TENS above the waist

I posted this on FetLife in October, but figured it could use some more coverage …

Here’s the thing. We’ve all heard the “rule” that you should never play with electricity above the waist. Some people are quite vehement about it.

On the other hand, I’ve never actually heard of above-the-waist electro-play, at least not using electro-stim or TENS units, killing anyone, or even putting them in danger. Medical supply outfits routinely sell TENS (transcutaneous electronic nerve stimulation) units for pain relief, and folks put the electrodes in all sort of places, mostly well above the waist. Nobody appears to have ever died from this.

Since for the most part, when we talk of electro-play, we’re really talking about TENS units or electro-stim units that work on fundamentally the same principle, this seems to fly in the face the oft-quoted advice. Somehow, if there was, I’d expect that somewhere along the line, at least some of the vast numbers of vanillas with a TENS unit and no clue about electricity or physiology would have managed to fry themselves by now.

It seems I’m not alone, the Kinkbusters have actually tried it, placing TENS electrodes front and back of willing participants, right across the heart, cranked it up to full power and watched as they twitched and expired on the floor. Oh dear.

Actually, they didn’t. They were fine. I’m guessing they were more masochistic than me, ’cause I’ve got a fair idea how much that’s gotta hurt. But no heart fibrillations, no trouble breathing. Here’s the video:

Yeah, that myth seems pretty much busted.

But to me, there’s a difference between a safe activity and pulling a stunt that you just managed to get away with, this time, and there’s always this nagging little feeling that just because someone got away with it once, well, twice, actually make that quite a few times, things might be close to the edge and they might not always make it. Or what didn’t kill these chaps might just kill you … or, much more importantly, me. The trouble with all of the above is that the evidence is basically anecdotal.

I’d rather work with facts and measurements. Stand back! I’m going to try SCIENCE!

Now, the particular case I’m interested in is TENS stimulation of the nipples. I acquired an electro-stim device a while ago that has a pair of clips, with a copper contact on it, which accept a standard electrode lead plug. With my basic TENS unit, is hooking up the leads to my buds dangerous? Because, I might be crazy, but I’m not insane, and I’m certainly not suicidal.

The mere fact that the electro-stim device with nipple clips exists suggests that it’s probably fine … the manufacturer wouldn’t want to kill their customers, would they? But it’s also a cheap nasty device, re-purposed from a muscle stimulation device, and frankly, some stuff I’ve seen come from China scares me.

(Note that these clips are uni-polar, not the bi-polar nipple clips sold for some commercial “BDSM” stim units, which theoretically don’t pass current anywhere but through the nipples. Theoretically.)

So, let’s start from first principles.

A bit of Googling suggests that a danger of electrocution starts with a current of around 60-100 milliamps (mA) passing through the chest cavity. Now, you actually need a really tiny current (microamps) to pass through the heart itself to cause fibrillation, but its seems that to get that much current to the heart itself, a much larger current has to be passing through and around the chest cavity. Obviously, the game changes if one or more of the electrodes is inside the chest in close proximity to the heart, but …. well, let’s just say I don’t plan to test that scenario.

I won’t get too hung up on how the lethal-current through-heart number relates to the lethal-current-through-chest number, but suffice to say, lots of good authorities seem to think that over 60 mA (some say more), applied across the chest cavity e.g. in an arm-to-arm or arm-to-leg shock, carries a risk of death. So let’s just run with that.

Right, so the next question is, can a TENS unit deliver 60 mA.

The TENS 3000 (my unit, a fairly typical one), claims its pulse amplitude is “0-80 mA”

Uh-oh!

But wait, there’s more! In full, it’s “0-80 mA peak into a load of 500 ohm load each channel”. And further down, it puts the voltage at “0-40 V (Load 500 ohm)”.

Now, a power supply provides volts, not amps. Amps – current flow – is a function of voltage and resistance. Ohm’s Law, E=IR, says voltage (E, “electromotive force”) equals current (I, in amps) times resistance (in ohms). And if we plug 40 volts and 500 ohms into the formula, we do indeed get 80 mA of current.

But we have some variables. The obvious one is the voltage. It’s controlled by the amplitude knobs on the TENS unit. But more important is the resistance. Because that’s the resistance of the whole system, including the electrodes, and the body the current is passing through.

A bit more Googling, and we find that the internal resistance of a human body is fairly low, hundreds of ohms. Maybe 500 ohms for a long path (limb-to-limb). But the skin is a different story. The surface of the skin, the epidermis, is basically dead; it contains relatively little moisture and no nerves or blood vessels (these are all in the dermis below), very little to conduct electricity with.

The resistance of the skin is hard to quantify, and what few numbers are available via the web seemed to vary enormously. So there’s nothing for it but to actually break out some test gear, and a test subject.

Me.

My first reaction was simply to put the electrodes into a multimeter and test. Palm to palm (the best conductivity I found using the electrode pads – and yes you, in the cheap seats, I thought of the sweaty palm jokes long before you did!), I get around six megaohms. Nipple to nipple on the bare probes (roughly equivalent to using the clips), about three.

And let’s take a worst-case scenario: across the tongue. Somewhere around fifty kiloohms, but that varied a lot.

But I’m not satisfied. Human bodies are notoriously non-linear things, and a digital multimeter uses very low voltages to measure resistance. Worse, it tends to tweak its voltage to get better readings of linear measurements. Perhaps it’s time to step things up. Let’s get a bit more direct.

We want to measure the current in an actual shock situation. The two volts or so that the meter puts out in its resistance mode doesn’t cut it. Time to crank up the volts.

Now, I’d like to have used a real TENS unit for this, but TENS doesn’t put out a constant current. Rather, it puts out pulses; for my unit, they’re from 30 to 260 microseconds long, and at 2 to 150 Hz. That is, even at the highest frequency and the longest pulses, the power is on less than 4% of the time. A multimeter can’t measure that accurately.

The next best thing I can do is measure the actual current from a constant supply. Now I don’t have a 40 volt supply handy, but I do have a 12 volt supply.

Cutting straight to the chase, I wire up the meter, set to measure in milliamps, in series with the nipple clips and the power supply. And turn on the juice. Now this hurts a bit. It’s not like the TENS, because it’s not pulsing; it’s just a steady sting.

0.6 mA. It wobbles around a bit before settling on that number.

So, doing the arithmetic, .6 mA @ 12V gives 20 k ohms. Apply that to the 40 V maximum pulse of the TENS unit, and we get 2 mA.

About a thirtieth of the minimum current required to kill someone.

But I’m not finished. You know how you’ve always touched a 9V battery to your tongue to see if it’s good?

Well, y’know, 12V hurts more. And because it’s perpetually wet, it conducts electricity rather better. Actually, it’s a pretty good worst-case scenario for the skin resistance test.

Now you touch that 9V battery for a few moments, I’m hanging onto 12V for about a minute to make sure I have a good reading. It’s not fun, but I get a steady result, at about 8 mA. Do the arithmetic again, and we’re at 1,500 ohms. And that about agrees with the worst case numbers I’ve seen on the web.

At 40 V, we’d be looking at 26 mA. Still not enough to kill you (and this is a path length of, oh about a centimetre, not across the heart, and some of the current is actually passing through the surface saliva and not through the tongue at all), but I’m gonna guess it’ll hurt like Hell. I’ll point out at this stage that the instructions for the TENS unit quite explicitly state that one should not place electrodes in the mouth.

Wetting the skin makes a small difference. I suspect that if you pruned out in a warm bath of salt water, you might drop a digit off the dry resistance. But short of stabbing probes into your flesh, you’re not going to get a heart-stopping current from a TENS unit, no matter where you put the electrodes. And it’s probably going to get too painful to take before that happens.

Further, for the nipples, or most other reasonable TENS applications (both therapeutic and play), very little current would actually pass through the heart even if there was 60+ mA involved. The bulk of the current will be in the flesh just below the skin, taking the shortest path through the best conductor. Below that flesh is a mess of rib cage, sternum and connective tissue, or rather higher resistance.

Now, before I close, note that I’ve tested only certain cases. The TENS unit instructions contain several safety don’ts, which I’ll paraphrase here:

• Don’t use if you suspect heart disease;
• Don’t use when pregnant;
• Don’t use with pacemakers and other implanted electronic devices;
• Don’t put electrodes on the throat, over the eyes, in the mouth or internally;

I’d endorse these; most of them have good reason, even if it’s “nobody’s tested this fully, so we can’t say it’s safe”. But they’re a heck of a long way from a blanket ban on all play above the waist!

And finally, the above is all based around the specifications of one device. Check the specifications of your device before applying the conclusions presented above. And of course it your device is not a TENS unit, all bets are off!