It is important to understand how three different myocardial cell types function and relate to arrhythmias. The first type, or pacemaker cells, that have intrinsic automaticity, spontaneous diastolic depolarization, when they fire, they generate an action potential that can spread to contiguous tissue. In the atria, these cells are concentrated primarily in the SA node and the AV junction. The His bundle and bundle branches are composed of a second morphologically distinct cell type, the Purkinje cell. These Purkinje cells extend to the very distal ramifications of the bundle branches and the endocardium of the left and right ventricles, where they are termed Purkinje fibers. The electrical impulse is typically insulated from the ventricles until the impulse reaches these Purkinje fibers, the ultimate activation link to the third cell type, contractile cell. Myocardial contractile cells are located throughout both the atria and the ventricles. Their primary purpose is contraction with force generation, but they also passively conduct the electrical forces to complete activation of the cardiac chambers. All three cell types can be involved in the two basic mechanisms for arrhythmia generation, automaticity and reentry.
Automaticity
Automaticity, intrinsic spontaneous diastolic depolarization of the pacemaker cells, can be dominant or latent. The SA node's intrinsic automaticity rate is from 50 to 100 depolarizations per minute and is normally dominant as the fastest rate of spontaneous diastolic depolarizations. [Animation]
When the sinus node slows sufficiently, latent pacemakers may become dominant. Latent pacemaker rates range from 35 to 50 beats per minute for the AV junction including the AV node and proximal His bundle; 30 to 40 beats per minute for the bundle branches; and then the slowest 20 to 30 beats per minute for the more distal Purkinje fibers. Under special conditions, atrial and ventricular contractile cells can also act as abnormal latent pacemakers.