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Influences of head positions and bite opening on collapsibility of the passive pharynx

Department of Anesthesiology (B1), Graduate School of Medicine, Chiba University, Chiba, 260-8670, Japan

Submitted 25 August 2003 ; accepted in final form 1 March 2004

	ABSTRACT

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION ACKNOWLEDGMENTS REFERENCES

 
A collapsible tube surrounded by soft material within a rigid box was proposed as a two-dimensional mechanical model for the pharyngeal airway. This model predicts that changes in the box size (pharyngeal bony enclosure size anatomically defined as cross-sectional area bounded by the inside edge of bony structures such as the mandible, maxilla, and spine, and being perpendicular to the airway) influence patency of the tube. We examined whether changes in the bony enclosure size either with head positioning or bite opening influence collapsibility of the pharyngeal airway. Static mechanical properties of the passive pharynx were evaluated in anesthetized, paralyzed patients with sleep-disordered breathing before and during neck extension with bite closure (n = 11), neck flexion with bite closure (n = 9), and neutral neck position with bite opening (n = 11). Neck extension significantly increased maximum oropharyngeal airway size and decreased closing pressures of the velopharynx and oropharynx. Notably, neck extension significantly decreased compliance of the oropharyngeal airway wall. Neck flexion and bite opening decreased maximum oropharyngeal airway size and increased closing pressure of the velopharynx and oropharynx. Our results indicate the importance of neck and mandibular position for determining patency and collapsibility of the passive pharynx.

obstructive sleep apnea; upper airway; closing pressure; neck positions; mouth opening

Structurally, the pharyngeal airway is surrounded by soft tissue such as the tongue, which is enclosed by bony structures such as the mandible and cervical vertebrae (Fig. 1). Consequently, a collapsible tube surrounded by soft material within a rigid box was proposed as a two-dimensional mechanical model for the pharyngeal airway (24). Pharyngeal bony enclosure size, anatomically defined as cross-sectional area bounded by the inside edge of the bony structures and being perpendicular to the airway, corresponds to the box size of the mechanical model. The contribution of obesity and craniofacial anomaly such as a small maxilla and mandible to the increased collapsibility of the passive pharynx was well explained by the mechanical model (24).

View larger version (26K): [in this window] [in a new window]   Fig. 1. Anatomic arrangements of the pharynx, tongue, mandible, and cervical vertebra (left) and a mechanical model for the structures (right). Plumen, pressure inside the collapsible tube; Ptissue, pressure surrounding the collapsible tube; Ptm, transmural pressure. In the mechanical model, the luminal size of the collapsible tube is determined by mechanical properties of the tube (tube law) and Ptm. Ptm is defined as the pressure difference between pressures inside (Plumen) and outside the tube (Ptissue). For a given Plumen, an increase in Ptissue results in reduction of Ptm and therefore narrows the lumen. Ptissue is determined by the balance between the amount of soft material inside the rigid box and the size of the surrounding rigid box.

	MATERIALS AND METHODS

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION ACKNOWLEDGMENTS REFERENCES

 
Subjects and overnight oximetry.   The study consisted of 24 male patients with SDB who were interested in undergoing uvulopalatopharyngoplasty and were scheduled to undergo endoscopic pharyngeal assessment to determine their indications for this procedure (4). All had histories of excessive daytime sleepiness, habitual snoring, and witnessed repetitive apnea. SDB was evaluated by a pulse oximeter (Pulsox-5; Minolta, Tokyo, Japan). All subjects were instructed to attach an oximetry finger probe before sleep and to remove the probe on awakening. Digital readings of arterial oxygen saturation (Sa_O2) and pulse rate were stored every 5 s in a memory card. The stored data were displayed on a computer screen to check the quality of the recordings. The computer calculated oxygen desaturation index, defined as the number of oxygen desaturation exceeding 4% from the baseline, and the percent of time spent at Sa_O2 <90%. Table 1 lists all nocturnal oximetry data and anthropometric characteristics. Although the oximetry evaluation alone does not clarify the nature of SDB, we believe that all patients can be safely diagnosed as having obstructive sleep apnea (OSA) on the basis of the oximetry results and the clinical symptoms (1).

Preparation of the subjects.   Each subject was initially premedicated with 0.5 mg of atropine and placed in the supine position on an operating table, where a modified tight-fitting nasal mask was attached. Care was taken to prevent air leaks from the mask, particularly when the airway was pressurized above 20 cmH₂O. General anesthesia was induced and maintained by intravenous infusion of propofol, and intravenous injection of a muscle relaxant (vecuronium 0.2 mg/kg) produced complete paralysis throughout the experiment while the subject was ventilated with positive pressure through an anesthetic machine. Sa_O2, electrocardiogram, and blood pressure were continuously monitored. The tip of a slim endoscope (FB10X, Pentax, Tokyo, Japan, 3 mm OD) was inserted through the modified nasal mask and the naris down to the upper airway to visualize the velopharynx (retropalatal airway), and the oropharynx (retroglossal airway). A closed-circuit camera (ETV8, Nisco, Saitama, Japan) was connected to the endoscope, and the pharyngeal images were recorded on a videotape. Reading of airway pressure (Paw), measured by a water manometer, was simultaneously recorded on videotape.

Experimental procedures.   To determine the pressure-area relationship of the pharynx, after disconnection from the anesthetic machine the nasal mask was connected to a pressure-control system capable of accurately manipulating Paw from –20 to 20 cmH₂O in a stepwise fashion. At cessation of mechanical ventilation of the subject under complete muscle paralysis, apnea resulted. Paw was immediately increased up to 20 cmH₂O to dilate the airway and then gradually reduced, within a 2- to 3-min span, from 20 cmH₂O to the closing pressure of the retropalatal airway in a stepwise fashion. The latter represented the pressure at which complete closure of the retropalatal airway occurred, as evident on the video screen. The apneic test was terminated when Sa_O2 fell below 95%. This procedure of experimentally induced apnea allowed construction of a pressure-area relationship of the visualized pharyngeal segment. The subject was manually ventilated for at least 1 min before and after the apneic test. Distance between the tip of the endoscope and the narrowing site was measured with a wire passed through the aspiration channel of the endoscope.

Each patient participated in either the head position study (n = 13) or the bite opening study (n = 11) (Fig. 2). In the head position study, in addition to the control measurement (head and neck in neutral position with bite closed by a chin strap), the apneic tests were repeated during both neck extension (neck maximally extended by placing cushions under the shoulders with bite closed by a chin strap) and neck flexion (neck maximally flexed by placing cushions under the head with bite closed by a chin strap) in seven patients, whereas the tests were performed either during only neck extension (n = 4) or neck flexion (n = 2) in the remaining six patients. The control measurement was performed once per patient. In the bite opening study, the bite was widened by inserting a mouthpiece between the upper and lower incisors in head and neck in the neutral position, producing a 15-mm distance between the incisors (Fig. 2). The apneic test was initiated immediately after establishment of each experimental condition. On the completion of the experiment time span of 30–60 min, atropine (0.02 mg/kg) and neostigmine (0.04 mg/kg) were administered to reverse muscle paralysis.

View larger version (40K): [in this window] [in a new window]   Fig. 2. Illustrative configuration of head, neck, and mandible positions for each experimental condition. Note that differences of the distance between the mentum and cervical column resulted from the configuration differences. A horizontal straight line for each condition represents the experimental table, and cushions were placed under the shoulders for the neck extension and placed under the head for the neck flexion.

View larger version (14K): [in this window] [in a new window]   Fig. 3. Accuracy of our area measurement for a constant distance. The measured area was systematically deviated from actual area.

Statistical analysis.   All values are expressed by median (25–75 percentiles). The Wilcoxon’s signed-rank test was used for comparison between the control and other conditions. Linear regression analysis was performed between observed and estimated closing pressures. P < 0.05 was considered to be significant.

	RESULTS

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION ACKNOWLEDGMENTS REFERENCES

 
Effects of neck extension on static pharyngeal mechanics.   Changes in static mechanical variables before and during neck extension are presented in Fig. 4. Neck extension approximately doubled A_max and significantly decreased both K and P'_close at the oropharynx. In addition to the significant influences on the oropharyngeal segment, P'_close at the velopharynx also significantly decreased during neck extension. Notably, there were tendencies of increase in A_max (P = 0.054) and decrease in K (P = 0.067) at the velopharynx, as shown in Fig. 4, although these are not statistically significant. The results indicate that neck extension dilates and stiffens the velopharyngeal and oropharyngeal airway, improving pharyngeal airway patency.

View larger version (27K): [in this window] [in a new window]   Fig. 4. Changes in static mechanical variables in response to neck extension (NE) at the velopharynx (A) and oropharynx (B). Each line represents a different patient. Amax, maximum cross-sectional area; K, a constant obtained by an exponential fitting the pressure-area relationship and representing stiffness of the pharyngeal airway wall; P'close, estimated closing pressure calculated from the fitted exponential function. Lower and upper boundaries of the box indicate 25th and 75th percentages. Solid line within the box marks the median, and vertical lines indicate the 90th and 10th percentages. *P < 0.05 vs. control.

View larger version (25K): [in this window] [in a new window]   Fig. 5. Changes in static mechanical variables in response to neck flexion (NF) at the velopharynx (A) and oropharynx (B). Each line represents a different patient. Lower and upper boundaries of the box indicate 25th and 75th percentages. Solid line within the box marks the median, and vertical lines indicate the 90th and 10th percentages. *P < 0.05 vs. control.

View larger version (26K): [in this window] [in a new window]   Fig. 6. Changes in static mechanical variables in response to bite opening (BO) at the velopharynx (A) and oropharynx (B). Each line represents a different patient. Lower and upper boundaries of the box indicate 25th and 75th percentages. Solid line within the box marks the median, and vertical lines indicate the 90th and 10th percentages. *P < 0.05 vs. control.

View larger version (14K): [in this window] [in a new window]   Fig. 7. Correlation between estimated closing pressure and observed closing pressure at a primary site of closure for all experimental conditions. , Control condition; , neck extension; , neck flexion; , bite opening.

	DISCUSSION

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION ACKNOWLEDGMENTS REFERENCES

Design and limitations of the study.   Although many previous studies have reported significant influences of neck positions (9, 10, 13, 16, 17, 25) and mouth opening on pharyngeal airway patency (11), this is the first study that purely evaluates regional structural changes of airway collapsibility by mechanical interventions under the elimination of neural mechanisms. Because techniques in obtaining closing pressures, study population, and amount of structural changes by the interventions differ between previous studies and this study, it is inappropriate to compare the amount of changes in closing pressures between the studies. However, it should be noted that the amount of increase in the closing pressure with bite opening in the active pharynx during sleep (3 cmH₂O) (23) is never smaller than that obtained in the passive pharynx (2–2.7 cmH₂O), suggesting no recruitment of the pharyngeal dilator muscles for compensation of the structural pharyngeal narrowing with bite opening during sleep. In contrast, the fact that bite opening did not influence upper airway collapsibility during wakefulness (23) strongly suggests the presence of neural compensatory mechanisms for the structural detrimental effects of bite opening during wakefulness.

Because of the systematic error of cross-sectional area measurement in our experimental setting, as shown in Fig. 3, the pressure-area relationship obtained in this study may systematically deviate from a true relationship. A true A_max is considered to be 10% greater than the measured A_max. Because A_max error mainly results from deformation of the endoscopic image at the outer area, statistical A_max differences before and during mechanical interventions may be valid. In contrast, P'_close may be minimally influenced by the measurement error due to the nature of the exponential curve. We consider that the significant discrepancy between observed P_close and estimated P'_close demonstrated in Fig. 7 is a result of cross-sectional measurement error, but rather of discontinuity of airway pressure changes (1-cmH₂O step changes) during the apneic test.

Mechanical model of the pharyngeal airway.   Assuming that neck extension increases the pharyngeal bony enclosure size, the two-dimensional model presented in Fig. 1 predicts reduction of pressure surrounding the collapsible tube (P_tissue) and increase of the airway size for a given pressure inside the collapsible tube (P_lumen). In contrast, neck flexion and bite opening increase P_tissue and decrease the airway size, with the assumption that these interventions decrease the pharyngeal bony enclosure size. In fact, the results of this study confirmed these predictions, although this does not indicate that change in the bony enclosure size is the only predominant mechanism for the observed changes in pharyngeal airway collapsibility during the mechanical interventions.

Reduction of pharyngeal airway compliance during neck extension.   One notable finding in this study is reduction of the oropharyngeal K value, i.e., reduction of the oropharyngeal airway wall compliance, during neck extension. This can be interpreted as flattening of the "tube law" curve of the pharyngeal airway in the two-dimensional mechanical model (Fig. 1). However, the mechanisms causing the tube law change are not clearly presented by the model. In anesthetized dogs, van de Graaff (22) previously demonstrated increase in the tracheal tension decreased upper airway resistance. Recently, Thut et al. (21) found significant changes of critical closing pressure during head positioning in association with changes in upper airway length in cats with isolated upper airways. In anesthetized humans, the distance between the tip of an endotracheal tube and the carina of the trachea increases during neck extension (20), suggesting airway lengthening, and therefore an increase in the longitudinal force during neck extension. Accordingly, significant reduction of the oropharyngeal airway compliance found in this study can result from an increase in longitudinal force during neck extension. A three-dimensional model including airway length and interaction between the soft tissue structures along the airway, which may significantly influence longitudinal force altering airway compliance, may be an advanced mechanical model for the pharyngeal airway and should be tested in the future by measuring P_tissue and longitudinal forces (Fig. 8). Furthermore, in a nonparalyzed condition like natural sleep, the role of pharyngeal airway dilator muscles also needs to be included in the model as an alternative mechanism.

View larger version (29K): [in this window] [in a new window]   Fig. 8. Advanced 3-dimensional mechanical model of the pharyngeal airway explaining the results of this study. Neck extension (left) possibly produces increase in box size and displacement of soft tissue into the box in addition to airway lengthening, resulting in decreasing Ptissue and increasing longitudinal force. In contrast, neck flexion and bite opening (right) may produce reduction of box size and displacement of the soft tissue out of the box, resulting in increasing Ptissue.

Clinical implications.   Craniocervical extension with a forward head posture is reported to be common in severe OSA patients during wakefulness (14, 19). Our results suggest that head posture compensates for increased collapsibility of the pharyngeal airway in these patients. Head position during sleep is mainly determined by the height and design of the pillow. Although use of a higher pillow often flexes the neck, which is not advantageous for airway maintenance, placement of a higher pillow with maintenance of the head straight up (sniffing position) produced higher cervical extension and improvement of pharyngeal airway patency in sedated children (18). Recently, Kushida et al. (8) found significant improvement of SDB in mild OSA patients with use of a pillow that promoted neck extension, whereas the improvement was not evident in severe OSA patients. Their finding agrees with our observation that 3 of 11 patients presented P'_close above atmospheric pressure even after neck extension, whereas neck extension significantly decreased P'_close. Accordingly, neck extension alone may not completely establish patent airway and normalize breathing in severe OSA patients.

Miyamoto et al. (12) reported that the total time spent with mouth opening during sleep was greater in OSA patients than in normal subjects, in accordance with previous observations by Hollowell and Suratt (2). They further reported that bite opening progressively increased during the apneic period and decreased at termination of apnea (12). Our results suggest that bite opening partly contributes to the occurrence and persistence of obstructive apnea; moreover, bite closure partly contributes to reestablishment of the patent airway. Furthermore, bite closure with the use of oral appliances can contribute to efficacy of oral appliances for treatment of SDB, although a recent clinical investigation did not support this (15).

We conclude that head positions and bite opening influence collapsibility of the passive pharynx in patients with SDB. The observations are well explained by a mechanical model for the pharyngeal airway (a collapsible tube surrounded by soft material within a rigid box).

	ACKNOWLEDGMENTS

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION ACKNOWLEDGMENTS REFERENCES

 
We appreciate the assistance of Sara Shimizu, M.D., who greatly helped to improve this manuscript.

	FOOTNOTES

 

Address for reprint requests and other correspondence: S. Isono, Dept. of Anesthesiology (B1), Graduate School of Medicine, Chiba University, 1-8-1 Inohana-cho, Chuo-ku, Chiba, 260-8670, Japan (E-mail: isonos-chiba{at}umin.ac.jp).

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.

	REFERENCES

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION ACKNOWLEDGMENTS REFERENCES

 

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This Article Abstract Full Text (PDF) Free All Versions of this Article: 97/1/339    most recent 00907.2003v1 Submit a response Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (11) Citing Articles via Google Scholar Google Scholar Articles by Isono, S. Articles by Nishino, T. Search for Related Content PubMed PubMed Citation Articles by Isono, S. Articles by Nishino, T.