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Background: In locations where oxygen and anesthesia gas supplies are limited, and where circle systems are not practical, means to reduce fresh gas flow during maintenance of inhalational anesthesia are of potential value. We investigated whether a common transport breathing apparatus could be modified to allow interchange between Mapleson D (Map-D) and Mapleson A (Map A) configurations. Methods: A common Map-D transport system was converted to a Map-A system by switching positions of the exhaust valve and the elbow connector where fresh gas is delivered; these two breathing systems were compared in this study. The key question was whether rebreathing of CO2 could be eliminated at a lower fresh gas flow rate (FGF) with the Map-A design. A structured protocol was followed. Results: A mean decrease in FGF of 2.8 l/min was seen with the Map-A apparatus when compared with the Map-D ( P = 0.003). With no significant differences in physiologic or anesthetic variables, FGF/VE was significantly lower with the Mapleson A configuration than with the Mapleson D system design (1.1 vs. 1.8; P = 0.007). The extent to which FGF could be lowered when switching between Mapleson D and A systems correlated strongly with the patients' respiratory rate while under anesthesia ( r = 0.45, P < 0.01). Conclusions: Cost and resource savings can be realized through the use of a breathing system modification that achieves appropriate ventilation at lower fresh gas flows. [ABSTRACT FROM AUTHOR]