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Abstract

Introduction: Alveoli in the acutely injured lungs may repetitively collapse and expand at expiration having a negative impact on ventilation/perfusion match. Hypothesis: Low tidal ventilation with PEEP optimized to maintain the end-expiratory transpulmonary pressure?0 cmH2O will improve oxygenation and shunt fraction, as compared to strategies that may allow for negative expiratory transpulmonary pressures. Methods: Ten Yorkshire pigs were instrumented and mechanically ventilated using transpulmonary pressure measurements. Baseline lung mechanics were obtained for the following parameters: Vt=10mL/kg, PEEP=5cmH2O, and FiO2 = 0. 21. Lungs were injured by instilling 5% Tween-20 (3ml/kg), and ARDS was established when PF ratios fell <200. After a single recruitment maneuver (40 cmH2O x 40sec.), the animals were randomized into a PtpPEEP Group with PEEP adjusted to maintain the end-expiratory transpulmonary pressure?0 cmH2O, or the Control Group where PEEP was adjusted based on the lower PEEP scale as per ARDSNet guidelines. Cardiovascular and respiratory parameters were recorded hourly for 24hr. Results: After 12hr, the PtpPEEP Group started separating from the Control Group, maintaining, as per protocol, PtpPEEP pressures?0 cmH2O, while the Control Group showed negative values 48% of time. Overall, the PtpPEEP Group had an odds ratio of 4.3 for normal oxygenation (PF ratio>300), and intrapulmonary shunting significantly lower than the Control Group (p<0.01). Regression analysis showed that shunt fractions increased with time (p<0.01) associated with decreasing PF ratios (<0.01); however, for the same PF ratios, the PtpPEEP Group showed lower shunt fractions than the Control (p<0.01). Conclusions: In our study, the extent of shunting was a consequence of the PEEP optimization strategy. The PtpPEEP Group improved the oxygenation and shunt fractions, indicating that maintaining a PtpPEEP?0 cmH2O may have a significant impact on ventilation/perfusion matching.