An implantable medical device system includes a pacemaker and an extravascular implantable cardioverter defibrillator (ICD). The pacemaker is configured to acquire a cardiac electrical signal, determine RR intervals from the cardiac electrical signal, apply ventricular tachycardia detection criteria solely to the RR intervals, detect ventricular tachycardia (VT) when the detection criteria are met; and deliver anti-tachycardia pacing in response to detecting the VT before the extravascular ICD delivers a shock therapy.