The upper airway during anaesthesia D.R. Hillman, P.R. Platt, P.R. Eastwood  British Journal of Anaesthesia  Volume 91, Issue 1, Pages 31-39 (July 2003) DOI: 10.1093/bja/aeg126 Copyright © 2003 British Journal of Anaesthesia Terms and Conditions

Fig 1 Starling resistor model of the upper airway with a collapsible (pharyngeal) segment between two rigid tubes (nasal and tracheal). When flow limitation is present, maximal inspiratory flow is defined by (Pupstream–Pcrit)/Rupstream, where Pcrit is the critical closing pressure (=tissue pressure); Pupstream=upstream pressure; Pdownstream=downstream pressure; Rupstream=upstream resistance, and Rdownstream=downstream resistance. See text for further details. British Journal of Anaesthesia 2003 91, 31-39DOI: (10.1093/bja/aeg126) Copyright © 2003 British Journal of Anaesthesia Terms and Conditions

Fig 2 Record from a patient during anaesthesia with isoflurane showing responses when mask pressure was rapidly reduced from a maintenance pressure (≈14 cm H2O) for five breaths and then returned to the maintenance pressure. This procedure was repeated with a range of positive and/or negative airway pressures, causing varying degrees of inspiratory flow limitation (a plateau in inspiratory flow while oesophageal pressure fell progressively, panels A–C, see also Fig. 3), including a pressure sufficient to cause complete airway collapse (no change in inspiratory flow despite a changing oesophageal pressure, panel D). Note that negative intrathoracic pressure swings are not transmitted to the nasopharynx, indicating collapse distal to this point. Pm=mask pressure; Pnp=nasopharyngeal pressure; Pop=oropharyngeal pressure; Php=hypopharyngeal pressure; Poes=oesophageal pressure. (From Eastwood and colleagues,11 with permission.) British Journal of Anaesthesia 2003 91, 31-39DOI: (10.1093/bja/aeg126) Copyright © 2003 British Journal of Anaesthesia Terms and Conditions

Fig 3 Relationship between maximal inspiratory flow and nasal mask (upstream) pressure for flow-limited breaths from two subjects breathing spontaneously at end-tidal isoflurane levels of 1.2%. The left panel demonstrates data from a subject with a relatively stable upper airway; note that the critical closing pressure (Pcrit) is subatmospheric. The right panel demonstrates data from a subject with an unstable airway; note that Pcrit exceeds atmospheric pressure. British Journal of Anaesthesia 2003 91, 31-39DOI: (10.1093/bja/aeg126) Copyright © 2003 British Journal of Anaesthesia Terms and Conditions