Editing Electrocardiography (section)

===Virtual Electrode===

The virtual electrode is used to obtain useful measurements from the precordial leads, and also allows the creation of the augmented limb leads.

The virtual electrode is known as '''Wilson's Central Terminal''' ('''WCT'''). For the precordial leads, WCT is formed by averaging the three standard limb leads (I, II, and III):
:<math>
V_W = \frac{1}{3}(RA+LA+LL)
</math>

WCT is therefore a virtual electrode which sits slightly [[posterior (anatomy)|posteriorly]] to the heart. It is a useful point, from which the electrical potential of the precordial leads is measured.<ref name="AHA_ECG_pt1" />

WCT used to be used as a reference for the virtual limb leads, however use in this way produced leads with very small amplitudes. Goldberger's modification is now used to produce each augmented limb lead, aVF, aVR, and aVL, which produces 50% larger amplitude leads than the standard WCT. Goldberger's WCT is formed according to the following:<ref name="AHA_ECG_pt1" />
:<math>
aVR = \frac{LA + LL}{2}
</math>
:<math>
aVL = \frac{RA + LL}{2}
</math>
:<math>
aVF = \frac{RA + LA}{2}
</math>

In a 12-lead ECG, all leads except the limb leads are assumed to be unipolar (aVR, aVL, aVF, V<sub>1</sub>, V<sub>2</sub>, V<sub>3</sub>, V<sub>4</sub>, V<sub>5</sub>, and V<sub>6</sub>).
The measurement of a voltage requires two contacts and so, electrically, the unipolar leads are measured from the common lead (negative) and the unipolar lead (positive).
This averaging for the common lead and the abstract unipolar lead concept makes for a more challenging understanding and is complicated by sloppy usage of "lead" and "electrode".
In fact, instead of being a constant reference, V<sub>W</sub> has a value that fluctuates throughout the heart cycle. It also does not truly represent the center-of-heart potential due to the body parts the signals travel through.<ref>{{cite journal |last1=Gargiulo |first1=GD |title=True unipolar ECG machine for Wilson Central Terminal measurements. |journal=BioMed Research International |date=2015 |volume=2015 |pages=586397 |doi=10.1155/2015/586397 |pmid=26495303 |pmc=460614|doi-access=free }}</ref> Because voltage is by definition a bipolar measurement between two points, describing an electrocardiographic lead as "unipolar" makes little sense electrically and should be avoided. The American Heart Association states "All leads are effectively 'bipolar,' and the term 'unipolar' in description of the augmented limb leads and the precordial leads lacks precision."<ref>{{cite journal |last1=Kligfield |first1=P|last2=Gettes |first2=LS |last3=Bailey |first3=JJ |last4=Childers |first4=R |last5=Deal |first5=BJ |last6=Hancock |first6=EW |last7=van Herpen |first7=G |last8=Kors |first8=J |last9=Macfarlane |first9=P |last10=Mirvis |first10=DM |last11=Pahlm |first11=O |last12=Rautaharju |first12=P |last13=Wagner |first13=GS. |title=Recommendations for the standardization and interpretation of the electrocardiogram: Part I: The electrocardiogram and its technology: A scientific statement from the American Heart Association Electrocardiography and Arrhythmia Committee, Council on Clinical Cardiology; the American College of Cardiology Foundation; and the Heart Rhythm Society. |journal=Journal of the American College of Cardiology |date=2007 |volume=49 |issue=10|pages=1109–1127 |doi=10.1016/j.jacc.2007.01.024 |pmid=17349896}}</ref>