Effects of a synthetic lung surfactant
on pharyngeal patency in awake human subjects
T. VAN DER TOUW, A. B. H. CRAWFORD, AND J. R. WHEATLEY
Department of Respiratory Medicine, Westmead Hospital,
Westmead, New South Wales 2145, Australia
Van der Touw, T., A. B. H. Crawford, and J. R.
Wheatley. Effects of a synthetic lung surfactant on pharyngeal patency in awake human subjects. J. Appl. Physiol.
82(1): 78–85, 1997.—We examined the effects of separate
applications of saline and a synthetic lung surfactant preparation (Surf; Exosurf Neonatal) into the supraglottic airway
(SA) on the anteroposterior pharyngeal diameter (Dap ) and
the airway pressures required to close (Pcl) and reopen
(Pop) the SA in five awake normal supine subjects. Dap, Pcl,
and Pop were determined during lateral X-ray fluoroscopy
and voluntary glottic closure when pressure applied to the
SA lumen was decreased from 0 to 220 cmH2O and then
increased to 120 cmH2O. After Surf application and relative
to control, Dap was larger for most of the applied pressures,
Pcl decreased (212.3 6 1.9 to 218.7 6 0.9 cmH2O; P , 0.01),
Pop decreased (13.4 6 1.9 to 26.0 6 3.4 cmH2O; P , 0.01),
and genioglossus electromyographic activity did not change
(P . 0.05). Saline had no effect. These observations suggest
that pharyngeal intraluminal surface properties are important in maintaining pharyngeal patency. We propose that
surfactants enhance pharyngeal patency by reducing surface
tension and adhesive forces acting on intraluminal SA surfaces.
upper airway physiology; closing pressure; opening pressure;
surface forces
have reported reductions in upper
airway resistance and snoring after application of
surfactants (surface-active agents that reduce surface
tension) into the oropharynx of anesthetized dogs (15,
28). In addition, the application of surfactant to the
upper airway can facilitate reopening of the occluded
upper airway in dogs (15). Little is known concerning
the effects of surfactants on upper airway patency in
humans, although Hoffstein and co-workers (11) reported reductions in both the incidence and maximum
sound level of snoring in sleeping human subjects after
applying a surfactant into the upper airway.
The mechanism by which surfactants may increase
upper airway patency is unknown. Widdicombe and
Davies (28) reported increased genioglossus muscle
electromyographic activity (EMGgg) after applying a
mixture of surfactants into the oropharynx of anesthetized dogs. This suggests that surfactants may influence the upper airway by increasing upper airway
muscle activity. However, an artificial lung surfactant
preparation has been shown to improve upper airway
patency in anesthetized dogs with bilaterally sectioned
hypoglossal nerves (15). Therefore, factors other than
upper airway muscle recruitment may be involved in
the improvement in upper airway patency after topical
surfactant application.
TWO ANIMAL STUDIES
78
In a recent study (27), we demonstrated apparent
occlusion of the oropharyngeal airway in awake normal
subjects when negative pressure was applied to the
upper airway lumen via a mouthpiece while the subject
voluntarily maintained a closed glottis. Furthermore,
the oropharynx still appeared occluded when airway
pressure had returned to atmospheric pressure after
the applied negative pressure was removed, suggesting
that intraluminal surface forces were involved in maintaining apposition of the airway walls. In addition,
reopening of the closed oropharyngeal airway only
occurred after activation of upper airway-dilating forces
during an active inspiratory effort. Therefore, mucosal
surface properties of the upper airway may be important in the development and/or maintenance of airway
occlusion during periods of negative intraluminal pressure. This suggests that surface tension and adhesive
forces in the upper airway may be relevant to the
pathogenesis of the obstructive sleep apnoea (OSA)
syndrome.
On the basis of these data, we hypothesized that the
application of surfactant into the supraglottic airway
(SA) would facilitate spontaneous reopening of the
occluded oropharyngeal airway in awake normal subjects. Therefore, we examined the patency of the SA
during exposure to negative and positive intraluminal
pressures both before and after the application of a
synthetic lung surfactant preparation into the SA of
normal subjects. In addition, the studies were repeated
while we measured the EMGgg to examine the effect of
the surfactant application on upper airway dilator
muscle activity.
METHODS
We measured anteroposterior (A-P) intraluminal SA diameters from X-ray fluoroscopic images in five awake supine
male subjects [age 36 6 1 (SE) yr; body weight 73 6 7 kg]
without a clinical history of sleep disturbance. During voluntary glottic closure, negative and positive pressures were
applied to the SA lumen before and after separate applications of saline and a synthetic lung surfactant preparation
into the SA. On a separate day, the protocol was repeated in
four of the subjects while EMGgg activity was measured
without using X-ray fluoroscopy. The protocol was approved
by the Westmead Hospital Human Ethics and Radiation
Safety Committees, and all subjects gave their informed
consent.
X-ray fluoroscopy. The entire SA was viewed by lateral
X-ray fluoroscopy by using a mobile X-ray C-arm image
intensifier (Phillips BV 25). A 1.0-mm-thick copper filter was
used to reduce the dose of radiation received at the skin and to
improve the resolution of the air-contrast image. No contrast
medium was used. The total thyroid radiation dose in each
subject was measured by thermoluminescent dosimetry with
the use of lithium fluoride chips and did not exceed 1.6 mGy.
0161-7567/97 $5.00 Copyright r 1997 the American Physiological Society
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SURFACTANT AND HUMAN PHARYNGEAL PATENCY
During X-ray screening, negative and positive pressures
from a pressure source were applied to the SA lumen during
voluntary glottic closure at end expiration (27). During the
maneuver, there was no flow through the glottis, and the
subjects were instructed to cease respiratory efforts. In four
subjects, pressures were applied via a mouthpiece with a
noseclip in situ. In the remaining subject, pressure was
applied via a modified nasal continuous positive-airwaypressure mask with the mouth closed. During each maneuver, a ramp of negative pressure to 220 cmH2O was applied to
the SA lumen, followed by a gradual return to 0 cmH2O and
then a ramp of positive pressure to 120 cmH2O, all for a total
time of 20 s. The maneuver was repeated two to three times
under each of the following experimental conditions: 1) before
application of saline or surfactant into the SA (control); 2)
within 3 min after application of 5 ml of hypotonic saline
(0.55%, wt/vol) into the SA; and 3) within 3 min after
application of 5 ml of a synthetic lung surfactant (Exosurf
Neonatal, Burroughs Wellcome Australia; each 5 ml containing 67.5 mg dipalmitoylphosphatidylcholine, 7.5 mg hexadecanol, 5.0 mg tyloxapol, and 29.2 mg NaCl) into the SA. The
order of the experimental conditions remained constant, with
control runs followed by saline and then surfactant. For each
application, 1 ml of saline or surfactant was applied via each
nostril and sniffed, and a further 3 ml were applied orally and
gargled. The hypotonic saline was intentionally prepared to
have the same osmolarity as the reconstituted Exosurf surfactant preparation (190 mosmol/l).
The subject’s head and neck were immobilized by the
mouthpiece or nasal mask and by the X-ray table, to which
was attached a pair of head callipers that were applied to the
subject’s temples. Pressure at the mouthpiece or nasal mask
was measured with a differential pressure transducer (Celesco, 6100 cmH2O). Airflow was measured with a pneumotachograph (Fleisch no. 2) coupled to a differential pressure
transducer (Celesco, 610 cmH2O) to determine whether the
glottis remained closed while pressure was applied to the SA.
The pressure and airflow signals were displayed on an
oscilloscope (Fig. 1). The oscilloscope signals were recorded by
using a television camera, the electrical output of which was
mixed with those from the image intensifier and a digital
timer. The mixed signals were displayed on a television
monitor and stored on videotape for subsequent analysis
(Fig. 1).
79
EMG. EMGgg activity was measured on a separate day in
four of the five subjects by using bipolar Teflon-coated finewire electrodes (40 gauge) inserted orally into the body of the
genioglossus muscle by using a 23-gauge hypodermic needle
(20, 29), with a grounding surface electrode placed on the
forehead. The raw EMGgg signal was displayed on an oscilloscope and monitored throughout the study. The raw EMGgg
signal was band-pass filtered (10021,000 Hz), amplified,
rectified, and passed through a leaky integrator (Neotrace NT
1900) with a time constant of 100 ms, to produce a moving
time average (MTA) EMGgg signal that was recorded together with mouth pressure and flow on a multichannel
strip-chart recorder (Hewlett-Packard 7758B). Electrode
placement was considered acceptable only if tongue protrusion resulted in recruitment of raw EMGgg action potentials
and increased MTA EMGgg activity.
During EMG studies, subjects were placed supine and
breathed via a mouthpiece with a noseclip positioned. Negative and positive pressures were applied to the SA lumen
during voluntary glottic closure at end expiration in the same
manner as for the fluoroscopy studies. The protocol for the
EMG study was identical to that for the fluoroscopy study,
except that X-ray screening was not performed.
Data analysis. Pharyngeal A-P diameters were related to
mouthpiece or nasal mask pressures by analyzing the simultaneous record of the SA fluoroscopic image and pressure
signal. Measurements of pharyngeal diameter were made
from a monitor screen with the use of a video analyzer
(Colorado Video, model 321) coupled to an analog-to-digital
converter (IOtech, ADC 488/16A, Cleveland, OH) and a
personal computer (27). Vertical and horizontal displacements of the video analyzer cursors on the monitor screen
were calibrated from the recorded fluoroscopic image of a
steel-ball marker of known dimensions taped to the subject’s
skin. The steel ball was placed anteriorly over the laryngeal
cartilage in the midsaggital line.
In the four subjects in whom pressure was applied via the
mouthpiece, the A-P oropharyngeal diameter was measured
from the recorded video image at the level corresponding to
the cranial limit of the third cervical vertebra (C3) (Fig. 2). In
the subject in whom pressure was applied via a nasal mask,
occlusion of the nasopharynx during negative pressure preceded and thereby prevented complete oropharyngeal closure. Consequently, in this subject, the A-P nasopharyngeal
Fig. 1. Schematic diagram illustrating experimental setup used to monitor and record X-ray fluoroscopic image of supraglottic airway, pressure (P)
applied to supraglottic airway, and airflow (V̇). See
text for details.
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80
SURFACTANT AND HUMAN PHARYNGEAL PATENCY
Fig. 2. Diagrammatic representation of lateral projection of human
supraglottic airway. SP, soft palate; T, tongue; C3, 3rd cervical
vertebra; horizontal arrows denote anteroposterior luminal diameter
of oropharynx at cranial limit of C3.
diameter was measured at a level 1 cm cranial to the caudal
limit of the uvulus. The A-P oropharyngeal diameters from
four subjects and the A-P nasopharyngeal diameter from one
subject were combined and referred to as Dap. In all subjects,
Dap was measured at pressure intervals of 5 cmH2O from 0 to
220 cmH2O and from 220 to 120 cmH2O. Dap was expressed
as a percentage of each subject’s control value at 0 cmH2O
before negative pressure. SA closing and reopening pressures
(Pcl and Pop, respectively) were determined by visual inspection of the recorded SA image and relating this to the recorded
pressure signal. Pcl was defined as the negative pressure
where A-P airway occlusion was first observed fluoroscopically at any point along the breathing route. Similarly, Pop
was defined as the pressure where the fluoroscopic SA image
first appeared patent along the entire breathing route. Measurements of Pcl, Pop, and Dap were averaged from two or
three X-ray screening runs for each experimental condition.
X-ray screening and EMG runs were not included in the
analysis if an inspection of the fluoroscopic images, airflow, or
EMGgg signals revealed swallowing, occlusion of the oral
cavity when the tip of the tongue was sucked onto the hard
palate, or failure to maintain glottic closure. The MTA
EMGgg was quantified in arbitrary units above electrical
zero. Measurements of MTA EMGgg activity were made when
mouth pressure was 0 (before negative pressure), 220, 0,
110, and 120 cmH2O. The EMG data from repeat runs under
the three experimental conditions were averaged for each of
the five levels of mouth pressure in each subject.
All values are expressed as means 6 SE. Statistical
analysis of the Pcl, Pop, and Dap data was performed with
one-way analysis of variance and the least significant difference test. Hysteresis of the Dap-pressure relationship was
assessed by comparing Dap at corresponding negative pressures as the applied pressure decreased from 0 to 215 cmH2O
and increased from 215 to 0 cmH2O (paired Students t-test).
Statistical analysis of MTA EMGgg data was performed with
the Wilcoxon signed-rank test for paired variates. The null
hypothesis for all statistical tests was rejected at P , 0.05
(two-tailed test).
The nasopharyngeal pressure-diameter relationship
and the Pcl and Pop data from the remaining nasal
mask subject were quantitatively similar to the data
from the four subjects in whom pressures were applied
via a mouthpiece. Therefore, the results obtained from
all five subjects were pooled.
For the mouthpiece-breathing subjects, the airway
level of initial occlusion was in the oropharynx, sometimes in association with occlusion of the oral cavity. No
hypopharyngeal closure was observed in any subject.
For all subjects, the sites of initial occlusion and
reopening were not always at the measured Dap level,
so that Pcl and Pop did not necessarily equal the
pressures where SA closure and reopening occurred, as
measured at the Dap level.
The effects of saline and surfactant on Pcl and Pop
are shown in Figs. 3 and 4. Saline had no consistent
effect on Pcl (control 212.3 6 1.9 cmH2O, saline 211.8 6
1.3 cmH2O; P . 0.75), whereas Pcl was consistently
reduced after surfactant (218.7 6 0.9 cmH2O; P , 0.01
relative to control and saline). Similarly, saline had no
consistent effect on Pop (control 13.4 6 1.9 cmH2O,
saline 8.3 6 3.6 cmH2O; P . 0.25), whereas Pop was
reduced after surfactant (26.0 6 3.4 cmH2O; P , 0.01
relative to control and saline values). Furthermore,
positive pressure was not required to reopen the occluded SA in any subject after surfactant (Fig. 4). In
contrast, positive pressure was required to reopen the
occluded SA in all subjects during control or after saline
except in one subject after saline (Fig. 4).
The mean Dap-pressure relationships before and after applications of saline and surfactant into the SA are
shown in Fig. 5. The Dap progressively narrowed as
airway pressure became more negative, and airway
closure occurred in all subjects by 220 cmH2O both
before and after saline or surfactant. However, before
220 cmH2O Dap was larger after surfactant at 215
cmH2O relative to control and saline (both P , 0.05).
RESULTS
Initial attempts to apply pressure via a nasal mask
were unsuccessful in two of three subjects because of
the occlusion of the nasopharynx, which was unrelated
to the application of negative pressure. Hence, we used
a mouthpiece to apply the pressure in four subjects.
Fig. 3. Supraglottic airway (SA) closing pressures before (control)
and after applying saline and surfactant into SA in 5 subjects. Open
symbols, individual subjects; r connected by bold lines represent
group means. Vertical lines represent 6 SE. * P , 0.01 relative to
control and saline. Note decrease in closing pressure after surfactant,
whereas saline had no consistent effect.
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SURFACTANT AND HUMAN PHARYNGEAL PATENCY
Fig. 4. SA opening pressures before (control) and after applying
saline and surfactant into SA in 5 subjects. Symbols as in Fig. 3. Note
decrease in opening pressures after surfactant, whereas saline had
no consistent effect. Positive pressure was not required to open
occluded airway in any subject after surfactant. In contrast, positive
pressure was required to open occluded airway in all subjects during
control and in all but 1 subject after saline. * P , 0.01 relative to
control and saline.
During control and saline maneuvers, as the pressure
was progressively increased from 220 cmH2O, the SA
generally remained occluded until positive pressure
was applied, which resulted in SA reopening. As the
positive pressure was increased during control and
saline maneuvers, there was progressive SA widening,
and Dap at 120 cmH2O did not differ from starting
values (at 0 cmH2O before negative pressure). The Dap
after saline did not differ from control values at any
level of applied pressure (all P values .0.1). In contrast, as the pressure was increased from 220 to 0
Fig. 5. Anteroposterior (A-P) diameter of human pharynx at cranial
limit of C3 (Dap ) as pressure applied to SA decreases from 0 to 220
cmH2O, then returns to 0 cmH2O and increases to 120 cmH2O. Data
are means from 5 subjects. Pharyngeal diameters are expressed as a
percentage of each subject’s control Dap at zero airway pressure
before pressure application. Vertical lines represent 1 SE, and where
not visible, SE is zero or very small. j, control; l, saline; p,
surfactant. * P , 0.05 for control vs. surfactant. † P , 0.05 for saline
vs. surfactant. Note the larger Dap after surfactant relative to control
and saline both before airway closure and after airway reopening.
81
cmH2O after surfactant, there was a spontaneous SA
reopening in all subjects. After reopening, there was a
progressive SA widening as the pressure increased to
120 cmH2O. Hence, Dap after surfactant was larger
relative to control at 215 (before 220 cmH2O), 25, 0,
15, 110, and 115 cmH2O (all P values ,0.05) and
larger relative to saline at 215 (before 220 cmH2O), 0,
15 and 110 cmH2O (all P values ,0.02).
As demonstrated in Fig. 5, a counterclockwise hysteresis was apparent in the Dap-pressure relationship at
pressures between 0 and 220 cmH2O. Consequently,
Dap during control and saline was significantly larger
as pressure decreased from 0 to 220 cmH2O than at
corresponding pressures during the increase from 220
to 0 cmH2O (P , 0.05 at 0 and 25 cmH2O, respectively).
In contrast, after surfactant was applied into the SA,
hysteresis appeared diminished (Fig. 5) and did not
reach statistical significance at airway pressures between 0 and 220 cmH2O (all P values .0.1).
MTA EMGgg activity did not change after application of saline or surfactant into the SA at any pressure
applied to the SA (all P values .0.05) (Fig. 6). However,
relative to control levels, MTA EMGgg activity tended
to be elevated after saline or surfactant at pressures of
110 and 120 cmH2O (Fig. 6).
DISCUSSION
The principal finding of this study in awake normal
subjects is that the pharyngeal airway is more resistant to collapse and closure from negative intraluminal
airway pressure after application of a synthetic lung
Fig. 6. Moving time average (MTA) electromyographic (EMG) activity of genioglossus muscle in arbitrary units (AU) as SA pressure
decreased from 0 to 220 cmH2O and then increased to 120 cmH2O
during voluntary glottic closure. Data are means 11 SE from 4
subjects. Stippled bars, control; open bars, saline; solid bars, surfactant. Note that genioglossus MTA EMG activity did not change
significantly after application of saline or surfactant into the SA at
any pressure applied to SA.
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SURFACTANT AND HUMAN PHARYNGEAL PATENCY
surfactant preparation into the SA. Furthermore, reopening of the occluded pharynx is facilitated after
surfactant. In contrast, application of saline into the SA
lumen had no consistent effect on SA patency, although
the SA was subjected to the same maneuvers during
the saline runs as during the surfactant runs. Measurements of pharyngeal airway diameter tended to be
larger after surfactant, relative to control and saline
values, at any given pressure applied to the SA within
the 220 to 120 cmH2O range (except at 220 cmH2O,
where the pharyngeal diameter invariably equaled
zero). In contrast, saline had no consistent effect on Pcl,
Pop, or Dap at any of the pressures examined in this
study. In addition, MTA EMGgg activity did not change
significantly after application of saline or surfactant
into the SA at any of the measured pressures applied to
the SA in this study.
We previously developed and evaluated an X-ray
fluoroscopic method for studying the pressure-diameter
relationship of the isolated human SA in the absence of
inspiratory effort (27). This method was employed in
the present study to determine Pcl, Pop, and the
pressure-Dap relationship. The method provided a lateral view of the entire SA that enabled us to identify the
oropharynx and nasopharynx as initial sites of apparent SA closure. One limitation of the technique was the
inability to examine changes in the pharyngeal crosssectional configuration, as our measurements of SA
diameter were only made in the A-P dimension. In
addition, intrapharyngeal or esophageal catheterization was not performed as an independent means of
determining airway closure, because we wished to
avoid any possible influence of upper airway catheterization on our results. Therefore, we cannot claim with
certainty that 0-mm-diameter A-P measurements during negative airway pressure represent total airway
closure.
The retropalatal and oropharyngeal airways lack
rigid or bony support and consequently are susceptible
to collapse. The collapsible nature of these upper
airway segments is clearly evident during obstructive
sleep apnea where they are the primary sites of inspiratory narrowing and closure (22, 23). It has been postulated that pharyngeal patency is dependent on the
balance between opposing forces generated by the
respiratory pump muscles and the upper airway dilator
muscles (18). The former promotes upper airway closure during inspiration by generating negative intraluminal pressure, whereas the latter opens or stabilizes
the pharyngeal airway. However, the factors that determine pharyngeal patency are not fully understood, and
the above model ignores the potential role of intraluminal surface forces.
Clinical lung surfactant preparations such as Exosurf reduce surface tension (5). Our observation that
application of Exosurf lung surfactant into the SA can
improve SA patency in awake human subjects is,
therefore, consistent with a substantial role for intraluminal surface forces in the maintenance of pharyngeal
patency when the airway lumen is exposed to modest
negative and positive pressures. The findings of this
study agree with previous reports of facilitated reopening of the occluded upper airway and reduced upper
airway resistance after application of surfactants into
the oropharynx of anesthetized dogs (15, 28). In addition, snoring is reduced after application of surfactants
into the upper airway of sleeping human subjects (11)
and anesthetized dogs (28). This also is consistent with
improved upper airway patency after instillation of
surfactant into the pharyngeal airway. However, our
study is the first to demonstrate that pharyngeal
collapse and closure from modest negative intraluminal pressure is attenuated after the application of
surfactant into the SA.
The mechanism(s) by which surfactants may promote upper airway patency is not known, and a number
of possibilities need to be considered. These include 1)
moistening of the upper airway intraluminal surfaces
by the liquid surfactant preparation; 2) increased upper airway muscle activity after surfactant; 3) the role
of chemical additives in the synthetic lung surfactant
preparation; 4) the role of surfactant in reducing surface tension in the upper airway; 5) the role of surfactant in decreasing adhesion between apposed intraluminal surfaces; and 6) the role of surfactant in reducing
friction between apposed intraluminal surfaces.
First, moistening of upper airway intraluminal surfaces does not improve SA patency, as application of
saline into the upper airway had no consistent effect on
upper airway patency. Little is known about the effects
of saline and surfactant on the rheological properties of
mucus lining the SA. However, the lack of consistent
effect of saline on SA patency in this study is consistent
with studies in anesthetized animals (15, 28). In addition, it is conceivable that the hypotonicity of the
surfactant preparation used in our study may have
improved upper airway patency by drawing fluid into
the pharyngeal lumen by osmosis. However, this seems
unlikely, as the osmolarity of the hypotonic saline used
in this study was intentionally matched to that of the
synthetic lung surfactant preparation, but the hypotonic saline had no consistent effect on SA patency.
Second, increased EMGgg activity has been reported
together with reductions in upper airway resistance
and snoring after applying surfactants into the oropharynx of anesthetized dogs (28). Because the genioglossus
muscle is an important pharyngeal dilator (2, 18), it is
feasible that surfactants may improve pharyngeal patency by recruitment of upper airway dilator muscles
such as the genioglossus. However, Miki et al. (15)
observed facilitated reopening of the occluded upper
airway and reduced upper airway resistance after
applying an artificial lung surfactant into the oropharynx of anesthetized dogs with bilaterally sectioned
hypoglossal nerves. This demonstrates that surfactants can improve upper airway patency in the absence
of reflex genioglossus muscle recruitment. In the present study, MTA EMGgg activity did not change after
application of surfactant into the SA. Although MTA
EMGgg activity tended to increase after surfactant at
110 and 120 cmH2O (Fig. 6), this would not have
directly influenced the surfactant-related decreases in
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SURFACTANT AND HUMAN PHARYNGEAL PATENCY
Pcl and Pop, as these pressures were invariably subatmospheric after surfactant. Furthermore, the lack of
surfactant-related changes in the resting Dap (before
application of pressure to the SA) argues against a
sustained recruitment of upper airway muscles by
surfactant in the absence of significant upper airway
pressures.
Although recruitment of raw EMGgg action potentials and increased MTA EMGgg activity consistently
occurred in each subject during tongue protrusion,
negative airway pressure failed to recruit EMGgg
activity in the present study. This differs from findings
of other studies that have shown recruitment of EMG
activity in the genioglossus and other upper airway
muscles during negative airway pressure in awake
human subjects and anesthetized animals (12, 14, 25,
26). However, Horner et al. (12) demonstrated substantial intersubject variation in the magnitude of EMGgg
recruitment during negative airway pressure, with
small levels of recruitment in some human subjects. In
addition, in the study by Horner et al., EMGgg recruitment was smaller during voluntary glottic closure than
when the glottis was open. Therefore, it is possible that
EMGgg recruitment was not observed during negative
airway pressure in the present study because EMG
measurements were only made in a small number of
subjects during voluntary glottic closure. Consequently, we cannot exclude the possibility that Exosurf
lung surfactant enhanced SA patency during negative
airway pressure as a result of increased recruitment
from the genioglossus muscle. In addition, EMG activity was only sampled from the genioglossus muscle, so
that involvement of other upper airway muscles cannot
be excluded.
Third, the surfactant preparation used in our study
contains a number of chemical additives. Exosurf is a
synthetic lung surfactant preparation that reduces the
severity of respiratory distress syndrome in human
infants (1, 3). This synthetic lung surfactant preparation is a mixture of the dominant phospholipid constituent of endogenous pulmonary surfactant (dipalmitoylphosphatidylcholine) and the additives hexadecanol
and tyloxapol, which have surfactant properties of their
own (5) and are believed to enhance dispersion and
adsorption of dipalmitoylphosphatidylcholine in the
lungs. The present study does not enable us to specifically identify which of the constituents of the Exosurf
surfactant preparation was responsible for the improved patency of the SA. Nevertheless, the results of
this and previous studies (11, 15, 28) have shown that a
variety of surfactant preparations with different constituents can improve upper airway patency, suggesting that the surface-active properties of the preparations are primarily responsible.
The role of surface tension forces within the pharyngeal airways has not been investigated. In general
terms, the Laplace equation describes the collapsing
pressure generated by surface tension within cylinders
as being inversely proportional to the internal radius.
This suggests that surface tension forces would exert
little collapsing pressure within the large pharyngeal
83
airway. However, the pharynx should not be regarded
as a simple uniform hollow cylinder. The crosssectional appearance of the human retroglossal and
retropalatal airways is variable and frequently shows
marked narrowing in the A-P direction (6, 13, 21, 23).
The narrowing is most apparent laterally and frequently gives rise to narrow pleats projecting into the
pharyngeal lumen. In awake supine human subjects,
the most lateral portions of the pharyngeal pleats may
normally be occluded with apparent apposition of the
pharyngeal surfaces (13). Theoretically, surface tension
forces acting to collapse the pharynx may be substantial at the most lateral patent sections of the narrow
pharyngeal pleats where the radius of curvature will be
very small. In these regions, pharyngeal collapse may
commence in the most lateral patent segment of the
pharyngeal pleats and advance medially because of the
influence of surface tension forces. In support of this,
changes in cross-sectional pharyngeal shape are greater
in the lateral than in the A-P direction during nasally
applied positive airway pressure (13) and resting tidal
breathing (21). We speculate that surface tension forces
in the pharyngeal pleats may exert a substantial
collapsing force on the pharyngeal walls and contribute
significantly to the decrease in the overall crosssectional area of the pharynx during negative airway
pressure. Under these circumstances, a decrease in
intrapharyngeal surface tension forces by surfactant
may sufficiently attenuate pharyngeal collapse to account for the surfactant-related changes in Pcl and Dap
observed in this study. In addition, the radius of
curvature of the most lateral patent region of the
pharyngeal pleats may decrease as the pharynx collapses, resulting in progressively greater surface tension forces acting on the lateral pharyngeal walls. This
could explain why surfactant did not significantly
influence Dap until the pharynx had partially collapsed
during the application of negative pressure (Fig. 5).
Furthermore, the cross-sectional diameter of patent
sections of the SA may be very small when the SA
begins to reopen, so that a reduction in intraluminal
surface tension by surfactant may significantly reduce
Pop. Thus decreased surface tension forces in the
pharyngeal airway after surfactant may account for the
improved pharyngeal patency during negative and
positive airway pressures and may play a role in
preventing upper airway collapse.
Another potential mechanism of action of surfactants
to be considered is that of reducing adhesion between
already apposed intraluminal pharyngeal surfaces. Both
the present and a previous study from our laboratory
(27) have demonstrated that airway pressures less
negative than the closing pressure are required to
reopen the occluded upper airway. This supports findings from other workers (16, 17, 19, 30) and suggests
that apposed intraluminal upper airway surfaces are
adherent. Interestingly, both endogenous and exogenous phospholipid surfactants have been shown to
reduce adhesion in vitro (4, 7). Furthermore, endogenous surfactants appear to directly bond to the epithelial surfaces of many organs (9, 10, 24), the adsorbed
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SURFACTANT AND HUMAN PHARYNGEAL PATENCY
surfactant lining being likened to a thin polyethylene
layer that resists adhesion and makes epithelial surfaces hydrophobic (8, 9, 10). Therefore, we postulate
that surfactants applied into the SA are adsorbed to
intraluminal SA surfaces and that this may facilitate
reopening of the occluded SA by reducing adhesion
between apposed intraluminal airway surfaces.
The final mechanism of action of surfactants to be
considered is that of lubrication of the intraluminal SA
surfaces. Surfactants have long been used as lubricants
to reduce friction, and surface-active phospholipids
may potentially be highly effective lubricants in vivo
(8). It is possible that apposed intraluminal pharyngeal
surfaces slide over one another during pharyngeal
collapse and reopening, with the friction between the
sliding surfaces opposing the forces acting to change
pharyngeal patency. If surfactant acts to decrease these
frictional forces, then this would facilitate both pharyngeal collapse and reopening. However, this is inconsistent with our observation that the pharynx was more
resistant to collapse after surfactant was applied into
the SA. This suggests that a lubricant action by surfactant is not a major mechanism responsible for increased pharyngeal patency during negative and positive airway pressures after surfactant has been applied
into the SA.
Although we cannot exclude a possible contribution
from upper airway muscle recruitment, it seems likely
from the foregoing discussion that the observed improvements in SA patency after surfactant in our study are
predominantly due to reductions in pharyngeal intraluminal surface tension and adhesion, which are effects
directly attributable to properties of the surfactant.
Hysteresis was evident in the control Dap-pressure
relationship (Fig. 5), so that Dap was larger as the
applied pressure decreased from 0 to 220 cmH2O than
at corresponding pressures as pressure increased from
220 to 0 cmH2O. This confirms findings from a previous
study (27) where we speculated that surface tension
may contribute to the hysteresis of the pharyngeal
diameter-pressure relationship. Indeed, hysteresis appeared diminished after application of the surfactant
preparation into the SA, supporting our view that
surface forces are a major factor accounting for the
hysteresis of the Dap-pressure relationship.
Our observation that an application of a synthetic
lung surfactant into the SA helps prevent collapse and
facilitates reopening of the pharyngeal airway suggests
that exogenous surfactants may potentially have a
therapeutic role in the treatment of upper airway
disorders such as the OSA syndrome. Airway resistance
increases disproportionately with decreases in intraluminal airway diameter. Consequently, even modest
surfactant-related increases in pharyngeal diameter
may limit the inspiratory pharyngeal collapsing pressures generated by OSA patients during sleep, hence
providing some protection against the occurrence of
airway obstruction. In addition, the enhancement of
pharyngeal reopening by surfactant may reduce the
duration of obstructive apneas and the degree of oxygen
desaturation.
The authors express their appreciation to K. Byth for statistical
advice.
This study was supported by the National Health and Medical
Research Council of Australia and by Wellcome, Australia.
Address for reprint requests: T. Van der Touw, Intensive Care
Unit, Westmead Hospital, Westmead NSW 2145, Australia.
Received 27 February 1996; accepted in final form 13 August 1996.
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