The correct positioning of the one-way valves is essential in draw-over anaesthesia:

• Non-return valve, between the vaporiser and the SIB/bellows. It prevents back flow into the vaporiser during SV or IPPV
• Non-rebreathing patient inflating valve, e.g. Laerdal valve (Fig 1), at the patient end of the breathing system, downstream of the SIB/bellows.
  
  It allows gas to be breathed in from the breathing system and breathed out to atmosphere. It prevents air from being drawn in via the exit port during spontaneous breathing. It allows spontaneous and controlled ventilation without adjustment. It needs regular cleaning to prevent stickiness or sterilisation if used in an infected patient without a filter in the circuit.
  
  Remember: if the bellows are used, the disc valve downstream of the bellows must be disabled with a magnet (Fig 2) when using this modern valve
  
• New valves. Gradian Health Systems (UAM) and Diamedica (DPA 02 and Glostavent) have designed newer breathing valves for use with their equipment. This eliminate the valve by the patient airway and allow for the use of positive end-expiratory pressure (PEEP).