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

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.