step 206 a check is made whether at least two a paces have already been generated if not then in step 208 a determination is made whether the current a pace is the second if not then it must be the first and in step 210 the measurement of the a r interval is stored and the system waits for the next a pace but if it is the second then in step 212 a comparison is made whether the first two a r intervals were similar e g within u00b1 50 ms if they were similar then in step 214 their average is taken and the system waits for the next a pace if the a r intervals are determined to be not similar in step 210 then in step 216 the system delivers a warning message and ends the procedure if in step 206 it is determined that the current a pace is the third or higher a pace in the series then in step 218 a check is made whether the a r interval is within a window around the average a r interval as previously determined in step 214 if the current a r interval is within u00b1 50 ms of the average a v interval then the a pace is classified as having captured the atria and the system waits for the next a pace but if the current a r interval does not fall within the u00b1 50.