How do you size a nasopharyngeal airway

Keith Roberts a,�, Keith Porter b

a Surgical Rotation, Heartlands Hospital, Birmingham, UKb Selly Oak Hospital, Birmingham, UK

Received 22 April 2002; received in revised form 2 May 2002; accepted 12 August 2002

Abstract

Objective: To measure an appropriately sized nasopharyngeal airway, it is taught that the size is related to the patients little finger

or nostril (anterior nares). This study has been designed to identify whether these comparisons are valid. Method: Direct comparison

of the dimensions of ten subjects’ little fingers and anterior nares with the internal anatomy of their nose as visualised on coronal

MRI scans. Results: Neither method correlated statistically with the nasal anatomy of that subject. Conclusions: The methods used

traditionally to size a nasopharyngeal airway do not correlate with the airway anatomy and are unreliable. It is more appropriate to

size the airway dependent upon the patient’s size, sex and race.

# 2002 Elsevier Science Ireland Ltd. All rights reserved.

Keywords: Airway; Airway management; Nasopharyngeal airway

Resumo

Objectivo: Ensina-se a selecionar o calibre da via aerea nasofarıngea, equiparando o calibre o dedo minimo do doente ao da sua

narina (porcao anterior das fossas nasais). Este estudo foi desenhado para verificar se estas comparacoes sao validas. Metodo:

Comparacao directa das dimensoes do dedo mınimo de dez indivıduos e das fossas nasais anteriores com a anatomia do seu nariz

quando visualizado em cortes coronais de scan MRI. Resultados: Nenhum dos metodos se correlacionava estatısticamente com a

anatomia nasal dos sujeitos estudados. Conclusoes: Os metodos tradicionalmente usados para medir uma via aerea nasofarıngea

nao se correlacionam com a anatomia da via aerea e nao sao fidedignos. E mais apropriado medir uma via aerea de acordo com a

idade, sexo e raca do doente.

# 2002 Elsevier Science Ireland Ltd. All rights reserved.

Palavras chave: Via aerea; Manuseio da via aerea; Via aerea nasofarıngea

Resumen

Objetivo : Para medir una vıa aerea nasofarıngea de tamano adecuado se ensena que se puede hacer una comparacion con el

menique o la narina del paciente (narina anterior). Este estudio ha sido disenado para identificar si acaso estas comparaciones son

validas. Metodo : Comparacion directa de las dimensiones de los meniques y las narinas de 10 sujetos, con la anatomıa interna de sus

narices visualizadas con tomografıas de resonancia nuclear magnetica( MRI). Resultados : Ninguno de los metodos se correlaciono

estadısticamente con la anatomıa nasal de ese sujeto. Conclusiones : Los metodos usados tradicionalmente para medir las vıas aereas

nasofarıngeas no se relacionan con la anatomıa de la vıa aerea y no son confiables. Es mas apropiado para calcular el tamano de la

vıa aerea dependiendo del tamano sexo y raza del paciente.

# 2002 Elsevier Science Ireland Ltd. All rights reserved.

Palabras clave: Vıa aerea; Manejo de vıa aerea; Canula nasofarıngea

� Corresponding author. Present address: 48 Broom Lane, Dickens Heath, Solihull B90 1SJ, UK. Tel./fax: �/44-7801-65-8505

E-mail address: dr_keith@hotmail.com (K. Roberts).

Resuscitation 56 (2003) 19�/23

www.elsevier.com/locate/resuscitation

0300-9572/02/$ - see front matter # 2002 Elsevier Science Ireland Ltd. All rights reserved.

PII: S 0 3 0 0 - 9 5 7 2 ( 0 2 ) 0 0 2 9 1 - 5

1. Introduction

Nasopharyngeal airways are of benefit in patients

who require airway support. They may be preferred overan oropharyngeal airway if the patient has an intact gag

reflex or in whom an oropharyngeal airway is contra-

indicated [1]. Such contraindications may include un-

stable fractures of the mandible, other major oral

trauma and trismus.

It is taught that to select the appropriate sized

nasopharyngeal airway a comparison can be made

with the patients little finger or their anterior nares.This applies to both paediatric [2] and adult patients [3].

A literature search of the MEDLINE, CINAHL and TRIP

databases was performed with ‘nasopharyngeal airway’

as the keyword phrase. No literature was found that

discussed how to size a nasopharyngeal airway. From

this we have inferred that the methods taught to size a

nasopharyngeal airway are anecdotal rather than based

on fact.This study has been designed to elucidate whether

there is a valid comparison between either of these two

traditionally taught methods of sizing a nasopharyngeal

airway and the actual nasal anatomy.

2. Method

Our measurements were derived from ten subjects

undergoing a MRI scan at the Queen Elizabeth Hospi-

tal, Birmingham UK, during January 2001. An addi-tional sequence of coronal T1 scans at right angles to the

nasal floor was performed for our use. Informed patient

consent was sought and given. Any patients being

scanned for pathology that could involve the nose

were excluded. Indications for the primary scan are

given in Table 1.

A nasopharyngeal airway passes along the floor of the

nose inferomedially to the inferior concha. It was,therefore, necessary to measure the narrowest point

along this bony path. To do this we took measurements

at 5 mm intervals and the scan that demonstrated the

narrowest area inferomedially to the inferior concha was

selected. This was measured using the software supplied

with the Siemens Magnetom Vision 1.5T MRI scanner.

It was necessary to measure distances between bony

landmarks as the soft tissues vary greatly in their

volume, in accordance with their normal erectile cycle.

The bony walls of this part of the nose are lined with

erectile tissue and respiratory epithelium. The measure-

ments taken, therefore, overestimate the dimensions.However, this erectile tissue is easily compressed and

can also be stripped from the bone along with the

periosteum. Inspection of a nasopharyngeal airway that

has been removed after use sometimes demonstrates

this.

The subjects little fingers were measured in three

places: at the level of the distal and proximal inter-

phalangeal joints and at the narrowest point between thetwo. Their anterior nares were measured anteroposter-

iorly and at the point of maximum width. These

measurements were achieved with Vernier callipers and

a scientific ruler with 0.5 mm divisions.

3. Results

The ten patients presented as out patients with

various indications for MRI scan. These are summarised

in Table 1.

The average age was 54.7 (32�/74). The average weight

was 72.3 Kg (57.6�/88.9). The average height was 172 cm

(165�/180). Six of the subjects were female. All patients

were Caucasian. One patient had a history of nasal

trauma (subject 2). He had been punched 5 yearspreviously which had deviated his septum.

From the MRI scans it was possible to measure the

bony constraints of the path that a nasopharyngeal

airway would take. This is depicted in Fig. 1.

The nasopharyngeal tube will pass inferiorly and

medially to the point a . To calculate the largest diameter

airway that would pass along this path it was required to

derive a calculation for this purpose. See Appendix Afor this derivation.

Our results are in Table 2 (all dimensions are in mm):

All three dimensions of the little finger and the

anteroposterior length of the nares overestimated the

airway size in every instance.

The width of the nares correlated to within 1 mm six

times on the right and three times on the left. Pearson

correlation was performed for each side. The r value onthe right was 0.02 with a p value of 0.9494. On the left

Table 1

Indications for MRI scan

Cerebral tumor 3

Pituitary cyst/tumor 2

Stroke 1

Paraesthesia/trigeminal neuralgia 2

Retro-orbital mass/tumor 2 Fig. 1. Diagram of a coronal section of the left nasal cavity showing

the nasal septum, lateral wall, floor and inferior concha.

K. Roberts, K. Porter / Resuscitation 56 (2003) 19�/2320

the r value was 0.11 and the p value 0.7646. An r value

of 0 indicates no correlation, a value of 1or �/1 complete

correlation (Fig. 2). No correlation has been achieved

with statistical significance.

The one subject (subject 2) with a history of nasal

trauma had the greatest difference in widths of his nares.

This did not correlate with the internal dimensions of his

nasal cavity.

Note that in Table 2 the diameter of the nasophar-

yngeal airway refers to the external diameter. The

commonly used PortexTM nasopharyngeal airways are

described in terms of the internal diameter. A size 7

airway has an internal diameter of 7 mm. Table 3 lists

internal and external diameters of PortexTM nasophar-

yngeal airways.

4. Discussion

From the results we conclude that neither of the

anecdotally taught methods to size a nasopharyngeal

airway are reliable. In addition these anecdotal methods

suggest that the diameter and not the length is the most

important measurement when choosing the correct

airway. This is not the case. Stoneham’s comprehensive

work [4] in 1993 detailed that the length of the airway

was a more important factor in determining appropriate

size than diameter. If too short the airway would fail to

separate the soft palate from the posterior wall of the

pharynx and if too long would enter either the larynx

and aggravate laryngeal reflexes or enter the space

between the epiglottis and the tongue (vallecula) where

the airway could become obstructed. Stoneham con-

cluded that the ideal length of a nasopharyngeal airway

was one which would lie within 1 cm of the epiglottis.

This corresponded to a nares-epiglottis length of 150

mm in men and 130 mm in women (corresponding to a

size 7 and 6 Portex nasopharyngeal airway, respec-

tively). Stoneham identified a clear relationship between

nares-epiglottis length and height. The average height of

the male subjects was 178 cm and the females 163 cm.

Gender, when compared with height, does not alter the

nares-epiglottis length [4,5].

The patients’ racial origin has also been shown to be

an important determinant of internal nasal anatomy.

Asians have smaller airways than Caucasians who in

turn have smaller airways than Negroes [5]. The number

Table 2

Results

Subject Measurement:

1 2 3 4 5 NP

1

R 17 15 18.5 19 9.5 10.5

L 16 15.5 17.5 15 9 6.5

2

R 18 19 20 12 8 9

L 15 15 20 13 4 10

3

R 12.5 14 15 14 7 6.5

L 13 14 15 12 6.5 5.3

4

R 14.5 13 15 14.5 7 10.5

L 14.5 14 15 13.5 7 10

5

R 14 13.5 16 15.5 9 9.5

L 13.5 12.5 16 15.5 9.5 9.5

6

R 16 16 17 13.5 8 7.6

L 17 17 17.5 13.5 8 7.6

7

R 15.5 14.5 17 22 10 8.4

L 15 14 17 20 10 9.4

8

R 16.5 16 18 18 8 8.2

L 18 17 19.5 18 8.5 10.7

9

R 15.5 16.5 1.9 16 6 10

L 15.5 16 19 17 6 8.1

10

R 14.5 14 16.5 13.5 5.5 9

L 14.5 14 16 16 5.5 7.2

Measurements: 1, Width of distal interphalangeal joint, medial to

lateral; 2, Width of interphalangeal space, medial to lateral; 3, Width

of proximal interphalangeal joint, medial to lateral; 4, Anterior to

posterior length of the nares; 5, Width of the nares. NP, Maximum

external diameter nasopharyngeal airway that would fit the subject (see

Appendix A); R, right; L, left.

Fig. 2. Width of right nares vs calculated maximum width of right NP

airway.

Table 3

Internal and external diameters of PortexTM nasopharyngeal airways

Internal diameter (mm) External diameter (mm)

6 8.8

7 10.2

8 11.6

9 12.9

K. Roberts, K. Porter / Resuscitation 56 (2003) 19�/23 21

of subjects in our study was too low to perform such

analysis.

If no obvious deformity of the nose is present we

would advise initial placement in the patients right nasal

cavity. This is due to the design of the nasopharyngeal

airway. The angle of the bevel is designed to cause less

damage to nasal mucosa when inserted into the right

nostril. However, if there is obvious deformity we would

advise placement in the nostril that appears most patent.

The subject with the history of nasal trauma had the

greatest difference in the dimensions of his nares. This

subjects internal anatomy was similar on both sides,

being within 1 mm of each other. This contrasts with our

above statement. Without further work we do not

regard this one case to provide sufficient evidence to

disregard the external sizes of the nose when deciding

which nostril to place the airway.

It is widely taught that a contraindication to naso-

pharyngeal airway insertion is a known or suspected

fracture of the base of skull. The ATLS manual and

course have played a key role in disseminating this. This

advice seems to be based on a single case report [6]

where a nasopharyngeal airway was placed intracra-

nially through a cribriform plate fracture. Clear evi-

dence of basal skull fracture is not always apparent in a

patient with an obstructed or partially obstructed

airway particularly with blood around the airways. In

the absence of any other alternative methods to open an

airway, both paramedics and doctors can be faced with

the hopeless scenario of a patient with progressive

hypoxia. In this scenario, recognising that technique of

insertion is absolutely critical, we regard nasopharyn-

geal airway insertion as a safe procedure, where the

benefits of the procedure far outweigh the small risk of

intracranial penetration [7].The nasopharyngeal airway is a valuable tool in

airway management. It is cheap, easy to insert, effective

and probably underused. Its use is not constrained to

hospital medicine. It is also of value in the pre-hospital

environment [8] and in the back of the ambulance [8]. In

the emergency situation time is valuable. Comparing

different sized nasopharyngeal airways with the little

finger or nose wastes time and now seems pointless. A

size 7 Portex nasopharyngeal airway should be used in

most males and a 6 in females. This choice of airway size

depends upon the patients height and should be adjusted

accordingly.

Appendix A: Derivation of an equation for a circle

Diagram of a coronal section of the left nasal cavity

showing the nasal septum, lateral wall, floor and inferior

concha.

The nasopharyngeal tube will pass inferiorly and

medially to the point a . y is the vertical displacement

of point a from the nasal floor while x is the long-

itudinal displacement from point a .

To calculate the largest nasopharyngeal airway that

could be inserted into each patient the following formulawas used:

R��n2 9

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffin2

2 � 4n1n3

p

2n1

This was derived from the equation of a circle: (x�/

a )2�/(y�/b )2�/R2.

In our example because we know that at certain

points of the circle x�/0 and y�/0 we know that a�/

b�/R . Therefore, we can simplify the equation of a

circle:

A.1. Derivation of the equation of a circle

(x�a)2�(y�a)2�a

x2�2ax�a2�y2�2ay�a2�a2

x2�2ax�a2�y2�2ay�0

Insert x and y values to create quadratic equation in

form of:

n219n2a9n3�0

and, therefore:

K. Roberts, K. Porter / Resuscitation 56 (2003) 19�/2322

R��n2 9

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffin2

2 � 4n1n3

p

2n1

References

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[2] Gwinnut C. Lecture notes on clinical anaesthesia, Blackwell

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[3] Greaves I, Porter KM, Ryan J. Trauma care manual. Arnold,

2001, p. 41.

[4] Stoneham MD. The nasopharyngeal airway. Anaesthesia

1993;48:575�/80.

[5] Morgan NJ, MacGregor FB, Birchall MA, Lund VJ, Sittampalam

Y. Racial differences in nasal fossa dimensions determined by

acoustic rhinometry. Rhinology 1995;33:224�/8.

[6] Muzzi DA, Losasso TJ, Cucchiara RF. Complication from a

nasopharyngeal airway in a patient with a basilar skull fracture.

Anesthesiology 1991;74:366�/8.

[7] Allison K, Porter K. Nasopharyngeal airways: an under-utilised

pre-hospital resource. Pre-Hospital Immediate Care

2000;4(4):192�/3.

[8] Jenkin A. The nasopharyngeal airway: a useful adjunct for the

accident and emergency patient. Accid Emerg Nurs 1996;4:16�/20.

K. Roberts, K. Porter / Resuscitation 56 (2003) 19�/23 23