Detection of pulmonary aspiration of gastric contents in an animal model by assay of peptic activity in bronchoalveolar fluid

Objective: To determine whether peptic activity in bronchoalveolar fluid, due to the presence of the gastric proteolytic enzyme pepsin, could serve as a biochemical marker for pulmonary aspiration of gastric contents. Design: Prospective, experimental trial. Setting: A university animal research laboratory. Subjects: Thirty-six New Zealand rabbits, weighing 2 to 4 kg. Interventions: New Zealand rabbits were anesthetized, intubated via tracheostomy, and mechanically ventilated. Pulmonary aspiration was induced by the intratracheal instillation of 2 mL/kg human gastric juice (pH 1.2 +/- 0.2; pepsin activity 0.02 +/- 0.006 mu g/mL; human gastric juice group, n = 24) or normal saline solution (pH 5.2 +/- 0.2; normal saline solution group; n = 12). Mechanical ventilation was continued. Bronchoalveolar lavage was performed at 15 mins (human gastric juice group, n = 8; normal saline solution group, n = 4), 30 mins (human gastric juice group, n = 8; normal saline solution group, n = 4), or 60 mins (human gastric juice group, n = 8; normal saline solution group, n = 4) postaspiration. Measurements and Main Results: Peak airway pressure and Pao(2) values were measured at baseline and 15 and 30 mins after aspiration. The pH of retrieved bronchoalveolar lavage fluid was measured and pepsin activity in sample fluid was determined. Changes from baseline in peak airway pressure and Pao(2) were significant in human gastric juice animals at 15 and 30 mins when compared with normal saline solution animals (Pao(2) -4% vs. -44%, peak airway pressure 20% vs. 36% at 15 mins; Pao(2) -16% vs. -79%, peak airway pressure 28% vs. 69% at 30 mins; normal saline solution group vs. human gastric juice group, p<.02). Bronchoalveolar lavage fluid pH was not significantly different between groups at any time postaspiration (6.6 +/- 0.7 vs. 6.0 +/- 0.4 at 15 mins; 7.4 +/- 0.9 vs. 6.5 +/- 0.4 at 30 mins; 7.2 +/- 0.5 vs. 6.4 +/- 0.4 at 60 mins, normal saline solution group vs. human gastric juice group, p = NS). No peptic activity was present in bronchoalveolar lavage fluid from normal saline solution animals at any time. In the human gastric juice group, peptic activity was detected in postaspiration bronchoalveolar lavage fluid in eight of eight animals at 15 mins, six of eight animals at 30 mins, and five of eight animals at 60 mins (normal saline solution group vs. human gastric juice group; p<.001 at 15 mins, p <.01 at 30 mins, p = NS at 60 mins). Peptic activity of bronchoalveolar ravage fluid varied; mean values were greater at 15 mins than at 30 or 60 mins (pepsin activity: 0.004 +/- 0.002 mu g/mL vs. 0.002 +/- 0.001 mu g/ml. vs. 0.0006 +/- 0.0001 mu g/mL, respectively, p<.05). Conclusions: The results of this study suggest that peptic activity in bronchoalveolar ravage fluid can be detected up to 60 mins after induced, experimental gastric juice aspiration and may prove a clinically useful biochemical marker for episodes of occult pulmonary aspiration of gastric contents.