC capacitive coupling. AC doesn't 'build up' a charge, at least not without a diode. Therefore, the assumptions upon which these Capacitive Coupling / Parasitic Capacitance Questions are based seem to be incorrect. This is assuming that the waveform really is AC as described.

It might be worth backing-up a bit and rechecking the assumptions of what's the root cause (or causes).

RE: Capacitive Coupling / Parasitic Capacitance Questions

(OP)
@itsmoked
Thanks! I figure I am going to have to pick up some equipment to test this! So these recommendations really help! Thank you! All I have at my disposal right now is an old hi-pot machine and a cheap harbor freight DMM. So I really appreciate the advice, knowing how to best setup the test is going to save money in the test environment.

@VE1BLL
I don't understand. It could be my own incompetence on these systems work, please help me. Is it the phrase "builds up"? Are you saying that as soon as the electrode is charged anything nearby that conducts is already at its max capacitance? If you could clarify a bit it would really help me understand. Otherwise, I would have to guess that something is at least acting like a diode because at least from what I read, the longer the surgery goes on and the more the Sawbones lays on the foot pedal, the more likely the patient is to catch a stray burn.

RE: Capacitive Coupling / Parasitic Capacitance Questions

(OP)
@itsmoked

What you're describing sounds a lot like bipolar electrosurgery, which seems all together safer to me!

I found this about the generators, although the source is old: "Most modern electrosurgical generators are isolated from earth ground, and have fault monitors that will disable the machine and sound an alarm if the ground electrode circuit is not intact. While these features minimize the incidence of alternate pathway burns, several measures can be taken to make them even less likely to occur."

This is type of arcing is odd to me. I wish I understood it better. They reported it as far back as the 70's but it was initially being reported as something that was theoretically possible but unlikely. But then you have photographic evidence of it happening and at a rate that correlates to the incident rate.

These are current arcs right? I think my understanding of capacitors is insufficient, I keep thinking of cameras and tasers. These seem like low voltage systems, but I don't know.

RE: Capacitive Coupling / Parasitic Capacitance Questions

"I don't understand. It could be my own incompetence on these systems work, please help me. Is it the phrase "builds up"? Are you saying that as soon as the electrode is charged anything nearby that conducts is already at its max capacitance? If you could clarify a bit it would really help me understand. Otherwise, I would have to guess that something is at least acting like a diode because at least from what I read, the longer the surgery goes on and the more the Sawbones lays on the foot pedal, the more likely the patient is to catch a stray burn."

This type of capacitor does not have "max capacitance" as if it were changing; it's not changing. More likely, it's a bad circuit design, and the machine is changing its output voltage or frequency or both as duty cycle increases or usage duration increases.

Your photos are indeed showing arcing, but I think that's just poor design or poor material. That level of arcing is showing a complete lack of isolation at the point of arcing, and the insulation is essentially acting as if it were non existent. I would seriously look at that particular probe to see if the insulation is either damaged or degraded. It's possible that something the process of sterilization or other preparation is damaging the material.

TTFN
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RE: Capacitive Coupling / Parasitic Capacitance Questions

(OP)
@IRstuff

The output voltage and frequency are controlled by the ESG. " the machine is changing its output voltage or frequency or both as duty cycle increases or usage duration increases" So something IS happening during duration. You're saying that it's likely voltage, frequency or both. If the generator is supplying the selected voltage at the selected frequency where is this increase coming from?

You're unknowingly repeating the same lines that we're stated in much earlier white papers, and were later deemed inaccurate. I know it seems unlikely but the scenario is playing out as I have described. I can assure you, that insulation is checked both before and after each use. Insulation failure or degradation is a real but different failure mode and it is accounted for. This is different. It seems like you're thinking of it more along the lines of direct coupling. It does seem like the most likely failure mode, but in these situations it isn't happening that way. If the insert were uninsulated and it touched something and arc'd it would be obvious, if the insulation were cracked or degraded it would be obvious. This is something else. The insulation checks good and yet there is arcing from the charged/hot/electrified(term?) insert through the insulation through the air through the patient back to the return electrode.

That was my understanding of why it was called capacitive coupling because the insulation and the air are acting as the dielectric part of the capacitor.

RE: Capacitive Coupling / Parasitic Capacitance Questions

An AC waveform will charge and discharge capacitors with equal vigor; there's nothing to break the symmetry. At 1 MHz (for example), this charge and discharge cycle will occur every microsecond.

So, if the description is that the problem is building up over a much longer period, then that's a strong hint that the root cause is something other than a fixed characteristic such as 'parasitic capacitance'.

Note - this logic is precariously balanced on the information provided and assumes that I've not missed anything.

RE: Capacitive Coupling / Parasitic Capacitance Questions

I don't know who or how these things were checked, but if the probe was shielded as the manufacturer suggests that it is, only a shield AND insulation failure can produce those arcs. Even relatively small currents would require 10x the capacitance that could possibly exist in the entire probe AND a complete failure of insulation AND a complete failure of the shielding.


"AORN recommends active electrode monitoring (AEM) as a way to minimize the risk of stray electrosurgical burns due to insulation failure and capacitive coupling. Under normal operating conditions, AEM technology delivers 100 percent of the power to the surgeon’s intended site. Capacitively coupled energy is safely drained to the generator via a protective shield built into 5mm AEM instruments. If primary insulation fails, or the level of capacitively coupled current becomes too much, AEM technology shuts down the generator, protecting the patient from a potentially life-threatening burn, and alerts the perioperative staff."

So, why are you attempting to do something different?

TTFN
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RE: Capacitive Coupling / Parasitic Capacitance Questions

(OP)
@VE1BLL

That could be my own misunderstanding, but Capacitance is stored electrical charge. What I have to go on is the scientists and engineers who deal with this everyday have called it Capacitive coupling.

RE: Capacitive Coupling / Parasitic Capacitance Questions

(OP)
@IRstuff

"Capacitive Coupling occurs whenever two conductors are separated by a non-conductor. An electrostatic field is created between the two conductors. As a result, a current in one conductor can, through this electrostatic field, induce a current in the second conductor. An "inadvertent capacitor" may be created by the composition and placement of surgical instruments:

Electrode (conductor) + Electrode Insulation (non-conductor) + Metal Cannula (conductor) = CAPACITOR

If we consider a metal cannula system, a capacitor occurs whenever a non-conductor separates two conductors. The conductive active electrode is surrounded by nonconductive insulation - in turn surrounded by a conductive metal cannula. An "inadvertent capacitor" may be created by the surgical instruments. If we consider a plastic cannula system, capacitance cannot be entirely eliminated with an all plastic cannula. The patient's conductive tissue completes the definition of a capacitor; capacitance is reduced, but is not entirely eliminated. Finally, if we finally consider a hybrid cannula system, this is the worst combination and occurs when metal cannula held in place by a plastic anchor, or when a metal reducing tube is placed through a plastic cannula. The metal cannula or reducer still creates a capacitor with the active electrode. However the plastic port prevents the current from dissipating through the abdominal wall. Capacitively coupled current may exit to adjacent tissue on its way to the patient return electrode can cause significant injury.

I'm not trying to do anything different. In fact, all of that is fine, what I'm concerned about is what is now going to happen in "Zone 4" The front part and what's going on in the patient, I'm not concerned about, I've got it covered, I'm looking at the back end of this. So if a surgeon is touching this newly designed handle, that he doesn't act like the new grounding pad.

BTW I agree with AORN but only 1 company actually makes tools like that right now, and they're the ones selling the AEM system. Meaning they've identified th

RE: Capacitive Coupling / Parasitic Capacitance Questions

Yes, capacitive coupling can cause things to become charged. But the Universe tends to be symmetrical. So for one-half cycle the other plate (the handle) can be charged positive, and then for the other one-cycle cycle, it can be charged negative. Lacking a diode to break the symmetry, ones is as good as the other. Rinse and repeat at 1 million time per second.

The axis of time is a rich vein, in terms of providing evidence. If something is building up over a period of minutes, then you need to find a mechanism that matches that time period. Capacitive coupling of 1MHz AC isn't it.

Usual disclaimer applies.

RE: Capacitive Coupling / Parasitic Capacitance Questions

(OP)
@VE1BLL

I get what you're saying I really do, so I'm looking for explanations. All I can tell you is the data that I have gathered or has been presented to me as best I understand it. I don't understand how it could be charged enough to discharge at half a micro second. In fact, I don't think that is what is going on, but it's also not what I am concerned with. Whatever is going on over in zones 2 and 3, it's real and it's happening, the people seeing it might not be describing it right, but I'm seeing instruments with sidewalls blown out and patients with burns on their insides.

All that aside, I want to make sure it doesn't happen in zone 4

RE: Capacitive Coupling / Parasitic Capacitance Questions

(OP)
As I said before all of these instruments undergo insulation testing before and after surgery

Here is an FDA event revolving the manufacturers of that double insulated AEM instrument. I'm not trying to create a new system, I'm not worried about zones 2 and 3 although it seems there is plenty of reason to be concerned. I want to know: If I have this standard electrode, and I have my insulator piece, and the handle around the insulator pieces is conductive, how do I test for capacitance accurately?

http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/...

RE: Capacitive Coupling / Parasitic Capacitance Questions

(OP)
Clarification: Side walls blown out was from insulation failure not from the capacitive coupling, I reread what I wrote, and what I wrote and what I was trying to convey don't match. I have seen the sidewalls blown out, with weak or degrade insulation. I know what that looks like, and results in burns. I have seen instruments that look good but have discharged through insulation that tested good and the burns those instruments have caused.

RE: Capacitive Coupling / Parasitic Capacitance Questions

(OP)
Best explanation I have seen so far
CAPACITIVELY COUPLED CURRENT

The phenomenon of capacitively coupled currents is more difficult to understand. It is the inducement of current through the intact insulation of active electrodes to surrounding cannulas or instruments.
Note: The term induced/inducement technically relates to inductive coupling, not capacitive coupling. However, the term will be used in this paper for the purpose of teaching the concept of the transfer of current through a nonconductor.

What is capacitance? It is a stored electrical charge. Capacitors are defined as two electrical conductors separated by an insulator. During minimally invasive procedures incorporating the passage of electrosurgical energy into the abdomen, a capacitor can be created. The active electrode (a conductor) is surrounded by insulation (a nonconductor) which is often passed through a metal cannula (conductor). A capacitor has been created. A capacitor can induce an electrical current into the metal cannula through the process of capacitance. This capacitively coupled current wants to complete the circuit by finding a pathway to the patient return electrode. The electrical charge will be stored in the metal cannula until either the generator is deactivated or a pathway to complete the circuit presents itself. As we begin our discussion of capacitively coupled current, it is important to remember that capacitively coupled current is increased with a high voltage coagulation waveform, as opposed to a low voltage cutting waveform. Additionally, activating the generator in open circuit greatly increases the level of capacitance that will occur. Also, by isolating the outer conductor (metal cannula or instrument) from the abdominal wall by a plastic nonconductor, the possibility for injury is increased. The level of capacitance is increased when the cannula is small (i.e. 5 mm vs. 10 mm) and the electrode is long.

The amount of electrical energy capacitively coupled onto the metal cannula will be dependent on a number of variables--first, when the generator is activated in open circuit. This, as previously described, causes the generator to go to maximum voltage. The higher the voltage, the more current that will be produced. Conversely, capacitance will be minimal if the generator is activated in closed circuit. This means that the active electrode is touching or in near proximity to the target tissue. This will help ensure that the generator’s output will travel through the target tissue on its way to the patient return electrode. Voltages will remain lower and the amount of capacitance will likewise remain low.

The second variable is the waveform selected by the surgeon. Since the cut waveform uses significantly less voltage, the amount of capacitive current is less. The cutting waveform can be used for both vaporization (cutting) and desiccation. The high voltage coagulation waveform should be reserved for fulguration. When using the coagulation waveform, the surgeon should be careful not to activate the generator in open circuit.

The type of cannula/trocar system chosen also plays an important role in the amount of capacitance produced and whether this current can be safely dispersed. An all metal system is appropriate in that any capacitive currents will be safely dispersed through the greater surface area provided by the chest or abdominal wall, thereby reducing current density. In normal circumstances, this surface area will be adequate to safely dissipate any current buildup on the cannula without significant heat production or tissue damage.

An all plastic cannula system is also appropriate. When using an all plastic system the definition of a capacitor has been eliminated. Instead of two conductors separated by a nonconductor, there is now the conductive electrode, covered by nonconductive insulation, surrounded by the nonconductive cannula. A capacitor no longer exists and, therefore, concerns over the capacitively coupled current have also been eliminated.

The big problem with capacitance occurs when a metal cannula is held in place by a plastic anchor (hybrid trocar/cannula system). (This complies with the definition of a capacitor through the metal portion of the cannula.) However, the coupled current is not able to disperse safely through the chest or abdominal wall because the cannula is held in place by a nonconductive plastic anchor.

Consequently, the current that is coupled onto the metal cannula can only complete the circuit by discharging to tissue that it may encounter within the cavity (Figure 3). The unknown variable is the amount of tissue that is in proximity to the metal cannula. If it is large, it is unlikely that the discharging current will cause any damage due to the principle of low current density. However, a small contact area will create high current density and potentially a significant injury. Often this occurrence is outside the field of vision and, therefore, is not detected as a problem until several days postoperatively.

RE: Capacitive Coupling / Parasitic Capacitance Questions

If everything is working you shouldn't be able to measure the capacitance, because there should be a shield in-between.

The last picture you posted is weird, in that I don't understand if the upper electrode tip is intentionally uninsulated and unshielded, in which case, the zap to the bowel is almost a given. But, if that "electrode tip" isn't being used, why is it powered, and if it's not, why isn't it grounded or insulated?

It seems to me that most of what I'm reading here is a consequence of bad decisions in the OR, rather than an inherent design issue, possibly. Why would someone be using this metal cannula if this is a known problem?

TTFN
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RE: Capacitive Coupling / Parasitic Capacitance Questions

(OP)
Hopefully this sheds some light on how this is happening with AC voltage.

So worst case scenario High Power Generator, Coagulation Open circuit, device is known to be a inadvertent capacitor. How do I measure how much of a capacitor it really is, and determine where it is likely to discharge?

LoL maybe should tell the surgeons just wear really thick glovesrofl2


http://www.dremed.com/catalog/documents/Lforcefx.p...

RE: Capacitive Coupling / Parasitic Capacitance Questions

The capacitive discharge as shown in that picture is specifically possible because of the metal cannula, since what the metal does is to put all of the capacitance in parallel. A probe without a metal cannula cannot discharge in this fashion, since the charge that could be generated cannot move to the point of closest approach, since the material is non conducting. Since the metal cannula is not participating in anything, I don't see why just a plastic sleeve wouldn't eliminate this problem.

As I stated before, a capacitance meter would be used to measure capacitance, but without a metal cannula, you would get meaningless measurements.

TTFN
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RE: Capacitive Coupling / Parasitic Capacitance Questions

(OP)
@IRstuff

That I get. Unfortunately the metal cannula does have other uses so getting rid of it isn't really an option (on this design anyway). Plus as I have stated before, I'm not really worried about zones 2 and 3 What I have is much more interesting. So in zone 4 (the handle) we have the "bovie post" connector going through Radel insulation pieces which are housed in a big 17-4SS shell.

After talking with you guys, I'm sure it's a capacitor; I'm trying to determine how bad it is. If I do like kevin / itsmoked said and wrap the handle in foil where is the worst case challenge to place the other probe?

RE: Capacitive Coupling / Parasitic Capacitance Questions

I think that providing a contactable metal surface will make things worse, because, as I mentioned above, you've now provided a conductive electrode for the capacitance, allowing the entire handle to participate, rather than just one tiny part.

In any case, for your measurement, the case is one electrode, and either of the two wires would be the other.

TTFN
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RE: Capacitive Coupling / Parasitic Capacitance Questions

The operating principles of this type of surgical instrument is similar to corona treaters which are described here http://www.enerconind.com/mediaLib/stml/techPaper/...

The "arc" in these instruments is a corona discharge where the air at the tip of the instrument is ionized by a very high electric field created by the sharp tip and high voltage. C

The manual that was linked to above said the instrument operated at 50 kHz. That is a long way from a MHz.

RE: Capacitive Coupling / Parasitic Capacitance Questions

(OP)
"The 'arc' in these instruments is a corona discharge where the air at the tip of the instrument is ionized by a very high electric field created by the sharp tip and high voltage. C

The manual that was linked to above said the instrument operated at 50 kHz. That is a long way from a MHz."

  1. You want these to discharge at the tip and nowhere else, this discussion was about capacitive coupling, and discharge through insulation materials. In this case, the handle is actually acting as a capacitor, and the capacitive charge is building. The insulation is not being damaged to the point of discharge.
  2. The majority of the tips are rounded. Regardless, in a perfect situation, the tip has the least resistance to the grounding pad placed on the other side of the patient, thus purposely creating an arc. However, as evidenced by Encision and numerous surgeons the instrument itself will act as a capacitor and discharge in zones 2 or 3 (or rarely zone 4) and arc through sections where the insulator is acting as part of the capacitor.
  3. You'll also notice in my above posts that I mentioned various ranges for the generator units. That particular unit is considered to be in the low range. However, as I also mentioned earlier Coag is the worst condition for this, and if you look at the manual, it states that Monopolar cut and coagulation occur at 390kHz, still far short of the 1MHz seen in the high range generators but more than 7X the number you're stating. Where did you get that?
In the end it doesn't matter, regardless of my calculations nor the fact that for less than $5K we could prototype and build five to play with and know for sure vs. $80K for tooling for an alternative design, the design and 4+ months of work were scrapped. curse

RE: Capacitive Coupling / Parasitic Capacitance Questions

(OP)
BTW Compositepro

Thank you for the article, it is very interesting. I once worked for a company where I designed a treatment chamber to apply a "chemical secret sauce" wink to various substrates to increase the surface tension chemically. It worked almost or as well as flame treatment.

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