FAULTY GAS MIXERS CAUSE RETROPOLLUTION OF MEDICAL GAS-SUPPLY PIPELINES

A defective Air/O-2 mixer of a ventilator located downstream of the ga s outlets of two pipelines is a potential cause of retropollution. Ret ropollution of O-2 with Air or vice versa carries a risk of either a) a hypoxic gas mixture delivery during anaesthesia, including O-2-N2O a dministration, when the O-2 pipeline supplies Air instead of O-2, Or b ) a hyperoxic gas mixture delivery in the intensive therapy unit for n eonates during administration of a Op-Air mixture, when the Air pipeli ne supplies O-2 instead of Air. A defective O-2/N2O flowmeter-mixer of an anaesthesia machine, with N2O flow control by O-2 through a differ ential pressure manometer, can cause retropollution of O-2 supply pipe line with N2O or vice versa. The prerequisite for retropollution is th e association of three events: build-up of a pressure difference betwe en the two gas lines; defective or absent back-flow check valve in the circuit of the gas at a lower pressure; one of the following defects: a) the pressure equilibrating valves of the mixer cannot amend the pr essure difference and allow a gas reflow at the gas mixture outlet; b) leak in the diaphragm of a pressure equilibrating valve; c) defective bypass supply valve. The optimal means for the recognition of a pipel ine contamination by another gas is the O-2 analyzer, especially in an aesthetic areas where the presence of N2O and Air carries the risk of a hypoxic gas mixture delivery. The mixer or flowmeter-mixer responsib le for retropollution can be recognized in plunging successively the v arious quick couplers underwater into a glass, while the others remain connected to their outlets and the mixer set at a O-2 concentration o f 50 vol% or the flowmeters set at a similar flow. In case of retropol lution, the gas reflow produces bubbles. It is recommended: a) in anae sthetic areas to set the O-2 pressure at about 0.2 bar above that of A ir and the latter of about 0.2 bar above that of N2O; b) in intensive therapy units for neonates, to set the Air pressure at about 0.2 bar a bove that of O-2; C) in all areas to disconnect from the gas outlets t he devices equipped with a mixer or a flowmeter-mixer when not in use.