Multiple respiratory gas monitoring causes changes of inspired oxygen concentration in closed anesthesia system

Summary

Effect of multiple respiratory gas monitoring (MRGM) on inspired concentration of oxygen in circuit system during closed anesthesia was studied in 51 adult patients scheduled for abdominal surgery. Required flow rate of fresh oxygen (OFR), inspired oxygen concentration (FiO₂) and oxygen saturation of pulse oximeter (SpO₂) were measured continuously. Patients were equally divided into three groups at random, group C (no MRGN used), group M₁ (using MRGM with its tail gas returned to circuit system), group M₂ (using MRGM without tail gas returned). The results revealed that during 180 min of closed anesthesia, OFR required in group C and M₁ were about 200 - 230 ml/min, and in group M₂ it was about 400 ml/min. In group C FiO₂ decreased by about 10 % after 60 min of closed anesthesia (P< 0. 01, 60 min vs 0 min) and then stayed stable at this level. In group M₁, FiO₂ decreased by 16 % at 60 min and 34 % at 180 min and the decrease was significantly greater than that in group C (P< 0.01).In group M₂, FiO₂ remained constant during closed anesthesia, which was much higher than those in group C and M₁. The tail gas of Capnomac Ultima MRGM contained less oxygen than its sample gas drawn from circuit system simultaneously.