In these examples, both lead II rhythm strips exhibit a long pause with missing P’s and QRS’. In the top rhythm strip, the duration of the pause (2.06 sec) is unrelated to the P-P interval - this is sinus arrest. The lower strip shows the pause duration to be exactly twice the duration of the P-P interval. This is SA block, due to one sinus depolarization not conducting out of the SA node to the atria.