The Two Lead Systems

The Hexaxial System

Now, let’s combine A and B from Figure 4-9. Using the same principle as before—that leads can be moved as long as the resultant lead is parallel and of the same polarity—we can produce the hexaxial system (Figure 4-10). Think of this as a system of analyzing vectors that cuts the center of the heart along a plane, creating a front half and a back half. It would be as if there were a glass sheet dividing the body from ear to ear. In anatomical terminology, this is called a coronal cut. Keep in mind as you proceed that what you are evaluating is how the vector would project on the two-dimensional glass sheet and not on the three-dimensional anterior or posterior parts of the heart.

Figure 4-10 The hexaxial system.

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The hexaxial system gives rise to the six limb leads I, II, III, aVR, aVL, and aVF. Traditionally, the side of the lead that has the positive electrode, or pole, is the one that has the lead name at its end (Figure 4-9). Hence, the positive pole of lead I is at the right side of the circle, the positive pole of aVF is down, and so on. Also, note that the leads are 30 degrees apart. This will be very useful when we talk about axis.

The Precordial System

Remember the precordial leads—the ones on the chest itself? Think of these leads as sitting on a plane that is perpendicular to the limb leads. Again imagine a glass sheet, this time splitting the body through the center of the heart, into top and bottom halves. This is called a transverse plane. The result is a cross section, with six leads produced by the six chest electrodes (Figure 4-11).

Figure 4-11 The six precordial leads.

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