The sinus cycle length, i.e. the SS interval, can be determined by determining the interval between the first P wave of each group, in this instance 1.74 seconds (0.72 + 1.02 seconds) and then dividing it by the number of P waves within this envelope which in this instance is 3. This calculates to a sinus cycle length of 0.58 seconds (1.74/3), a rate of 103. The presumed relationship of the sinus node discharge to the onset of atrial activation can then be determined by setting calipers to this interval (0.58 seconds), placing one arm of the calipers at the onset of the first P wave in the group and then “walking” the calipers through the ECG strip as demonstrated in this figure. This demonstrates the progressive prolongation of the SA interval prior to the block.