Sensors - Application and Multidisciplinary Aspects of Wireless Sensor Networks - page 28

2.10 Electrocardiograph

An electrocardiograph is a device for recording of the electrical activity of the heart
over time, usually in a noninvasive recording via skin electrodes.
An electrocardiogram is obtained by measuring electrical potential between
various points of the body using a biomedical instrumentation amplifier. A lead
records the electrical signals of the heart from a particular combination of recording
electrodes which are placed at specific points on the patient's body.
There are two types of leads - unipolar and bipolar. The former has an indifferent
electrode at the center of the Einthoven's triangle (which can be likened to the “neu-
tral” of a wall socket) at zero potential. The direction of these leads is from the
“center” of the heart radially outward. These include the precordial (chest) leads and
augmented limb leads - VR, VL, and VF. The bipolar type, in contrast, has both
electrodes at some potential, with the direction of the corresponding lead being from
the electrode at lower potential to the one at higher potential, e.g., in limb lead I, the
direction is from left to right. These include the limb leads - I, II, and III.

2.10.1 Leads

A lead refers to a combination of electrodes that forms an imaginary line in the
body along which the electrical signals are measured. The standard 12-lead ECG
consists of 10 electrodes.

2.10.2 Limb Leads

Leads I, II, and III are the so-called limb leads because at one time, the subjects of
electrocardiography had to literally place their arms and legs in buckets of salt water
in order to obtain signals for Einthoven's string galvanometer. They form the basis
of what is known as Einthoven's triangle.
Lead I is a dipole with the negative electrode on the right arm and the positive
electrode on the left arm. Lead II is a dipole with the negative electrode on the right
arm and the positive electrode on the left leg. Lead III is a dipole with the negative
electrode on the left arm and the positive electrode on the left leg.

2.10.3 Augmented Limb

Augmented limb leads (aVR, aVL, and aVF) are derived from the same three elec-
trodes as leads I, II, and III. However, they view the heart from different angles