An implantable medical device, comprising: a plurality of electrodes sensing cardiac signals, the plurality of electrodes forming a first sensing vector on a first channel and a second sensing vector on a second channel; a processor (194, 142) processing an interval associated with a cardiac cycle of the sensed cardiac signals; and a control unit controlling the device to advance from a first state to a second state in response to an R-R interval associated with the cardiac cycle of the sensed cardiac signals being indicative of a cardiac event and to advance from the second state in response to processing of a cardiac signal sensed during a predetermined sensing window; wherein the processor determines, while the device is in the first state, a first heart rate corresponding to the first sensing vector and a second heart rate corresponding to the second sensing vector in response to the R-R interval associated with a cardiac cycle of the sensed cardiac signals being indicative of the cardiac event, and the control unit causes the device to advance from the first state to the second state in response to both the first heart rate and the second heart rate being greater than a predetermined threshold; and wherein the processor determines, while the device is in the second state, whether a cardiac signal sensed during a plurality of predetermined sensing windows corresponds to the cardiac event, and the control unit causes the device to advance from the second state in response to the processor determining that a predetermined number of the plurality of predetermined sensing windows correspond to the cardiac event; and wherein the control unit causes the device to advance from the second state to the first state in response to the predetermined number of the plurality of predetermined sensing windows not corresponding to the cardiac event, from the second state to a third state in response to the predetermined number of the plurality of predetermined sensing windows corresponding to the cardiac event, remain in the second state in response to the predetermined number of the plurality of predetermined sensing windows not corresponding to the cardiac event and the first heart rate and the second heart rate being greater than the predetermined threshold subsequent to the advancing from the first state to the second state. a plurality of electrodes sensing cardiac signals, the plurality of electrodes forming a first sensing vector on a first channel and a second sensing vector on a second channel; a processor (194, 142) processing an interval associated with a cardiac cycle of the sensed cardiac signals; and a control unit controlling the device to advance from a first state to a second state in response to an R-R interval associated with the cardiac cycle of the sensed cardiac signals being indicative of a cardiac event and to advance from the second state in response to processing of a cardiac signal sensed during a predetermined sensing window; wherein the processor determines, while the device is in the first state, a first heart rate corresponding to the first sensing vector and a second heart rate corresponding to the second sensing vector in response to the R-R interval associated with a cardiac cycle of the sensed cardiac signals being indicative of the cardiac event, and the control unit causes the device to advance from the first state to the second state in response to both the first heart rate and the second heart rate being greater than a predetermined threshold; and wherein the processor determines, while the device is in the second state, whether a cardiac signal sensed during a plurality of predetermined sensing windows corresponds to the cardiac event, and the control unit causes the device to advance from the second state in response to the processor determining that a predetermined number of the plurality of predetermined sensing windows correspond to the cardiac event; and wherein the control unit causes the device to advance from the second state to the first state in response to the predetermined number of the plurality of predetermined sensing windows not corresponding to the cardiac event, from the second state to a third state in response to the predetermined number of the plurality of predetermined sensing windows corresponding to the cardiac event, remain in the second state in response to the predetermined number of the plurality of predetermined sensing windows not corresponding to the cardiac event and the first heart rate and the second heart rate being greater than the predetermined threshold subsequent to the advancing from the first state to the second state.