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Right-to-left shunt (Qs/Qt) was measured by the SF6 and oxygen methods in 13 anaesthetized closed-chest dogs intubated with a double-lumen endobronchial tube. Collapse of the left lung increased Qs/Qt from 10% to 23%, suggesting that blood flow to the left lung had been reduced by about 60%. Increasing right lung mean airway pressure by the alteration of the inspiratory:expiratory time ratio or the application of PEEP produced a small but non-significant increase in Qs/Qt with significant increases in arterial and mixed venous carbon dioxide tensions, and arterial to right lung end-tidal carbon dioxide tension difference. Fluid loading during collapse increased cardiac output and pulmonary vascular pressures, but Qs/Qt did not differ significantly from the normovolaemic collapsed state. Increasing the right mean airway pressure in this condition had no effect on Qs/Qt or carbon dioxide tensions.

Saline lavage of the lungs of eight anaesthetized New Zealand White rabbits was performed to produce a surfactant-depletion model of the neonatal respiratory distress syndrome. A characteristic change in the shape of the pressure-volume curve occurred, with the appearance of an inflection point on the inspiratory limb. The animals were treated with high frequency oscillation (HFO) at 15 Hz and the results compared with those of a previously studied group of eight similar rabbits treated with controlled mechanical ventilation (CMV) with end-expiratory pressure equal to the pressure at the inflection point, but with the same mean airway pressure of 15 mm Hg. The animals treated with HFO had significantly higher arterial oxygen tensions and end-expiratory lung volumes than those treated with CMV, but there was no significant difference between hyaline membrane scores. Although there was no significant difference in mean survival times, five of the HFO treated animals showed reversion of the shape of their pressure-volume curves towards the prelavage configuration, suggesting a recovery of surfactant function.

A study was undertaken to compare CPAP/IMV systems shown to be in common use in the U.K. The most frequently used system (Servo 900B) has characteristics which are likely to increase the work of breathing. Recommendations are made regarding the theoretically most acceptable systems.