Atelectasis is inversely proportional to transpulmonary pressure during weaning from ventilator support in a large...

In mechanically ventilated, lung injured, patients without spontaneous breathing effort, atelectasis with shunt and desaturation may appear suddenly when ventilator pressures are decreased. It is not known how such a formation of atelectasis is related to transpulmonary pressure (P_L ) during weaning from mechanical ventilation when the spontaneous breathing effort is increased. If the relation between P_L and atelectasis were known, monitoring of P_L might help to avoid formation of atelectasis and cyclic collapse during weaning. The main purpose of this study was to determine the relation between P_L and atelectasis in an experimental model representing weaning from mechanical ventilation.

Dynamic transverse computed tomography scans were acquired in ten anaesthetized, surfactant-depleted pigs with preserved spontaneous breathing, as ventilator support was lowered by sequentially reducing inspiratory pressure and positive end expiratory pressure in steps. The volumes of gas and atelectasis in the lungs were correlated with P_L obtained using oesophageal pressure recordings. Work of breathing (WOB) was assessed from Campbell diagrams.

Gradual decrease in P_L in both end-expiration and end-inspiration caused a proportional increase in atelectasis and decrease in the gas content (linear mixed model with an autoregressive correlation matrix; P < 0.001) as the WOB increased. However, cyclic alveolar collapse during tidal ventilation did not increase significantly.

We found a proportional correlation between atelectasis and P_L during the 'weaning process' in experimental mild lung injury. If confirmed in the clinical setting, a gradual tapering of ventilator support can be recommended for weaning without risk of sudden formation of atelectasis.