Upper limits of resistance of apparatus for inhalation analgesia during labour: Response of mothers to increasing apparatus resistance

Abstract

In 37 mothers at the end of the first stage of labour, the effects were studied of deliberately increasing the inspiratory resistance of two types of inhalation apparatus, the Entonox apparatus and the Cardiff Penthrane Inhaler. The additional resistance consisted of one of four circular metal discs with a central hole of diameter varying from 5 mm to 9.5 mm. No mother rejected the apparatus when any of the resistances was added. Only on direct questioning did a small proportion of those who breathed through the higher resistances admit that they noticed any difference. The apparatus with the lowest resistance (9.5 mm orifice) satisfied all the physiological parameters studied. Changes in intrathoracic pressure were small and there was no significant effect upon frequency of breathing, mean ventilation, or peak flow. Mean ventilation was significantly reduced only when the highest resistance was added. It would seem that the Entonox apparatus and the Cardiff Penthrane Inhaler, even with added resistance, would satisfy the mothers and be acceptable physiologically. There is therefore no evidence that the resistance of the present inhalation apparatus is excessive. © 1974 John Sherratt and Son Ltd.