Leads
The term "lead" in electrocardiography causes much confusion because it is used to refer to two different things. In accordance with common parlance, the word lead may be used to refer to the electrical cable attaching the electrodes to the ECG recorder. As such, it may be acceptable to refer to the "left arm lead" as the electrode (and its cable) that should be attached at or near the left arm. Usually, 10 of these electrodes are standard in a "12-lead" ECG.
Alternatively (and some would say properly, in the context of electrocardiography), the word lead may refer to the tracing of the voltage difference between two of the electrodes and is what is actually produced by the ECG recorder. Each will have a specific name. For example "lead I" is the voltage between the right arm electrode and the left arm electrode, whereas "Lead II" is the voltage between the right arm and the feet. (This rapidly becomes more complex as one of the "electrodes" may in fact be a composite of the electrical signal from a combination of the other electrodes). Twelve of this type of lead form a "12-lead" ECG.
To cause additional confusion, the term "limb leads" usually refers to the tracings from leads I, II and III rather than the electrodes attached to the limbs.
Read more about this topic: Electrocardiography
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