Effects of mean airway pressure and tidal excursion on lung injury induced by mechanical ventilation in an isolated perfused rabbit lung model

Objective: To study the relative contributions of mean airway pressure (mPaw) and tidal excursion (V(T)) to ventilator-induced lung injury under constant perfusion conditions. Design: Prospective, randomized study. Setting: Experimental animal laboratory. Subjects: Fifteen sets of isolated rabbit lungs. Interventions: Rabbit lungs were perfused (constant flow, 500 mL/min; capillary pressure, 10 mm Hg) and randomized to be ventilated at identical peak transpulmonary pressure (pressure control ventilation [30 cm H₂O and frequency of 20/min]) with three different ventilatory patterns that differed from each other by either mPaw or V(T): group A (low mPaw [13.4 ± 0.2 cm H₂O]/large V(T) [55 ± 8 mL], n = 5); group B (high mPaw [21.2 ± 0.2 cm H₂O]/small V(T) [18 ± 1 mL], n = 5); and group C (high mPaw [21.8 ± 0.5 cm H₂O]/large V(T) [53 ± 5 mL], n = 5). Measurements and Main Results: Continuous weight gain (edema formation), change in ultrafiltration coefficient (ΔK(f), vascular permeability index), and histology (lung hemorrhage) were examined. In group A, ΔK(f) (0.08 ± 0.08 g/min/cm H₂O/100 g) was less than in group B (0.28 ± 0.19 g/min/cm H₂O/100 g) or group C (0.41 ± 0.29 g/min/cm H₂O/100 g) (p = .05). Group A experienced significantly less hemorrhage (histologic score, 5.4 ± 2.2) than groups B (10.3 ± 2.1) and C (11.1 ± 3.0) (p < .05). A similar trend was observed for weight gain. In contrast to tidal excursion, mPaw was found to be a significant factor for lung hemorrhage and increased K(f) (two-way analysis of variance; p < .05). Weight gain (r²=.54, p = .04) and lung hemorrhage (r² = .65, p = .01) correlated with the mean pulmonary artery pressure changes that resulted from the implementation of the ventilatory strategies. The difference between the changes in mPaw and mean pulmonary artery pressure linearly predicted ΔK(f) (p = .005 and .05, respectively, r² = 0.73). Conclusions: Under these experimental conditions, mPaw contributes more than tidal excursion to lung hemorrhage and permeability alterations induced by mechanical ventilation.