In embodiments, a wearable medical (WM) system includes ECG electrodes, and a support structure that can be worn by an ambulatory patient so as to maintain the ECG electrodes on the patient's body. When thus maintained, the ECG electrodes can be configured to sense an ECG signal of the ambulatory patient. The WM system further includes a memory that stores a reference template, and an output device. The reference template can be made from one or more early portions of the ECG signal of the patient. Then later portions of the ECG signal are sensed and compared against the reference template, to determine if there is a specific problem. If it is so determined, the output device can output an alert.
