title: An unusual PR interval alternans during 2:1 atrio-ventricular block.
abstract: In a case of a 2:1 second degree A-V block during sinus rhythm, PR intervals of two different durations occur, either separately or alternating beat-by-beat. The longer intervals are not caused by concealed conduction in the A-V junction of the preceding blocked impulses and the shorter ones are not due to supernormal conduction induced by the blocked impulses. This primary PR interval alternans is alternating from a pathophysiological point of view; it only concerns every other impulse alternately traveling along the faster and slower A-V junctional pathway.
mesh_terms: D004562:Electrocardiography; D005260:Female; D006327:Heart Block / Q000175:diagnosis / Q000503:physiopathology*; D006801:Humans; D008875:Middle Aged; D009200:Myocardial Contraction; D010138:Pacemaker, Artificial; D012804:Sick Sinus Syndrome / Q000628:therapy; D012849:Sinoatrial Node / Q000502:physiology; D013599:Systole