The sinus node is the dominant pacemaker in the normal human heart. Rhythms arising from any part of the heart other than the sinus node are considered ectopic.1 Most tissues in the conduction system can manifest pacemaker activity. When the dominant pacemaker slows or fails (default, see below) or when conduction is interrupted (block, see below), these pacemakers have the potential to escape and become the dominant pacemaker of the heart. The more distally located a subsidiary pacemaker, the slower its escape rate (Lewis' "law of the heart"). Under pathologic conditions, these ectopic pacemakers can accelerate and become the dominant pacemaker of the heart (usurpation, see below).
Under usual circumstances, the atria and ventricles are driven by a single pacemaker. When the atria and ventricles are driven by different pacemakers, atrioventricular (AV) dissociation is said to be present. Pick and Langendorf2 have described four basic mechanisms for AV
Duncan E, Silverman I, Swiryn S. Left Posterior Fascicular Escape Rhythm With Isorhythmic Atrioventricular Dissociation. Arch Intern Med. 1987;147(5):977–979. doi:10.1001/archinte.1987.00370050169028