Mechanical ventilation strategies influence pulmonary mechanics in atelectasis-prone rabbits

Introduction: Ventilation strategies can modify the course of acute lung injury. Strategies that permit alveolar de-recruitment and cyclic overdistension, such as conventional mechanical ventilation with low PEEP (CMV-LP) can induce progressive injury, whereas strategies that promote lung recruitment, such as high frequency oscillatory ventilation (HFOV), are associated with better short term outcomes. We hypothesized that HFOV and CMV with high PEEP (CMV-HP) would be associated with more favorable pulmonary mechanics in comparison to a CMV-LP strategy. Methods: Thirty-three New Zealand White rabbits were anesthetized and instrumented with a tracheostomy and vascular catheters, and ventilated with a FiO₂ of 1.0. Lung injury was induced by repeated saline lavage. Following injury (PaO₂<100torr), all animals underwent HFOV (Paw: 16cmH₂O, IT: 33%) with periods of dynamic sustained inflation (Paw: 30cmH₂O) for 15 sec, until PaO₂>300 torr. The animals were then randomized to 1) continue HFOV (n=8), 2) CMV-HP (n=9, TV: 10ml/kg, PEEP: 10 cm/H₂O) or 3) CMV-LP (n=8, TV: 10ml/kg, PEEP: 2cm/H₂O). A group of uninjured animals (n=8) ventilated with a PEEP of 5 cm/H₂O served as controls. Static pressure-volume curves were obtained at randomization and at the end of the experiment. Prior to sacrifice, animals underwent chest fluoroscopy at a distending airway pressure of 16cmH₂O. A lung area index (a 2-d projection image of lung volume) was derived by measuring the lung area on the digital images using dedicated software. Results: Values are means ± SD. Comparisons made by Kruskall Wallis one-way analysis of variance on ranks, with post-hoc multiple comparisons by the Dunn's method.*p<0.05 vs control. vs control. 1 p<0.05 vs CMV-LP Control HFOV CMV-HP CMV-LP Static compliance (ml/cmH₂O) 1.15±0.1 0.69±0.1 1 0.57±0.2*1 0.3±0.06*Lung area index (cm²/kg) 10.1±1.7 8.79±1.5 1 8.83±2.2 1 6.15±1.1*Conclusions: Strategies that promote lung recruitment, such as HFOV and CMV-HP, result in more favorable pulmonary mechanics than CMV-LP in acutely injured, atclectasis-pronc animals after 4 hours of mechanical ventilation. HFOV animals exhibited better lung compliance than those in the CMV-HP group.