Surface tension can markedly influence upper airway patency

The effects of reducing upper airway (UA) surface tension on UA closing and opening pressures (Pcl and Pop) were examined by removing the air-liquid interface within the UA lumen. Methods: Ten adult male Wistar rats were euthanased (200 mg sodium pentobarbitone, i.p.) and studied supine. Pressure transducers were connected to both ends of the surgically isolated UA (tracheal cannula and airtight face mask with mouth sealed and oesophagus ligated). Positive and negative pressures were applied to the UA via the mask. Pcl and Pop were measured before and after filling the UA with isotonic saline. Results: The saline filled UA consistently failed to close in 2 animals although mask pressure was decreased to below -70 cm H2O. In the 8 animals where closure of the saline filled UA was achieved, Pcl and Pop were lower after saline filling [Pcl = -17.6 ± 3.4 cm H2O (air filled) and -25.2 ± 5.1 cm H2O (saline filled), mean ± SD, p = 0.0159; Pop = 3.1 ± 4.0 cm H2O (air filled) and -7.1 ± 9.5 cm H2O (saline filled), p = 0.0027]. However, the saline filled UA in 2 of these 8 animals intermittently failed to close when mask pressure was decreased to below -50 cm H2O. Conclusions: The results suggest that abolishing surface tension in the UA, although difficult to achieve, makes the rat UA markedly resistant to closure during negative airway pressure. More effective methods for reducing UA surface tension may be therapeutically beneficial in promoting UA patency.