The Newton valve revisited: an in-vitro study of ventilator circuit dead space

A laboratory study was conducted to investigate the volume (length) of the ventilator circuit dead space (VCD) tubing at which dilution of an inspired gas by ventilator driving gas first occurs using three lung models. Various lengths of two VCD tubing materials [Portex (Sims Portex Ltd, Kent, UK) 10 mm bore smooth-walled silicon and Intersurgical (Wokingham, Berks, UK) 22 mm corrugated plastic] were interposed between a T-piece circuit and Nuffield 200 ventilator (Penlon, Abingdon, Oxon, UK) with a Newton valve attached. Dilution of inspired gas by the ventilator driving gas was first detected during ventilation of infant and child lung models when the VCD was 7 ml (6 cm) and 77 ml (102 cm), respectively, using Portex tubing, and 24 ml (6.5 cm) and 105 ml (29 cm), respectively, using Intersurgical tubing. No dilution occurred using the neonatal model. Dilution of the inspired anaesthetic gases by ventilator driving gas may occur in paediatric practice if the VCD volume (length) is inadequate. This risk is greatest in the child.