title: Letter: Second degree A-V block.
mesh_terms: D000818:Animals; D002036:Bundle of His / Q000503:physiopathology; D004285:Dogs; D004562:Electrocardiography; D006327:Heart Block / Q000145:classification / Q000175:diagnosis / Q000503:physiopathology; D006329:Heart Conduction System / Q000503:physiopathology*; D006801:Humans; D011690:Purkinje Fibers / Q000503:physiopathology