Lead II ECG Rhythm Strip Recorded from a Dog Showing Sinus Tachycardia (175 bpm) and First-Degree Atrioventricular Conduction Block. Because the Heart Rate is Rapid, P Waves are Superimposed on the Downslope of the Preceding T Waves. The Pr Interval Exceeds 0.16 second (Paper Speed = 50 mm/S).