A method discriminates between ventricular arrhythmia and supraventricular tachycardia by detecting an earliest arriving electrical signal following anti-tachycardia pacing. An implantable cardiac defibrillator provides simultaneous atrioventricular anti-tachycardia pacing bursts and detects an earliest arriving electrical signal. This discrimination capability reduces the incidence of inappropriate shocks from dual-chamber implantable cardiac defibrillators to near zero and provides a method to differentially diagnose supraventricular tachycardia from ventricular tachycardia.