Effect of reducing alveolar surface tension on stress failure in pulmonary capillaries

We previously showed that when pulmonary capillaries are exposed to high transmural pressures, stress failure of the blood-gas barrier occurs. It has been suggested that the surface tension of the alveolar lining layer may protect against stress failure because at high transmural pressures the capillaries bulge into the alveolar spaces. To test this hypothesis, we abolished the gas-liquid surface tension of the alveoli by filling rabbit lungs with normal saline. The lungs were then perfused at capillary transmural pressures of 32.5 or 52.5 cmH2O for 1 min with autologous blood, the blood was washed out with a saline-dextran mixture (3 min), and the lungs were fixed for electron microscopy with buffered glutaraldehyde; all perfusions were done at the same pressure. The frequency of breaks was measured in the capillary endothelial layer, alveolar epithelial layer, and basement membranes, and the data were compared with those in air-filled lungs at the same capillary transmural pressure and lung volume. We found that the frequency of breaks in the endothelium was not significantly different between air and saline filling and that there were fewer breaks in the outer boundary of the epithelial cells. By contrast, after saline filling, a larger number of breaks were seen in the inner boundary of the epithelium. The frequency of disruptions of the inner boundary of the epithelium was closely correlated with the volume of edema fluid collected at the trachea during the perfusion. These breaks in the inner boundary of the epithelium had not previously been seen in air-filled lungs exposed to the same pressures. The results suggest that abolishing the surface tension of the alveolar lining layer removes support from parts of the blood-gas barrier when the capillaries are subjected to a high transmural pressure but that not all portions of the barrier are subjected to the same forces.