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Faculty of Medicine, University of Calgary, Calgary, Alberta, Canada T2N 4N1
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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We described the dynamic
mechanical properties of the passive pharynx in Vietnamese pot-bellied
pigs and the effects of caudal tracheal displacement. During general
anesthesia and neuromuscular blockade, airflow through the upper airway
(
) and pharyngeal cross-sectional area were measured during
ramp decreases in pressure downstream from the pharynx (Pdown).
Measurements were made with 0, 1, and 2 cm of caudal tracheal
displacement. Airflow limitation and/or negative pressure dependence
(NPD) were observed in all animals. Tracheal displacement (2 cm)
increased maximal (max) by
205.1 ± 105.1% (P < 0.05) relative to the value
with no displacement and increased the magnitude of NPD, expressed as
percent decrease in from max, from
22.9 ± 27.4 to 56.6 ± 37.5% (P < 0.05). Initial decreases in Pdown narrowed all levels of the
pharynx, but, once max was reached, further
decreases in Pdown narrowed the hypopharynx but not the nasopharynx and
oropharynx. We conclude that the hypopharynx is the flow-limiting site
in the pig pharynx. Tracheal displacement not only improved airflow
dynamics as max increased but also resulted in
pronounced NPD.
caudal tracheal displacement; sleep apnea; airflow limitation; negative-pressure dependence
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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OBSTRUCTIVE SLEEP-DISORDERED breathing (OSDB) is characterized by airflow limitation and dynamic pharyngeal collapse during inspiration. A fundamental mechanism underlying pharyngeal collapse during sleep is sleep-related decrements of pharyngeal dilator muscle activity. However, why some patients with OSDB display repeated occlusion of the pharynx and others display high upper airway (UAW) resistance is unclear. Also, little is known about the mechanical behavior of the pharynx during flow limitation and dynamic collapse. The first systematic observations of pharyngeal mechanics, including cross-sectional area (A), under dynamic conditions in people with OSDB were provided by Isono et al. (6). They investigated the interrelationships between A, driving pressure, and airflow of the hypotonic velopharynx of obstructive sleep apnea (OSA) patients. During flow limitation, a progressive decrease in A of the velopharynx was observed.
Dynamics of airflow through the UAW () are often interpreted
by using a Starling resistor model (5, 8, 10, 13-15). Flow limitation occurs in a Starling resistor when intraluminal pressure equals the pressure external to the tube, the critical pressure (Pcrit). Experimentally, Pcrit is measured as the intraluminal pressure at the site of pharyngeal collapse when flow limitation first
occurs. The flow-limiting site in this model is defined in terms of
intraluminal pressure as the most downstream point where intraluminal
pressure does not vary with driving pressure. Studies using the
Starling resistor model have shown that maximal
(max) is modulated by Pcrit as well as
resistance upstream from the flow-limiting site (Rup). The Starling
resistor model has provided insight into pharyngeal airflow dynamics,
such as the importance of collapsibility of the flow-limiting site.
This model, however, does not explain the mechanism of flow limitation, nor does it predict the geometry of the airways responsible for flow limitation.
Wave speed theory, as presented by Dawson and Elliot (2),
provides a more quantitative framework for understanding the mechanics
of flow limitation. This theory recognizes that collapsible tubes, such
as airways, cannot carry a greater flow than the flow for which the
fluid velocity equals wave speed at some point in the system. The
pertinent wave speed is the speed at which a small disturbance travels
in a compliant tube filled with fluid. The point in the tube at which
local fluid velocity first reaches wave speed is called the "choke
point." The wave speed (c) is related to the choke point
area (A), the choke point area's rate of change with
transmural pressure (P), and gas density (
).
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(1) |
This study investigates airflow dynamics and pharyngeal A in the isolated UAW of Vietnamese pot-bellied pigs. When obese, these animals exhibit OSDB characterized by high UAW resistance, snoring, and inspiratory flow limitation but not obstructive apneas (16). The first objective of this study was to describe the relationship between pressure, airflow, and pharyngeal A during ramps in negative pressure applied to the passive pharynx. We hypothesized that flow limitation would occur with formation of a choke point in the pharynx and that location of the choke point could be predicted from the local static mechanical properties of the pharyngeal segments described in the accompanying study (17). The second objective was to investigate the effects of caudal tracheal displacement on the dynamic mechanics of the passive pharynx. Caudal displacement of the trachea decreases pharyngeal collapsibility in the passive pig pharynx (17) as well as other preparations (10, 15) and contributes to the decrease in UAW resistance during spontaneous breathing in conjunction with inspiratory activity of pharyngeal dilator muscles (18). We hypothesized that caudal tracheal displacement would increase maximal airflow through the pharynx by stiffening and/or increasing cross-sectional area of the choke point according to Eq. 1.
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Isolated UAW preparation.
Eight mature Vietnamese pot-bellied pigs were studied (5 females and 3 males). Animal characteristics are provided in Table 1 of the
accompanying study (17); pig 6 from Table 1 was
not included in this study. All protocols were approved by the Animal Care Committee at the University of Calgary. As described in detail in
the accompanying study (17), an isolated UAW preparation was used. Briefly, during general anesthesia, the cervical trachea was
transected and the animal was ventilated via the caudal trachea. A
laryngeal cannula was inserted in the rostral trachea with the tip
placed just rostral to the vocal cords. In contrast to the accompanying
study (17), the mouth of the animal was sealed but the
nose was unimpeded. The animal was paralyzed throughout the experiment
by intravenous administration of pancuronium bromide (0.1 mg/kg every
15 min). Atropine (0.04 ml/kg iv) was administered as needed to
minimize airway secretions.
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Experimental procedures.
Ramps in negative pressure lasting 12 s and reaching approximately
50 cmH2O were generated by the blower and applied to the UAW. Pharyngeal images were recorded on videotape, and and Pdown were digitally acquired at a sampling frequency of 100 Hz by
using commercial data acquisition software (DataPac, Run Technologies). A time-code generator (model VC-405, Thalner Electronics Laboratories) synchronized the videotaped images with the Pdown and
signals. Negative pressure ramps were repeated in each animal, while
the nasopharynx, oropharynx, and hypopharynx were imaged at 0, 1, and 2 cm of caudal tracheal displacement as described in the accompanying study (17); under any given condition, ramps were repeated
three times.
Data analysis.
max reached during the ramp and the value of
Pdown at the time of max were calculated for
each ramp, and the mean value of the three ramps was used for analysis.
Flow limitation was defined as no change in with decreasing
Pdown, and negative pressure dependence (NPD) was defined as a decrease
in with decreasing Pdown. The magnitude of NPD was
expressed as the percent decrease in from
max. In animals in which Pup was measured, Rup
at the onset of max was calculated by dividing
max by Pup.
max and magnitude of NPD from ramps performed
with the endoscope in the nasopharynx (scope in) were compared with
ramps without the endoscope or the endoscope in the larynx (scope out).
According to wave speed theory, the choke point will occur in the
airway where A · dP/dA (where P is airway
pressure) is at a minimum.
A · dP/dA of the pharyngeal segments at
the onset of flow limitation was estimated as follows: Pdown at the
onset of flow limitation (Pdown at max) was used
as an estimate of intraluminal pressure at the choke point as airflow
first became limited. A of each pharyngeal segment at Pdown
at max was calculated from the static
A-airway pressure relationship from the accompanying study
(17) and used to calculate
A · dP/dA.
Statistical analysis.
The effects of tracheal displacement on the dependent variables
(max, Pdown at max, and
the magnitude of NPD) were analyzed by using a repeated-measures,
two-factor ANOVA. Tukey's test was used for post hoc analysis. The
relationship between static and dynamic mechanics of the pharynx was
analyzed by determining the strength of association between compliance
(dA/dP) and closing pressure (Pclose) from the accompanying
study (17) and max by using Pearson
product-moment correlation.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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UAW pressure-flow relationships.
We hypothesized that flow limitation would occur during application of
negative pressure ramps to the passive pharynx as indicated by a
plateau in with increasingly negative Pdown. Flow
limitation was frequently observed, but, unexpectedly, NPD was also a
common observation. Four typical patterns of during
negative pressure ramps are shown in Fig.
1. Two patterns were characterized by an
increase in to a maximal value followed by flow limitation for the remainder of the ramp (type A) or for some of the
ramp followed by NPD (type B). The other two patterns were
characterized by an increase in to a maximal value followed
by gradual NPD (type C) or abrupt NPD (type D).
Of the ramps analyzed at 0 cm of tracheal displacement, all animals
demonstrated either flow limitation or NPD; two of eight animals had
flow limitation alone (type A), two NPD alone (types
C and D), and four had flow limitation and NPD
(type B).
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Effect of endoscope placement on max and
magnitude of NPD.
The possibility that the presence of the endoscope in the pharynx could
affect airflow dynamics was investigated. In three animals, we compared
the results of pressure ramps with the endoscope in the nasopharynx
(scope in) and the endoscope removed or the tip of the endoscope in the
larynx (scope out). Ramps performed with 2 cm of tracheal displacement
applied to the UAW were compared because the magnitude of NPD was
greatest during this condition (see below). In two of three animals,
the magnitude of NPD was significantly less with scope in compared with
scope out (P < 0.05) (Fig.
2); in one of three animals,
max was significantly greater with scope in
compared with scope out (P < 0.05; Fig. 2). Because
the presence of the endoscope in the pharynx appeared to affect airflow
dynamics, only data from ramps performed without the endoscope in the
pharynx, or with the endoscope in the larynx, were used for comparisons
of the dependent variables (max, Pdown at
max, magnitude of NPD).
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Effect of tracheal displacement.
We hypothesized that tracheal displacement would increase
max. As expected, tracheal displacement
significantly increased max (2 vs. 0 cm of
displacement, P < 0.05; Fig.
3). The Pdown at which
max was reached tended to decrease with tracheal
displacement, although this was not statistically significant.
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from max. An
example of a pronounced increase in NPD with tracheal displacement in one animal is shown in Fig. 4. To
determine whether tracheal displacement consistently increased the
magnitude of NPD, the average change in the magnitude of NPD with 1 and
2 cm of displacement was calculated for all animals. The magnitude of
NPD in all animals was significantly greater with 2 cm of
tracheal displacement compared with 0 cm (magnitude of NPD was
22.9 ± 27.4, 50.0 ± 36.9, and 56.6 ± 37.5% for 0, 1, and 2 cm of tracheal displacement respectively).
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Pharyngeal A.
Adequate images of the nasopharynx during negative pressure ramps were
attained in two animals, of the oropharynx in two animals, and of the
hypopharynx in three animals. Representative examples of pharyngeal
A during negative pressure ramps are shown in Fig. 5. The behaviors of the nasopharynx and
oropharynx were similar in that changes in A mirrored
changes in ; A quickly decreased with the rapid
increase in at the beginning of the ramp, and minimum
A was reached at the same time as
max. Once minimum A was reached,
A of the nasopharynx and oropharynx either did not change or
increased as Pdown continued to decrease. Thus, once
max was reached, the downstream pressure
disturbance produced by lowering Pdown was unable to propagate to these
segments, indicating that the choke point was caudal to these segments.
In contrast to the nasopharynx and oropharynx, A of the
hypopharynx decreased abruptly at the beginning of the ramp and
continued to decrease throughout the ramp, with A
approaching 0. Because the hypopharynx narrowed as Pdown
decreased, even after the onset of flow limitation, the choke point was
either at, or just caudal to, the hypopharynx. An example of
hypopharyngeal A during pronounced NPD is shown in Fig.
6; similar to flow limitation, NPD was
associated with pronounced narrowing of the hypopharynx. Nasopharyngeal
and oropharyngeal A did not decrease during NPD (data not
shown), suggesting that the choke point location during flow limitation
and NPD were similar.
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max was correlated with
dA/dP and Pclose of the pharyngeal segments from the
accompanying study (17), a negative correlation was found
between max and dA/dP of the hypopharynx (r2 = 0.85, P = 0.07) but not other pharyngeal segments for measurements made with 0 cm of tracheal displacement. Thus a more compliant choke point led to
lower values of max. No significant correlations were found between max and dA/dP or
Pclose measured at 2 cm of tracheal displacement for any pharyngeal segments.
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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We described the dynamic mechanics of the passive pharynx in
Vietnamese pot-bellied pigs by using wave speed theory as a framework. The passive pharynx exhibited flow limitation and NPD during negative pressure ramps. Both direct observation of pharyngeal A
during negative pressure ramps and estimation of
A · dP/dA are consistent with the choke
point occurring at, or just rostral, to the hypopharynx. Application of
caudal tracheal displacement to the pharynx increased max, likely due to increased choke point
stiffness, as predicted by wave speed theory. Interestingly, another
effect of tracheal displacement was to increase the magnitude of NPD.
Methodological issues. The mouth of the animal was sealed, allowing nasopharyngeal airflow only during negative pressure ramps. Limiting airflow to the nasopharyngeal route may mimic the normal route of breathing in the intact animal because of the retropalatal configuration of the epiglottis in the pig (11). A study using a rat isolated airway preparation did report a significant difference in airflow dynamics during medial XII nerve stimulation with the oropharyngeal airway sealed compared with unsealed (5). However, these effects were attributed to differences in activation of pharyngeal musculature, and thus they may not be relevant in a paralyzed preparation.
Dynamic mechanics of the pharynx in other species have been investigated with use of similar isolated UAW preparations (5, 8, 10, 15). In these experiments, measurements of Pup were made with the catheter tip carefully positioned just rostral to the flow-limiting site. The flow-limiting site was determined as the most downstream point in the pharynx at which Pup did not vary with changes in Pdown at the onset of flow limitation; the flow-limiting site was located just rostral to the rim of the soft palate in the rat and cat. The nadir in Pup reached at the onset of flow limitation was used as a measure of Pcrit. Our Pup measurements were not made at, or just rostral to, the flow-limiting site (the hypopharynx), but rather they were made upstream from the hypopharynx at the nasopharynx. Thus our Pup measurements are not analogous to Pcrit as an additional pressure drop likely occurred across the oropharynx. The presence of the endoscope in the pharynx during measurements may have influenced airflow dynamics because the magnitude of NPD tended to be less during ramps with the endoscope in the nasopharynx compared with ramps without the endoscope in the pharynx. Perhaps the pronounced narrowing of the hypopharynx observed during NPD was mechanically impeded by the endoscope, minimizing the decrease in. To
avoid the possible confounding effects of the endoscope, only data from
ramps performed without the endoscope in the pharynx were used for
comparisons of dependent variables.
Dynamic mechanics. The passive pharynx of the pig exhibited flow limitation during ramps in negative pressure. According to wave speed theory, when airflow velocity reaches wave speed (predicted by Eq. 1), the pressure disturbance, in this case a decrease in Pdown, can no longer propagate upstream and affect flow; the change in Pdown is no longer "seen" by upstream segment and flow becomes independent of, or uncouples from, Pdown. The choke point is defined by wave speed theory in terms of local properties of the tube, specifically, where A · dP/dA is at a minimum. This is equivalent to the "flow-limiting site" of the Starling resistor model, which is defined in terms of pressure: the most downstream point in the tube where intraluminal pressure does not vary with driving pressure. In this study, we used pharyngeal A instead of intraluminal pressure to determine choke point location and estimated A · dP/dA from static A-airway pressure relationships determined in the accompanying study (17).
Flow limitation in the passive pharynx of these animals was associated with narrowing of the hypopharynx but not of the oropharynx or nasopharynx. Because A is determined by intraluminal pressure, the most downstream point in the tube where A, and thus intraluminal pressure, did not vary with driving pressure was caudal to the oropharynx but not caudal to the hypopharynx. Our estimates of A · dP/dA for the three pharyngeal segments also predicts choke point formation at the hypopharynx as A · dP/dA was most often at a minimum for this segment. Therefore, the onset of flow limitation in the passive pig pharynx is associated with choke point formation at or near the hypopharynx. Because only discrete portions of the pharynx were investigated, the choke point location cannot be precisely pinpointed within the pharynx. Also, we cannot exclude the possibility of some degree of axial movement of the choke point that may occur during flow limitation or with tracheal displacement. Because hypopharyngeal narrowing was always observed after the onset of flow limitation but never naso- or oropharyngeal narrowing, rostral migration of the choke point was likely minimal. NPD was also a frequent feature of airflow dynamics in the passive pharynx of these animals and was associated with pronounced narrowing of the hypopharynx. NPD has been described previously in isovolume pressure-flow curves of expiratory flow (4), isolated tracheae (3), and the hypotonic human pharynx (6). However, little is known about mechanisms of NPD. Although elucidating mechanisms of NPD was not an original objective of this study, NPD was a prominent feature of airflow dynamics in this preparation and warrants speculation as to its origins. Wave speed theory offers an explanation of NPD in collapsible tubes via the effects of longitudinal tension on choke point properties during flow limitation (19). Longitudinal tension in a tube contributes to the radial force balance if there is a curvature in the wall in the axial direction. For example, longitudinal tension in a tube with a constricted segment contributes an outward radial force acting on the tube wall at the constriction that is proportional to the magnitude of the tension and the degree of longitudinal curvature. Thus, in the presence of longitudinal tension, when flow through a tube just becomes limited and choke point area is small, the longitudinal curvature at the choke point in a tube may be large enough to cause a significant outward radial force. After the initial onset of initial flow limitation, a continued fall in Pdown can compress the tube just downstream from the constriction, reducing the curvature at the choke point. As the longitudinal curvature at the choke point is reduced, so is the magnitude of the outward radial force, decreasing choke point area, decreasing A · dP/dA, and thus decreasing wave speed, with a consequent reduction in from
max .
An alternate explanation for the observed NPD could be a shift from a
wave speed mechanism of flow limitation to viscous flow limitation.
Viscous flow limitation results from a coupling between tube compliance
and dissipative pressure losses as opposed to convective acceleration
of gas in the case of wave speed flow limitation (20).
Viscous flow limitation is typically associated with very small tubes,
such as the peripheral airways of the lung, where the maximal flow
allowed by wave speed mechanisms is larger than that allowed by viscous
flow limitation. In the case of the hypopharynx, perhaps the initial
onset of flow limitation was due to wave speed mechanisms, when
hypopharyngeal A was relatively large. Because
hypopharyngeal A became very small with continued decreases
in Pdown, it may be possible that viscous flow limitation occurred with
a maximal flow much lower than that due to wave speed, resulting in a
fall in airflow from the initial maximal value.
Although NPD was commonly observed in this isolated UAW preparation,
NPD was not a feature of airflow dynamics in the intact sleeping animal
(16). Thus the pronounced NPD observed in this study may
be characteristic of the passive pharynx only.
Effects of tracheal displacement.
In the accompanying study, we demonstrated that caudal tracheal
displacement decreased compliance of the hypopharynx in the passive pig
pharynx (17). According to wave speed theory, the local
properties of the choke point, the hypopharynx, determine the airflow
mechanics. Equation 1 states that wave speed velocity, and
thus max, is determined by the area and
compliance of the choke point; decreased compliance of the choke point
will increase max. In this study, tracheal
displacement did indeed increase max of the
passive pharynx. These findings of increased max with tracheal displacement are in agreement with studies using a feline
isolated UAW preparation (10, 15).
max when 2 cm of tracheal displacement were
applied to the UAW compared with a decrease of ~20% in the absence
of tracheal displacement (Fig. 4). Tracheal displacement would be
expected to increase longitudinal tension in the airway walls, and, as
discussed in the previous section, the presence of longitudinal tension
in the airway walls could result in NPD. However, significant NPD was
also observed in some animals with 0 cm of tracheal displacement when
very little longitudinal tension in the airway walls would be expected.
Effects of luminal shape. The hypopharynx was characterized not only by pronounced decreases in A during negative pressure ramps but also by changes in luminal shape. As the hypopharynx narrowed, the lumen changed from ovoid to slitlike with the long axis in a transverse plane. Such changes in luminal shape may affect airflow dynamics through the hypopharynx independent of changes in A.
Changes in luminal shape may affect airflow dynamics by altering the amount of energy dissipation due to friction. Frictional losses associated with fluid flow through a tube depend on A of the tube but also on cross-sectional shape of the tube such that deviations from circularity are associated with greater frictional losses (1). Thus an elliptical-shaped lumen would have greater frictional losses than a circular lumen of equal A. However, energy losses due to friction depend not only on A and cross-sectional shape of the tube but also on tube length and roughness, density and viscosity of the fluid itself, velocity of fluid flow, and quality of flow (laminar or turbulent) (1). Thus the impact of luminal shape changes on airflow dynamics of the pharynx cannot be predicted without consideration of these other factors. Such an analysis would be interesting but is beyond the scope of this study. Differences in luminal shape of the pharynx between normals and OSA patients have been acknowledged (9, 12), although the significance of this to the pathogenesis of OSA is unclear. Leiter (7) speculates that differences in luminal shape may affect the effectiveness of pharyngeal dilator muscle activity; we speculate that differences in luminal shape may also affect airflow dynamics independent of neuromuscular factors.Relationship between static and dynamic mechanics.
This study, in combination with the accompanying study
(17), is unique in that both the static and dynamic
mechanics of the passive pharynx were characterized in the same
animals. When static compliance of the pharyngeal segments was used to
predict the location of the choke point during dynamic airflow,
estimated A · dP/dA was most often at a
minimum at the hypopharynx. Choke point behavior of the hypopharynx was
corroborated by direct observation of pharyngeal A during
the negative pressure ramps. Dynamic behavior of the entire airway was
related to the static mechanical properties of the choke point; as
dA/dP of the hypopharynx increased,
max decreased. This relationship is predicted by
wave speed theory because wave speed, and therefore
max, is inversely related to wall stiffness of
the choke point (2). Thus the static mechanical properties
of the individual pharyngeal segments are consistent with the dynamic
behavior of the pharynx, including choke point location and effects of
tracheal displacement.
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ACKNOWLEDGEMENTS |
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The authors acknowledge the technical assistance of Jesse Charlton.
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FOOTNOTES |
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Address for reprint requests and other correspondence: J. E. Remmers, Faculty of Medicine, Univ. of Calgary, 3330 Hospital Dr. NW, Calgary, AB, Canada T2N 4N1 (E-mail: jeremmer{at}ucalgary.ca).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
First published February 8, 2002;10.1152/japplphysiol.00762.2001
Received 23 July 2001; accepted in final form 4 February 2002.
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REFERENCES |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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,
Outliers. * Significantly different from 0 cm of tracheal
displacement, P < 0.05.
, 1 cm of tracheal displacement; 
, 2 cm
of tracheal displacement. Negative value of
A · dP/dA occurred when Pdown at