The Science Journal of the American Association for Respiratory Care
BACKGROUND: Smaller tidal volumes have been suggested to minimize ventilator associated lung injury in patients with ARDS. The acute effects of such an approach upon measures of gas exchange and lung mechanics is not certain in humans. Moreover, the interaction of differing tidal volumes and levels of PEEP are also unclear, primarily due to the variability between patients and over time in patients with ALI. In this study, we sought to measure the effects of different tidal volume-PEEP combinations upon oxygenation and Cst and we hypothesized that PaO2 would continue to improve after Cst had been maximized.
Methods: Based upon a PaO2/FIO2 of <250, bilateral infiltrates on CXR, and PCWP <18, five patients were enrolled. PEEP (5, 10, 15 or 20 cmH2O) and tidal volume (6 or 10 ml/kg) combinations were applied in random order and measurements (PaO2, Vd/Vt. Cst) made at each setting after a 5-minute stabilization period. FiO2 was held constant (80-90%) throughout the experiments. Upon completion, the patient was returned to previous ventilator settings. Statistical comparisons were made between the tidal volume settings at each PEEP level using the Wilcoxon Signed Rank test and ANOVA was used to compare Cst, and P/F ratio across increasing PEEP levels.
Results: The P/F ratio was slightly higher at 10ml/kg compared to 6ml/kg tidal volumes at each level of PEEP. Cst was significantly higher with 10 ml/kg with PEEP = 5 cmH2O (p = 0.03). Only at the highest PEEP level was Cst better with 6ml/kg tidal volume (p = 0.14). The trends of increasing Cst and P/F ratio with increasing PEEP were not statistically significant.
Conclusions: Our data suggest that the potential lung protective benefits associated with small tidal volumes may only be realized at higher levels of PEEP. The clinical implications of this is of considerable importance given the trend towards lower tidal volume ventilation in patients with ALI/ARDS. In keeping with our hypothesis, maximizing oxygenation may result at a cost to pulmonary mechanics that may in turn result in lung injury and increased time on the ventilator. These results must be cautiously interpreted because of the small sample size reported here.
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