Measurement Of The Nasal Airway 

Introduction

Nasal airway measurement is done to get objective evidence to show how patent or how blocked the nasal airway is. Nasal obstruction is one of the most common complaints, but treatment is usually initiated without any objective measurement of nasal airflow or nasal patency. This is the reason why we need to measure the nasal airway. 

Why is it Difficult to Establish a Normal Range of Nasal Patency?

Nose is subjected to a lot of environmental changes like temperature, pressure, microorganisms, and humidification. But when airflow passes through the nose and reaches the lungs, most of the humidification and purification is done by the nose, and constant, regulated air reaches the lungs. So it is very difficult to get standardized normalcy or standardized measurement of nasal airflow. 

Nasal cycle: whenever there is vasodilation, the turbinates become boggy, and whenever there is vasoconstriction, the turbinates shrink. When turbinates become boggy or bulge, when there is increased discharge from the nose, the patient will have nasal obstruction. After some time, there can be a shrinkage of nasal turbinate, causing relief. This cycle of congestion and decongestion is called the nasal cycle. 

Also read: Mucoceles of the Paranasal Sinuses: Causes and Treatment

Anatomical Nasal Patency

Normal nasal patency and anatomical nasal patency are two different terminologies. 

'Anatomical' nasal patency: Amount of airflow that goes into the nose when only bones and cartilages are taken into account. It is the nasal patency measured after decongestion of the nasal blood vessels by application of a topical nasal decongestant or by standard exercise. When decongestants are applied, nasal mucosa will shrink, and only hard structures will play an important role in nasal airflow.

Objective Measurement for Nasal Airway

• Rhinomanometry 
• Acoustic rhinometry 
• Rhinosteriometry 
• Peak nasal flow 
• Nasal spirometry

Rhinomanometry

Rhinomanometry is the simultaneous measurement of airflow through the nose and pressure across the nose during breathing. Nasal resistance to airflow is calculated from two measurements: nasal airflow and trans-nasal pressure. Rhinomanometry gives the functional measure, while the acoustic rhinometry gives the anatomical measure of the cross-sectional area of nasal volume. It is of 2 types. 

• Active rhinomanometry involves the generation of nasal airflow and pressure with normal breathing. 
• Based upon the location of the pressure sensing tube, it is divided into 2 types.

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Anterior: A Pressure Sensing Tube Is Placed Anteriorly. 

In active anterior rhinomanometry, the pressuresensing tube is normally taped to one nasal passage. The sealed nasal passage measures the pressure in the posterior nares. 

With this method, nasal airflow is measured from one nostril at a time, and the pressure-sensing tube is moved from one side of the nose to the other. Nasal resistance is determined separately for each nasal passage, and the total resistance is then calculated by the summing values in the formula.

Posterior: A Pressure Sensing Tube Is Placed In The Nasopharynx.

A pressure-sensing tube is held in the mouth and detects the posterior nares pressure when the soft palate allows an airway to the mouth. Disadvantage: As the tube is placed behind the palate, there is a possibility of gag reflex. As the mouth is open, it may interfere with the result. 

Advantage: bilateral nasal airflow

Passive rhinomanometry involves the generation of nasal airflow and pressure from an external source, such as a fan or pump, to drive air through the nose. 

• Rarely done. 
• Passive rhinomanometry involves the direction of an external flow of air through the nose and out of the mouth. 
• The method may involve either measurement of a driving pressure at a constant flow or measurement of the flow at a constant pressure. 
• Passive rhinomanometry is particularly useful if it is necessary to separate the upper and lower airways for experimental work. 

While calculating nasal resistance, the patient is asked to breathe for 8–10 cycles. During inspiration, a downward graph is obtained, and during expiration, an upward graph is obtained. 

Nasal resistance to air flow may be calculated from the following equation: 

• R = δP/V 
• R = resistance to air flow/cm H₂O/litre per second of Pa/cm3 per second 
• P = trans-nasal pressure, in cm H₂O or Pa V = nasal air flow, in litre/s or cm3/s

Nasal air flow increases with the increase of trans-nasal pressure, but at higher pressures there is a limitation of flow due to the increased frictional effects of turbulent airflow. The right and left nasal airflows are normally asymmetrical due to nasal cycle, and therefore a single pressure value may relate to two different airflows.

Also read: Granulomatous Conditions of the Nose

Normal Nasal Airflow

Nasal patency in health is unstable and maybe even more variable in disease; therefore, it is difficult to give a normal range. The variability is normal between the two nasal passages; one nasal passage is obstructed and the other can be patented as a normal phase of the nasal cycle. In adult subjects free from signs of nasal disease, mean total resistance was -0.23 Pacm³s (0.2-0.3). If the nose is decongested by exercise or application of a topical decongestant, then this eliminates any physiological variation in resistance and allows one to investigate the anatomical factors influencing resistance.

Acoustic Rhinomanometry

Acoustic rhinometry: The Method consists of generating an acoustic pulse from a spark source or speaker, and the sound pulse is transmitted along a tube into the nose. The sound pulse is reflected from inside the nose according to changes in the local acoustic impedance, which are related to the cross-sectional area of the nasal cavity. The reflected sound is detected by a microphone, which transmits the sound signal to an amplifier and computer system for processing into an area distance graph. The cross-sectional area measurements obtained with acoustic rhinometry correlate very well with area measurements made by CT scans and nasal airway resistance measured by rhinomanometry, but the accuracy of acoustic rhinometry is unreliable in the posterior part of the nose, especially when the nasal passage is congested. 

A major advantage of the technique of acoustic rhinometry is that it provides a measure of nasal cross-sectional area and volume along the length of the nasal passage. The accuracy of acoustic rhinometry, like rhinomanometry, is dependent on the interface between the equipment and the nose. 

For example, the cross-sectional area of the nasal vestibule is susceptible to distortion if a tube is inserted into the nose. Malpositioning of the nasal tube and air leaks are just likely to give spurious measurements.

Also read: Nasal Tip Surgery and Its Deformities

Peak Nasal Flow

The peak inspiratory or expiratory airflow through the nose associated with maximal respiratory effort can be used as a measure of nasal conductance. The measurement is effort-dependent and is less sensitive than rhinomanometry or acoustic rhinometry in determining small changes in conductance. Expiratory measurements are likely to cause the expulsion of nasal secretions into the measuring instrument, and inspiratory flow measurements are likely to cause nasal alar collapse and flow limitation.

Nasometry

It is a procedure where the acoustic output from the nose is measured. It is mainly responsible for the resonance of sound.

Subjective Measurements

Subjective sensation is influenced by mood, air, temperature, cold receptors, and menthol.

It can also be affected by other factors. 

• Congestion of ethmoid bone 
• Congestion of ostia of paranasal sinuses. 
• Congestion of the eustachian tube.

The reason for the lack of correlation between the perception of nasal airflow and nasal resistance as measured by rhinomanometry may be because the resistance to nasal airflow is primarily determined by the nasal valve area. Whereas the symptoms of nasal obstruction may be influenced by other areas of the nose as well as the nasal valve area. 

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Measurement Of The Nasal Airway By The Rhinologist

The rhinologist may be interested in measuring the patency of the nasal airway for two reasons: 

• Assess the severity of any nasal obstruction and select patients for medical and/or surgical treatment 
• Assess the efficacy of any treatment in increasing nasal airflow. 

Nasal surgery for the treatment of nasal obstruction should be guided by objective measurement of nasal airway patency pre- and post-operatively

Important Points 

• Objective and subjective measurements of nasal conductance do not correlate well for bilateral measures but do correlate for unilateral measures. 
• Subjective measurements are important as they relate directly to symptoms. 
• Decongestion of the nose eliminates the effects of the nasal cycle and allows the measurement of anatomical nasal airflow. 
• Acoustic rhinometry provides anatomical measurement rather than functional measurements of the nasal airway. 
• Rhinomanometry is generally accepted as the gold standard for the measurement of nasal airway resistance. 
• Acoustic rhinometry and rhinomanometry in their current forms have not found a routine place in the day-to-day assessment of patients in the minology clinic.

Also read: Medical Management Of Chronic Rhinosinusitis

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