a pacemaker - mediated tachycardia is detected by calculating the correlations for typically two to six heart cycles between , for example , spontaneous atrial beats and the pacemaker generated av delays , which are varied in a predictable manner . the correlation is low when no pmt is present because the possible variation in the interval between spontaneous atrial beats is independent of the variation of the av delay . correlation is high when a pmt is present , because the interval between spontaneous atrial beats will then equal the pmt cycle time , i . e ., the av delay plus the retrograde conduction time in the heart tissue .