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1 Westmead Hospital and University of Sydney, Ludwig Engel Centre for Respiratory Research, Westmead, NSW, Australia
2 Department of Pulmonary Physiology, Sir Charles Gairdner Hospital, Perth, WA, Australia; Department of Human Movement and Exercise Science, University of Western Australia, Perth, WA, Australia
3 Department of Pulmonary Physiology, Sir Charles Gairdner Hospital, Perth, WA, Australia
4 Department of Anaesthesia, Sir Charles Gairdner Hospital, Perth, WA, Australia
5 Department of Pulmonary Physiology, Sir Charles Gairdner Hospital, Perth, WA, Australia; Department of Anaesthesia, Sir Charles Gairdner Hospital, Perth, WA, Australia
* To whom correspondence should be addressed. E-mail: jason_kirkness{at}wmi.usyd.edu.au.
Upper airway (UA) patency may be influenced by surface tension (
) operating within the liquid lining the upper airway (UAL). We examined the role of of UAL in the maintenance of UA patency in 8 isoflurane anesthetized supine human subjects breathing via a nasal mask connected to a pneumotachograph attached to a pressure delivery system. We evaluated: 1) mask pressure at which the UA closed (PCRIT); 2) UA resistance up-stream from the site of UA collapse (RUS); and 3) mask pressure at which the UA re-opened (PO). A multiple pressure-transducer catheter was used to identify the site of airway closure (velopharyngeal in all subjects). UAL samples (0.2µl) were collected and the of UAL determined using the "pull-off force" technique. Studies were performed before and after the intra-pharyngeal instillation of 5ml of exogenous surfactant (Exosurf® GSK). The of UAL decreased from 61.9±4.1mN/m (Control) to 50.3±5.0mN/m (Surfactant; p<0.02). 
PO, RUS, and PO-PCRIT (=Control-Surfactant) were positively correlated with (r2>0.6; p<0.02) PCRIT was not (r2=0.4; p>0.9). In addition, mean peak inspiratory airflow (no flow-limitation) significantly increased (p<0.04) from 0.31±0.06l/s (Control) to 0.36±0.06l/s (Surfactant). These findings suggest that of UAL exerts a force on the upper airway wall that hinders airway opening. Instillation of exogenous surfactant into the UA lowers the of UAL, thus increasing upper airway patency and augmenting re-opening of the collapsed airway.
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