The valve system in the mouth piece of two spacer devices was analysed. Pressures required to open and close the non-rebreathing valve were very low (less than 0.1 kPa). Inspiratory flow requirements were within physiological limits for infants' normal tidal breathing. Expiratory flow requirements varied significantly, and the flow required to prevent rebreathing from the chamber may exceed the physiological flow limits for normal tidal breathing.
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