An example of alternating 3:2 and 2:1 SA block is shown in this ECG and ladder diagram. This diagnosis is more difficult than that of 3:2 and 2:1 AV block because the PR intervals are constant and because there are no P waves during the pauses. Note, however, that as in the example of the 3:2 and 2:1 AV block, the pauses are of different durations (1.38 and 1.48 seconds). This is the clue to the diagnosis. If the longer pause of 1.48 seconds between the 3rd and 4th QRS complexes is assumed to represent 2:1 SA block, then an SS interval of 0.74 seconds can be postulated. Applying this interval throughout the strip establishes the location of the sinus node discharges relative to the P waves and reveals the presence of 3:2 block in the groups preceding and following the 2:1 block.