LEAD SYSTEMS
Standard Limb and Precordial Leads
The small electric currents produced by the electrical activity
of the heart spread throughout the body, which behaves as a volume conductor, allowing
the surface ECG to be recorded at any site. The standard leads are bipolar leads,
because they measure differences in potential between pairs of electrodes. The electrodes
are placed on the right arm, the left arm, and the left leg. The leads are formed
by the imaginary lines connecting the electrodes, and the polarities correspond to
the conventions of the Einthoven triangle ( Fig.
34-2
). They are labeled leads I, II,
Figure 34-3
Unipolar limb lead circuit (VR). (From Thys
DM, Kaplan JA: The ECG in Anesthesia and Critical Care. New York, Churchill Livingstone,
1987.)
Figure 34-4
Goldberger modification to the unipolar lead aVR. (From
Thys DM, Kaplan JA: The ECG in Anesthesia and Critical Care. New York, Churchill
Livingstone, 1987.)
and III. If the three electrodes of the standard leads are connected through resistances
of 5000 ohms each, a common central terminal with zero potential is obtained. When
this common electrode is used with another exploring electrode, the potential difference
between them represents the actual potential ( Fig.
34-3
). On a standard 12-lead ECG, three unipolar limb leads are usually
recorded: aVR, aVL, and aVF. The letter a indicates
that they are augmented limb leads and were obtained using the Goldberger modification
( Fig. 34-4
). In this modification,
the resistors are removed from the circuit, and the exploring electrode is disconnected
from the central terminal. This modification produces larger electrocardiographic
deflections.
Additional information on the heart's electrical activity is obtained
by placing electrodes closer to the heart or around the thorax. In the precordial
lead system, the neutral electrode is formed by the standard leads, and an exploring
electrode is placed on the chest wall. The ECG is normally recorded with the exploring
electrode in one or more of six precordial positions. They are indicated by the
letter V, followed by a numeral from 1 to 6 indicating
the location of the electrode on the chest wall ( Fig.
34-5
).
