What is the role of airway resistance in nasal aerodynamics?

Updated: Jul 13, 2021
  • Author: Samuel J Lin, MD; Chief Editor: Arlen D Meyers, MD, MBA  more...
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Nasal airway resistance accounts for more than 50% of total airway resistance. [6] The nasal cavity has been modeled as 2 resistors in parallel. [1, 9] The 3 components of nasal resistance are as follows: the nasal vestibule, nasal valve, and nasal cavum. [6]

The term nasal valve most often refers to the internal valve, which is the limiting region of airflow. The nasal valve is defined as the lower edge of upper lateral cartilages incorporating the anterior ends of the inferior turbinates adjacent to the nasal septum. [6] The angle between the septum and the upper lateral cartilage is 10-15°. [10] The nasal valve is usually located less than 2 cm distal in the nasal passageway, approximately 1.3 cm from the naris. The average cross-sectional area is 0.73 cm2. [6] Nasal resistance is composed of two structural elements; the first layer is composed of underlying bone, cartilage, and muscle, while the second layer consists of the overlying mucosa.

Both environmental and intrinsic factors affect nasal resistance. Factors decreasing resistance include exercise, sympathomimetics, rebreathing, atrophic rhinitis, and erect posture. [1, 11]  Exercise causes sympathetic vasoconstriction and contraction of the alae nasi, increasing the capacity of the nasal passages. Rebreathing has been shown to increase arterial carbon dioxide levels, causing nasal vasoconstriction and a reduction in nasal resistance. [11] Going from a supine to an upright position decreases jugulovenous distention and nasal airway resistance. [11]

Causes of increased nasal resistance include infective rhinitis, allergic rhinitis, vasomotor rhinitis, hyperventilation, supine posture, alcohol, aspirin, and cold air. [12] In vasomotor rhinitis, vagal overactivity causes increased resistance. Nasal resistance increases markedly in the first 2-3 cm of the nasal airway. [1]

The nasal vestibule is the first component of nasal resistance. The nasal vestibule is composed of compliant walls that are liable to collapse from the negative pressures generated during inspiration. [1] The vestibule has been termed the external nasal valve. Studies have shown 30 L/min is the limiting flow during inspiration at which nasal airway collapse occurs in this area. [1] The nasal vestibule is primarily supported by alar cartilage and musculofibrous attachments. Despite the tendency, airway collapse is prevented by activation of the dilator naris muscles during inspiration. During expiration, positive pressure is the driving force for nasal vestibule dilation.

A study by Silva demonstrated that cephalic malposition of the lower lateral nasal cartilages is closely associated with external nasal valve insufficiency, finding 17 patients who presented with insufficiency among 23 cases of cephalic malposition (74%). [13]

Facial nerve paralysis can cause loss of active contraction and contribute to airway obstruction. In suspected facial nerve damage, activity of the alae nasi muscle may be tested. [14] Loss of innervation can result in alar collapse even in quiet respiration. The voluntary flaring of the naris has been attributed to a possible 20% reduction of nasal resistance, a product of facial nerve contribution to nasal airway resistance. [1] In patients with nasal alar collapse, the size of the external nasal valve can, in comparison with controls, shrink by over 40% during forced inspiration, significantly impeding nasal airflow. [15] Active dilation of the dilator naris occurs during exercise, reducing airway resistance. [16]

A major area of resistance occurs at the anterior tip of the inferior turbinate at the entrance to the piriform aperture. This important area is called the internal nasal valve. The nasal valve represents the narrowest segment of the airway. [1] In total, the valve area includes the distal end of the upper lateral cartilage, the head of the inferior turbinate, the caudal septum, the floor of the nose, the frontal process of the maxilla, the lateral fibrofatty tissue, and the piriform aperture. [9] The nasal valve area is considered as a region rather than an oblique cross-sectional area of the nasal passageway. In 1983, Haight and Cole demonstrated that the anterior end of the inferior turbinate could advance as much as 5 mm with the administration of histamine. [12]

Bachmann and Legler (1972) stated that the nasal valve area occurs at the entrance of the pyriform aperture, which corresponds to a major site of resistance anterior to the tip of the inferior turbinate. [17] The valve area is dynamic; venous erectile tissue of the turbinate and septum can cause marked obstruction. The clinical relevance of the nasal valve relates to its location. Treatment directed at the inferior turbinate will have marked effects on nasal airway resistance; trimming of the septum posterior to the valve area has fewer effects on resistance. A positive Cottle test result may signify resting narrowing of the nasal valve. A Cottle test result is considered positive if the soft tissue and nasal vestibule are lateralized, increasing the valve angle and airflow.

The nasal cavum is located posterior to the piriform aperture. Its overall contribution to total airway resistance is small. The component of nasal cavum resistance is determined by degree of vascular engorgement of tissues. Acoustic rhinometry demonstrates that the tip of the inferior turbinate narrows the airway immediately posterior to the nasal valve, while the turbinated regions of the nasal passage have relatively large cross-sectional areas. [1]

Plotting the nasal cycle, Tan et al used unilateral peak nasal inspiratory flow (UPNIF) and unilateral minimal cross-sectional area (UMCA) readings to determine airflow rates and resistance values, respectively. With the ratios between right and left UPNIF and UMCA calculated, the investigators found a directly proportional relationship between 1/resistance ratio and airflow rate ratio. The study indicated that in persons with a normal nasal cycle, data points are situated close to the regressed line, with the points occurring a significant distance from the line in patients with nasal dysfunction. [18]

A prospective cohort study by Kaura et al indicated that employment of the Nasal Obstruction Balance Index (NOBI) leads to better correlation of scores for the unilateral PNIF, acoustic rhinometry, and visual analog scale for nasal obstruction (VAS-NO). The NOBI was derived by dividing the difference between left and right nasal airway measurements by the maximum unilateral measurement. It was found to aid in predicting septal deviation by helping to identify the more obstructed side of the nose, revealing this, for example, in 92.4% of cases when used with the PNIF, post decongestant. [19]

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