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

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Abstract
To study the relative contributions of mean airway pressure (mPaw) and tidal excursion (VT) to ventilator-induced lung injury under constant perfusion conditions. Prospective, randomized study. Experimental animal laboratory. Fifteen sets of isolated rabbit lungs. 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 H2 O and frequency of 20/min]) with three different ventilatory patterns that differed from each other by either mPaw or VT: group A (low mPaw [13.4 +/- 0.2 cm H2 O]/large VT [55 +/- 8 mL], n = 5); group B (high mPaw [21.2 +/- 0.2 cm H2 O]/small VT [18 +/- 1 mL], n = 5); and group C (high mPaw [21.8 +/- 0.5 cm H2 O]/large VT [53 +/- 5 mL], n = 5). Continuous weight gain (edema formation), change in ultrafiltration coefficient (Delta Kf, vascular permeability index), and histology (lung hemorrhage) were examined. In group A, Delta Kf (0.08 +/- 0.08 g/min/cm H2 O/100 g) was less than in group B (0.28 +/- 0.19 g/min/cm H2 O/100 g) or group C (0.41 +/- 0.29 g/min/cm H2 O/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 f Under these experimental conditions, mPaw contributes more than tidal excursion to lung hemorrhage and permeability alterations induced by mechanical ventilation. (Crit Care Med 1999; 27:1533-1541)