Arrhythmia Recognition: The Art of Interpretation

The depolarization of the actual sinus node takes place very shortly before the onset of the P wave. From there, the electrical impulse itself does not have to travel through the atria by direct cell-to-cell transmission in order to reach the AV node. Instead, the impulse is conducted very quickly through a specialized tract of cells known as the internodal pathways. This rapid system allows the impulse to reach the AV node while the slow process of the depolarization of the atrial myocytes takes place. The P wave itself is formed by the depolarization of the actual atrial myocyte themselves. These simultaneously occurring processes are graphically represented in Figure 28-3 by the blue dashed line seen under the P wave.

Additional Information

The “Blocks”

Very often, beginners have some difficulty identifying and understanding the AV blocks. This is partly because the word “block” is used to describe many different processes.

As mentioned, the AV blocks are typically caused by a conduction problem and typically show characteristic ECG patterns that involve either a prolongation of the PR interval, dropped QRS complexes, or a complete pathologic lack of communication between the atria and the ventricles. The conduction defect in the AV blocks can actually take place at any level of the ventricular portion of the electrical conduction system, but the more common ones involve mainly the AV node and the bundle of His or the bundle branches themselves, and can be either functional or pathologic.

The bundle branch blocks, on the other hand, are caused by true pathologic and anatomic blockade at the level of the left or right bundle branches of the electrical conduction system. The block causes an obstruction to the propagation of the impulse in either one ventricle or the other. In order to depolarize the ventricle in question, the depolarization wave must proceed by direct cell-to-cell contact. The slow transmission of the impulse creates wide QRS complexes that will have one of two morphologic presentations, either a right bundle branch block pattern or a left bundle branch block pattern.

Likewise, the hemiblocks are caused by localized pathologic “blocks” to impulse conduction in either the left anterior or the left posterior fascicles. The hemiblocks mainly alter the direction of the main ventricular axis on the ECG.

After conduction of the impulse through the atria and the AV node, the impulse then moves through the His bundles, the bundle branches, and the Purkinje system. These events are still all occurring during the PR interval. Notice that the PR interval represents conduction throughout the entire electrical conduction system. The PR interval is measured as the distance from the beginning of the P wave to the beginning of the QRS complex.

A first-degree AV block is defined as a delay in the conduction of the supraventricular impulse to the ventricles, serious enough to prolong the PR interval above 0.20 seconds (the upper limit of normal). The delay most commonly occurs at the level of the AV node or the bundle of His. (Rarely, the delay can occur distal to the bundle of His.) Note that first-degree AV block is a delay or an incomplete block and every P wave will conduct to the ventricles. It may be late, but it will conduct every time.

So, how does a delay in conduction cause a PR prolongation? As we saw in Figure 28-3, transmission of the impulse through the electrical conduction system takes place during the PR interval. As we can further deduce from Figure 28-3, and clearly see in Figure 28-4, the prolongation of any one of the components of the PR interval will prolong the entire interval. Therefore, a serious delay in conduction through the AV node or the bundle of His will lead to a serious prolongation of that section of the PR interval, and hence, the whole thing.

As stated earlier, a first-degree AV block is electrocardiographically represented by a PR interval that is greater than 0.20 seconds, or one big block on the ECG paper. In general, a first-degree AV block will have a PR interval in the 0.21 to 0.40 range. The width of the PR interval, however, can be quite impressive, reaching levels of 0.60 or even 1 second wide. Figure 28-5 shows various examples of PR prolongation with various widths. Note that sometimes the P wave may actually fuse with the previous T wave. In very rare instances, the P wave of one complex may actually be before the previous QRS. Talk about a confusing presentation!

In first-degree AV block, the width of the QRS complexes should be normal, except in the case of a preexisting bundle branch block or the presence of aberrancy. Remember, ventricular depolarization is represented on the ECG by the QRS complex. The QRS complexes are normal width in first-degree AV block because ventricular depolarization is not directly affected, the onset merely occurs a little later, time-wise. In other words, the delay in conduction occurs only at the level of the electrical conduction system (represented by the PR interval) and not at the level of ventricular depolarization (represented by the QRS complex).

Figure 28-5 highlights one additional, critical fact about first-degree AV blocks: Each P wave has its own QRS complex. In other words, the conduction ratio is 1:1 (one P wave for every QRS complex). This is because first-degree AV blocks are not really true blocks but merely reflect delays in conduction. As we shall see, the rest of the AV blocks are associated with dropped QRS complexes or complete blocks, with separate atrial and ventricular rhythms, and their conduction ratios will not be 1:1.

Finally, the width of the PR prolongation in any one patient with first-degree AV block may change slightly from time to time. This variation in PR interval may be due to various factors. For example, the heart rate itself will affect the PR interval, with bradycardias having longer intervals and tachycardias having shorter intervals. The vagal tone of the patient can also influence the PR interval, with vagal stimulation lengthening it. The presence of dual-AV nodal pathways, as we mentioned in Chapter 25, AV Nodal Reentry Tachycardia may be associated with varying PR intervals, depending on which tract the impulse takes to reach the AV node.

In this chapter, we are going to depart from our traditional method of waiting to show the actual patient rhythm strips until the end of the chapter and present them after each section. This will help you become familiar with each type one at a time. We hope that this presentation will help you form a clearer picture of this sometimes confusing topic.

Additional Information

Wide QRS Complexes and AV Blocks

In general, any of the AV blocks can occur in the AV node proper or in the bundle of His before the bifurcation. These complexes will have narrow QRS complex associated with them (provided that there is no preexisting bundle branch block or aberrancy). Any AV block that originates below the bifurcation of the bundle of His typically has wide QRS complexes. Remember, blocks don’t always have to have wide complexes.

As we shall see when we get to the more serious kinds of AV blocks, narrow complexes typically occur if the block is above the bifurcation of the bundle of His. This is because, in the cases when there is conduction, all of the ventricular depolarization will proceed along the normal electrical conduction system. Wide QRS complexes, on the other hand, are caused by infra-Hisian (below the bundle of His) blocks. This occurs because these blocks will always have some transmission of the impulse in the ventricles by direct cell-to-cell conduction. This process is slow and leads to the formation of wide complexes on the ECG.

This discussion is not very pertinent to first-degree AV blocks, but will become more important with the more advanced blocks to follow.