Does Airway Pressure Release Ventilation Alter Lung Function After Acute Lung Injury

Background During airway pressure release ventilation (APRV), tidal ventilation occurs between the increased lung volume established by the application of continuous positive airway pressure (CPAP) and the relaxation volume of the respiratory system. Concern has been expressed that release of CPAP may cause unstable alveoli to collapse and not reinflate when airway pressure is restored. Objective To compare pulmonary mechanics and oxygenation in animals with acute lung injury during CPAP with and without APRV. Design Experimental, subject-controlled, randomized crossover investigation. Setting Anesthesiology research laboratory, University of South Florida College of Medicine Health Sciences Center. Subjects Ten pigs of either sex. Interventions Acute lung injury was induced with an intravenous infusion of oleic acid (72 µ g/kg) followed by randomly alternated 60-min trials of CPAP with and without APRV. Continuous positive airway pressure was titrated to produce an arterial oxyhemoglobin saturation of at least 95% (FIo 2 =0.21). Airway pressure release ventilation was arbitrarily cycled to atmospheric pressure 10 times per minute with a release time titrated to coincide with attainment of respiratory system relaxation volume. Measurements Cardiac output, arterial and mixed venous pH, blood gas tensions, hemoglobin concentration and oxyhemoglobin saturation, central venous pressure, pulmonary and systemic artery pressures, pulmonary artery occlusion pressure, airway gas flow, airway pressure, and pleural pressure were measured. Tidal volume (VT), dynamic lung compliance, intrapulmonary venous admixture, pulmonary vascular resistance, systemic vascular resistance, oxygen delivery, oxygen consumption, and oxygen extraction ratio were calculated. Main results Central venous infusion of oleic acid reduced PaO 2 from 94 ± 4 mm Hg to 52 ± 9 mm Hg (mean ± 1 SD) (p 2 O to 20 ± 6 mL/cm H 2 O (p=0.002) and increased venous admixture from 13 ± 3% to 32 ± 7% (p 2 O) with or without APRV at 10 breaths per minute with a release time of 1.1 ± 0.2 s. Although mean transpulmonary pressure was significantly greater during CPAP (11.7 ± 3.3 cm H 2 O) vs APRV (9.4 ± 3.8 cm H 2 O) (p 2 was decreased and pHa was increased during APRV vs CPAP (p=0.003 and p=0.005). PaO 2 declined from 83 ± 4 mm Hg to 79 ± 4 mm Hg (p=0.004) during APRV, but arterial oxyhemoglobin saturation (96.6 ± 1.4% vs 96.9 ± 1.3%) did not. Intrapulmonary venous admixture (9 ± 3% vs 11 ± 5%) and oxygen delivery (469 ± 67 mL/min vs 479 ± 66 mL/min) were not altered. After treatment periods and removal of CPAP for 60 min, PaO 2 and intrapulmonary venous admixture returned to baseline values. Discussion Intrapulmonary venous admixture, arterial oxyhemoglobin saturation, and oxygen delivery were maintained by APRV at levels induced by CPAP despite the presence of unstable alveoli. Decrease in PaO 2 was caused by increase in pHa and decrease in PaCO 2 , not by deterioration of pulmonary function. We conclude that periodic decrease of airway pressure created by APRV does not cause significant deterioration in oxygenation or lung mechanics.