Extreme hypoventilation reduces ventilator-induced lung injury during ventilation with low positive end-expiratory pressure in saline-lavaged rabbits

Abstract

Objective: To compare the degrees of ventilator-induced lung injury caused by two ventilation protocols.

Design: Randomized trial.

Setting: University animal laboratory.

Subjects: Sixteen New Zealand white rabbits.

Interventions: After five sequential saline lung lavages, eight pairs 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 adjusted for a PaCO2 of 35 to 45 torr (4.7 to 6 kPa). Study group animals were ventilated through an intratracheal catheter, with inspiratory time of 1.5 secs, ventilatory rate of 20 breaths/min, and peak inspiratory pressure of 4 to 8 cm H2O, adjusted to maintain PaCO2 at 150 to 250 torr (20 to 33 kPa).

Measurements and main results: Arterial blood gases were measured every 30 mins. After 4 hrs, a final lung lavage was performed. Physiologic parameters, cell counts and protein concentration in the final lavage, and lung histology were compared between groups. The alveolar-arterial oxygen tension gradient was higher in the study group over the first 1.5 hrs, but the time profile showed significantly (p = .001) greater improvement in the study group. After 4 hrs, the mean alveolar-arterial oxygen tension gradient was lower in the study group (94 torr [12.5 kPa] vs. 201 torr [26.8 kPa]). The increase 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 lavage (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.

Conclusion: These findings suggest that a very low tidal volume reduces ventilator-induced lung injury in saline-lavaged rabbits during ventilation at low lung volume.