J Physiol Volume 515, Number 1, 293-298, February 15, 1999
The Journal of Physiology (1999), 515.1, pp. 293-298
© Copyright 1999 The Physiological Society
Oral airway flow dynamics in healthy humans
T. C. Amis, N. O'Neill and J. R. Wheatley
Department of Respiratory Medicine, Westmead Hospital, and University of Sydney, Westmead, NSW 2145, Australia
MS 8501 Received 15 July 1998; accepted after revision 9 November 1998.
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ABSTRACT |
- Oral airway resistance (RO) is an important determinant of oro-nasal partitioning of airflow (e.g. during exercise and sleep); however, little is known of factors influencing its magnitude and measurement.
- We developed a non-invasive standardized technique for measuring RO (based on a modification of posterior rhinomanometry) and examined inspiratory RO in 17 healthy male subjects (age, 36 ± 2 years (mean ± s.e.m.); height, 177 ± 2 cm; weight, 83 ± 3 kg).
- Inspiratory RO (at 0·4 l s-1) was 0·86 ± 0·23 cmH2O l-1 s-1 during resting mouthpiece breathing in the upright posture. RO was unaffected by assumption of the supine posture, tended to decrease with head and neck extension and increased to 1·22 ± 0·19 cmH2O l-1 s-1 (n = 10 subjects, P < 0·01) with 40-45 deg of head and neck flexion. When breathing via a mouth-mask RO was 2·98 ± 0·42 cmH2O l-1 s-1 (n = 7) and not significantly different from nasal airway resistance.
- Thus, in awake healthy male subjects with constant jaw position, RO is unaffected by body posture but increases with modest degrees of head and neck flexion. This influence on upper airway patency may be important when oral route breathing is associated with alterations in head and neck position, e.g. during sleep.
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INTRODUCTION |
At rest, approximately 80 % of healthy subjects breathe exclusively via the nose (Uddstromer, 1940; Camner & Bakke, 1980; Niinimma et al. 1981). During exercise, the demand for increased ventilation is normally associated with a shift in the breathing route from nasal to oronasal, with the major component of ventilation occurring via the mouth (Niinimma et al. 1981; Wheatley et al. 1991a). The oral breathing route also represents a vital alternative pathway in disease states involving nasal (e.g. allergy/infection) or retropalatal (e.g. snoring and obstructive sleep apnoea) obstruction. While the airflow resistance of the nasal airway (RN) has been investigated extensively (Cole, 1988, 1992) measurement of oral resistance (RO) has received little attention and there is currently no established standard methodology for the assessment of RO.
While RN is restricted to a relatively narrow range of values, RO has the potential to vary from infinity (mouth closed) to something approaching zero (mouth widely open). Thus, while the oral pathway is generally regarded as a low resistance breathing route, this is not always the case. For example, the use of pursed lip breathing by some patients with chronic obstructive pulmonary disease results in an RO that is higher than RN (Rodenstein & Stanescu, 1983).
Beyond the effect of mouth opening and closing, little is known about the factors affecting RO. For example, while it is known that total upper airway resistance increases with head and neck flexion (Spann & Hyatt, 1971; Liistro et al. 1988), the effect of head and neck position, or even body posture, on RO is uncertain. There is also a paucity of studies examining the role of RO (as opposed to pharyngeal resistance) in disease states, particularly those involving upper airway dysfunction, such as obstructive sleep apnoea. This is related, at least in part, to the lack of a standardized methodology for measuring RO in a manner which allows comparisons to be made between different individuals or in the same individual on different occasions.
Thus, the aims of the present study were to: (1) develop techniques for measuring RO in awake human subjects, (2) determine a range of normal values for RO during quiet tidal breathing, (3) assess the between subject variability and the within subject reproducibility of this measurement, (4) determine the influence of body posture and head and neck position on RO, and (5) evaluate the influence of standardizing jaw position on RO by comparing measurements made with both a mouth-mask and a mouthpiece.
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METHODS |
General
We measured inspiratory RO in both the upright and supine posture in 17 healthy male subjects (age, 36 ± 2 years (mean ± S.E.M.); height, 177 ± 2 cm; weight, 83 ± 3 kg). In ten of these subjects the influence of head and neck position on inspiratory RO was also assessed. In addition, in a group of seven healthy male subjects (age, 38 ± 4 years), five of whom were included in the posture studies mentioned above, a comparison was made between RO measurements made with a mouthpiece and those made with a mouth-mask. Each subject completed a questionnaire to determine his suitability for the study. Only subjects reporting no current symptoms of nasal, oropharyngeal, sleep or cardio-respiratory disorders and who were non-snorers (or rarely snored) were included. Informed consent was obtained from each subject. The protocol was performed according to the Declaration of Helsinki and was approved by the Western Sydney Area Health Service Human Ethics Committee.
Measurement of inspiratory RO
Mouthpiece studies. Each study was performed with the subject breathing quietly via a standard mouthpiece (internal cross-sectional area, 300 mm2; Vacuumed Tri-Seal, SensorMedics, Middle Park, Victoria, Australia). The mouthpiece was connected to a heated pneumotachograph (Fleisch no. 2; Gould, Bilthoven, Netherlands) which was coupled to a differential pressure transducer (± 10 cmH2O; Celesco Transducer Products, IDM Instruments, Dandenong, Victoria, Australia) for the measurement of oral airflow. An occluded nasal CPAP mask (Sullivan, ResMed, Sydney, Australia) was placed over the nose, and checked to ensure the absence of leaks. With the occluded nasal mask in place there was no nasal route airflow and pressure measured inside the mask reflected oropharyngeal pressure (Fig. 1). Transoral pressure was measured using a differential pressure transducer (± 100 cmH2O; MP 45, Validyne, Northridge, CA, USA), one side of which was connected to the mouthpiece while the other side was connected to the nasal mask. Both flow and pressure signals were digitized using a sampling frequency of 50-100 Hz and recorded directly on a computer. Four to five consecutive, stable, representative breaths were analysed for each run.
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Figure 1. Measurement of oral resistance
Schematic diagram of the experimental set-up for measuring oral resistance. A pneumotachograph was used to measure oral airflow (V). Pressures were measured at the mouthpiece (Pmouth) and in the nasal mask (Poropharyngeal). Since the nasal mask was occluded, mask pressure corresponded to pressure in the oropharyngeal airway.
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Subjects were studied first in the upright (seated) posture and then supine. Care was taken to ensure subjects maintained alertness throughout the study especially when supine. Neck position was maintained constant throughout by the use of a neck brace (Stifneck extrication collar, small; California Medical Products, Sacramento, CA, USA). Jaw (via the mouthpiece) and neck (via the neck brace) position were deliberately maintained constant during the posture studies, in order to examine the influence of gravity on the intra-oral and soft tissue structures posterior to the teeth. Use of the neck brace resulted in a tip of chin to manubrio-sternal notch measurement of 13·9 ± 0·03 cm. In a subgroup of four subjects, the protocol was repeated on four separate days over a 2 week period in order to assess the between session variability for RO within a given subject.
On a separate occasion ten subjects were studied in the upright posture (without the neck brace) with varying degrees of head and neck flexion and extension. During this part of the study, a laser pointer was attached horizontally to the subject's head just above the left ear. The subject flexed and extended the head and neck so that the laser beam highlighted the degree of flexion or extension on a calibrated scale attached to the laboratory wall at a fixed distance in front of the subject. Measurements were taken in a neutral control position (chosen by the subject and set at 0 deg), and then randomly at 25 deg of flexion, 25 deg of extension, maximum flexion (40-45 deg), and at the maximum extension measurable (45-50 deg). In calibrating the wall scale, the point of rotation of the laser pointer was assumed to be at the back of the head.
Mouth-mask studies. In order to examine the influence of the mouthpiece (i.e. standardization of jaw position) on RO measurements, the upright tidal breathing protocol was repeated with subjects (in the seated position and wearing a neck brace as described above) breathing via a mouth-mask (ResMed, Sydney, NSW, Australia) and then via the standard mouthpiece. In these subjects RN was also measured concurrently (i.e. during the same session) via conventional posterior rhinomanometry (Wheatley et al. 1991b).
Data analysis
Inspiratory RO was calculated directly from transoral pressure- flow plots reconstructed from the stored data in a manner similar to that previously described by Wheatley et al. (1991b). Three to five separate runs were obtained for each condition in each subject. Firstly, an inspiratory transoral pressure-flow plot was constructed from data obtained during four to five consecutive stable representative breaths from each run. Since measurement of transoral pressure using our approach requires a patent nasopharyngeal airway, this was tested by examining the transoral pressure-flow plots from each run for evidence of any phase lag between the pressure and airflow signals, a phenomenon which occurs when the nasopharyngeal airway is occluded. Only data exhibiting no looping of the pressure-flow plot at zero flow (and therefore no phase lag between pressure and flow) were accepted for analysis.
A power function of the form P = a b (where P represents transoral pressure, represents oral flow and a and b are constants) was then fitted to the inspiratory pressure flow curve by the method of least squares. Inspiratory RO was then calculated from this relationship at flow rates of 0·2 and 0·4 l s-1. Results from repeated runs were then averaged to give individual mean values. Between and within (i.e. from repeated runs on same day) subject coefficients of variation were also calculated.
Inspiratory RO values in the upright and supine posture, in different head and neck positions, and measured with mouthpiece or mouth-mask were compared using Student's t test for paired samples with Bonferroni corrections for multiple comparisons where required. A components of variance analysis was also performed on the data from those subjects with repeat studies on different days in order to examine the between session variability in RO measurements as compared to the within session reproducibility. P < 0·05 was considered significant, except where Bonferroni corrections were used, then P < 0·005 was considered significant.
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RESULTS |
Upright vs. supine measurements during mouthpiece breathing
The power function P = a b fitted the inspiratory pressure- flow data with an r 2 value of 0·98 ± 0·01 for all the curve fitting procedures performed in both postures. The exponent b was 1·63 ± 0·04 in the upright posture and increased significantly when supine (1·76 ± 0·07, P < 0·05). The values for a did not differ significantly between the upright (1·54 ± 0·36) and supine postures (1·77 ± 0·27, P > 0·05).
Mean RO values for both the upright and supine postures are summarized in Table 1 together with the within and between subject coefficients of variation. There was no significant effect of posture on RO at either flow rate (P > 0·7). Measurements made in the upright posture at a flow of 0·4 l s-1 tended to be the most reproducible within a subject while measurements made in the supine posture at a flow of 0·2 l s-1 were the least reproducible (P < 0·02, compared with the upright value at 0·4 l s-1).
Table 1. Oral airway resistance in healthy subjects
| Posture | Flow
(l s-1) | RO
(cmH2O l-1 s-1) | c.v.W
(%) | c.v.B
(%) |
| Upright | 0·2 | 0·61 ± 0·16 | 25·5 ± 3·0 | 108·9 |
| 0·4 | 0·86 ± 0·23 | 20·9 ± 3·4 | 110·2 |
| Supine | 0·2 | 0·62 ± 0·12 | 30·7 ± 3·1 * | 81·3 |
| 0·4 | 0·90 ± 0·16 | 24·5 ± 2·9 | 73·7 |
Mean (± S.E.M.) inspiratory RO measurements from 17 healthy subjects in upright and supine positions, at oral airflows (Flow) of 0·2 and 0·4 l s-1, together with the associated within and between subject coefficients of variation (c.v.W and c.v.B, respectively). *P < 0·02 compared with upright value at 0·4 l s-1.
The components of variance analysis for repeated testing of RO is summarized in Table 2. Between session variability was greater than within sesssion variability for all conditions, although significance was not achieved in the supine posture at a flow of 0·2 l s-1.
Table 2. Components of variance for repeated testing of oral airway resistance
| Posture | Flow
(ls-1) | Between
session
component | Within
session
component | F value |
| Upright | 0·2 | 0·009 | 0·005 | 1·8 * |
| 0·4 | 0·015 | 0·006 | 2·5 * |
| Supine | 0·2 | 0·025 | 0·016 | 1·5 |
| 0·4 | 0·079 | 0·041 | 1·9 * |
A one-way ANOVA was performed on the RO values to determine the components of variance in order to compare the between and within session variability. The between and within session components of variance values reflect the magnitude of the between and within session variability. The F value is the ratio of the larger to the smaller component. A significant F value indicates a significant difference between the contributions of the two components. *P < 0·05.
Head and neck flexion vs. extension during mouthpiece breathing
When head and neck position were altered in ten subjects the power function, P = a b, fitted the inspiratory transoral pressure-flow data with an r2 value of 0·99 ± 0·01 for all the curve fitting procedures performed. Figure 2 shows the influence of head and neck position on the transoral pressure-flow relationships of the oral pathway in a representative subject. For the group as a whole, the a values of the fitted power functions were greater with both 25 deg (1·86 ± 0·32) and 40-45 deg flexion (2·39 ± 0·45) compared with control (1·42 ± 0·34; P < 0·005). The a value also increased significantly from 45-50 deg (1·16 ± 0·21) extension to 40-45 deg flexion (P < 0·005), but did not change significantly from control to either level of head and neck extension.
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Figure 2. Transoral pressure-flow curves
Representative example of power functions P = a V b fitted to the inspiratory transoral pressure-flow data from one subject in the neutral control position (A) and at 45 deg flexion (B) and 45 deg extension (C). Note that the flexion curve (B) is steeper than both A and C as reflected by the larger a constant of the power function. Transoral pressures (horizontal dashed lines) were calculated at an oral flow of 0·4 l s-1 (vertical dotted line) from each fitted power function and used to compute oral resistance values.
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The b exponent did not change significantly with 25 deg (1·72 ± 0·08) or 40-45 deg (1·70 ± 0·06) flexion or with 25 deg (1·85 ± 0·04) or 45-50 deg (1·90 ± 0·05) extension when compared with control (1·83 ± 0·06). However, when compared with 45-50 deg extension, b values during 40-45 deg flexion were significantly lower (P < 0·005).
Calculated mean inspiratory RO values (at 0·4 l s-1) were significantly greater with both 25 deg and 40-45 deg of head and neck flexion when compared with the neutral control position (P < 0·005, Fig. 3). Inspiratory RO values also increased significantly from both 25 deg and 45-50 deg extension to 40-45 deg of flexion (P < 0·005) but did not change significantly between either level of extension and the control position (Fig. 3).
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Figure 3. Effect of head and neck posture on oral airway resistance
Inspiratory oral airway resistance values (at 0·4 l s-1) for different head and neck positions in ten normal subjects. In A the different symbols represent individual subjects, while B shows group mean values. Bars represent +1 S.E.M. *P < 0·005 compared with control.  P < 0·005 compared with 40-45 deg flexion.
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Mouthpiece vs. mouth-mask measurements
During quiet tidal upright breathing, RO (at 0·4 l s-1) was greater when breathing via the mouth-mask than when breathing via the mouthpiece in all seven subjects studied. The group mean value for RO when breathing via the mouth-mask was 2·98 ± 0·42 cmH2O l-1 s-1, which was significantly greater than when breathing via the standard mouthpiece (0·74 ± 0·22 cmH2O l-1 s-1; P < 0·02). However, RO with mouth-mask was not significantly different to RN (3·70 ± 1·08 cmH2O l-1 s-1; n = 6, P > 0·4), whereas RO with mouthpiece was substantially less than RN (P < 0·02).
The between subject coefficient of variation for RO with mouth-mask was 36·9 % compared with 78·3 % with the mouthpiece and 71·5 % for RN. However, the within subject coefficient of variation was significantly less for RN (9·4 ± 3·9 %) than for either RO with mouth-mask (36·4 ± 2·2 %) or mouthpiece (31·8 ± 10·4 %, both P < 0·05).
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DISCUSSION |
There are very few published studies addressing the resistance of the oral airway and, with the exception of our previous study, which focused on the effects of exercise (Wheatley et al. 1991b), none used the same approach or method of analysis, and none was as large and as comprehensive as the present study. Our values for RO during upright mouthpiece breathing are slightly higher than those reported by Cole et al. (1982) (0·2- 0·5 cmH2O l-1 s-1), Spann & Hyatt (1971) (0·5 cmH2O l-1 s-1) and Schiratzki (1965) (0·5 cmH2O l-1 s-1). This may reflect a difference in subjects, in the conditions of measurement (e.g. neck brace used in the present study), in the mouthpiece used, or in the way RO was measured or calculated.
While assumption of the supine posture is known to increase RN (Olsen et al. 1988), the effect of posture on RO, as opposed to pharyngeal resistance, has not been studied. There have been a number of studies focusing on postural effects on pharyngeal patency (Fouke & Strohl, 1987; Yildirim et al. 1991; Jan et al. 1994; Pae et al. 1994) which suggest that in awake humans the effective cross-sectional area of the upper airway decreases as the subject moves from upright to supine, despite increased upper airway muscle activation. This decrease in upper airway dimensions is accompanied by an increase in supraglottic airway resistance (Anch et al. 1982). However, the present study demonstrates that moving from upright to supine does not lead to an increase in the oral component of upper airway resistance, provided the head, neck and jaw position remain constant.
In the present study, we also demonstrated that RO increases with head and neck flexion and tends to decrease with 45-50 deg extension. This is similar to reported findings for the resistance of the whole upper airway (Spann & Hyatt, 1971; Liistro et al. 1988). In their study Spann & Hyatt (1971) found that head and neck flexion of 24 deg was associated with an increase in upper airway resistance of about 1 cmH2O l-1 s-1 during quiet breathing. We conclude that RO is increased by modest degrees of head and neck flexion and speculate that this may be associated with the tongue occupying more of the intra-oral space or narrowing of the palatoglossal junction. This influence on upper airway patency may be important when oral route breathing is associated with alterations in head and neck position, as may occur during sleep.
Inspiratory oral pressure-flow relationships were described in the present study by the power function P = a b. The exponent b describes the curvilinearity of the pressure-flow relationship and has been used to infer the flow regime (Wheatley et al. 1991b). There are three classical types of flow regime: laminar, turbulent and orifice. Laminar flow is characterized by a b value of 1·0, whereas the b value for turbulent flow is 1·75 and for orifice flow is 2·0 (Jaeger & Matthys, 1968/1969).
The association of lower b values with head and neck flexion compared with those values obtained during head and neck extension suggests somewhat less turbulent airflow as the head and neck are flexed despite an increase in RO. This finding may be caused by changes in the uniformity of oral airway lumenal geometry (i.e. shape changes) related to head and neck position that are not associated with the decrease in cross-sectional area that results in an increased RO. However, it is evident that all b values remain in the range associated with turbulent airflow.
The use of a mouthpiece leads to a fairly wide opening of the mouth and probably causes an artificial reduction in oral airway resistance compared with the more natural position of mouth opening. In a study by Cole et al. (1982), using a head-out body plethysmograph, breathing via a mouthpiece caused reductions in RO of 70-88 % when compared with control values. The findings in the present study confirm that breathing with a mouth-mask results in a reproducibly increased RO (with no relative increase in measurement variability) when compared with mouthpiece breathing. Under the conditions of the present study RO with mouth-mask breathing was not significantly different from RN. There is potential for the mouth-mask itself to have influenced RO. However, despite the fact that the mouth-mask provided more scope for mouth opening than the mouthpiece, subjects chose to increase RO to a level equivalent to RN. This latter finding is similar to that reported by Cole et al. (1982) whose use of a head-out body plethysmograph avoided the need for a face-mask. It appears, therefore, that overall upper airway resistance may not necessarily decrease during mouth-only breathing at rest when compared with nasal breathing. Hence, we speculate that in healthy subjects during quiet tidal breathing, upper airway resistance is tightly controlled and linked to a preset level (i.e. RN) even with alterations in the breathing route.
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Acknowledgements
The authors wish to thank Michael Rynn for assistance with computer programming, Emily Di Somma for assistance with preparation of the manuscript, and K. Byth for statistical advice. This study was supported by the NH&MRC of Australia, the Community Health & Anti-Tuberculosis Association of New South Wales and the Garnett Passe & Rodney Williams Foundation.
Corresponding author
T. C. Amis: Department of Respiratory Medicine, Westmead Hospital, Westmead NSW 2145, Australia.
Email: terencea{at}westgate.wh.usyd.edu.au
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Introduction
