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 (204, 780) 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 (208, 210) 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 (204, 780) 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 (208, 210) 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 (208, 210) 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.