Age effects susceptibility to pulmonary barotrauma in rabbits. OBJECTIVE: We studied the effect of age on the development of pulmonary barotrauma after mechanical ventilation with high peak inspiratory pressures (PIP). DESIGN: Young (4 to 6 wk old) and adult rabbits were ventilated for 1 hr at PIPs of 15, 30, and either 45 cm H2O (young group) or 55 cm H2O (adult group). MEASUREMENTS AND MAIN RESULTS: The pulmonary capillary filtration coefficient (Kf,c) was measured in an isolated lung perfusion system after the animals were killed. In young rabbits, Kf,c increased significantly from the 15 cm H2O PIP value in both the 30 cm H2O (55%) and 45 cm H2O (507%) PIP groups, whereas Kf,c was increased in adult rabbits only in the 55 cm H2O (113%) PIP group. Kf,c was significantly (p less than .01) higher in young rabbits than in adult rabbits after ventilation, with every level of PIP being 91% higher at 15 cm H2O PIP and 440% higher at 45 to 55 cm H2O PIP. Also, a greater incidence of pneumothorax and airleaks was observed in the young rabbits. Pressure-volume loops demonstrated that the young rabbits had more compliant lungs and chest wall than adult rabbits. CONCLUSIONS: These data indicate that the lungs of young rabbits had a higher baseline microvascular permeability and were more susceptible to the development of ventilator-induced increased microvascular permeability. More compliant lungs and chest wall and the larger distending volumes attained at each peak airway pressure appear to be the mechanisms. 