The nature of the PR interval.
D000109:Acetylcholine / Q000494:pharmacology; D000818:Animals; D001285:Atropine / Q000494:pharmacology; D002297:Cardanolides / Q000502:physiology; D004285:Dogs; D004558:Electric Stimulation; D004562:Electrocardiography; D006329:Heart Conduction System / Q000502:physiology*; D009638:Norepinephrine / Q000494:pharmacology; D010477:Perfusion; D012849:Sinoatrial Node / Q000502:physiology
D002297:Cardanolides; D001285:Atropine; D000109:Acetylcholine; D009638:Norepinephrine