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(or vectors) because the negative electrode for these leads is a modification of
Wilson's central terminal, which is derived by adding leads I, II, and III together and
plugging them into the negative terminal of the EKG machine. This zeroes out the
negative electrode and allows the positive electrode to become the “exploring elec-
trode” or a unipolar lead.
The precordial leads (V1, V2, V3, V4, V5, and V6) are placed directly on the chest.
Because of their close proximity to the heart, they do not require augmentation.
Wilson's central terminal is used for the negative electrode, and these leads are
considered to be unipolar. The precordial leads view the heart's electrical activity
in the so-called horizontal plane. The heart's electrical axis in the horizontal plane
is referred to as the Z axis.
Leads V1, V2, and V3 are referred to as the right precordial leads and V4, V5,
and V6 are referred to as the left precordial leads.
Lead V1 is placed in the fourth intercostal space to the right of the sternum. Lead
V2 is placed in the fourth intercostal space to the left of the sternum. Lead V3 is
placed directly between leads V2 and V4. Lead V4 is placed in the fifth intercostal
space in the midclavicular line (even if the apex beat is displaced). Lead V5 is
placed horizontally with V4 in the anterior axillary line. Lead V6 is placed horizon-
tally with V4 and V5 in the midaxillary line.
An additional electrode (usually green) is present in modern four-lead and
twelve-lead ECGs. This is the ground lead and is placed on the right leg by con-
vention, although in theory it can be placed anywhere on the body. With a three-
lead ECG, when one dipole is viewed, the remaining lead becomes the ground
lead by default.
1. Wilson JS (2005) Sensor technology handbook. Handbook, Elsevier Inc, Burlington, MA
2. Ripka P, Tipek A (2007) Modern sensors handbook. ISTE Ltd, UK
3. Sinclair IR (2001) Sensors and transducers, 3rd edn. Elsevier
4. Buck RP, Lindner E, Kutner W, Inzelt G (2004) Piezoelectric chemical sensors. Pure Appl Chem