Safety system for breathing apparatus

In an anesthesia apparatus having a breathing circuit gas supply means and rebreathing means comprising alternately selectable powered ventilator means and rebreathing bag means and means for selecting said rebreathing means, said apparatus being arranged for supplying gas to a spontaneously breathing patient using said bag means during first mode of operation or to artificially ventilate said patient by manually squeezing said bag means during a second mode of operation or to artificially ventilate said patient automatically by said powered ventilator during a third mode of operation, said bag means being selected by said selecting means during either said first or second mode of operation, said powered ventilator means being selected by said selecting means during said third mode of operation, said gas supply means and said selected rebreathing means being connected together by said selecting means for enabling gas from said supply means and from said selected rebreathing means to flow together at a first point, conduit means coupled to the lungs of a patient, inspiratory valve means connected to said gas supply and said selected rebreathing means for enabling gas from said first point to flow downstream through said conduit means to said lungs, expiratory valve means coupled between said conduit means and said rebreathing means for enabling gas to flow downstream from said lungs through said conduit means to said selected rebreathing means, and pressure relief means for venting gas out of said selected rebreathing means, the improvement comprising a safety system comprising alarm means and monitoring means comprising first pressure responsive switch means for monitoring gas pressure downstream of said first point and for establishing a first threshold pressure in excess of pressures normally encountered during spontaneous breathing but below a level at which patient injury could occur, second pressure responsive switch means for monitoring gas pressure downstream of said first point and for establishing a second threshold pressure greater than or equal to said first threshold pressure but below a level at which patient injury could occur, said first pressure responsive switch means enabling said alarm means only if the pressure monitored exceeds said first threshold pressure, said alarm means, when enabled, being responsive to said second pressure responsive switch means for providing an alarm signal only in the event that the pressure monitored exceeds said second threshold pressure for a predetermined period of time, in all three modes of operation of said apparatus, said first pressure responsive switch means disabling said alarm means whenever the pressure monitored is below said first threshold pressure.