Impacts of spontaneous breathing during lung protective ventilation in experimental acute lung injury: Spontaneous breathing does not always good

INTRODUCTIONS. Spontaneous breathing (SB) preserved during lung protective ventilation might act differently, depending on different transpulmonary pressure (PTP). Furthermore, even though some investigators have claimed that SB might generate potentially injurious PTP, we have no convincing data to prove the downside of SB, so far. OBJECTIVES. The goal of this study was to test our hypothesis that (1) compared with low tidal volume ventilation (LTV) preserved SB gently, LTV with strong SB may lead to improve lung aeration, resulting in better respiratory function and reduction of lung injury; and (2) potentially injurious PTP might be generated by SB, which could lead to lung injury, even though lung protective ventilation is applied. METHODS. Thirty-two lavages-injured rabbits were randomized to receive 4-h lung protective ventilations with (a) LTV group: tidal volume (VT) of 6 ml/kg, (b) LTV+ SBstrong group: VT of 6 ml/kg with SB preserved substantially, (c) moderate VT ventilation (MTV) group: VT of 7-9 ml/kg, and (d) MTV+ SBstrong group: VT of 7-9 ml/kg with SB preserved substantially. All groups had the same positive end-expiratory pressure (PEEP) of 8 cmH2O and plateau pressure (Pplateau) of<30 cmH2O. Respiratory variables including gas exchange, lung compliance and hemodynamic variables were measured every 60 min. Differential cell counts in bronchoalveolar fluid (BALF) and distribution of lung aeration and collapse using lung histology were analyzed. RESULTS. LTV+ SBstrong had the most favorable oxygenation (285 ± 108 mmHg at 4 h, p<0.05) and lung compliance (p<0.05), associated with better lung aeration by minimizing atelectasis (atelectasis in diaphragmatic slice; 24.2 ± 16.2%, p<0.05). Whereas, MTV+ SBstrong yielded significant increased atelectasis with hyperinflation (atelectasis in diaphragmatic slice; 83.2 ± 6.9%, p<0.05), associated with extremely increased neutrophils in BALF (p<0.05), even when delivered with comparable PEEP and Pplateau<30 cmH2O. Although all groups except MTV+ SBstrong had PTP of<30 cmH2O, MTV+ SBstrong showed the highest PTP of[33 cmH2O (p<0.05). CONCLUSIONS. Our results demonstrate that strong SB during LTV, rather than gently, improves lung aeration by decreasing atelectasis, which contribute to less lung injury and, when SB is preserved, injurious PTP could be generated even when Pplateau is limited <30 cmH2O. These results indicate that SB preserved during mechanical ventilation has a potential risk of deteriorating lung injury, and neither VT nor Pplateau targeted lung protective ventilation is sufficient to reduce lung injury.