Spontaneous changes in ventricular tachycardia cycle length. Understanding spontaneous fluctuations in ventricular tachycardia cycle length is required to develop algorithms for ventricular tachycardia detection and termination. Variations in cycle length, time to stable cycle length and the range of RR intervals during ventricular tachycardia were analyzed in 74 episodes of sustained monomorphic ventricular tachycardia induced in patients not taking antiarrhythmic medication. Linear regression demonstrated cycle length variability to decrease over time (41 +/- 24 to 17 +/- 19 ms, p less than 0.001). Slower ventricular tachycardia had more cycle length variability than faster ventricular tachycardia (p less than 0.001). Ventricular tachycardia that was initially more variable tended to remain more variable (p less than 0.001). Fifty-four percent of episodes stabilized within the first 15 beats, 75% by 30 beats and 93% by 50 beats. The number of beats to stable cycle length was independent of ventricular tachycardia rate. The average range in cycle length per episode was 127 +/- 72 ms; 12% of ventricular tachycardia episodes varied by less than 50 ms and 45% by less than 150 ms. The maximal range in RR intervals from a single episode of ventricular tachycardia was 290 ms. Therefore, ventricular tachycardia demonstrates a wide range of cycle lengths and has time-dependent changes in variability and stability. These cycle length changes should be considered in the algorithms for ventricular tachycardia detection and termination by automatic antitachycardia devices. 