How to Properly Measure a Nasopharyngeal Airway

Measuring a nasopharyngeal airway (NPA) correctly means estimating the distance from the nostril to the back of the pharynx so the device tip sits above the epiglottis without dropping past it. The most widely recommended technique is measuring from the tip of the nose to the tragus of the ear and subtracting about 10 millimeters, though research shows that even this method is only an approximation. Getting it right matters more than many clinicians realize: one fibreoptic study found that 60% of placed NPAs ended up too deep, sitting beyond the tip of the epiglottis, and nearly half of those patients showed signs of airway obstruction.

Why the Old Sizing Rules Are Unreliable

For decades, clinical teaching told providers to size an NPA by comparing it to the diameter of the patient’s nostril or the width of their little finger. The logic seemed intuitive: if the tube fits the nostril, it should fit the nasal passage. But MRI data have shown that these methods do not reliably predict where the airway tip will end up once inserted. The external width of a nostril tells you almost nothing about the internal length of the nasal passage or the distance to the pharynx.¹ Those traditional rules conflate diameter sizing (which affects whether the tube fits through the nostril without trauma) with length sizing (which determines whether the tip lands in the right spot). The two dimensions are governed by different anatomy, and confusing them is a recipe for misplacement.

The Nares-to-Tragus Method

The most commonly taught measurement today uses an external landmark: the distance from the nares (the opening of the nostril you plan to insert through) to the tragus of the ear on the same side. You hold the NPA alongside the face, placing the flanged end at the nostril and checking where the tip falls relative to the tragus. The device should reach to about the tragus, then you subtract roughly 10 millimeters from that length. This nares-to-tragus minus 10 mm approach (often abbreviated NT-10) outperforms the older nares-to-mandible-angle method because the tragus sits closer to the anatomical plane of the posterior pharynx.²

A separate study looking at adults under general anesthesia used a slightly different external landmark: the philtrum-to-tragus distance, measured from the groove between the nose and upper lip to the ear tragus. Researchers found this measurement had the smallest average deviation from the actual nares-to-epiglottis distance when compared to several competing techniques, and it predicted optimal insertion depth with fewer patients showing more than a one-centimeter error.³ The philtrum-to-tragus method is not yet as widely taught as the nares-to-tragus technique, but it is worth knowing about, especially if your department or training program references it.

In practice, both methods aim for the same target zone. You want the NPA tip to rest in the posterior pharynx, past the soft palate but above the epiglottis. If it stops too short, the tube sits in the nasal passage and does nothing useful. If it goes too deep, it can push past the epiglottis into the laryngeal inlet, provoking gagging, laryngospasm, or obstruction.

Height-Based Sizing as a Rapid Alternative

When time is short, patient height offers a surprisingly practical shortcut. Nasal endoscopy data have shown that the ideal NPA length correlates strongly with a person’s height, independent of sex. For an average-height woman, a size 6 Portex NPA is appropriate; for an average-height man, a size 7 fits best.¹ This is useful in emergency settings where you may not have time to measure distances on a patient’s face. Glancing at whether the person in front of you is short, average, or tall gets you to a reasonable size faster than fumbling with a tape measure mid-resuscitation.

Height-based sizing has limits, of course. It does not account for unusual facial proportions, prior nasal surgery, or anatomical variants like a deviated septum. But as a rapid first estimate, it outperforms the nostril-width or little-finger methods that are still taught in some courses.

How the Numbers Break Down in Adults

To understand why sizing errors happen, it helps to know the actual internal distances involved. In a fibreoptic study measuring airway landmarks in adults, the average distance from the nares to the epiglottis was about 159 mm in men and 140 mm in women. The distance from the nares all the way to the larynx was roughly 209 mm in men and 180 mm in women.⁴ The sweet spot for the NPA tip is in that gap between the epiglottis and the larynx, ideally closer to the epiglottic end. That is a relatively narrow corridor. An NPA that is just two centimeters too long crosses from “well placed” to “past the epiglottis,” which is exactly the error the same study documented in 60% of placements.

These numbers also explain why a single NPA size cannot work for everyone. A 20-mm difference in nares-to-epiglottis distance between the average man and average woman means a tube sized for one will not work for the other. Factor in individual variation and the standard deviations involved (about 11-12 mm for each measurement), and you can see why even correctly applied sizing rules will occasionally be off by a clinically meaningful amount.

Measuring and Sizing in Children

Pediatric NPA sizing is harder. Children’s airways are shorter, more variable relative to external landmarks, and more prone to tissue trauma. A prospective study that compared several landmark-based formulas in children using MRI found that none of them reliably placed the NPA tip in the ideal target corridor. The NT-10 method (nares-to-tragus minus 10 mm) came closest, but even it only served as a rough approximation.⁵

The practical takeaway for pediatric patients is to use NT-10 as your starting estimate but pay close attention during insertion. If you meet resistance, hear abnormal sounds, or notice the child’s breathing pattern worsen rather than improve, the device may be too long or too short. The researchers who studied this problem explicitly recommended evaluating airway patency and confirming that air is actually flowing through the NPA lumen during and after insertion, rather than trusting any single formula.⁵

In pediatric emergency medicine, some providers keep a range of NPA sizes pre-measured and labeled by age or weight group so that selection can happen quickly. This is a reasonable workaround given the limitations of external measurement in small patients.

Preparation Before Insertion

Measuring the correct length is only the first step. The nasal passage is vascular, lined with fragile mucosa, and curves in ways that can catch a rigid tube tip. Preparation makes a real difference in whether the NPA goes in smoothly or causes epistaxis and trauma.

Vasoconstriction of the nasal mucosa is standard when time permits. In clinical settings, providers often apply a topical vasoconstrictor such as xylometazoline drops to shrink the blood vessels in the nasal lining. The NPA itself should be thoroughly lubricated with a water-soluble gel. Some protocols also apply topical anesthetic, such as lidocaine gel, to reduce discomfort and reflex gagging.⁶ In one case report, a provider used a lidocaine-based gel for lubrication along with a separate mucosa surface anesthetic applied minutes before insertion, which allowed the NPA to be placed in a sedated but spontaneously breathing patient without triggering airway reflexes.⁷

The preferred insertion path follows the inferior meatus, the space between the inferior turbinate and the floor of the nasal cavity. This is the widest natural corridor through the nose.⁸ That means directing the NPA along the floor of the nose, not angling it upward toward the bridge. A common mistake, especially under stress, is pointing the tube toward the top of the nasal cavity, which sends it into the middle or superior meatus where the passage is narrower and more vascular. Inserting with a gentle posterior and slightly inferior angle, with the bevel facing the septum, follows the path of least resistance.

What Misplacement Looks Like

If the NPA is too long, the tip can slip past the epiglottis and sit against or within the laryngeal structures. The fibreoptic study that examined NPA position in anesthetized adults found that 13% of devices ended up lodged in the vallecula, the small depression just in front of the epiglottis. Altogether, 42% of the patients in that study showed clinical signs of respiratory obstruction despite having an NPA in place.⁴ These are not minor inconveniences. A misplaced NPA can turn what should be a simple airway adjunct into a source of obstruction, making the patient worse rather than better.

Signs that an NPA may be poorly positioned include persistent snoring or stridor after placement, no improvement in oxygen saturation, visible gagging or retching (suggesting the tip is too deep and stimulating the larynx), or worsening respiratory effort. If you placed an NPA and the patient’s breathing does not improve or gets worse, the first thing to reassess is length. Withdraw the device slightly and see if airflow improves. If the NPA is clearly too short and sitting in the nasal passage without reaching the pharynx, you need a longer one.

When Not to Use a Nasopharyngeal Airway

The most critical contraindication is suspected basilar skull fracture. If the bony floor of the skull is disrupted, a tube pushed through the nose can potentially pass through the fracture site into the cranial vault. This is rare, but it has been documented in case reports involving patients with severe facial trauma.⁹ Classic signs of basilar skull fracture include bruising behind the ears (Battle’s sign), raccoon eyes (periorbital bruising), clear fluid draining from the nose or ears, and hemotympanum. If any of these are present, reach for an oropharyngeal airway or another alternative instead.

Other relative contraindications include known or suspected nasal fractures, significant midface trauma, a history of nasal surgery (which may have altered internal anatomy), and coagulopathy or anticoagulant therapy (which raises the bleeding risk from any nasal instrumentation). In patients taking blood thinners, the decision to place an NPA should weigh the airway benefit against the risk of difficult-to-control epistaxis.

Newer Device Designs and Adjustability

Traditional NPAs are cut to length from a fixed tube or selected from a set of pre-sized devices. Once you pick a size, that is what you get. Newer designs are trying to address the inherent imprecision of external measurement. One recently described device uses a flexible catheter with a spring core that conforms to nasal anatomy, combined with a restrictive nose plug that controls how deep the catheter can advance. The plug secures the device at the nostril and prevents it from migrating deeper over time.¹⁰ The flexible construction also aims to reduce mucosal abrasion, which is one of the most common complications of NPA use even when sizing is correct.

Adjustable-depth designs are an acknowledgment that no single measurement method is precise enough to guarantee perfect placement in every patient. The idea is to insert the device, assess airway patency in real time, and then lock the depth at whatever position works. This is especially appealing for prolonged use in settings like post-anesthesia recovery, where an NPA might stay in place for an extended period and where small shifts in position could gradually turn a well-placed tube into a poorly placed one.

Putting It All Together at the Bedside

When you need to place an NPA quickly, a practical sequence looks like this:

• Select the nostril: Choose the more patent side. If the patient can communicate, ask which side feels clearer. Look for visible septal deviation and avoid that side.
• Estimate length: Measure from the nares to the tragus of the ear on the same side. Subtract about 10 mm. If you cannot measure, use height-based selection: a size 6 for an average-height woman, a size 7 for an average-height man.
• Prepare the passage: Apply a vasoconstrictor if available. Lubricate the NPA generously with water-soluble gel. Consider topical anesthetic for a conscious patient.
• Insert along the floor: Direct the tube posteriorly and slightly inferiorly, following the inferior meatus. Keep the bevel facing the septum. Advance gently. If you meet firm resistance, do not force it; try the other nostril or a smaller diameter.
• Assess position: Once placed, listen for air movement through the NPA. Watch for improvement in breathing pattern and oxygen saturation. If the patient gags continuously or breathing worsens, withdraw the device slightly and reassess.

The final check is the most underrated part of the process. Measurement gets you in the right range, but clinical assessment after placement is what confirms the NPA is actually doing its job. A tube that measures perfectly on the outside of the face can still end up mispositioned because of individual anatomical variation inside the nose and pharynx. Trust your measurement as a starting point, then trust the patient’s response as your confirmation.

Diameter Sizing and the Forgotten Second Dimension

Most sizing discussions focus on length, but diameter matters too. An NPA that is too wide for the nasal passage causes mucosal blanching, bleeding, and pain. One that is too narrow may not provide enough airflow to be useful. The internal diameter of the NPA should be large enough to allow adequate ventilation but small enough to pass through the nostril without excessive force. In adults, common internal diameters range from about 6 mm to 8 mm (roughly 24 to 34 French). The general approach is to select the largest diameter the nostril will accept without blanching or significant resistance at the naris.

For clinical practice involving nasotracheal procedures, one research group used a standardized approach of 7.5 mm (32 Fr) for women and 8.0 mm (34 Fr) for men, treating diameter as relatively fixed while adjusting other variables.⁶ This simplification works in controlled settings but may need adjustment in patients with naturally narrow nares, nasal polyps, or chronic inflammation that narrows the passage. When in doubt about diameter, start smaller. A slightly undersized NPA that sits in the right position is far more useful than one that traumatizes the nasal mucosa on the way in.