A medical device, comprising: - a first electrode pair (62, 38); - a second electrode pair (36, 38) having a different distance between its electrodes than the first electrode pair (62, 38); and - a controller connected with the first electrode pair and the second electrode pair and configured to: - use the first electrode pair (62, 38) to determine a first cardiac signal of a heartbeat; - use the second electrode pair (36, 38) to determine a second cardiac signal of the heartbeat; - apply a first comparison algorithm to the first cardiac signal to determine whether the first cardiac signal is indicative for a supraventricular tachycardia or indicative for a ventricular tachycardia, and comprising comparing the first signal with a first template to determine if changes of QRS-wave morphology parameters are indicative for a supraventricular tachycardia; - apply a second comparison algorithm to the second cardiac signal to determine whether the second cardiac signal is indicative for the supraventricular tachycardia or indicative for the ventricular tachycardia, the second comparison algorithm being different from the first comparison algorithm and comprising comparing the second signal with a second template to determine if changes of S-wave morphology parameters are indicative for the supraventricular tachycardia; and - classify the heartbeat as being indicative for the ventricular tachycardia when at least one of the first cardiac signal and the second cardiac signal is determined to be indicative for the ventricular tachycardia. - a first electrode pair (62, 38); - a second electrode pair (36, 38) having a different distance between its electrodes than the first electrode pair (62, 38); and - a controller connected with the first electrode pair and the second electrode pair and configured to: - use the first electrode pair (62, 38) to determine a first cardiac signal of a heartbeat; - use the second electrode pair (36, 38) to determine a second cardiac signal of the heartbeat; - apply a first comparison algorithm to the first cardiac signal to determine whether the first cardiac signal is indicative for a supraventricular tachycardia or indicative for a ventricular tachycardia, and comprising comparing the first signal with a first template to determine if changes of QRS-wave morphology parameters are indicative for a supraventricular tachycardia; - apply a second comparison algorithm to the second cardiac signal to determine whether the second cardiac signal is indicative for the supraventricular tachycardia or indicative for the ventricular tachycardia, the second comparison algorithm being different from the first comparison algorithm and comprising comparing the second signal with a second template to determine if changes of S-wave morphology parameters are indicative for the supraventricular tachycardia; and - classify the heartbeat as being indicative for the ventricular tachycardia when at least one of the first cardiac signal and the second cardiac signal is determined to be indicative for the ventricular tachycardia. - use the first electrode pair (62, 38) to determine a first cardiac signal of a heartbeat; - use the second electrode pair (36, 38) to determine a second cardiac signal of the heartbeat; - apply a first comparison algorithm to the first cardiac signal to determine whether the first cardiac signal is indicative for a supraventricular tachycardia or indicative for a ventricular tachycardia, and comprising comparing the first signal with a first template to determine if changes of QRS-wave morphology parameters are indicative for a supraventricular tachycardia; - apply a second comparison algorithm to the second cardiac signal to determine whether the second cardiac signal is indicative for the supraventricular tachycardia or indicative for the ventricular tachycardia, the second comparison algorithm being different from the first comparison algorithm and comprising comparing the second signal with a second template to determine if changes of S-wave morphology parameters are indicative for the supraventricular tachycardia; and - classify the heartbeat as being indicative for the ventricular tachycardia when at least one of the first cardiac signal and the second cardiac signal is determined to be indicative for the ventricular tachycardia.