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Background : EAR circuit of Enclosed Afferent Reservior is a circuit modified from the Ampleson A (Magill) system. It can be used effectively in both the control and spontaneous ventilation circuit has been studied, however, no study on the spontaneous ventilation mode has been conducted.
Objective : To find the proper ratio of fresh gas flow (FGF) to minute ventilation (MV) in the EAR-spontaneous ventilation mode.
Design : Descriptive study.
Setting : Srinagarind Hospital
Material&Method : We used a hand-made EAR circuit to test of an artificial lung to produce carbon dioxide (CO2). While the FGF per MV was gradually reduced from 2 times to 0.25 times, ETCO2 and PIminCO2 obtained from the capnograph was recorded. The FGF:MV and PIminCO2 were calculated to find the proper FGF:MV ratio to produce rebreathing by the Kain & Nunn and Humphrey criteria.
Results: When decreasing the FGF:MV ratio from 2 times to 0.25 times, the ETCO2 and PIminCO2 increased from 33 mmHg to 97 mmHg and from 0 mmHg to 9 mmHg. According to the Kain and Nunn criteria, rebreathing occurred when the FGF:MV ratio was 0.83, while it was 0.42 by the Humphrey criteria.
Conclusion : Our hand-made EAR spontaneous ventilation circuit in the artificial lung model contributed to rebreathing when the FGF:MV ratio was 0.83 by the Kain and Nunn criteria and 0.42 the Humphrey criteria. Nevertheless, before recommending this system for use in patients, further study is required to find the proper ratio for humans.
Key words: Equipment, EAR system, CO2 elimination, Rebreathing.