Page 15 - Rapid Review of ECG Interpretation in Small Animal Practice, 2nd Edition

Principles of Electrocardiography

The limb leads form the points of what is known as THE LIMB LEADS
There are six limb leads which assess the cardiac
Einthoven’s triangle (Fig. 1.2). The positive pole for
VetBooks.ir lead I is on the left arm, the positive pole for lead depolarization in the frontal plane. A modified
version of the frontal plane in the dog is shown in
II on the left leg and the positive pole for lead III
is on the left leg. An imaginary line connecting the Figure 1.3. Leads I, II, and III directly record the
two electrodes is the lead axis. Each lead looks at electrical activity between two limb electrodes (Table
the heart from a different angle within the animal’s 1.1) and are thus referred to as bipolar limb leads.
frontal plane. There are 12 standard ECG leads used The unipolar (augmented) limb leads use the same
in veterinary medicine, which provide information electrodes as lead I, II, and III, and also measure
regarding the magnitude of the electrical activity the electrical activity between two terminals, but
of the heart and the direction of a moving the recording electrode is always positive and the
depolarization wavefront in multiple orientations. negative terminal is made up of the sum of the
A wavefront traveling toward the positive terminal electrodes attached to the right arm, left arm, and
of a lead results in a positive deflection of the ECG left leg (Fig. 1.4). These limb leads are named after
in that lead. When a wavefront travels away from their positive electrode, located on the left arm
the positive electrode, a negative deflection occurs. (aVL), the right arm (aVR), and the left foot (aVF),
A lead axis in parallel to the direction a wavefront where the “a” stands for augmented, and “V” stands
is moving results in a large deflection, while a lead for vector (Fig. 1.5). Together with leads I, II, and III,
axis perpendicular to the direction of a moving augmented limb leads aVR, aVL, and aVF form the
wavefront results in a small (or no) deflection on basis of the hexaxial reference system, which is used
the ECG. to calculate the heart’s electrical axis in the frontal
plane. The positive and negative terminals of the six
limb leads are listed in Table 1.1.

Transverse
– I +
RA LA
– –

III
II
+ + Frontal
LL

Fig. 1.3 The limb leads display the cardiac
Fig. 1.2 The limb leads (I, II, III) form the depolarization in the frontal plane. The chest leads
Einthoven triangle. display the depolarization in the transverse plane.
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