EXTREME HYPOVENTILATION REDUCES VENTILATOR-INDUCED LUNG INJURY DURINGVENTILATION WITH LOW POSITIVE END-EXPIRATORY PRESSURE IN SALINE-LAVAGED RABBITS

Objective: To compare the degrees of ventilator induced lung injury ca used by two ventilation protocols. Design: Randomized trial. Setting: University animal laboratory. Subjects: Sixteen New Zealand white rabb its. Interventions: After five sequential saline lung ravages, eight p airs of anesthetized rabbits were allocated randomly to receive either of two ventilation protocols for 4 hrs during neuromuscular blockade. Both groups received 3 cm H2O of positive end-expiratory pressure and 100% oxygen. Control group animals received an estimated tidal volume of 12 mL/kg, an inspiratory time of 0.7 sec, and a ventilatory rate a djusted for a Paco(2) of 35 to 45 torr (4.7 to 6 kPa). Study group ani mals were ventilated through an intratracheal catheter, with inspirato ry time of 1.5 sees, ventilatory rate of 20 breaths/min, and peak insp iratory pressure of 4 to 8 cm H2O, adjusted to maintain Paco(2) at 150 to 250 torr (20 to 33 kPa). Measurements and Main Results: Arterial b lood gases were measured every 30 mins. After 4 hrs, a final lung rava ge was performed. Physiologic parameters, cell counts and protein conc entration in the final lavage, and lung histology were compared betwee n groups. The alveolar-arterial oxygen tension gradient was higher in the study group over the first 1.5 hrs, but the time profile showed si gnificantly (p =.001) greater improvement in the study group. After 4 hrs, the mean alveolar-arterial oxygen tension gradient was lower in t he study group (94 torr [12.5 kPa] vs. 201 torr [26.8 kPa]). The incre ase in neutrophil count from the initial to the final lung lavage was lower in the study group (0.27 x 10(7) cells/L vs. 2.01 x 10(7) cells/ L, p = .037), as was the absolute value of the neutrophil count in the final ravage (1.33 x 10(7) cells/L vs. 3.02 x 10(7) cells/L, p = .04) . The median hyaline membrane score was lower in the study group (0.5 vs. 3.0) but the difference was not statistically significant. Conclus ion: These findings suggest that a very low tidal volume reduces venti lator-induced lung injury in saline lavaged rabbits during ventilation at low lung volume.