PACs and the PR Interval

The PR interval is composed of quite a few components (Figure 13-13). It is composed of the interval of time it takes to complete atrial depolarization and the physiologic block that occurs at the AV node, as well as the transmission time through the His bundle, the bundle branches, and finally, the Purkinje cells themselves. Take a look at Figure 13-13. The time it takes for the depolarization wave to travel from the sinus node to the AV node is represented by the bracketed area. Once the impulse reaches the AV node, the time interval of the physiologic block begins. This is a set interval of time that is innate in the AV tissue; the tissue will not let the impulse travel through until that time is up.

Figure 13-13 Components of the PR interval.

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So, what does this have to do with PACs? Well, the time interval in the brackets is affected by the location of the ectopic pacemaker. If the pacemaker site is close to the AV node, the time it takes for the depolarization wave to reach the AV node is shorter. Since that time is shorter, the whole PR interval will be shorter. If the pacemaker site is farther from the AV node than the distance from the SA node to the AV node, then the depolarization wave takes longer to reach the AV node and the PR interval is longer. Let’s look at some examples.

In Figure 13-14, the ectopic pacemaker is in the right atria close to the AV node. Would you expect the PR interval during a PAC to be prolonged or shorter than that seen during a sinus beat in the same patient? Well, since the distance is shorter, the PR interval should be shorter as well. The P wave axis of the PAC is headed superiorly, anteriorly, and to the left. What would you expect the P wave of a PAC from this focus to look like in lead II? It would be negative in lead II.

Figure 13-14 PAC with shorter PR interval.

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In Figure 13-15, the ectopic pacemaker is in the left atria and is pretty far from the AV node. You would expect the PR interval of the PAC to be longer than the one seen in NSR, and it is. The P wave of the PAC is positive in lead II, as you would expect. Don’t worry about the fact that it is biphasic; that is not important. The important thing is that the P wave is positive in lead II. In this case, the P wave axis didn’t help you much, but the early arrival of the PAC, the differing morphology of the P wave, and the different PR interval all point to the presence of a PAC. Note, however, that a strip from lead I or V₅ or V₆ would have shown negative Ps.

Figure 13-15 PAC with longer PR interval.

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