A new dual chamber ventricular tachyarrhythmia detection algorithm has been incorporated into an external prototype dual chamber implantable cardioverter defibrillator, the Model AMD 7250, for the purpose of acute human testing of the algorithm. Testing has been performed during ICD implants and electrophysiologic studies. The detection algorithm uses a hierarchical rule-based approach to combine pattern and timing information to reach a rhythm decision. Rules for atrial fibrillation (AF) and atrial flutte/tachycardia (AT), sinus tachycardia (ST) and other 1:1 SVT are designed to reject these rhythms that would otherwise be detected by rate-based defection. RESULTS: Thirty-five patients were studied during EP study (N=32) or ICD implant (N=3). Appropriate detection occurred in 23/23 episodes of VT and 8/8 episodes of VF. Inappropriate detection of VT was prevented in 11/15 episodes of ST and 23/32 episodes of AF/AT.