A medical device (e.g., an automated external defibrillator) automatically detects and reports cardiac asystole by first obtaining ECG data and calculating one or more ECG measures from the ECG data. The defibrillator evaluates the ECG data by classifying the ECG data into a class indicative of cardiac condition, wherein one class is indicative of cardiac asystole. If the defibrillator classifies the ECG data into the class indicative of cardiac asystole, the defibrillator reports the asystole classification on a display. The defibrillator may classify the ECG data into a rhythm class associated with a cardiac rhythm, such as asystole, and report the rhythm class of the ECG data on the display. The defibrillator may also suggest a procedure to undertake, such as a therapy (e.g., defibrillation for a shockable cardiac rhythm), based on the classification of the ECG data. The defibrillator may reserve reporting the detection of asystole until a predetermined number of ECG classifications over at least a predetermined period of time have resulted in an asystole classification. The defibrillator may also reserve reporting the detection of asystole until an ECG classification resulting in asystole has been performed outside of a predetermined period of time after application of a defibrillation pulse. A statistical binary classification and regression tree may be used to classify the ECG data according to cardiac rhythm. Other signal data, such as impedance or phonocardiographic signal data, may also be obtained and classified with the ECG data.