A defibrillator configured to assess chest recoil of a subject during cardio pulmonary resuscitation carried out by a rescuer on the subject and provide feedback to the rescuer, comprising: a biosignal measurement system configured to measure biosignals of the subject, determine when CPR is required and produce a CPR start signal and determine when CPR is to be ceased and produce a CPR stop signal, an impedance measurement system configured to measure impedance signals of the subject, a CPR assessment system connected to the biosignal measurement system to receive the CPR start signal and the CPR stop signal, connected to the impedance measurement system to receive impedance signals and configured to perform the steps: (i) receive the CPR start signal and produce a first feedback signal, (ii) receive an impedance signal of the subject measured during a plurality of CPR chest compressions by the rescuer, (iii) use the impedance signal to assess chest recoil of the subject during the plurality of CPR chest compressions, (iv) compare the chest recoil of the subject with a chest recoil threshold, (v) when the chest recoil of the subject is greaterthan the chest recoil threshold, produce a second feedback signal and go to step (vii), (vi) when the chest recoil of the subject is less than the chest recoil threshold, produce a third feedback signal and go to step (vii), (vii) when the CPR stop signal is not received, return to step (ii), (viii) when the CPR stop signal is received, produce a fourth feedback signal, and a feedback unit connected to the CPR assessment system and configured to receive the feedback signals and issue CPR feedback to the rescuer. a biosignal measurement system configured to measure biosignals of the subject, determine when CPR is required and produce a CPR start signal and determine when CPR is to be ceased and produce a CPR stop signal, an impedance measurement system configured to measure impedance signals of the subject, a CPR assessment system connected to the biosignal measurement system to receive the CPR start signal and the CPR stop signal, connected to the impedance measurement system to receive impedance signals and configured to perform the steps: (i) receive the CPR start signal and produce a first feedback signal, (ii) receive an impedance signal of the subject measured during a plurality of CPR chest compressions by the rescuer, (iii) use the impedance signal to assess chest recoil of the subject during the plurality of CPR chest compressions, (iv) compare the chest recoil of the subject with a chest recoil threshold, (v) when the chest recoil of the subject is greaterthan the chest recoil threshold, produce a second feedback signal and go to step (vii), (vi) when the chest recoil of the subject is less than the chest recoil threshold, produce a third feedback signal and go to step (vii), (vii) when the CPR stop signal is not received, return to step (ii), (viii) when the CPR stop signal is received, produce a fourth feedback signal, and (i) receive the CPR start signal and produce a first feedback signal, (ii) receive an impedance signal of the subject measured during a plurality of CPR chest compressions by the rescuer, (iii) use the impedance signal to assess chest recoil of the subject during the plurality of CPR chest compressions, (iv) compare the chest recoil of the subject with a chest recoil threshold, (v) when the chest recoil of the subject is greaterthan the chest recoil threshold, produce a second feedback signal and go to step (vii), (vi) when the chest recoil of the subject is less than the chest recoil threshold, produce a third feedback signal and go to step (vii), (vii) when the CPR stop signal is not received, return to step (ii), (viii) when the CPR stop signal is received, produce a fourth feedback signal, and a feedback unit connected to the CPR assessment system and configured to receive the feedback signals and issue CPR feedback to the rescuer.