An automatic external defibrillator including: a sensor for detecting when a rescuer is delivering a CPR chest compression to the patient; electrodes for application to the thorax of the patient for delivering a defibrillation shock to the patient and for detecting an ECG signal; defibrillation circuitry for delivering a defibrillation shock to the electrodes; and a processor and associated memory for executing software that controls operation of the defibrillator. The software provides: ECG analysis for analyzing the ECG signal to determine if the cardiac rhythm is shockable; CPR detection for analyzing the output of the sensor to determine when a CPR chest compression has been delivered, and integration of the ECG analysis and CPR detection so that the determination of whether the cardiac rhythm is shockable is based only on time periods of the ECG signal during which there has not been a CPR chest compression delivered.