Repetitive high-pressure recruitment maneuvers required to maximally recruit lung in a sheep model of acute respiratory distress syndrome

Abstract

Objective 

To compare the effects of two different recruitment maneuvers repeated multiple times on gas exchange lung injury, hemodynamic, and lung mechanics.

Design 

Randomized prospective comparison.

Settings 

Animal research laboratory.

Subject 

Nineteen fasted Hampshire sheep.

Interventions 

In 15 27-kg sheep with saline lavage lung injury, we compared the effects of two recruitment maneuvers: 40 cm H₂O continuous positive airway pressure for 60 secs and 40 cm H₂O positive end-expiratory pressure with 20 cm H₂O pressure control, rate 10 breaths/min, inspiratory to expiratory ratio 1:1 for 2 mins. Each recruitment maneuver was repeated four times, every 30 mins after a 30-sec ventilator disconnection. An additional group received no recruitment maneuvers. Animals were assigned randomly to the three groups and ventilated with 20 cm H₂O positive end-expiratory pressure, pressure control 15 cm H₂O, rate 20 breaths/min, inspiratory to expiratory ratio 1:1, and Fio₂ 1.0 between recruitment maneuver periods.

Measurements and Main Results  

Significant and marked increases in Pao₂ were observed in the pressure control recruitment maneuver group but only after the second recruitment maneuver. In both the control group and continuous positive airway pressure groups, Pao₂ did not significantly increase after any recruitment maneuver compared with baseline injury. There was a significant decrease in cardiac output immediately after some continuous positive airway pressure recruitment maneuvers and a significant increase in mean pulmonary artery pressure in both continuous positive airway pressure and pressure control groups immediately after recruitment maneuvers, but these changes resolved within 10 mins. There were no marked histologic differences between groups and no volutrauma.

Conclusion 

In this model, maximal lung recruitment was obtained with 40 cm H₂O positive end-expiratory pressure and 20 cm H₂O pressure control applied repetitively every 30 mins for 2 mins without physiologic or histologic harm. Multiple recruitment maneuvers in some animals were required for maximum effect.