Tracheal acid or surfactant instillation raises alveolar surface tension

Abstract

Whether alveolar liquid surface tension, T, is elevated in the acute respiratory distress syndrome (ARDS) has not been demonstrated in situ in the lungs. Neither is it known how exogenous surfactant, which has failed to treat ARDS, affects in situ T. We aim to determine T in an acid-aspiration ARDS model before and after exogenous surfactant administration. In isolated rat lungs, we combine servo-nulling pressure measurement and confocal microscopy to determine alveolar liquid T according to the Laplace relation. Administering 0.01 N (pH 1.9) HCl solution by alveolar injection or tracheal instillation, to model gastric liquid aspiration, raises T. Subsequent surfactant administration fails to normalize T. Further, in normal lungs, tracheal instillation of control saline or exogenous surfactant raises T. Lavaging the trachea with saline and injecting the lavage solution into the alveolus raises T, suggesting that tracheal instillation may wash T-raising airway contents to the alveolus. Adding 0.01 N HCl or 5 mM CaCl--either of which aggregates mucins--to tracheal lavage solution reduces or eliminates the effect of lavage solution on alveolar T. Following tracheal saline instillation, liquid suctioned directly out of alveoli through a micropipette contains mucins. And, alveolar injection of gastric mucin solution raises T. We conclude that (i) tracheal liquid instillation likely washes T-raising mucins to the alveolus and (ii) even exogenous surfactant that could be delivered mucin-free to the alveolus might not normalize T in acid-aspiration ARDS.

The Contribution of Surface Tension-Dependent Alveolar Septal Stress Concentrations to Ventilation-Induced Lung Injury in the Acute Respiratory Distress Syndrome

“…Surprisingly, however, this effect was not pH dependent. Tracheal instillation of pH 5.0 normal saline caused the same increase in alveolar T (Nguyen and Perlman, 2018). The T-raising effect of HCl or saline instillation was not due to surfactant dilution as direct alveolar injection of saline, which should cause greater dilution, did not alter T from normal.…”

“…In a PBS, pH as low as 2 did not alter T MIN of the hydrophobic fraction of calf lung surfactant (Haddad et al, 1994). In addition, when we administered pH 1.9 HCl solution by direct micropuncture to surface alveoli of isolated rat lungs, the solution raised alveolar T over a period of ∼30 min (Nguyen and Perlman, 2018). The time delay in the lungs suggests that low pH might raise T indirectly, perhaps by damaging the alveolar epithelium and releasing cell components that subsequently interfere with surfactant function.…”

“…In gastric aspiration, the effects of pH and mucins should tend to balance one another. If gastric contents mix significantly with airway liquid, then they should be partially buffered but also collect mucins (Kim et al, 2014;Nguyen and Perlman, 2018). The higher pH might not be injurious, but the mucins might elevate T (Nguyen and Perlman, 2018).…”

“…If gastric contents mix significantly with airway liquid, then they should be partially buffered but also collect mucins (Kim et al, 2014;Nguyen and Perlman, 2018). The higher pH might not be injurious, but the mucins might elevate T (Nguyen and Perlman, 2018). Alternatively, if there is relatively little mixing with airway liquid, there will be relatively little buffering, and pH will remain low.…”

“…Alternatively, if there is relatively little mixing with airway liquid, there will be relatively little buffering, and pH will remain low. The low pH will tend to aggregate mucins such that the mucins might not contribute to elevated T (Hong et al, 2005;Nguyen and Perlman, 2018). The low pH, itself, should lead to elevated T, albeit perhaps indirectly via debris from damaged epithelial or other cells.…”