Effects of intermittent positive-pressure ventilation on lungs of normal rabbits

We ventilated 10 healthy young rabbits with warm moist air at 20 cm H2O peak airway pressure and compared their pulmonary histology to that of 10 controls. The histological changes were analysed using the Leitz Texture Analysis System. Four hundred high-power fields (240 micrometers X 240 micrometers) were scanned from each section and the mean proportions of air space, interstitium and oedema determined. Calculated bicarbonate decreased significantly in both groups (P < 0.05); this was accompanied by a modest decrease in pHa in the ventilated group. The PaO2 and PaCO2 were maintained within the normal range. The blood pressure decreased in both groups. Seven of 10 ventilated rabbits had interstitial widening while only 1 of 10 controls had a similar lesion (chi 2 = 5.21, P = < 0.05). The area occupied by interstitium ranged from 26 to 46% (mean 37.6%) in controls and 18 to 81% (mean 45.4%) in ventilated rabbits. There was a lack of correlation between clinical and histopathological findings. While 6 of 10 ventilated rabbits had intra-alveolar oedema and a decrease in air space to less than 50% of total area, none developed hypoxaemia. Two of these 6 rabbits also had pHa less than 7.35 by the end of the study, yet all were clinically stable. These data indicate that intermittent partial-pressure ventilation with warm moist air may result in pulmonary parenchymal damage within 6 h in healthy rabbits.