An implantable medical device, IMD, , comprising: electrodes (101, 122, 124, 126, 132-138) configured to be located in or about a heart, the electrodes (101, 122, 124, 126, 132-138) including a bipolar electrode combination (122, 124, 126, 132-138) that defines a bipolar sensing vector, the electrodes (101, 122, 124, 126, 132-138) including a unipolar electrode combination (101, 122, 124, 126, 132-138) that defines a unipolar sensing vector; sensing circuitry configured to define a first sensing channel coupled to the bipolar electrode combination (122, 124, 126, 132-138) and configured to define a second sensing channel coupled to the unipolar electrode combination (101, 122, 124, 126, 132-138); memory to store program instructions; one or more processors configured to implement the program instructions to: collect cardiac activity, CA, signals over the first and second sensing channels for a series of beats; and identify a sensed interval delta, SID, from the CA signals for at least a portion of the series of beats, the SID representing a timing interval between i) a characteristic of interest, COI, in the CA signals collected over the first sensing channel and ii) the same COI in the CA signals collected over the second sensing channel , characterized in that the one or more processors are further configured to implement the program instructions to: designate the series of beats to correspond to be a polymorphic ventricular tachycardia, PVT, or a monomorphic ventricular tachycardia, MVT, based on the SID. electrodes (101, 122, 124, 126, 132-138) configured to be located in or about a heart, the electrodes (101, 122, 124, 126, 132-138) including a bipolar electrode combination (122, 124, 126, 132-138) that defines a bipolar sensing vector, the electrodes (101, 122, 124, 126, 132-138) including a unipolar electrode combination (101, 122, 124, 126, 132-138) that defines a unipolar sensing vector; sensing circuitry configured to define a first sensing channel coupled to the bipolar electrode combination (122, 124, 126, 132-138) and configured to define a second sensing channel coupled to the unipolar electrode combination (101, 122, 124, 126, 132-138); memory to store program instructions; one or more processors configured to implement the program instructions to: collect cardiac activity, CA, signals over the first and second sensing channels for a series of beats; and identify a sensed interval delta, SID, from the CA signals for at least a portion of the series of beats, the SID representing a timing interval between i) a characteristic of interest, COI, in the CA signals collected over the first sensing channel and ii) the same COI in the CA signals collected over the second sensing channel , characterized in that the one or more processors are further configured to implement the program instructions to: designate the series of beats to correspond to be a polymorphic ventricular tachycardia, PVT, or a monomorphic ventricular tachycardia, MVT, based on the SID. collect cardiac activity, CA, signals over the first and second sensing channels for a series of beats; and identify a sensed interval delta, SID, from the CA signals for at least a portion of the series of beats, the SID representing a timing interval between i) a characteristic of interest, COI, in the CA signals collected over the first sensing channel and ii) the same COI in the CA signals collected over the second sensing channel , characterized in that the one or more processors are further configured to implement the program instructions to: designate the series of beats to correspond to be a polymorphic ventricular tachycardia, PVT, or a monomorphic ventricular tachycardia, MVT, based on the SID. <EOS>