Effects of breathing circuit insulation on inspired gas conditioning and water vapour condensation: an in vitro study.

Abstract

Purpose: During general anesthesia, physiologic conditioning of inspired gases is bypassed. Mechanical ventilation with dry and cold gas from the central gas supply may lead to dehydration of the mucus membranes, cilia dysfunction, retention of secretions, and atelectasis. The use of metabolic fresh gas flow improves the conditioning of inspiratory gases but increases water vapour condensation within the breathing system. We sought to investigate the effects of breathing circuit insulation on the conditioning of inspired gases and the condensation of water vapour.

Methods: In this in vitro study, we used a mechanical nonheated, nonhumidified lung model with carbon dioxide (CO₂) insufflation. We tested foam, cotton, and polyester insulation (FOI, COI, and PEI) against control (noninsulated regular tubing). We measured temperature, absolute humidity (AH), and water vapour condensation after 120 min. We performed 8 measurements per group (total N = 32) and adjusted P values and confidence intervals (CIs) for multiple testing using Bonferroni-Holm adjustment.

Results: Regarding mean AH, FOI performed better than control. The mean (standard deviation [SD]) differences in AH between control and insulation were -0.63 (0.52) g·m^-3 H₂O for PEI (adjusted 95% CI, -1.42 to 0.17; P = 0.26), -0.63 (0.74) g·m^-3 H₂O for COI (adjusted 95% CI, -1.42 to 0.17; P = 0.26), and -1.13 (0.35) g·m^-3 H₂O for FOI (adjusted 95% CI, -1.92 to -0.33; P < 0.001). The mean temperature was higher in insulated circuits. The mean (SD) difference compared to control was 0.42 (0.28) °C for PEI (adjusted 95% CI, 0.05 to 0.79; P = 0.002), 0.62 (0.26) °C for COI (adjusted 95% CI, 0.25 to 0.99; P < 0.001), and -1.07 (0.14) °C for FOI (adjusted 95% CI, 0.70 to 1.44; P < 0.001). Condensation of water vapour was lower in insulated breathing circuits compared with control.

Conclusion: Foam-based insulation was the most effective form of insulation of the breathing circuit to increase temperature and AH of inspired gases and to reduce water vapour condensation. Overall, the results of this in vitro study support the principle of breathing circuit insulation as a method for inspired gas conditioning during the use of metabolic flow anesthesia.