Before we were born, our particular genetic material was created by taking some of the genetic material of our mothers and some of the genetic material of our fathers. We are, in essence, a fusion of the genetic material of both of our parents. We can resemble one parent more than another, but we will never be identical to either one. Keep this concept in mind as you read on.
In this section, we are going to be addressing an issue that will come up again and again throughout the book, the concept of fusion. Fusion refers to the merging together or melding of two or more waves and 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, but they are never morphologically identical to either. The fusion complexes thus formed are different in morphology from the rest of the strip as each of the parents adds input to the final product.
The confusion in electrocardiography among beginners is that, in reality, fusion is actually two different electrocardiographic phenomena with one name. The first is an isolated electrocardiographic summation or fusion of waves occurring from two different complexes. An example of this type of fusion is when the T wave from one complex fuses with the P wave that causes the complex immediately after it. This leads to the buried P phenomenon, which we will discuss at great length later on in the book. This type of fusion occurs mostly during premature or fast rhythms and is fairly evident on the ECG because of the events that are occurring around it.
The second type of fusion is an actual fusion of two entire complexes that are occurring simultaneously. An example of this type of fusion is one in which two ectopic foci in the atria, the ventricles, or both are firing at the same time. The result is that both depolarization 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 completely ectopic beats.
Please be aware that nowhere else have we seen fusion broken down in this manner. However, these issues are constantly being questioned by students as they begin to learn electrocardiography and arrhythmia interpretation. To our knowledge the literature just has never separated the two concepts. We feel breaking it down simplifies learning the two individual concepts.
The Isolated Electrocardiographic Type of Fusion
We have touched on this type of fusion before, when we talked about the buried P waves and how they alter the appearance of either the QRS complex or the T wave (depending on the actual burial site of the P wave). We want you to notice, however, that this type of fusion is actually just an electrocardiographic phenomenon. This is not an actual physical fusion of two waves occurring during the same complex but rather the waves are actually part of two different complexes adding themselves up on the ECG (Figure 6-24).
Figure 6-24 Buried P waves. Two different waves are occurring simultaneously. One is the ventricular repolarization wave of complex 1 (blue vector) and the other is an atrial depolarization wave of complex 2 (green vector). The net result of the two vectors is an electrocardiographic fusion complex. The fusion of the two waves alters the morphology of the T wave in the rhythm strip.
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DescriptionTo put it another way, the fusion of the two waves in this type of isolated fusion occurs only at the level of the strip because the two separate events are occurring at the same time. The result is that the two electrical vectors are interacting on the ECG only, but the forces of one vector are not affecting the actual shape of the other vector; the forces are just adding up on the ECG.
The other type of fusion is when the waves actually merge and fuse in the heart mechanically. In this case, the ECG lead then picks up that information and transfers it to the strip. This is a subtle difference from isolated electrocardiographic fusion. We call this type of fusion “actual fusion” to help distinguish it. Graphically, an example of this is represented by Figure 6-25, where two atrial foci are firing at the same time.
Figure 6-25 Two different depolarization waves are occurring simultaneously, one arising from the SA node, the other from an ectopic atrial focus. The net result of the two waves is a fusion complex. The fusion complex has characteristics of both individual waves.
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DescriptionSuppose you had a complex that originated in the SA node (Figure 6-25, green vector). The depolarization wave would begin in the SA node and begin to spread normally throughout the atria. This depolarization wave would give rise to a normal P wave. Now, suppose that you had another depolarization wave start elsewhere in the atria or AV junctional area (Figure 6-25, blue vector). This wave would have no way of knowing that there already was a wave coming toward it from the SA node. This wave would start to give rise to an inverted P wave with an abnormal P-wave axis and differing morphology from the sinus P wave.
Eventually, the two waves would crash into each other and cancel each other out. But, before they did that they would have had to create some sort of an electrocardiographic representation. The problem lies in that both of these waves are occurring at the exact same time. Electrocardiographically, the waves would fuse or mix with each other to create a complex that has some of the characteristics of the sinus wave and some of the characteristics of the ectopic wave. How much the fusion wave resembles either of the two parent waves depends on the timing of the two waves and their proximity to the leads. The depolarization wave that started the earliest would have the closest morphologic appearance to the start of the final fusion wave. The middle and the end would be up for grabs.
The exact same concept of fusion is at work when a normally transmitted atrial complex fuses with an ectopic ventricular complex (Figure 6-26) or when a junctional complex fuses with an ectopic ventricular complex (Figure 6-27). Fusion complexes between an atrial P wave and a junctional ectopic pacemaker are quite common, and we shall see some examples when we start to examine actual patient strips.
Figure 6-26 Two different depolarization waves are occurring simultaneously, one arising from the SA node and then spreading to the ventricles via the AV node, the other originating from a ventricular ectopic focus. The net result of the two ventricular waves is a fusion complex. The fusion complex has characteristics of both individual waves. Note that the fusion did not occur with the P wave, just the QRS complexes.
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Description
Figure 6-27 Two different depolarization waves are occurring simultaneously, one arising from the AV node, the other from a ventricular ectopic focus. The net result of the two waves is a fusion complex of the QRS complexes, just like the one we saw in Figure 6-26. The fusion complex has characteristics of both individual waves. Note that there is no visible P wave in this Figure when compared to Figure 6-26.
© Jones & Bartlett Learning.
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