2 leads is misleading

[Andrew Walsh]

V5 needs the Wilson Central Terminal so by definition needs the left arm electrode, hence leads I and III and all the augmented leads.

It is not possible to obtain lead V5 without the additional electrode so even if the result of this competition is that "two will do" it is meaningless as it is impossible to acquire without LA as well.

[physionet-challenges]

Dear Tsardoz,

I believe you have confused the notion of leads and electrodes here. A two-lead system can include three, four or even five electrodes depending on how you configure your system. Many people confuse the concept of lead and electrode (because a physical wire, or colloquially, a 'lead' is attached to the electrode pad). However, a 'lead' is one of the standard 12 tracings we see on the diagnostic ECG, and is formed by measuring the potential difference between two points. Although the standard diagnostic recording is called a 12-lead ECG, it uses only 10 electrode pads, some of which are active or passive reference points. Certain electrodes are used in more than one pairs/ combination and thus provide data on more than one lead. In other words, there is some redundancy in the ECG. (As an aside you might ask why do we do this? Generally, this is because the heart is not a point source at infinity, and so has near-field effects - the varying locations of the leads allow you to 'image' different portions of the heart.) This book is a great reference to read more about this concept: https://www.bem.fi/book/

It is also important to note that, by definition, Wilson's Central Terminal (WCT) is the simple average of the three exploring electrodes connected to the right arm (RA), left arm (LA) and left leg (LL) and it is assumed to be steady and of negligible amplitude during the cardiac cycle. You can therefore approximate the WCT from averaging any three electrode points if they are placed around the heart. (The vector sum cancels.) Of course - the WCT has problems itself. See here for example: https://www.hindawi.com/journals/bmri/2015/586397/

So we would respectfully disagree that '2 leads is misleading'. 

Leads II and V5 are indeed two leads, but can be formed in many ways,  although they will require *at least* three electrodes (outside true unipolar capacitive electrodes). If you argue this is misleading, then you should argue all named lead systems in cardiology are misleading - and ask the entire field to redefine what it means by a lead (i.e. an electrode).  We would not have the hubris to do so, and we therefore stick with the clinical convention. 

We thank you for the opportunity to clear up this confusion about how leads are named with respect to electrodes - it is not an uncommon mistake to make. 

All the best,

-Gari

https://physionetchallenges.github.io/

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