A method of operating an implantable cardiac stimulus system comprising an implantable device having operational circuitry coupled to a plurality of sensing electrodes disposed within a patient and defining a plurality of sensing vectors for performing cardiac event detection, the method comprising: the implantable device capturing data using at least a first sensing vector and a second sensing vector of the plurality of sensing vectors; the implantable device analyzing the captured data and calculating a quality metric related to the quality of signal captured using each of the first sensing vector and the second sensing vector; and the implantable device selecting a default sensing vector from the plurality of sensing vectors for use in cardiac event detection by the implantable device using the quality metric; calculating the quality metric for each of the first sensing vector and the second sensing vector being performed in the following manner: a set of detections is captured in the respective sensing vector, each detection occurring at an interval following a preceding detection and having an amplitude; the set of detections is analyzed using the intervals between detections to determine if a respective interval exceeds an interval threshold, thereby indicating a detection represents a cardiac event, and, if the intervals indicate at least a threshold portion of the set of detections represents cardiac events, a score is calculated using the set of detections; and if the intervals do not indicate at least a threshold portion of the set of detections represents cardiac events, amplitudes of the detections are analyzed in conjunction with intervals, and, if the amplitudes and intervals indicate at least a threshold portion of the set of detections represent cardiac events, a score is calculated using the set of detections; and, if neither the intervals nor the intervals and amplitudes indicate at least a threshold portion of the set of detections represents cardiac events, possible scores are generated using the set of detections, the possible scores corresponding to at least first and second resolutions of ambiguity within the set of detections. the implantable device capturing data using at least a first sensing vector and a second sensing vector of the plurality of sensing vectors; the implantable device analyzing the captured data and calculating a quality metric related to the quality of signal captured using each of the first sensing vector and the second sensing vector; and the implantable device selecting a default sensing vector from the plurality of sensing vectors for use in cardiac event detection by the implantable device using the quality metric; calculating the quality metric for each of the first sensing vector and the second sensing vector being performed in the following manner: a set of detections is captured in the respective sensing vector, each detection occurring at an interval following a preceding detection and having an amplitude; the set of detections is analyzed using the intervals between detections to determine if a respective interval exceeds an interval threshold, thereby indicating a detection represents a cardiac event, and, if the intervals indicate at least a threshold portion of the set of detections represents cardiac events, a score is calculated using the set of detections; and if the intervals do not indicate at least a threshold portion of the set of detections represents cardiac events, amplitudes of the detections are analyzed in conjunction with intervals, and, if the amplitudes and intervals indicate at least a threshold portion of the set of detections represent cardiac events, a score is calculated using the set of detections; and, if neither the intervals nor the intervals and amplitudes indicate at least a threshold portion of the set of detections represents cardiac events, possible scores are generated using the set of detections, the possible scores corresponding to at least first and second resolutions of ambiguity within the set of detections. a set of detections is captured in the respective sensing vector, each detection occurring at an interval following a preceding detection and having an amplitude; the set of detections is analyzed using the intervals between detections to determine if a respective interval exceeds an interval threshold, thereby indicating a detection represents a cardiac event, and, if the intervals indicate at least a threshold portion of the set of detections represents cardiac events, a score is calculated using the set of detections; and if the intervals do not indicate at least a threshold portion of the set of detections represents cardiac events, amplitudes of the detections are analyzed in conjunction with intervals, and, if the amplitudes and intervals indicate at least a threshold portion of the set of detections represent cardiac events, a score is calculated using the set of detections; and, if neither the intervals nor the intervals and amplitudes indicate at least a threshold portion of the set of detections represents cardiac events, possible scores are generated using the set of detections, the possible scores corresponding to at least first and second resolutions of ambiguity within the set of detections.