Dynamics of re-expansion of atelectasis during general anaesthesia

A major cause of impaired gas exchange during general anaesthesia is atelec tasis, causing pulmonary shunt. A 'vital capacity' (VC) manoeuvre (i.e. inf lation of the lungs up to 40 cm H2O, maintained for 15 s) may re-expand ate lectasis and improve oxygenation. However, such a manoeuvre may cause adver se cardiovascular effects. Reducing the time of maximal inflation may impro ve the margin of safety. The aim of this study was to analyse the change ov er time in the amount of atelectasis during a VC manoeuvre in 12 anaestheti zed adults with healthy lungs. I.v, anaesthesia with controlled mechanical ventilation (VT 9 (SD I) ml kg(-1)) was used. For the VC manoeuvre, the lun gs were inflated up to an airway pressure (Paw) of 40 cm H2O. This pressure was maintained for 26 s. Atelectasis was assessed by analysis of computed x-ray tomography. The amount of atelectasis, measured at the base of the lu ngs, was 4.0 (SD 2.7) cm(2) after induction of anaesthesia. The decrease in the amount of atelectasis over time during the VC manoeuvre was described by a negative exponential function with a time constant of 2.6 s. At an ins pired oxygen concentration of 40%, Pa-O2 increased from 17.2 (4.0) kPa befo re to 22.2 (6.0) kPa (P = 0.013) after the VC manoeuvre. Thus in anaestheti zed adults undergoing mechanical ventilation with healthy lungs, inflation of the lungs to a Paw of 40 cm H2O, maintained for 7-8 s only, may re-expan d all previously collapsed lung tissue, as detected by lung computed tomogr aphy, and improve oxygenation. We conclude that the previously proposed tim e for a VC manoeuvre may be halved in such subjects.