Embodiments of the present concept are directed to medical devices for use by a rescuer who is caring for a patient and includes a bottom device for use with a top device to measure the depth of Cardio Pulmonary Resuscitation (CPR) chest compressions delivered to the chest of a patient. The top device is intended for placement on the chest of the patient and has a top mechanism that is moveable up and down as the chest compressions are delivered to the patient. The bottom device includes a generally elongate member having a handle at one end and a bottom mechanism near the opposite end. The elongate member is structured to be placed under the patient during delivery of CPR. The top mechanism and the bottom mechanism cooperate to generate a value for a net depth of the compressions of the patient chest with reference to each other.