Resistance, reverse flow and opening pressure of unidirectional valves.

Based on a new preliminary standard of the 'Comité Européen de Normalisation', the following unidirectional valves were tested with regard to resistance, opening pressure, reverse flow and dislocation: the Dräger inspiratory and expiratory valves, the Engström inspiratory valve, the Ohmeda valve, the Siemens Ventilator 710 inspiratory and expiratory valves, the Siemens Ventilator 900C unidirectional valve of the absorber and the Megamed 700 inspiratory and expiratory valves of the circle system 219 (Megamed 700 CS 219). The following valves fulfilled all Comité Européen de Normalisation requirements: Dräger inspiratory and expiratory valves, Siemens 900 absorber valve and Megamed 700 CS 219 inspiratory valve. The Siemens 710 valve and the Megamed 700 CS 219 expiratory valve did not meet the requirements for flow resistance. The Ohmeda and Siemens 710 valves and the Engström inspiratory valve did not fulfil the Comité Européen de Normalisation requirements for reverse flow. In addition, the Engström inspiratory valve did not comply with the dislocation test. The requirements for the opening pressure were met by all the valves tested. Valves with the disc in a horizontal position achieved better results than those with the disc in a vertical position. These measurements, showing the differences in the performance of various types of valves confirm the feasibility of the standards proposal.