It's always nice (I guess I mean interesting) when you come across something "new", and the other day I came across ESIS for the first time.

Whilst looking up some part numbers, we noticed ESIS ECG cables in a catalogue. It seems that this stands for "Electrosurgical Interference Suppression". I also noticed that they are even more expensive than normal (shielded) ECG cables.

Upon digging a little deeper, we came across ECG trunk cables containing 100 mH RF chokes in series with a 1K resistor in each lead line. It seems that the chokes are located within the
block at the business end of the trunk cable (that is, where the individual lead wires plug in). And I'm guessing that the resistors are at the other end (where the cable plugs into the monitor). But this may not be typical, so more information please!

I was wondering why those chokes aren't put inside the ECG monitor itself, but then realised that the ECG leads themselves probably act as an antenna to the RF output from the electrosurgical unit.

Has anyone actually been in theatres and seen the results of all this? Can anyone comment on what happens at the monitor display when the surgeon is blasting away with:-

1) Normal ECG leads being used
2) ESIS ECG leads being used

And (one last question) when you actually encounter such cables ... how are they marked (some special symbol(s), perhaps)?

Geoff,
In the past the HP (Philips) ones were orange as opposed to grey.
The current Draeger monitors have a blue insert that goes between the leads and the trunk cable.
If and ESU is used without these chokes (inductors for the youngsters) it tends to saturate the front end of the ECG amplifier or even, if you are unlucky, blow the front end amplifier.
The main problems with these leads is that they also block the high frequency signal put down the limb leads to measure respiration.
RoJo

Thanks for that info, Robert.

I'm wondering what went on in the (more distant) past. Maybe the monitors were switched off (or the ECG cable yanked out) for a minute or two?

That's an interesting comment about respiration monitoring. But perhaps that's not such a big deal in the theatre (anaesthetised) environment. And (obviously - we hope) these special trunk cables should not be used elsewhere! So yes, clear (vivid) colour coding must be a good idea.

On a related issue, does anyone have a more-or-less definitive list of the frequencies dished out by electrosurgical units these days?

I believe that they all fall within the Medium Frequency (MF) band (being 300 kHz to 3 MHz). That's Radio Frequency, of course (anything over 3 kHz and all the way up to 300 GHz or so). In fact some of them would most likely appear at the low end of the dial on an AM radio!

My own notes indicate that whilst some units apparently stick with one frequency (eg, 510 kHz), others employ a range, depending upon basic application (Cut, Co-ag, Bipolar etc.).

Typically a 10 ľA current at 100 kHz or so, I believe. And using RA (R) and LA (L) (that is, Lead II) as positioned for monitoring.

I just came across some stuff about ESIS in a manual for a Datascope 3000 published back in 1989 (the days of quality kit, and informative manuals)! So it seems that I should have been trying harder to keep up all that time!

The suppression circuit is contained in an optional filter block for the Datascope 3000 (where it always should* be, in my opinion). I believe that the Propaq Encore has ESIS built in (that is, actual circuitry) but have yet to come across the diagram.

Out of interest, the 3000 uses (used) the venerable Intel 80C196 processor (plus an Intel 80978H on the NIBP board). The '196 was also used in engine controllers, apparently.

Meanwhile, check out Figure 3 as an interesting test rig!

And how's this for Acceptance Testing?

Lastly, here's another little piece about "signals, spikes and artifacts".

* But less money gets made selling special ECG cables that way, of course. But I could be wrong there, maybe when they say "attach the leads to the patient cable using the optional ESIS filter block" they are actually talking about a special trunk cable (or special connector that by this time would surely have been lost - just like the "blue insert" that RoJo has mentioned). Oh well, back to the drawing board!