Utilization of morphology discrimination after undersensing determination for underlying rhythms in the therapy zone

A medical device and method for detecting and classifying cardiac rhythm episodes that includes a sensing module to sense cardiac events, a therapy delivery module, and a detection module configured to determine intervals between the sensed cardiac events, determine a predetermined cardiac episode is occurring in response to the determined intervals, determine whether a ventricular rate is greater than an atrial rate in response to the determined intervals, determine whether undersensing is occurring in response to the ventricular rate being greater than the atrial rate, perform a supraventricular tachycardia (SVT) discrimination analysis in response to undersensing occurring, and control the therapy delivery module to deliver therapy in response to the SVT discrimination analysis.