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Any circuit through the heart is enough for (at least a single-lead) ECG. For example, the AliveCor is a hand-held mobile phone case—when your left hand and right hand are both touching it, you can get an ECG.

Chest ECGs done at the hospital are often 12-lead, and those give different "angles" through the heart, which gives cardiologists more clues on exactly which parts of cardiac tissue are causing a problem.

2 comments

Well the 12 lead is more finnessed than that - essentially without a 12 lead your ability to definitively rule out an acute heart attack is completely impaired. 5 lead ECGs will still miss (some high percentage, above 10% and below 50%) of heart attacks.

A one (two?) lead ecg as you describe is not going to tell anyone what is going on with ST segments, particularly as you would be only looking at your aVR or aVL(depending on which way you sum the currents).

Yeah, but anyone who's trying to dx (or, worse, rule out) an AMI on the basis of what their Apple Watch can report is ... yeah, I don't even know what they are. Not the sharpest pencil in the box, for a start.

The augmented leads (aVR, aVL, and aVF) require a minimum of three electrodes (multiple electrodes are connected to ground).

An electrode on each arm gives you Lead I.

Lead I is not as useful as Lead II, but it's still plenty useful for diagnosing all sorts of cardiac issues (there's a lot more that can go wrong with your heart than just an MI).

Presumably a single lead would still be able to detect certain arrhythmias? I.e. if there are no QRS complexes, then there may be something wrong with the person wearing the device. There are benign explanations for why the signal might be weak but also lethal causes like Vfib -- with a few additional, I don't think it's implausible for a device to be able to make the decision to ask the user "hey, are you okay?" and then potentially seek help if the user doesn't confirm that they are OK within some set period.

Absolutely. Consider the fact that an AED only has one lead to work with, and it is able to recognize a couple different lethal arrhythmias in order to deliver a shock (and more importantly, _not_ deliver a shock at any other time).

To be clear, by "lead" here, I mean a pair of electrodes. A single electrode (like you might put on a watch) is useless.

12-lead ECGs are really only done with 10 physical leads. One lead is ground, six leads are each single-channel measures (V1 -V6) that wrap around the heart and the remaining three leads are used to create six channels - three single-channel measures, and the last three channels are calculated as diffs between them. V1-V6 have specific spots on the chest to live, but the remaining four leads (including ground) get one limb each, and generally work wherever you want on that limb.

(I used to be an EEG tech, and for us, the ECG was the primary thing we wanted to get rid of...)

I'm actually really curious -- why would you want to get rid of the ECG as an EEG tech? Do the signals interfere? (I don't know too much about EEG -- happy to learn more!)

The short form is that EEG is brain and ECG is heart, and you only want to see brain waves, not heart waves - it's pure interference, that changes the shape of the wave. It's simply 'not the thing we're looking for'.

Most clinical EEGs are related to epilepsy or similar, which usually has a particular pattern with spikes in it, but sometimes that pattern is subtle in the EEG. The spiky nature of ECG 'bleeding through' can make it a bit harder to determine the true EEG spikes from the artifact ECG patterns. Usually you will record a single-lead ECG along with the EEG, so you can see where the ECG spikes occurred and account for them in diagnosing the EEG.

This link has some clear spike-and-wave epileptic waveforms in it, but it's not always so clear-cut: https://en.wikipedia.org/wiki/Spike-and-wave

Finally, there's not a lot of overlap between neurological and cardiac patients - there's rarely any need for an EEG tech to do a full ECG (we never did, and we did approx 3200 patients/year), and full ECGs are pretty commonly done in most other places in a hospital anyway.