Reduced surface tension of upper airway lining liquid promotes upper airway patency in anaesthetised humans

Upper airway (UA) patency in unconscious humans may be influenced by surface tension forces (ST) operating within the liquid lining the upper airway (UAL). Methods: We examined the role of ST of UAL in the maintenance of UA patency in 8 anaesthetised supine human subjects (2 male. 6 female). Subjects breathed via a nasal mask connected to a pneumotachograph and attached to a pressure delivery system capable of maintaining a constant positive pressure (no flow limitation) or a decreased mask pressure (3-5 breaths). For each subject we evaluated: 1 ) mask pressure at which the UA closed (Peril); 1) UA resistance up-stream from the site of UA collapse (Rus); and 3) mask pressure at which the UA re-opened (Po). A multiple pressuretransducer catheter, passed through the nasopharyngeal airway into the eosophagus. was used to identify1 the site of airway closure (the velopharynx in all subjects). 0.2uL of UAL was collected via a nasal catheter advanced to the site of UA collapse and the ST of UAL determined using the "pull-off force" technique. Studies were performed before and after the instillation (at the site of U A collapse) of 5mL of exogenous surfactant (Exsurf®, Glaxo Wellcome). Results: The airway remained hypotonie (absent intramuscular genioglossus EMG activity) in all subjects throughout each study. The ST of UAL decreased from 61.9±4.1 mN:m (Control) to 50.3±5.0 mN/m (Surfactant: p<0.02). However. APo. ARus, and APo-Pcrit (A=Control-Surfactant) were positively correlated with AST (R:=0.8.0.9.0.6 respectively: p<0.02) but APcrit w-as not (R²=0.4|. In addition, mean peak inspirator)' airflow (no flow limitation) significantly increased (p0.04. paired t-test) from 0.31 ±0.06 L/s (Control) to 0.36±0.06 L-'s with surfactant. Conclusion: These findings suggest that decreasing ST of UAL reduces UA resistance and augments re-opening of the collapsed airway. This implies that surface tension of UAL is a factor in determining patency of the relaxed human UA.