A CPR machine (100) is configured to perform compressions on a patient's (182) chest that alternate with releases. The CPR machine includes a compression mechanism (148), and a driver system (141) configured to drive the compression mechanism. A compression force may be sensed, and the driving is adjusted accordingly if there is a surprise. For instance, driving may have been automatic according to a motion-time profile, which is adjusted if the compression force is not as expected (850). An optional chest-lifting device (152) may lift the chest between the compressions, to assist actively the decompression of the chest. A lifting force may be sensed, and the motion-time profile can be adjusted if the compression force or the lifting force is not as expected. An advantage is that a changing condition in the patient or in the retention of the patient within the CPR machine may be detected and responded to.