In a clinical setting, not all 12 leads are displayed at the same time, and most often, not all leads are measured simultaneously. A common setup used is a five-wire system consisting of the two arm leads, which are actually placed on the shoulder areas, two leg leads placed where the legs join the torso, and one chest lead. This arrangement allows display of any of the limb leads (I, II, III, aVR, aVL, and aVF) and one of the precordial leads, depending on where the chest electrode is placed. Figure 10 shows the positioning of these five electrodes on the patient's body.
Nevertheless, the exact anatomical placement of the leads is very important for obtaining accurate ECG traces for clinical evaluations; moving an electrode even slightly away from its so-called correct position could cause dramatically different traces and possibly lead to misdiagnosis. A slight exception to this rule is the limb leads, which do not necessarily need to be placed at the proximal portion of the limb as described in the preceding paragraph. However, the limb leads for the most part need to be equidistant from each other relative to the heart (i.e., for determination of the electrical axis to be accurate).
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