The pathophysiology of aspiration pneumonitis

Aspiration pneumonitis is an important source of morbidity and mortality accounting for a significant proportion of both anesthetic and nonsurgical deaths and complications. A review of 104 cases of aspiration pneumonitis showed that the majority of patients aspirated liquid gastric contents, presumably containing hydrochloric acid. There was an associated 71 percent mortality. Experimental aspiration pneumonitis was produced in 50 dogs by instilling hydrochloric acid (pH 1, 2, or 3) or tube feeding in volumes up to 4 c.c./Kg. The effect of this aspiration was studied by serial observations of right heart pressure, arterial pressure, pCO₂, pO₂, pH, hematocrit, and the pH of acid composition of bronchial secretions. Plasma volume determinations were also done. Three methods of treatment were compared. The instillation of HCl resulted in massive bronchial congestion with plasma-like fluid leading to hemoconcentration, shock, and hypoxia with rapid neutralization of intrabronchial acid. No sustained increase in pulmonary artery or right ventricular pressures occurred. Plasma volume decreased 35 percent; pO₂ and blood pH were low. Treatment with steroids or saline lavage did not favorably influence the parameters studied. Positive pressure respiration reversed blood gas abnormalities and delayed but did not prevent death. These data indicate that, in the dog, hypoxia and hemoconcentration are paramount problems in aspiration pneumonitis. On the basis of these observations it is suggested that appropriate, immediate management of aspiration pneumonitis should include: (1) clearing of the airway, (2) positive pressure respiration to fully use any uninvolved segments of lung, and (3) appropriate plasma volume replacement.