Spontaneous ventilation with the Bain anaesthetic system.

Measurements of ventilation and of inspired gas composition were made while volunteers breathed a non-anaesthetic gas through a Bain anaesthetic system. It was found that rebreathing occurred when the fresh gas flow was between two-and-a-half and three times the minute volume. Fresh gas flows at least three times the minute volume appear to be necessary to prevent rebreathing when using this system.