A medical device to monitor and treat a patient includes at least one ECG sensing electrode configured to sense a patient ECG signal, a plurality of therapy electrodes configured to deliver one or more therapeutic shocks, and a controller. The controller is configured to select a first ECG template based on a first ECG signal received during a first period of time, select a second ECG template based on determining from a second ECG signal received during a second period of time that the second ECG template represents a better fit, determine whether the patient is experiencing a treatable cardiac event based in part on one or more morphological differences between the second ECG signal and one or more of the first and second ECG template, and cause the medical device to initiate a treatment sequence in response to determining that the patient is experiencing the treatable cardiac event.