Source:http://linkedlifedata.com/resource/umls/id/C0033036
MSH: A type of cardiac arrhythmia with premature atrial contractions or beats caused by signals originating from ectopic atrial sites. The ectopic signals may or may not conduct to the HEART VENTRICLES. Atrial premature complexes are characterized by premature P waves on ECG which are different in configuration from the P waves generated by the normal pacemaker complex in the SINOATRIAL NODE.,NCI: Ectopic impulses originating in the atria.,NCI: Ectopic impulses originating in the atria. (NCI)