Paroxysmal atrioventricular block: Are phase 3 and phase 4 block mechanisms or misnomers?

The era of deductive analysis of the electrocardiogram was prolific in its ability to yield accurate inferences regarding the pathophysiology of a vast number of heart rhythm disorders. However, occasionally there was a tendency to infer hypothetical electrophysiological mechanisms without the benefit of direct cellular information. This turned out to be misleading in some cases in which suggested mechanisms were subsequently found to be incorrect, and it also unintentionally hampered the search for more basic mechanisms. Two cases in point are the use of the terms “phase 3” and “phase 4” block in reference to tachycardia-dependent (TD) and pause-dependent (PD) paroxysmal atrioventricular block (PAVB), 1 respectively. Not only did basic studies definitely show that TD-PAVB is not related to phase 3 block, but also significant basic studies demonstrated that PD-PAVB need not depend on a phase 4 depolarization mechanism. Here we revisit the problems of TD- and PD-PAVB and illustrate by clinical and basic science examples possible pathophysiological mechanisms.