An apparatus for determining when a patient is susceptible to defibrillation comprises a plurality of electrodes (12, 14) for obtaining an ECG signal from a patient, and data processing means (30, 42) for determining a region of the ECG signal where the signal passes from a first threshold to a second threshold at least equal in magnitude but of opposite polarity to the first threshold while the gradient of the signal remains within certain limits, detecting the next following ECG signal peak, and providing an output signal upon such detection.