Diagnostic Imaging Enhances Management Of Acute Respiratory Distress Syndrome

Oura TJ, Hanel RM, Davies J, et al.

American College of Veterinary Radiologists 2012:112.

Introduction/Purpose: Hypoxemia secondary to acute respiratory distress syndrome (ARDS) often necessitates mechanical ventilation that can cause further parenchymal damage during alveolar recruitment. High resolution computed tomography (HRCT) was used as the gold standard in a porcine model of ARDS to evaluate a novel technique for assessing ventilator induced lung injury. HRCT techniques used during this study were then applied to two mechanically ventilated canine patients with ARDS.

Methods: ARDS was created in five mechanically ventilated swine using a saline lavage model. Inspiratory and expiratory HRCT and lung function testing using a novel, three gas technique of methane, carbon monoxide, and acetylene were performed at variable positive end expiratory pressure (PEEP) settings. Quantification of alveolar recruitment/derecruitment was calculated as the percentage of total voxels that changed from >-500 HU to <-500 HU between expiration and inspiration. Overdistension was calculated as the percentage of voxels at full inspiration having <-800 HU. The HRCT results were compared to alveolar volume, diffusion capacity, and capillary pulmonary capillary blood flow as calculated by the three gas technique. The technique for HRCT of mechanically ventilated patients was applied to two canine patients with ARDS that underwent HRCT within 12 hours of initiating mechanical ventilation and then again within 24-36 hours.

Results: In the porcine model, increased PEEP settings resulted in a reduced mean percentage of recruitment/derecruitment (4.7% ± SD 5.3) compared to a lower PEEP setting (33.0% ± SD 6.3). Higher PEEP settings resulted in an increased percentage of overdistended alveoli (33.7% ± SD 11.3) compared to lower PEEP settings (19.3% ± SD 4.7). Diffusion and perfusion parameters increased at the highest PEEP compared to the lowest PEEP as evaluated by pulmonary function testing. In the canine patients, there was minimal evidence of recruitment/derecruitment or overdistension; however, HRCT allowed monitoring of poorly aerated and normally aerated alveoli over time.

Discussion/Conclusion: HRCT can be used in an animal model of ARDS to assess efficacy of mechanical ventilation, and these results correlate to a novel pulmonary function testing method. Similar techniques can be applied to clinical veterinary patients with ARDS/ALI to monitor lung response to mechanical ventilation.