Warning : Use the following information at your own risk. While accuracy is one my goals, there is always the possibility that some of the information could be wrong. There could be typos. I could also be severely mistaken in some of my knowledge. This site is meant to help clarify certain concepts of ECG and at no point should any life-or-death decision be made based upon the information contained within. Remember, this is just some page on the internet. (If you do find errors, please notify me by feedback.)
I'm frequently asked if there is some way that I can explain the concept of the human heart's premature ectopic complexes using waterfowl. In response to these requests, I have put together in this section an extended metaphor that addresses the topics common to premature atrial, junctional, and ventricular complexes.
The normal mallard rhythm
Figure 10-1 shows a number of mallards marching along. Each mallard represents the ECG complex above it. Let's assume there exists some commander of the ducks (who is not shown in the diagram). He is very picky when it comes to marching, and he requires that each duck maintain a specified distance between himself and the duck before him. This distance is the length of a single ruler (shown below ducks).
Enter the goose
Now let's look at the duck-duck-goose patterns. In figure x-x, you see that a goose (representing a ventricular complex) has joined the parade. He took ("stole") the place of one of the "regularly scheduled" ducks. In addition to this, the goose is following a little too closely (much like bad drivers do). Thus, we call this goose premature.
Remember, each of the ducks was originally given the order to march at a distance of one ruler's length behind the duck directly in front of him. This puts the duck that follows the goose in a dilemma. He has two options :
In the heart, the "option" is usually decided by where the premature complex originates. Those complexes that cause a compensatory pause are those whose impulse does not reach the normal (sinus) pacemaker. Lack of the compensatory pause is generally attributed to the impulse from the premature complex conducting retrograde towards the sinus node and resetting it. Although this is by no means a fixed rule, PACs (and PJCs) tend to reset the sinus while PVCs tend not to reset the sinus. Thus, if a premature complex is followed by a compensatory pause, you should suspect a PVC.
Sometimes, a goose may be able to squeeze between two ducks without messing with their pattern at all. When this happens with an PVC, we call it an interpolated PVC. An interpolated complex is a premature complex that is early enough so that no complex is skipped. Thus, in figure 10-4, there are no missing ducks.
Unifocal versus multifocal
Remember the following generalization : different shape means different origin.
The QRS shape often reflects where the impulse entered the ventricular conduction system. As far as the ventricles are concerned, pacemakers in the sinus node, atria, and the junction (normally) all share the same path. Thus a sinus complex, a PAC, and a PJC are all likely to have similarly shaped QRS complexes.
Pacemakers in the ventricular conduction system are not limited to one location; they can occur in a variety of places. These different places should produce different QRS complexes. For example, in figure 10-5, we see two different PVCs. We should assume, because of their dissimilarity, that these PVCs each originated from a different part of the ventricular conduction system. Thus, we designate them multifocal. If all of the PVCs had the same general shape, we would refer to them as unifocal.
Patterns of premature beats
These terms describe how often the premature complexes appear. If their appearance seems random, none of the following terms are applicable.
Multiple premature beats
The term salvo (as in barrage) is used to describe the occurrence of multiple premature complexes in a row. (Many consider "multiple" in this case to mean three or more.) When many premature complexes occur in a row, you should probably start looking at them less in terms of premature complexes and more in terms of tachycardia.