Open scavenging systems.
A mathematical model was developed to cover the dynamic characteristics of open scavenging systems. The performance of such a device depends on the expiratory tidal volume (VT), the scavenging flow (Vs) and the mode of exhalational volume build-up. A widely used open scavenging system was tested by using carbon dioxide in oxygen as a marker gas. Capnography revealed gross turbulence in the expiratory reservoir, which tends to increase the polluted gas volumes and loads extra requirements onto the scavenging equipment. Increases in the scavenging flow or the expiratory reservoir volume are the aids in obtaining satisfactory safety margins for pollution-free scavenging of surplus anaesthetic gases. The theoretical basis of a new method to determine the expiratory time constant of the respiratory system is also presented.