The Electrical Axis

Up until now, we have touched on the concept of the electrical axis in various chapters. This tells you that just about everything in electrocardiography is related to the electrical axis and its graphic representation in the different leads. Before you begin this chapter, you should go back and review the information on vectors in Chapter 4, Vectors and the Basic Beat.

The electrical axis, as we discussed there, is the sum total of all the vectors generated by the action potentials of the individual ventricular myocytes. We cannot evaluate the ventricular axis directly. Instead, we have to measure the way the vector looks as it travels under each of the various electrodes. The “pictures” generated by each of the leads give us different views of this axis as it relates to the three-dimensional state, as shown in Figure 5-8. When we examine how those pictures appear, we can piece together the vector because we know the locations from which they were taken.

An illustration shows the image of the heart captured from various angles, corresponding to different leads.

Figure 5-8 Locations of the leads determine the direction of the vector.

© Jones & Bartlett Learning.

Description

How do we use the axis clinically? Suppose there is hypertrophy of one of the ventricles. That ventricle would alter the ventricular axis in such a way as to assist us in diagnosing the problem. Now, imagine that an area of the heart infarcts. The ventricular axis would definitely be altered by the lack of electrical activity generated in that dead zone. Suppose a section of the electrical conduction system is diseased or blocked. Do you think that would alter the electrical axis of the ventricle? It certainly would.

For now, we are going to be reviewing the ventricular axis. Remember that there is an axis generated by each of the waves and intervals of the complex. The way they interact will reflect pathology.

In arrhythmia recognition, we will be using the P-wave axis extensively (see Chapter 8, Normal Sinus Rhythm, as an example). You can calculate the P-wave axis exactly the same way as you calculate the ventricular axis. For simplicity, we will look at the ventricular axis, since the QRS complexes are much bigger than the P waves and it is much easier to learn the concepts we will be talking about on larger waves.