Increasing fresh gas flows of oxygen in a circle absorber system for preoxygenation. Is it really efficacious?

Introduction: Preoxygenation is a routine practice prior to induction of general anaesthesia. The fresh gas flow (FGF) of oxygen used is an important factor for an effective preoxygenation, and the end tidal oxygen fraction (FeO2) is the most sensitive indicator for its adequacy. This study determines the ideal time for preoxygenation at different FGFs, and the effect of increasing the FGF on the time for preoxygenation. Materials and Methods: Ninety patients of ASA grade I/II, aged 18-65yr, weighing 50-80 kg and scheduled for elective surgery under general anaesthesia were divided by a computer generated random allocation table into three study groups (30 each) - Group 8, Group 10 and Group 12, according to the FGF used. A circle absorber system and a face mask were used for preoxygenation with one of the FGFs and tidal volume breathing technique. The values of FeO2 and blood peripheral oxygen saturation (SpO2) were recorded at prefixed intervals. The primary outcome variable i.e., time (sec) required for achieving FeO2 of ?90% was recorded. The secondary outcome variable i.e., the effect on the time to reach FeO2 ?90% with change in FGF was noted. All continuous variables across the 3 groups were compared by applying ANOVA test. Results: All the groups showed statistically significant difference among each other for the time to reach FeO2 ?90% (P= 0.000). The Mean (±SD) time taken to achieve FeO2 ?90% was, Group 8= 284.07±36.92 sec, Group 10= 199.87±20.71 sec, and Group 12= 140.67±27.88 sec. The secondary outcome is that the Group 12 took the least time and Group 8 took the maximum time. Conclusions: The time for preoxygenation ranges between 2 to 5 min depending on the FGF, and this time decrease with an increase in the FGF.