Effects of different fresh gas flows with or without a heat and moisture exchanger on inhaled gas humidity in adults undergoing general anaesthesia: A systematic review and meta-analysis of randomised controlled trials

Background: The minimum inhaled gas absolute humidity level is 20 mgH2O l-1 for short-duration use in general anaesthesia and 30 mgH2O l-1 for long-duration use in intensive care to avoid respiratory tract dehydration. Objective: The aim is to compare the effects of different fresh gas flows (FGFs) through a circle rebreathing system with or without a heat and moisture exchanger (HME) on inhaled gas absolute humidity in adults undergoing general anaesthesia. Design: Systematic review and meta-analyses of randomised controlled trials. We defined FGF (l min-1) as minimal (0.25 to 0.5), low (0.6 to 1.0) or high (≥2). We extracted the inhaled gas absolute humidity data at 60 and 120 min after connection of the patient to the breathing circuit. The effect size is expressed as the mean differences and corresponding 95% confidence intervals (CI). Data Sources: PubMed, EMBASE, SciELO, LILACS and CENTRAL until January 2017. Results: We included 10 studies. The inhaled gas absolute humidity was higher with minimal flow compared with low flow at 120 min [mean differences 2.51 (95%CI: 0.32 to 4.70); P = 0.02] but not at 60 min [mean differences 2.95 (95%CI: -0.95 to 6.84); P = 0.14], and higher with low flow compared with high flow at 120 min [mean differences 7.19 (95%CI: 4.53 to 9.86); P < 0.001]. An inhaled gas absolute humidity minimum of 20 mgH2O l-1 was attained with minimal flow at all times but not with low or high flows. An HME increased the inhaled gas absolute humidity: with minimal flow at 120 min [mean differences 8.49 (95%CI: 1.15 to 15.84); P = 0.02]; with low flow at 60 min [mean differences 9.87 (95%CI: 3.18 to 16.57); P = 0.04] and 120 min [mean differences 7.19 (95%CI: 3.29 to 11.10); P = 0.003]; and with high flow of 2 l min-1 at 60 min [mean differences 6.46 (95%CI: 4.05 to 8.86); P < 0.001] and of 3 l min-1 at 120 min [mean differences 12.18 (95%CI: 6.89 to 17.47); P < 0.001]. The inhaled gas absolute humidity data attained or were near 30 mgH2O l-1 when an HME was used at all FGFs and times. Conclusion: All intubated patients should receive a HME with low or high flows. With minimal flow, a HME adds cost and is not needed to achieve an appropriate inhaled gas absolute humidity.