Important Principles

Knowledge of the following three concepts (with which you are already acquainted) is almost all that is necessary to predict the normal morphology, or shape, of the ECG tracing in each lead:

1. The principle that impulses coming toward an electrode produce positive deflections, whereas impulses going away from an electrode produce negative deflections.

2. The positions from which the various electrodes "look" at the heart.

3. The sequence, direction, and relative magnitude of the four major vectors of cardiac depolarization and repolarization.

There is just one more principle of which you need to be aware, which is only a slight amplification of principle number 1. Figure 4.1 illustrates the principle that the more directly an impulse comes toward an electrode, the greater the amplitude of the positive recorded deflection will be. Vector A in the illustration is coming directly toward the recording electrode and produces a very tall deflection in recording A. Vector B, on the other hand, is not coming as directly toward the electrode, and therefore produces a deflection of less magnitude in recording B; that is, the positive deflection is shorter.

The same is true of impulses going away from an electrode, only the deflections are negative instead of positive. The more directly an impulse goes away from an electrode, the deeper will be the negative deflection, and the less directly an impulse goes away from an electrode, the shallower will be the negative deflection. Vector C is going directly away from the electrode, and so produces a deep negative deflection. Vector D, on the other hand, is not going as directly away and so produces a deflection of lesser amplitude; that is, the deflection is shallower.

Figure 4.1. Diagram of five force vectors of equal magnitude, but different directions, as recorded by a single exploring electrode,

Finally,when an impulse is traveling exactly perpendicular to an electrode, that is, when it is neither coming toward nor going away from the electrode, the recorded deflection will be either isoelectric or biphasic, with positive and negative deflections of equal amplitude. Vector E illustrates this result.

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