1. An automatic method for detecting heartbeats of a patient, the method comprising: sensing two or more selected ECG signals; and automatically processing the ECG signals with a programmable processor configured to: determine a velocity for each of the selected signals; sum together absolute values of each of the velocities; compare the sum with a threshold T having a value about one-half of an expected maximum value of the sum; and detect a heartbeat at a time t D of the velocity determinations when the sum is greater than the threshold T and when elapsed time since an immediately-previous heartbeat detection is greater than a preset refractory period t R .