Proposed upper limit of respiratory resistance for inhalation apparatus used in labour.

A specification for the upper limit of the inspiratory resistance for inhalation-analgesia apparatus used in labour has been derived by considering the additional respiratory power required to overcome the apparatus resistance. It is recommended that the additional power required to inspire through the apparatus, with the ventilation occurring in mothers having analgesia, should not be greater than the additional power involved in breathing without apparatus, at the greater ventilation found in mothers without analgesia. From data in the literature it is deduced that this general requirement will be satisfied for 99.9% of the population if the power dissipated in the apparatus is less than 1.68 watts at a ventilation of 40 l./min. This specific requirement is just met by a laminar-flow resistance of 0.26 cm H20/(l./min). The Entonox apparatus and the Cardiff Penthrane Inhaler meet this requirement easily although they do not conform to British Standard Specification 4272, Part 2 (1968).