Nasopharyngeal Airway Sizing
The most accurate method to determine nasopharyngeal airway size is to measure from the patient's nares to the tragus of the ear, then subtract 10mm for the optimal insertion length.
Primary Sizing Methods
For Pediatric Patients (Under 12 Years)
Use the nares-to-tragus distance minus 10mm 1. This measurement has been validated through 3D MRI reconstruction studies and shows strong correlation with the nares-to-epiglottis distance, which is the anatomical target for proper NPA placement.
The traditional nares-to-mandible measurement is less accurate and should be avoided 1.
For Adult Patients
Use the philtrum-to-ear tragus distance 2. This external measurement provides the best correlation with the optimal insertion depth (nares-to-epiglottis minus 1cm) and shows minimal deviation from the ideal length.
Alternative validated approach: Base sizing on patient height 3:
- Average height females: Size 6 (Portex sizing)
- Average height males: Size 7 (Portex sizing)
This height-based method is independent of sex and more accurate than traditional teaching.
What NOT to Use
Do not use the patient's little finger width or nostril diameter 3, 4. These traditional methods have been scientifically disproven through MRI studies and show no statistical correlation with actual nasal anatomy. Despite being widely taught, they are unreliable and should be abandoned.
Guideline-Recommended Sizes
According to established emergency department guidelines 5, 6:
Stock nasopharyngeal airways in these sizes:
The French (F) sizing system indicates the outer circumference in millimeters (e.g., 24F = 24mm circumference ≈ 7.6mm diameter).
Clinical Pitfalls
Common Misplacement Issues
Studies using fiberoptic laryngoscopy reveal that 60% of NPAs are placed too deep (beyond the epiglottis tip) and 13% lodge in the vallecula 7. This misplacement contributes to the 42% rate of respiratory obstruction seen clinically.
Key Measurements from Anatomical Studies
Mean distances from nares 7:
- To epiglottis: Males 159mm (±12mm), Females 140mm (±11mm)
- To larynx: Males 209mm (±11mm), Females 180mm (±11mm)
The NPA tip should ideally rest at or just above the epiglottis, not beyond it.
Practical Algorithm
Measure the external landmark:
- Children <12 years: Nares to tragus, subtract 10mm
- Adults: Philtrum to tragus (direct measurement)
- Alternative: Use height-based sizing (females size 6, males size 7 for average height)
Select diameter based on French sizing:
- Start with age-appropriate or patient-size-appropriate diameter
- The tube should pass without force through the nostril
Verify after insertion:
- Assess airway patency clinically
- Ensure adequate air movement
- Check for signs of obstruction (tongue protrusion, continued obstruction)
Why Traditional Methods Fail
The nostril width and little finger methods persist in training despite zero scientific validation 4. MRI studies definitively show these measurements do not correlate with internal nasal anatomy. The perpetuation of these methods represents a gap between evidence and practice that should be corrected in training programs 8.
Length-based resuscitation tapes are recommended for pediatric equipment sizing when available, as they provide more accurate estimates than age-based formulas 9.