Bedside to Bench

Arrhythmia Grand Rounds, June 2015, Volume 1, Issue 2: 79-86
DOI: 10.12945/j.agr.2015.033-15

Basic Foundations of Clinical Electrophysiology. Electrophysiological Mechanism of Phase 4 Conduction Block in The His Purkinje System: A Comparison With Phase 3 Block

Andrew L. Wit1, Mark E. Josephson2

1 Department of Pharmacology, Columbia University College of Physicians and Surgeons, New York, New York, USA
2 Harvard-Thorndike Electrophysiology Institute, Cardiovascular Division, Department of Medicine, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts, USA


We describe the mechanism underlying paroxysmal AV block that follows a sudden long cycle length. The phases and characteristics of the Purkinje fiber action potential are reviewed. Phase 4 block occurs when an impulse conducts into regions of Purkinje cells that have undergone spontaneous (phase 4) depolarization causing a reduction in the level of the membrane potential during diastole. This results in a decrease in inward sodium current during the depolarization phase (0) of the action potential, slow conduction and conduction block. The mechanisms involved are discussed in detail and compared to phase 3 block.

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