A second example of 3:2 SA block is shown here. Again, note that the P wave morphology and the PR interval are constant; that the RR interval encompassing the pause between the two groups (1.32 seconds) is less than twice the preceding RR interval (2 x 0.96 = 1.92 seconds) and that there is no P wave during the pause. These features suggest type I SA block with Wenckebach periodicity and a 3:2 conduction ratio. Try to determine the sinus node cycle length (the SS interval) and the relationship of the sinus node discharge to the P waves as demonstrated on the preceding page.