Lung mechanics and airway reactivity in sheep during development of oxygen toxicity

The causes of respiratory distress in O₂ toxicity are not well understood. The purpose of this study was to better define the airway abnormalities caused by breathing 100% O₂. Sheep were instrumented for measurements of dynamic compliance (Cdyn), functional residual capacity by body plethysmography (FRC), hemodynamics, and lung lymph flow. Each day Cdyn and FRC were measured before, during, and after the application of 45 min continuous positive airway pressure (CPAP) at 15 cmH₂O. The amount of aerosol histamine necessary to reduce Cdyn 35% from baseline (ED₃₅) was measured each day as was the response to aerosol metaproterenol. Cdyn decreased progressively from 0.083 ± 0.005 (SE) l/cmH₂O at baseline to 0.032 ± 0.004 l/cm H₂O at 96 h of O₂. Surprisingly, FRC did not decrease (1,397 ± 153 ml at baseline vs. 1,523 ± 139 ml at 96 h). The ED₃₅ to histamine did not vary among days or from air controls. Metaproterenol produced a variable inconsistent increase in Cdyn. We also measured changes in Cdyn during changes in respiratory rate and static pressure-volume relationships in five other sheep. We found a small but significant frequency dependence of compliance and an increase in lung stiffness with O₂ toxicity. We conclude that in adult sheep O₂ toxicity reduces Cdyn but does not increase airway reactivity. The large reduction in Cdyn in O₂ toxicity results from processes other than increased airway reactivity or reduced lung volume, and Cdyn decreases before the development of lung edema.