QRS Morphology in a Paced Rhythm

As we saw in Figure 39-4, the morphology of the QRS complex in the paced rhythms is wide and bizarre. The morphology is due to the slow ventricular depolarization wave that is created by the direct cell-to-cell transmission of the impulse. As mentioned, the morphology of the QRS complexes is similar to those found in ventricular rhythms.

The morphology of the paced complexes also has something else in common with the ectopically formed ventricular rhythms—the morphology of the complexes will depend on where they started and the vectors formed as the ventricles are depolarized. Since the most common site of pacing wire insertion is in the right ventricle, many paced rhythms will have a right bundle branch block morphology. However, this is not always the case, as some paced rhythms will have a left bundle branch block morphology.

Even though this is a text on arrhythmias, we should mention that you should not make any comments on axis or chamber enlargement and hypertrophy in any paced rhythm. There are some criteria for infarct changes in paced rhythms, but great care should be taken in interpreting them unless you are very experienced in electrocardiography. Unless you are skilled in electrocardiography, we suggest using 12-lead ECGs and multiple strips to strictly evaluate only the patient’s rhythm.