1. A system for predicting the onset of postoperative atrial fibrillation (AF) from an electrocardiogram (ECG) signal representing a patient, comprising: a signal processing component that determines parameters representing the activity of the heart of the patient from the ECG data; a feature extraction component that calculates a plurality of features useful in predicting postoperative AF from the determined parameters; and a classification component that determines an AF index for the patient, representing the likelihood that the patient will experience AF, from the calculated plurality of features.