An implantable medical device system includes an implantable cardioverter defibrillator (ICD) for detecting and treating ventricular tachycardia (VT). The ICD includes a sensing module for sensing a cardiac signal from selected cardiac signal sensing vectors. A control module generates morphology templates of the cardiac signals for multiple patient postures for each of the available sensing vectors. A cardiac signal received during an unknown cardiac rhythm is compared to the morphology templates without determining a current posture of the patient. The unknown cardiac rhythm is detected and classified as supraventricular tachycardia in response to the cardiac signal matching at least one of the morphology templates.