Lead II ECG Rhythm Strip Recorded from a Dog With Both First- and second-Degree Atrioventricular Block. The second-Degree AV Block is High Grade With Both 2:1 and 3:1 Block Resulting in Variation in the Rr Intervals. The Pr Interval for the Conducted beats is Prolonged but Constant (0.28 second) (Paper Speed = 25 mm/S).