Lead Systems

The exact placement of the leads in the standard 12-lead ECG is the result of general agreement that has evolved over the years among electrocardiog-raphers. Many additional lead locations other than the usual are possible. We will avoid, however, a discussion of the actual physical placement of leads and the theory of Einthoven's Triangle because it is not necessary to your understanding of ECG morphology and frequently results in confusion. Suffice it to say that the simpler description of lead systems that follows is entirely sufficient to support an accurate interpretation of the ECG.

We saw in Chapter 1 that the strip chart recordings of electrical events in a strip of muscle look different, depending on the position of the electrode that views the events. The 12 leads of the standard 12-lead ECG were selected to offer a wide variety of "views" of the heart.

Although a technical oversimplification, the following description of the leads in the 12-lead ECG is a useful, and clinically quite accurate, method of conceptualizing lead systems.

The 12-lead ECG "looks" at the heart in two different planes (Figure 3.3). The three so-called standard limb leads (I, II, and III), and the three so-called augmented limb leads (aVR, aVL, and aVF) all look at the heart from the "edges" of the frontal plane, as if the body was flat or unidimensional. It is common practice to refer to all six leads, collectively, simply as the limb leads.

The six V leads (VrV6) across the precordium look at the heart in the horizontal or transverse plane (Figure 3.3). These leads allow us to look at the front and left side of the heart and complete a more three-dimensional perspective.

Note in Figure 3.4 that to describe from which direction each of the limb leads looks at the heart in the frontal plane, we have again used our modified compass rose. It is useful to think of each lead as being an exploring elec-

Lead Systems 15

Transverse Plane

Transverse Plane aVF

Frontal Plane

Figure 3.3. The 12-lead electrocardiogram "looks" at the heart in two different planes. The limb leads examine the heart in the frontal plane, while the V leads examine the heart in the transverse plane.

trode (an electrode that explores the heart) located in the positions shown in the figure. In actuality, of course, this is not the case. Rather, the ECG machine manipulates signals in its internal circuitry to give us tracings that look much as if electrodes were placed in those positions.

One can readily see then that aVR "looks down" on the heart from above and to the right, at a position of-150 degrees. Lead aVL looks down on the heart from above and to the left, at -30 degrees. Lead I looks at the heart on the horizontal, directly from the left side, at 0 degrees. Finally, leads II, aVF, and III all look "up" at the heart from below and, together, are called the inferior leads because they look at the inferior wall of the heart from the angles shown in Figure 3.4. By the same token, leads I and aVL are said to look at the lateral and high lateral walls of the heart, respectively.

This system of superimposing each of the limb leads on our modified compass rose is called the hexaxial reference system. You are urged to aVR

Figure 3.4. The hexaxial reference system showing the direction from which each of the limb leads "looks" at the heart in the frontal plane.

Figure 3.4. The hexaxial reference system showing the direction from which each of the limb leads "looks" at the heart in the frontal plane.

memorize these positions and their assigned degree values because the hexaxial reference system will become your primary tool later on when you learn to determine the electrical axis of the heart.

In the same manner as the limb leads, the V leads, across the precordium of the chest, "look" at the heart in the transverse plane from the positions shown in Figure 3.5. In the case of the V leads,however, we do not use degrees on a compass rose to assign positions. Rather positions are assigned on the basis of actual location of the exploring electrodes as follows:

Vi—right sternal border, 4th interspace V2—left sternal border, 4th interspace V3—midway between V2 and V4 V4—midclavicular line, 5th interspace V5—anterior axillary line, 5th interspace V6—midaxillary line, 5th interspace

You will learn in later chapters that, particularly in diagnosing AMI, small changes in the height of the R wave on the V-lead recordings across the precordium can be important. Artifactual changes in R-wave height can be produced merely by slightly moving the positions of the V electrodes. Therefore, it is important when following serial ECG tracings to make certain that the V leads are placed in exactly the same position each time an ECG is performed. To this end, it is a good idea to mark the positions on the chest wall with indelible ink pen at the time of the first tracing so that subsequent tracings can be done with the V leads in exactly the same positions.

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