an implantable anti - tachycardia pacemaker having the capability of switching from a first pacing regimen to a second pacing regimen , without waiting to determine whether the first pacing regimen actually is successful in terminating the tachycardia . the device first delivers a short series of pacing pulses at the defined parameters of the first pacing regimen and then interrupts delivery of pacing pulses and measures the return cycle of the subsequent spontaneous depolarization . the device then resumes delivery of the first pacing regimen for a second , greater number of pacing pulses and again measures the return cycle . in the event that no increase in the return cycle occurs following the delivery of the longer series of pacing pulses , the device terminates the pacing regimen presently underway , and initiates the next scheduled therapy . in the event that an increase in the return cycle is detected , the device may simply deliver the programmed number of pacing pulses defined for the first pacing regimen .