A method of controlling the amount of compressed gas used for driving a reciprocating apparatus for cardio-pulmonary resuscitation (CPR) comprising a valve means for controlling the provision of driving gas comprises operation of the valve means during the compression phase to stop provision of driving gas, which operation is separated in time from the venting of the driving gas from the apparatus at the end of the compression phase. Also disclosed are a CPR apparatus operated by the method a method of compression depth sensing.
