Chest
Volume 107, Issue 3, March 1995, Pages 805-808
Journal home page for Chest

Laboratory and Animal Investigations: Articles
Does Airway Pressure Release Ventilation Alter Lung Function After Acute Lung Injury?

https://doi.org/10.1378/chest.107.3.805Get rights and content

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 µg/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% (FIo2=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 PaO2 from 94 ± 4 mm Hg to 52 ± 9 mm Hg (mean ± 1 SD) (p<0.001) and dynamic lung compliance from 40 ± 6 mL/cm H2O to 20 ± 6 mL/cm H2O (p=0.002) and increased venous admixture from 13 ± 3% to 32 ± 7% (p<0.001) in ten swine weighing 33.3 ± 4.1 kg while they were spontaneously breathing room air. After induction of lung injury, the swine received CPAP (14.7 ± 3.3 cm H2O) 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 H2O) vs APRV (9.4 ± 3.8 cm H2O) (p<0.001), there were no differences in hemodynamic variables. PaCO2 was decreased and pHa was increased during APRV vs CPAP (p=0.003 and p=0.005). PaO2 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, PaO2 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 PaO2 was caused by increase in pHa and decrease in PaCO2, 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.

Section snippets

Methods

After approval from the University of South Florida Institutional Animal Care and Use Committee, swine of either sex were sedated with ketamine hydrochloride (30 mg/kg intramuscularly) and anesthetized with enflurane and oxygen to facilitate tracheal intubation and instrumentation. After instrumentation, anesthesia was maintained with an intravenous infusion of ketamine hydrochloride (28 mg/kg/h). A sample tube from a capnometer was attached between the pneumotachograph and breathing circuit

Results

Central venous infusion of oleic acid reduced PaO2 from 94 ± 4 mm Hg to 52 ± 9 mm Hg (p<0.001) and dynamic lung compliance from 40 ± 6 mL/cm H2O to 20 ± 6 mL/cm H2O (p=0.002) and increased intrapulmonary venous admixture from 13 ± 3% to 32 ± 7% (p<0.001) while ten pigs breathed spontaneously at atmospheric pressure (FIo2=0.21). Pulmonary and hemodynamic data obtained after acute lung injury while pigs breathed spontaneously at atmospheric pressure before and after alternating periods of CPAP

Discussion

Optimizing alveolar ventilation with application of the least possible positive airway pressure is an important goal in the mechanical ventilatory management of patients with impaired pulmonary function. Airway pressure release ventilation is a ventilatory support technique designed to augment alveolar ventilation of subjects who require ventilatory assistance, despite improved pulmonary mechanics and oxygenation with CPAP.1, 5, 6 During APRV, airway pressure is decreased, gas exits the lungs,

ACKNOWLEDGMENTS

The authors appreciated use of the computerized pulmonary work station with physiologic pattern recognition software developed by Mauricio Leon, MD, technical assistance of Michael R. Hodges, and editorial assistance of John B. Downs, MD.

References (6)

There are more references available in the full text version of this article.

Cited by (14)

View all citing articles on Scopus
View full text