A Cardiac Pulmonary Resuscitation device arranged to perform Cardiac Pulmonary Resuscitation on a patient, the device comprising: - a supporting structure (L1, L2, F) comprising first and second legs (L1, L2) shaped to accommodate space for the patient's thorax between the first and second legs (L1, L2), wherein the first and second legs (L1, L2) each has a clamp mechanism (CL1, CL2) arranged for engagement with corresponding clamp mechanisms on each side of an associated backboard, so as to allow an operator to clamp the supporting structure (L1, L2, F) onto the associated backboard, - a compression box comprising an enclosure containing: - a plunger mechanism with a contact pad projecting downwards from the enclosure, and arranged to provide a chest compression force to the patient's chest, and - a processor arranged to control the plunger mechanism to perform Cardiac Pulmonary Resuscitation on the patient, and the device further comprising: - a height adjustment mechanism arranged to fix a height (h) of the compression box relative to the supporting structure (L1, L2, F), wherein in a first operating state of the device, upon clamping of the supporting structure (L1, L2, F) onto the associated backboard, the height adjustment mechanism is arranged to allow the compression box to slide by gravity in relation to the supporting structure (L1, L2, F), so as to press the contact pad against the patient's chest with a force determined by the mass of the compression box, characterised in that the device further comprises - a spring mechanism arranged to force the compression box downwards, so as to press the contact pad against the patient's chest with a predetermined force, in the first operating state. - a supporting structure (L1, L2, F) comprising first and second legs (L1, L2) shaped to accommodate space for the patient's thorax between the first and second legs (L1, L2), wherein the first and second legs (L1, L2) each has a clamp mechanism (CL1, CL2) arranged for engagement with corresponding clamp mechanisms on each side of an associated backboard, so as to allow an operator to clamp the supporting structure (L1, L2, F) onto the associated backboard, - a compression box comprising an enclosure containing: - a plunger mechanism with a contact pad projecting downwards from the enclosure, and arranged to provide a chest compression force to the patient's chest, and - a processor arranged to control the plunger mechanism to perform Cardiac Pulmonary Resuscitation on the patient, and - a plunger mechanism with a contact pad projecting downwards from the enclosure, and arranged to provide a chest compression force to the patient's chest, and - a processor arranged to control the plunger mechanism to perform Cardiac Pulmonary Resuscitation on the patient, and - a height adjustment mechanism arranged to fix a height (h) of the compression box relative to the supporting structure (L1, L2, F), wherein in a first operating state of the device, upon clamping of the supporting structure (L1, L2, F) onto the associated backboard, the height adjustment mechanism is arranged to allow the compression box to slide by gravity in relation to the supporting structure (L1, L2, F), so as to press the contact pad against the patient's chest with a force determined by the mass of the compression box, characterised in that the device further comprises - a spring mechanism arranged to force the compression box downwards, so as to press the contact pad against the patient's chest with a predetermined force, in the first operating state.