Respiratory effects of pressure support ventilation in spontaneously breathing patients under anaesthesia: Randomised controlled trial

Background: Lung volume loss is a major risk factor for postoperative respiratory complications after general anaesthesia and mechanical ventilation. We hypothesise that spontaneous breathing without pressure support may enhance the risk for atelectasis development. Therefore, we aimed at characterising whether pressure support prevents changes in lung function in patients breathing spontaneously through laryngeal mask airway.Methods: In this randomised controlled trial, adult female patients scheduled for elective gynaecological surgery in lithotomy position were randomly assigned to the continuous spontaneous breathing group (CSB, n = 20) or to the pressure support ventilation group (PSV, n = 20) in a tertiary university hospital. Lung function measurements were carried out before anaesthesia and 1 h postoperatively by a researcher blinded to the group allocation. Lung clearance index calculated from end-expiratory lung volume turnovers as primary outcome variable was assessed by the multiple-breath nitrogen washout technique (MBW). Respiratory mechanics were measured by forced oscillations to assess parameters reflecting the small airway function and respiratory tissue stiffness.Results: MBW was successfully completed in 18 patients in both CSB and PSV groups. The decrease in end-expiratory lung volume was more pronounced in the CSB than that in the PSV group (16.6 +/- 6.6 [95% CI] % vs. 7.6 +/- 11.1%, p = .0259), with no significant difference in the relative changes of the lung clearance index (-0.035 +/- 7.1% vs. -0.18 +/- 6.6%, p = .963). The postoperative changes in small airway function and respiratory tissue stiffness were significantly lower in the PSV than in the CSB group (p < .05 for both).Conclusions: These results suggest that pressure support ventilation protects against postoperative lung-volume loss without affecting ventilation inhomogeneity in spontaneously breathing patients with increased risk for atelectasis development.