HEALTHY LUNGS TOLERATE REPETITIVE COLLAPSE AND REOPENING DURING SHORTPERIODS OF MECHANICAL VENTILATION

The possible occurrence of lung damage if alveolar units are allowed t o collapse and reopen breath by breath during mechanical ventilation w ith normal tidal volumes was investigated. Anaesthetised, paralysed, o pen chest rabbits were subjected to either intrathoracic negative (NEE P; n=6) or positive (PEEP; n=6) end-expiratory pressure during volume controlled mechanical ventilation. Both experimental settings were pre ceded by a 30 min control period and followed by a 30 min recovery per iod during which a PEEP of 0.2 kPa was maintained. Pao(2) and pulmonar y compliance deteriorated significantly in the NEEP group during the e xperimental period and compared to ventilation with PEEP. Partial rest oration of lung mechanics and blood gases was achieved during the reco very period. After an alveolar recruitment manoeuvre, this recovery wa s complete. Lung clearance studied by depositing an aerosol of technet ium-99m-lahelled dieth) lenctriaminc pentaacetate (Tc-99m-DTPA in the alveoli, was significantly faster during ventilation with NEEP compare d to the PEEP group (P=0.0002) as well as the control period (P=0.0029 ). It did not recover completely during the recovery period but remain ed significantly faster. Light microscopic histology was normal in bot h groups with no evidence of inflammation or epithelial disruption. We conclude that previously healthy rabbit lungs show only a transient d isturbance of lung mechanics and blood gases with repetitive collapse and re-expansion. The integrity of the alveolar microstructure is pres erved. The disturbance in the alveolo-capillary permeability persists and may indicate surfactant related alveoio-capillary barrier dysfunct ion.