Does keeping vulnerable lungs expanded during ventilation reduce injury in surfactant-deficient rabbits?

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The study addressed a controversy: should high frequency oscillatory ventilation with an active expiratory phase (HFO-A) use low ventilator pressures or high alveolar volumes to minimize injury in lungs prone to atelectasis, or collapse?

In a controlled animal experiment, researchers ventilated 20 anesthetized, tracheostomized rabbits whose lungs had been made surfactant-deficient by lung lavage. They used one of three approaches: HFO-A at a high lung volume (HFO-A/HI), HFO-A at a low lung volume (HFO-A/LO), or conventional mechanical ventilation (CMV). All rabbits received 100% oxygen for 7 h. The researchers examined oxygenation, lung mechanics, and lung pathology.

In the HFO-A/HI rabbits, arterial oxygen tension was kept greater than 350 mm Hg. Their mean lung volume above functional residual capacity (FRC) was 23.4 ml/kg. In the HFO-A/LO and CMV rabbits, arterial oxygen tensions were 70 to 100 mm Hg. Mean lung volumes were 7.8 and 4.3 ml/kg, respectively.

After 7 h of ventilation, total respiratory system pressure-volume curves (P-V curves) showed no change from baseline in the HFO-A/HI group. The low-lung-volume groups—HFO-A/LO and CMV—showed diminished hysteresis of their P-V curves, lower total respiratory system compliance, more hyaline membranes, and severe airway epithelial damage. All these changes were statistically significant, with p < 0.05.

The researchers concluded that maintaining alveolar volume is a key mechanism for preventing lung injury during mechanical ventilation of lungs prone to atelectasis. They stated that, for optimal outcomes with high frequency oscillatory ventilation, alveoli must be actively reexpanded and then kept expanded using appropriate mean airway pressures.