generating sense signals representing electrical activity in a heart chamber and detecting a chamber sense when a sense signal exceeds a specified intrinsic detection threshold;

detecting a tachycardia in the heart chamber when a rate at which chamber senses are detected exceeds a specified tachycardia threshold value;

upon detection of a tachycardia, delivering a burst of pacing pulses in accordance with an anti-tachycardia pacing protocol, where the burst is output after a specified coupling interval with respect to a chamber sense;

determining if a pacing pulse has captured the heart chamber by detecting whether an evoked response occurs during a capture detection window following the output of a pacing pulse; and,

determining the extent to which the heart chamber has been entrained by counting the number of consecutive pacing pulses during the burst that achieve capture without an intervening loss of capture to thereby determine the number of successive pacing pulses achieving capture during the burst.