BIPOLAR LIMB LEADS.

Bipolar limb leads record the potential differences between two limbs,  As bipolar leads, the output is the potential difference between the limbs serving as positive and negative inputs. Lead I represents the potential difference between the left arm (positive electrode) and the right arm (negative electrode), lead II displays the potential difference between the left foot (positive electrode) and the right arm (negative electrode), and lead III represents the potential difference between the left foot (positive electrode) and the left arm (negative electrode). The electrode on the right foot that is not included in these leads serves as a ground connection.

Top, Electrode connections for recording the three bipolar limb leads I, II, and III. R, L, and F indicate locations of electrodes on the right arm, the left arm, and the left foot, respectively. Bottom, Electrode locations and electrical connections for recording a unipolar precordial lead. Left, The positions of the exploring electrode (V) for the six precordial leads. Right, Connections to form the Wilson central terminal for recording a precordial (V) lead.

The electrical connections for these leads are such that the potential in lead II equals the sum of potentials sensed in leads I and III. That is,

I + III = II.

This relationship is known as Einthoven's law.

NIPOLAR PRECORDIAL LEADS AND THE WILSON CENTRAL TERMINAL.

The unipolar precordial leads register the potential at each of the six designated torso sites in relation to a theoretical zero reference potential. To do so, an exploring electrode is placed on each precordial site and connected to the positive input of the recording system.

The negative or reference input is composed of a compound electrode (that is, a configuration of more than one electrode connected electrically) known as the Wilson central terminal. This terminal is formed by combining the output of the left arm, right arm, and left leg electrodes through 5000-W resistances. The result is that each precordial lead registers the potential at a precordial site with reference to the average potential on three limbs. The potential recorded by the Wilson central terminal remains relatively constant during the cardiac cycle, so the output of a precordial lead is determined predominantly by changes in the potential at the precordial site.

AUGMENTED UNIPOLAR LIMB LEADS.

The three augmented limb leads aV_r, aV_l, and aV_f are modified or augmented unipolar leads. The exploring electrode is the right arm electrode for lead aV_r, the left arm electrode for lead aV_l, and the left foot electrode for aV_f. It is the reference electrode that is modified. Instead of consisting of a full Wilson central terminal composed of the output from three limb electrodes, the reference potential is the mean of the potentials sensed by only two of the three limb electrodes; the electrode used for the exploring electrode is excluded from the reference electrode. For lead aV_l, for example, the exploring electrode is on the left arm and the reference electrode is the mean output of the electrodes on the right arm and the left foot. Similarly, for lead aV_f, the reference potential is the mean of the output of the two arm electrodes.

Electrode locations and electrical connections for recording the three augmented unipolar leads aV_r, aV_l, and aV_f. Dotted lines indicate connections to generate the reference electrode potential.

This modified reference system was designed to increase the amplitude of the output. The output of the limb leads without augmentation tended to be small, in part because the same electrode potential was included in both the exploring and reference potential input. Eliminating this duplication results in a theoretical increase in amplitude of 50 percent.

OTHER LEAD SYSTEMS.

Other lead systems may be used for specific purposes. For example, additional unipolar right precordial leads may be used to assess right ventricular lesions, and locations posterior to V₆ may be used to detect posterior infarctions, Such posterior locations include lead V₇ with the exploring electrode at the left posterior axillary line at the level of V₆, lead V₈ with the exploring electrode on the left midscapular line, lead V₄R with the exploring electrode on the right midclavicular line in the 4th intercostal space, and so forth. A vertical parasternal bipolar pair may facilitate detection of P waves for diagnosing arrhythmias.Precordial and anterior-posterior thoracic electrode arrays of up to 150 (or more) electrodes may be used to display the spatial distribution of body surface potentials as body surface isopotential maps. Modified lead systems are also used in ambulatory ECG recording and exercise stress testing, as described elsewhere, and for bedside cardiac monitoring.

Other lead systems that have had clinical utility include those designed to record a vectorcardiogram (VCG). The VCG depicts the orientation and strength of a single cardiac dipole or vector at each instant during the cardiac cycle. Lead systems for recording the VCG are referred to as orthogonal systems because they record the three orthogonal or mutually perpendicular components of the dipole moment—the horizontal (x axis), frontal (y axis) and sagittal or anteroposterior (z axis) axes. Clinical use of the VCG has waned in recent years. However, the VCG may be useful in certain situations, and as described below, vectorial principles remain essential to understanding the physiology and pathology of ECG waveform genesis.