Abstract: A pacemaker-mediated tachycardia (PMT) is detected by calculating the correlations for typically two to six heart cycles between, for example, spontaneous atrial beats (PP-intervals) 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.