The P Wave in Ventricular Rhythms

Some people never pay attention to the P waves in ventricular rhythms. That is a big mistake. The P waves are very useful in evaluating the wide-complex rhythms (and ventricular tachycardia in particular). In particular, we need to concentrate on the morphology, axis, and rate of the P waves. In addition, we need to pay particular attention to the P:QRS relationship.

P-Wave Morphology in Ventricular Rhythms

Would you expect any P waves in a ventricular rhythm, and, if so, what would they look like? The answer is that retrograde P waves can occasionally occur because of isolated ventricular firing. We say “occasionally” because the retrograde transmission of the ventricular impulse to the atria is very variable. Many times, the atrioventricular (AV) node allows the impulse to spread retrogradely to the atria. Many times, it completely blocks the retrograde transmission and the atria are totally oblivious to the occurrences in the ventricle. Other times, the atria and the ventricles are completely isolated from each other because of a third-degree AV nodal block. In these cases, the atria and ventricles are firing at their own intrinsic rates without any form of interaction.

The antegrade block or retrograde transmission of the ventricular impulse through the AV node into the atria is dependent on many variables, and it is impossible to predict when either will occur. Drugs, ischemia, intrinsic conduction system disease, electrolytes, sympathetic and parasympathetic influences, and so on, all influence the AV node in this respect.

Let’s look at what happens when there is retrograde conduction through the AV node. When an ectopic ventricular pacer fires, it stimulates the ventricles by direct cell-to-cell contact. This causes a wide, bizarre-looking QRS complex to develop. When the impulse reaches the AV node, the AV node can either allow the impulse to travel retrogradely into the atria or it can block the impulse completely (Figure 29-7).

Figure 29-7 Ventricular conduction through the AV node.

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If the impulse is spread retrogradely into the atria, a P wave will be created. Since the P-wave vector is heading from inferior to superior, the P wave will be inverted on the ECG. Also, in an isolated ventricular complex, the inverted P wave can either be buried in the QRS complex, be immediately after the QRS, or have a prolonged RP interval. The RP interval can often be long because it takes time for the cell-to-cell transmission of the ventricular impulse to reach the AV node. The farther the ectopic pacer is from the AV node, the longer the RP interval (Figure 29-8).

Figure 29-8 The closer the ectopic focus is to the AV node, the shorter the RP interval. The extra distance traveled by the ventricular impulse created by the ectopic focus represented in green causes the inverted P wave to occur farther along the complex.

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Additional Information

Fusion

Fusion of the QRS complexes and the other component waves occurs in every ventricular rhythm we will be discussing. We took our first look at fusion in Chapter 6, Electrocardiography and Arrhythmia Recognition. We will now include a short review of this topic for your convenience, but we strongly urge you to spend the time to go back and review this concept thoroughly. Fusion will greatly alter the morphology and appearance of ventricular complexes and will often lead you to misdiagnose some malignant ventricular rhythms.

Fusion refers to the merging together of two or more waves and/or vectors that are occurring at the same, or nearly the same, moment in time. The final result is that the complexes that are formed have characteristics of both of the parent waves or vectors. The complexes formed are different in morphology from the rest of the strip, as each of the parents adds its input into the final product.

In reality, fusion is actually two different electrocardiographic phenomena with one name. The first is an isolated electrocardiographic summation of waves from two different complexes. The fusion is on the ECG, but not actually in the heart. This occurs mostly during premature or fast rhythms and is fairly evident on the ECG because of the events occurring around it. It also occurs in most cases of AV block.

The second type of fusion is actual fusion of two depolarization waves occurring simultaneously. An example of this type of fusion is one in which a normally occurring ventricular depolarization begins to occur, and an ectopic pacer fires and begins to form its own depolarization wave somewhere else in the ventricle. The result is that both waves begin to form and eventually crash into one another. The complexes formed during this type of fusion are more difficult to differentiate and can be easily mistaken for complexes originating at completely different ectopic sites.

Both types of fusion occur in ventricular rhythms. We will look at fusion in the next segment of this chapter, and we will look at it throughout the rest of this section. In particular, fusion complexes can be very useful in evaluating ventricular tachycardia, and we will revisit this topic in more detail when we get to that chapter.

The P:QRS Relationship in Ventricular Rhythms

Most ventricular complexes do not have a P wave associated with them. Others have an inverted P wave with a prolonged RP interval. In this section, we will concentrate on the concepts involved in the dissociation of the atria from the ventricles.

As we pointed out in Chapter 1, Anatomy and Basic Physiology, the AV node is the only area of communication between the atria and the ventricles under normal circumstances. We talked about the AV node functioning as the gatekeeper between the atria and the ventricles and have seen various examples of the arrhythmias that can develop because of malfunctions. Now, suppose that the AV node shuts down all communication completely between the atria and the ventricles. What do you think would happen?

Once again, in order to answer that question, we have to go back to our basic knowledge. If you remember, we talked about the sequential order of pacemakers in the heart, which act as a fail-safe against asystole or cardiac standstill. We saw that the order went from sinus node, to atrial muscle, to AV node, to His bundles, to the bundle branches, to the Purkinje system, to the ventricular myocytes. Failure of an earlier group means that the next group would take over. So, what do you suppose would happen if the ventricles were cut off from above? The ventricular pacers would think that the upper groups have failed and they need to begin to take over the pacing function themselves. In other words, the isolation of the ventricles causes a ventricular pacer to take over ventricular pacing (Figure 29-9).

Figure 29-9 When the AV node is not conducting, it creates, functionally, a complete block at the level of the interventricular septum. The atria and the ventricles are completely oblivious to each other, and two separate pacemakers develop. Each pacemaker will keep its own intrinsic rate and control its respective chamber.

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Description

Each respective pacemaker, one atrial and one ventricular, would beat at its own intrinsic rate and create its own rhythm (see Figure 29-9). These two rhythms are occurring simultaneously, and would appear in the electrocardiogram as a series of atrial complexes and ventricular complexes occurring completely oblivious to each other. However, the ECG measures all waves sensed at any one time. Since the atrial and ventricular rates are occurring at different rates, they will often overlap. During those times, the ECG machine will fuse the two waves on each other mathematically by altering the vectors it senses. Note, however, that in complete or third-degree AV block, none of the atrial complexes make it through the AV node to depolarize the ventricles. In other words, they function completely separately from each other. Therefore, there is no true fusion of the waveforms occurring in the heart itself. Instead, what we typically see is an overlap that the machine interprets as buried P waves and changes in the PR or RP intervals (Figure 29-10).

Figure 29-10 The atria and the ventricles are completely oblivious to each other and there are two separate pacemakers. Each pacemaker will keep its own intrinsic rate and control its respective chamber. The ECG fuses these two rhythms into one strip (see strip in black). This is a third-degree heart block with a ventricular escape.

© Jones & Bartlett Learning.

In addition to complete or third-degree AV block, we also see AV dissociation. In this rhythm, the atria and ventricles beat to their own drummers, but occasionally, when the atrial rates are in perfect alignment, they can trigger a ventricular complex or at least influence it partially, leading to true fusion complexes. Note that in this case, the atria and ventricles function separately but can still influence each other if the timing is right. More on this later. This type of AV block is complete and is, therefore, known as a third-degree or complete AV block.

Always keep the P:QRS relationship in mind when approaching any wide-complex rhythm. It is one of the most useful tools in determining whether a wide-complex tachycardia is caused by a ventricular pacemaker or if it is an aberrant form of supraventricular tachycardia due to another reason.