Time to a 90% change in gas concentration: a comparison of three semi-closed anesthesia breathing systems

The speed with which gas concentration can be changed in the anesthesia breathing system affects the rate of denitrogenation, anesthesia induction, and emergence. Breathing system design also affects the speed at which gas concentration can be changed during maintenance. In this study, we sought to determine the speed of changes in gas concentration in modern semi-closed breathing systems. We hypothesized that equilibrium would be reached most quickly in breathing systems with smaller volume, and at high fresh gas flows.

Three anesthesia workstations were studied in vitro: the ADU (Datex-Ohmeda, now a division of GE Medical, Madison, WI), the Fabius GS with a COSY-1 breathing system (Draeger Medical, Telford, PA), and the Aestiva (Datex-Ohmeda, now a division of GE Medical, Madison, WI). The breathing systems were connected to a test lung and ventilated with air. The fresh gas flow was then changed to oxygen at rates of 1, 2, 4, 6, or 8 L/min, and times to 50%, 63%, 66%, 75%, and 90% change in oxygen concentration within the test lung were recorded. Ten trials were performed for each breathing system, at each fresh gas flow. The results were analyzed with a split-plot analysis of variance followed by post hoc tests with a Bonferroni correction.

At flows of 6 or 8 L/min, times to equilibration did not differ among the three breathing systems. At flows of 1 to 2 L/min, the gas concentration changed faster with the ADU than with the Aestiva or Fabius (P < 0.001). At 4 L/min, the ADU was faster than Aestiva (P < 0.001), but not Fabius.

We concluded that, other than fresh gas flow rate, breathing system volume has the biggest effect on time to equilibrium when the composition of the fresh gas inflow is changed. The position of components (e.g., valves, carbon dioxide absorber, fresh gas inlet, ventilator bellows or piston) within the breathing system has a less pronounced effect.