Ventricular rhythms are the most clinically feared cardiac rhythms because of the hemodynamic instability that they cause. As we shall see, there is no such thing as a completely benign ventricular rhythm. Even premature ventricular contractions, which are generally considered benign, can cause significant hemodynamic compromise in the right clinical setting.
A rhythm is considered to be of ventricular origin if the ectopic pacemaker is found distal to the bifurcation of the His bundles. The pacemaker could be found in the bundle branches, the anterior or posterior fascicle, the Purkinje system, or in the ventricular muscle. Typically, ventricular depolarization is spread through direct cell-to-cell contact. However, if the impulse originates in or near the conduction system, the impulse conduction can be conducted at least partly through the electrical conduction system. Therefore, the site of origin is critical in determining the exact electrocardiographic morphology of the complexes.
In Chapter 27, Narrow-Complex Supraventricular Tachycardia, we discussed how symmetrical, synchronous contraction of the ventricles is critical to maintaining a good cardiac output. In ventricular rhythms, symmetrical and synchronous contraction of the ventricles is almost nonexistent. The typical direct cell-to-cell transmission of the impulse leads to ventricular contractions that follow very bizarre, abnormal patterns. These patterns reflect defects in mechanical contraction that significantly alter the ejection fraction of the ventricles and, therefore, the cardiac output. If you add the lack of atrial kick into the equation, the problem is compounded significantly.
In this chapter, we begin by taking a look at how the morphology of a ventricular depolarization develops, and then we will review some clinically important aspects of these rhythms. A clear understanding of the mechanisms will make discussion of the individual rhythms much easier.