In this example, the sinus cycle length, calculated by dividing the interval between first P waves of each group (0.98+1.30=2.28 seconds) by the number of P waves within this envelope (3) is 0.76 seconds, indicating a sinus rate of 80. Walking the calipers set at this interval through the ECG strip, beginning with the first P wave of the group, demonstrates the progressive prolongation of the interval between the sinus node discharge and the P wave prior to the block. Once again, note that the duration of the pause (1.30 seconds) is less than twice the preceding PP(and RR)interval(2 x 0.98 = 1.96 seconds).