Abstract: Clinical studies of heart patients have demonstrated that ventricular tachyarrhythmia often is preceded by a foreshortened cardiac cycle length followed by a relatively long compensatory pause, thus producing in an abrupt short-to-long cycle length change. An implantable apparatus for preventing tachyarrhythmia measures the cardiac cycle length and detects the occurrence of a foreshortened cardiac cycle length more than a predefined amount between consecutive cycles. When a normal heart beat does not occur within a predefined period of time after such an abrupt change in cycle length, the resulting compensatory pause is eliminated by a cardiac pacer applying an appropriately timed electrical pulse to produce a contraction of the heart. The apparatus also includes a defibrillator to shock the heart in the event that the preventive pacing is not effective.