On Recording the Unipolar ECG Limb Leads via the Wilson's vs the Goldberger's Terminals: aVR, aVL, and aVF Revisited

The augmented unipolar limb leads aVR, aVL, and aVF, introduced by Goldberger in 1942, are an integral part of the 12-lead ECG.1,2 Leads I, II, and III have 2 dedicated electrodes, but the other 9 leads have a single dedicated electrode, and another one constructed from the averaged inputs of multiple electrodes. This Viewpoint discusses whether an indifferent pole for the recording of unipolar limb leads is best provided by the Wilson's central terminal (WCT), or by inputs from 2 limb electrodes (Goldberger's central terminal) (GCT), as done currently, and whether the latter have any advantages over the former. The term "unipolar", popularized by Wilson, is a misnomer, since no leads can be truly "unipolar", all requiring positive and negative poles. Thus the term unipolar is used herein in the quasi-unipolar sense, as when first introduced by Wilson and Goldberger, who also realized that such leads were not truly unipolar. The popularity of the unipolar leads reflected the quest of recording the ECG from various vantage points of the body, considering the limitations of the 3 bipolar leads, introduced by Einthoven,3 which register the difference of 2 ECG curves recorded at the 2 poles of these leads, and no variation in potential at each of these poles.4 In contrast the unipolar leads were thought to register such variation of absolute potential, something not really true. Initially the WCT was used to record the unipolar limb leads,5 but the amplitude was low, and the inscribed ECGs, then, and for many decades later,6 were thick-lined (≥2 mm) (Figure 1). Goldberger thought that the three 5,000 Ω resistances of the WCT were not necessary, and they could be substituted by 3 plain wires to form a terminal for recording of unipolar limb leads. He also reported that his limb leads ECGs were identical in morphology to the ones obtained via the WCT.1,2 However his innovation consisted of disconnecting the electrode from the composition of his GCT of the unipolar lead he was recording. Thus when aVR is recorded the GCT consists of a connection of left arm and left leg; when aVL is recorded the GCT consists of a connection of right arm and left leg; when aVF is recorded the GCT consists of a connection of right arm and left arm.1,2 Thus the GCT is variable, consisting of the mean of the potentials of the 2 (different for the 3 recordings) limb leads, in contrast to the WCT which is unvariable.1,2,5-8 This modification of Goldberger leads to the augmentation of the recorded limb leads by 50%, as can be shown mathematically, and thus the aVR, aVL, and aVF came into being.1,2,7-9