1. A method of estimating an actual ECG signal of a patient while performing chest compressions with an automatic chest compressions device, wherein the method comprises the steps of: providing an ECG sensor capable of measuring an ECG signal of the patient, said ECG sensor producing a measured ECG signal having an actual component and a noise component; providing an automatic chest compression device disposed to provide chest compressions to the patient, said chest compression device having a load sensor capable of determining the presence of a chest compression when the load sensed by the load sensor exceeds a predetermined value, said load sensor producing a compression signal corresponding the presence a chest compression; performing compressions; providing the measured ECG signal to a system identifier; providing the compression signal to the system identifier; estimating the noise component of the measured ECG signal with the system identifier by processing the measured ECG signal and the compression signal; providing the measured ECG signal and the estimated noise component of the measured ECG signal to a means for combining signals; calculating the estimated actual ECG with the means for combining signals by combining the measured ECG signal and the noise component of the measured ECG signal.