Rhinomanometry

Definition

Principle

1. Trans-nasal pressure (ΔP) - the pressure difference between the posterior nares and the nostril entrance (atmospheric or mask pressure)
2. Nasal airflow (V) - measured by a flow head consisting of a mesh resistance inside a tube; the pressure drop across the mesh is proportional to airflow

• R = nasal airway resistance (Pa/cm³/s, or cm H₂O/L/s)
• ΔP = trans-nasal pressure (Pa or cm H₂O)
• V = nasal airflow (cm³/s or L/s)

Classification of Rhinomanometry

1. Active vs. Passive

2. Anterior vs. Posterior (Active rhinomanometry)

• The pressure-sensing tube is taped to one nostril (sealing it), which then acts as an extension to measure posterior nares pressure
• Airflow is measured from the other (open) nostril
• The sides are tested one at a time (tube moved from side to side)
• Resistance is determined separately for each side, then total resistance calculated using the above formula
• Most commonly used clinical method, recommended by international standardization committees
• Limitation: Cannot be used when nasal septal perforation is present (the sealed nostril cannot accurately sense posterior pressure)

• The pressure-sensing tube is held in the mouth, detecting posterior nares pressure when the soft palate is open
• Total nasal airflow from both nostrils simultaneously can be measured
• Total nasal resistance is determined directly from total airflow and trans-nasal pressure
• Advantage: Total resistance measured directly without calculations
• Limitation: Not all patients can maintain an open soft palate airway to mouth; requires coaching; achievable in ~90% of subjects with training

Standard Measurement Points

• Unilateral (anterior method): sample pressure point of 150 Pa
• Bilateral (posterior method): sample pressure point of 75 Pa
• For Asian populations: 100 Pa and 50 Pa respectively (due to smaller body size and smaller lung volumes making the standard 150 Pa unachievable during quiet breathing)

The Pressure-Flow Curve

• Plotting ΔP (x-axis) vs. flow (y-axis) produces a curvilinear (sigmoid-shaped) relationship
• Nasal airflow increases with rising pressure, but at higher pressures turbulent airflow effects limit further flow increase
• Because the curve is non-linear, resistance cannot be read from the slope alone - it is defined at a fixed standard sample pressure point
• A more obstructed airway produces a curve lying closer to the pressure axis (greater pressure required to generate a given flow)
• Flattening of the curve distally at a single flow rate suggests nasal valve/alar collapse

Normal Values

4-Phase Rhinomanometry (4PR)

• Classic AAR uses a single fixed pressure point (150 Pa) for NAR calculation
• 4PR calculates NAR across all four phases of the breathing cycle (acceleration/deceleration of inspiration and expiration)
• Parameters measured:
  • Vertex Resistance (VR) - resistance at peak flow point; best correlates with symptom of nasal obstruction
  • Effective Resistance (Reff) - across entire breath
  • ReffIn / ReffEx - inspiratory and expiratory components
• Advantage: A result can be obtained even in patients who cannot reach 150 Pa during quiet breathing
• 4PR and classic RM do not significantly differ in outcomes

Clinical Applications

1. Pre- and post-operative assessment - septoplasty, turbinoplasty, functional rhinoplasty; objective documentation of surgical improvement
2. Nasal challenge tests - allergen, exercise, or pharmacological challenges to assess mucosal reactivity
3. Drug trials - assessing efficacy of decongestants, antihistamines, nasal steroids
4. Diagnosis of nasal obstruction - distinguishing structural vs. mucosal causes
5. Nasal cycle research - physiological alternating congestion/decongestion
6. Positional assessment - can be performed supine to evaluate recumbent nasal obstruction
7. Pediatric assessment - feasible from ~2 years of age with smaller face mask

Sources of Error and Standardization

• Mask leak is the most common error - multiple measurements with repositioning of the mask between readings are mandatory
• A single measurement of nasal airway resistance is unreliable
• A standard operating procedure (SOP) must be implemented to prevent investigator bias
• Equipment must be accurately calibrated before measurements
• The International Standards Committee (Clement 1984) has defined standardized methods for reporting

Comparison with Acoustic Rhinometry

Key Points for Exam

1. Rhinomanometry = simultaneous measurement of nasal airflow + trans-nasal pressure to compute NAR
2. Formula: R = ΔP / V (resistance = pressure / flow)
3. Anterior active = most used clinically; one nostril at a time; pressure tube taped to contralateral closed nostril
4. Posterior active = pressure tube in mouth; measures both sides simultaneously; cannot be used if patient cannot keep soft palate open
5. Standard sample pressures: 150 Pa (unilateral), 75 Pa (bilateral)
6. P-F curve is curvilinear/sigmoid; resistance reported at a fixed pressure point
7. Total resistance formula: 1/R(total) = 1/R(left) + 1/R(right)
8. 4-phase rhinomanometry analyzes all four phases of breathing - better for patients who can't reach 150 Pa
9. Important for pre/post-op objective documentation in nasal surgery