Atrial versus sinus rhythm on electocardiogram

An ECG demonstrating an intraatrial reentrant tachycardia/atrial flutter in a patient with congenital heart disease. Viewing lead V1, only a single p-wave is visible (arrows), and it is slightly positive with a negative terminal component, consistent with sinus rhythm/sinus tachycardia and potentially misleading. Lead II, however, reveals notching of the T-wave (a second p-wave, dashed arrows), and the third p-wave is entirely hidden within the QRS complex (not shown) in a 3:1 atrioventricular ratio.