Just like in premature junctional complexes (PJCs), PVCs can have fixed or nonfixed coupling intervals. To review, the coupling interval is the distance from the previous QRS complex to the PVC. A fixed coupling interval (Figure 30-2) is a fairly common occurrence when the PVCs originate in the same ectopic focus and the depolarization wave takes the same route. Fixed coupling intervals should not be off by more than 0.08 seconds. Variable coupling distances (Figure 30-3), on the other hand, are more common in PVCs that originate from multiple ectopic pacemaker and these PVCs will each have a different morphologic appearance.
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R-on-T Phenomenon
While we are on the topic of coupling intervals, there are a couple of special types of PVCs that we should look at more closely. These include R-on-T phenomenon and end-diastolic PVCs.
R-on-T Phenomenon
PVCs typically occur after the previous complex has finished repolarization—in other words, after the previous T wave is finished. In certain cases, however, the PVC starts during the relative refractory period of the previous complex’s T wave. This is known as the R-on-T phenomenon (Figure 30-4). As we saw in Chapter 1, Anatomy and Basic Physiology, this can create the potential for some very serious reentry loops and circus movements within the ventricles. These loops can lead to serious life-threatening rhythms, including ventricular tachycardia.
There has been a lot of controversy in the literature as to the true clinical importance of the R-on-T phenomenon. For a while, the thought was that the R-on-T phenomenon was very dangerous and that these PVCs had to be treated emergently. Then the pendulum swung in the direction of completely ignoring the R-on-T phenomenon, and the thought was that these PVCs were completely benign. Recently, the pendulum is beginning to swing back in the direction of potential danger.
Here is our thought on this. If there is a potential for serious clinical consequences, you should monitor the patient closely for these serious arrhythmias. In the meantime, a clinical decision based on a sound evaluation of the risk-benefit ratio of the various pharmacologic agents and your patient can be made. Consultation with a cardiologist or electrophysiologist should be obtained quickly, if necessary.
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End-Diastolic PVC
Sometimes, when there is an underlying slow sinus bradycardia, a PVC can fall in such a way that it falls after the next normally occurring sinus P wave (Figure 30-5). These PVCs are known as end-diastolic PVCs, because they occur during the late diastolic phase of the previous complex.
End-diastolic PVCs have no additional clinical significance. They can, however, cause frequent misdiagnoses and are often mistaken for aberrantly conducted premature atrial contractions (PACs) and PJCs. Luckily, this type of PVC is not frequently found.