Decreased surface tension of upper airway mucosal lining liquid increases upper airway patency in anaesthetised rabbits

Abstract

The obstructive sleep apnoea syndrome (OSA) is a disorder characterised by repetitive closure and re-opening of the upper airway during sleep. Upper airway luminal patency is influenced by a number of factors including: intraluminal air pressure, upper airway dilator muscle activity, surrounding extraluminal tissue pressure, and also surface forces which can potentially act within the liquid layer lining the upper airway. The aim of the present study was to examine the role of upper airway mucosal lining liquid (UAL) surface tension (gamma) in the control of upper airway patency. Upper airway opening (PO) and closing pressures (PC) were measured in 25 adult male, supine, tracheostomised, mechanically ventilated, anaesthetised (sodium pentabarbitone), New Zealand White rabbits before (control) and after instillation of 0.5 ml of either 0.9 % saline (n = 9) or an exogenous surfactant (n = 16; Exosurf Neonatal) into the pharyngeal airway. The gamma of UAL (0.2 microl) was quantified using the 'pull-off' force technique in which gamma is measured as the force required to separate two curved silica discs bridged by the liquid sample. The gamma of UAL decreased after instillation of surfactant from 54.1 +/- 1.7 mN m-1 (control; mean +/- S.E.M.) to 49.2 +/- 2.1 mN m-1 (surfactant; P < 0.04). Compared with control, PO increased significantly (P < 0.04; paired t test, n = 9) from 6.2 +/- 0.9 to 9.6 +/- 1.2 cmH2O with saline, and decreased significantly (P < 0.05, n = 16) from 6.6 +/- 0.4 to 5.5 +/- 0.6 cmH2O with surfactant instillation. Findings tended to be similar for PC. Change in both PO and PC showed a strong positive correlation with the change in gamma of UAL (both r > 0.70, P < 0.001). In conclusion, the patency of the upper airway in rabbits is partially influenced by the gamma of UAL. These findings suggest a role for UAL surface properties in the pathophysiology of OSA.

Figure 1. Raw data recording opening and closing pressures

Raw data recording showing pressure in the face mask (P_M), pressure recorded at the caudal end of the upper airway (P_UA), and change in upper airway volume (ΔV). As air is added to the upper airway P_UA increases immediately, whereas P_M does not change (airway closed) until a critical P_UA is reached i.e. the upper airway opening pressure (P_O). During the withdrawal of air from the upper airway a P_UA is reached where P_M no longer changes in parallel with P_UA, i.e. the upper airway closing pressure (P_C). Phasic ∞EMG activity was absent for both left (LSH) and right sternohyoid (RSH) muscles throughout the measurement.

Figure 2. Upper airway wall compliance and recoil pressures

Representative upper airway pressure (P_UA) volume (ΔV) recording during control conditions in one rabbit. Upper airway wall compliance was calculated as the slope of the linear regression lines (continuous lines) fitted to the inflation and deflation limbs of the pressure-volume relationship over the same volume range. P_C closing pressure, P_O opening pressure, P_DR deflation recoil pressure, P_IR corresponding inflation recoil pressure. Hysteresis (H_UA; dashed line) was calculated as P_IR−P_DR.

Figure 3. Effect of exogenous surfactant and saline on γ of UAL

Individual data for γ of UAL under control vs. saline (A) and control vs. surfactant (B) conditions. Note that increases in γ of UAL occurred in the majority of rabbits with saline, while decreases occurred in most rabbits with surfactant. Different lines represent individual rabbits. ^*P < 0.05vs. control. Bars represent group mean values.

Figure 4. Effect of exogenous surfactant and saline on P_O and P_C

Individual data for P_O (A and C) and P_C (B and D) under control, saline and surfactant conditions. Note that increases in both P_O and P_C occurred in the majority of rabbits with saline, while decreases occurred in most rabbits with surfactant. This was a significant change for all conditions except for P_C with surfactant. Different lines represent individual rabbits. ^*P < 0.05vs. control. Bars represent group mean data.

Figure 5. Influence of changing γ on upper airway mechanics

Change (control minus saline (triangles) or surfactant (circles) P_C (in ΔP_C; A, filled symbols) and P_O (in ΔP_O; B, open symbols) for each rabbit plotted against change in γ of UAL (Δγ;control minus saline or surfactant). Linear regression lines are shown. Note the strong positive correlations between Δγ and both ΔP_C and ΔP_O.