Knowledge of underlying mechanisms and the site of origin of arrhythmias is particularly important. All cardiac tissues have an inherent conduction velocity that is normally faster in the His-Purkinje system compared with the atrial and ventricular myocardium.
Conduction velocity is slowest in the AV and sinoatrial nodes.
Abnormally slow or ineffective propagation of atrial impulses across the AV junction can produce various degrees of AV block. Such a mechanism has also been incriminated in production of sinoatrial exit block when abnormalities exist at the sinoatrial junction. It is difficult to recognize tachyarrhythmias that are due to slow conduction. However, slow conduction is commonly encountered during propagation of premature impulses due to incomplete recovery, where conduction may be normal during the resting state. If premature impulses block along certain fibers or pathways and conduct slowly along others, the impulse can return via fibers not previously used.