M-mode Diagnosis of Tachyarrhythmia Can be Erroneous Owing to "Pseudo 1:1 Atrioventricular Movement" of the Atrial Wall Adjacent to the Atrioventricular Valve Possibly due to Atrioventricular Constraint: A Case of Neonatal Atrial Flutter With 2:1 Atrioventricular Conduction.