EFFICIENCY, ARGON ACCUMULATION, AND ACCUR ACY OF ANESTHETIC GAS MONITORING DURING MECHANICAL VENTILATION IN A CIRCLE SYSTEM USING AN OXYGENCONCENTRATOR

Oxygen (O-2) for clinical application is generally provided from eithe r a central gas supply via a hospital pipeline system or is delivered to the working place in cylinders as compressed gas. An alternative so urce is the one-site generation of O-2 from air using O-2 concentrator s based on molecular sieve technology. Whereas O-2 concentrators for a naesthesia in remote areas or underdeveloped countries are wide-spread , in Germany their use is common in neither hospitals nor anaesthesiol ogical practice. The maximum O-2 content produced by concentrators is 96% with about 4% argon (Ar) and minimal amounts of nitrogen and other noble gases. The total O-2 production is systematically limited, and therefore, the delivered concentration decreases with higher flows. Th ere is also a potential possibility of Ar accumulation in rebreathing anaesthesia systems with reduced fresh gas flow. We investigated the e fficiency and potential disadvantages of using O-2 concentrators in an aesthesia and the influence of Ar on the accuracy of anaesthetic gas m onitors. Methods. The efficiency of the concentrator was characterised as O-2 concentration depending on delivered gas flow. The degree of A r accumulation in rebreathing anaesthesia systems was obtained with an O-2-consuming and CO2-producing metabolic lung model consisting of a water-cooled burning chamber with an adjustable gas jet. The expirator y CO2 content was set to approximate ly 7%, representing an O-2 consum ption of 350 ml/min while ventilating the model with 500 mi tidal volu me and 10 breaths/min. The inspiratory O-2 concentration was adjusted to 35% or 70%; the fresh gas flow was set to 0.5 or 1 1/min. The accur acy of different types of anaesthesia monitors for O-2, CO2, volatile anaesthetics, and nitrous oxide in the presence of Ar was checked in c omparison with data obtained with a mass spectrometer. To evaluate the usefulness of O, concentrators for anaesthetic practice, the function of a respirator-concentrator unit was investigated in clinical routin e for 8 weeks. Results. The efficiency of the concentrator is flow-rat e dependent: O-2 concentrations higher than 90% are only achieved with flow rates below 51/min and decrease to values lower than 50% at 121/ min or more. Ar accumulation occurred in rebreathing circuits but exce eded values higher than 10% only under minimal-flow conditions (fresh gas flow 0.51/min). Ar did not influence the accuracy of common anaest hetic gas monitors. In clinical practice, the performance of anaesthes ia using O-2 from an O-2 concentrator generated no additional problems .Conclusions. For the future, the use of O-2 concentrators for anaesth esia seems to be a practicable alternative to compressed O-2 from cyli nders. The main application could be in small operating units or anaes thesia practices. The method is safe and without additional risk of hy poxia, even in rebreathing systems and closed circuits, when the O-2 c oncentration in the inspired gas is measured.