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Poiseuille’s Law
Poiseuille’s law states that ow through a tube is directly proportional to the difference in pressure (dP) multiplied by radius to the fourth power (r). Further
more, ow is inversely proportional to the length of the tube:
Flow 5 Constant (K) 3 dP 3 r/Length
e cross-sectional area is variable throughout the nasal airway. It is smallest at
the level of the internal nasal valve, measuring to mm. Posterior to the internal nasal valve, there is much greater cross-sectional area and lower resistance
to airow. e internal nasal valve is a bottleneck to airow in the nasal cavity
and is commonly a site for nasal airway obstruction.
,-
e internal nasal valve is a bottleneck to airow in the nasal cavity and is commonly a site for nasal airway obstruction.
-
Venturi Eect
e Venturi eect was initially applied as a law governing uid dynamics; it relates to a reduction in pressure when ow increases through a constricted section of a tube. Again, this eect may be seen most at the narrowest segments of
the nasal airway, including the internal nasal valve.
Particle Filtration
Particle ltration begins with air passage through the nasal vestibule. Nasal vibrissae in this location provide the initial mechanical barrier to larger particles.
Particles that escape the vestibule can also become suspended in nasal air and are
deposited on the nasal mucosal surfaces. Airway architecture, which is composed
of curves and sharp deections, allows improved deposition. Ninety percent of
particulate matter greater than or equal to microns in size is deposited primarily at the internal nasal valve and nasopharynx, where the airstream is changed
from a column to a sheet. is method of ltration is termed impingement. e
mucosal lining produces a bilayered mucous blanket with a thin deep layer and
a thicker, more viscous supercial layer. e mucous blanket possesses an overall
positive ionic charge. Negatively charged particulate matter becomes attached
and is removed through mucociliary clearance. Cilia stemming from the deep
layer beat at beats per minute and move the more viscous supercial layer.
is mechanism allows movement of particulate matter toward the nasopharynx,
along with drainage from the paranasal sinuses.
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Air Humidication
Inspired air undergoes humidication as it passes through the nasal cavity. Such
modication in air characteristics provides a moisturized environment, which
is important for proper sinonasal function. Overall, the vast majority of air humidication occurs in the nasal airway compared with the remaining upper airway tract. At introduction into the alveoli, air is typically at % humidication.
e process of expiration results in the cooling of air. Since cool air possesses less
moisture than warm air, a small volume of water is recovered during expiration;
the net balance for human respiration is a water loss of to ml per day.
Temperature Regulation
In addition to humidication, air also undergoes temperature regulation in the
nasal cavity. is process is closely related to air humidication. is allows inspired air to reach temperatures approximating that of core body temperature
by the time the air reaches the larynx.
Olfaction
Olfaction is an important role of the nose and nasal cavity. Smell allows the body
to obtain information about the external environment. Such information serves
key roles in taste, memory, and even avoidance of potential danger. Patients with
chronic conditions such as allergic rhinitis or rhinosinusitis may have partial or
complete loss of olfaction. Olfaction may also be compromised by medications,
trauma, mechanical obstruction, disorders of ciliary function, endocrine disorders, and congenital disorders such as Kallmann’s syndrome (congenital hypogonadism with anosmia).
On inspiration, air particles contact olfactory epithelium, located primarily on
the roof of the nasal cavity. Olfactory transduction occurs, converting odor molecules into electrical signals that are perceived as smells by the brain.
Phonation
Both the nasal cavity and paranasal sinuses contribute to resonance during phonation. Nasal consonants such as m and n require nasal airway patency. Nasal
airway obstruction may result in hyponasality. When septal perforations are present, patients may create a whistling sound when breathing.
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Chemoreception
In many animals, the anterior nasal septum houses the vomeronasal organ (Jacobson’s organ). is organ occurs as bilateral paired blind ducts in the anterior
septal mucosa.
,
Within the organ is specialized epithelium that may serve as
pheromone chemoreceptors. e presence of these structures in humans continues to be debated.
THE NASAL CYCLE
e nasal cycle is a normal physiologic process of alternating mucosal constriction and dilation of each side of the nasal cavity. Approximately % of people
experience this, and the nasal cycle can be aected by emotion, exertion, and
external temperature. Total airow and resistance remain constant throughout
the process, which requires minutes to hours per cycle. Although this is a
normal physiologic process, many patients may be unaware of this before surgery and may confuse this process with postoperative nasal airway obstruction.
CLINICAL EVALUATION
Primary assessment of the nasal airway is accomplished through a focused history and physical examination. Relevant factors include the onset, timing, duration, and severity of symptoms. Exacerbating and alleviating factors should also
be detailed, and any history of prior trauma or nasal surgery should be ascer
tained. Nasal airway obstruction that is constant suggests an anatomic problem,
whereas uctuating symptoms are more commonly associated with a physiologic
cause. Symptoms of unilaterality, epistaxis, or a progressive worsening warrant
exclusion of a neoplasm.
Physical examination should begin with evaluation of the external nose. e external nasal valve is assessed by observing the nasal base during quiet and forced
inspiration. Collapse of the nostril, which may be unilateral or bilateral, suggests
a weak lateral crus. Similarly, collapse of the lateral nasal wall may suggest weak
upper lateral cartilage and possibly internal valve dysfunction. Dorsal nasal deviation or collapse suggests a septal cause, with the potential for internal valve
dysfunction. Collapse of the nasal midvault may be seen in patients displaying
an inverted-V deformity or distorted dorsal aesthetic lines.
-
Internal examination of the nose is of critical importance and is facilitated with
a nasal speculum and proper lighting. e nasal cavity should be assessed at
rest and during respiration. At rest, the septum can be assessed. Deviation, sep-
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tal spurs, or perforation of the nasal septum should be noted. Crusting or dried
blood may be a sign of an underlying pathologic condition. Turbinate status
should also be investigated. Large, boggy turbinates with a bluish hue suggest allergic changes to the mucosa. Application of a topical nasal decongestant such as
oxymetazoline can help distinguish between mucosal swelling and bony hypertrophy. If the turbinate responds with resolution of engorgement, then mucosal
ase is the primary problem.
dise
Mucosal disease is less responsive to surgical intervention compared with bony
hypertrophy or malposition. Middle turbinates should also be assessed for the
existence of concha bullosa. e internal nasal valve is evaluated both at rest and
during respiration. e caudal border of the upper lateral cartilage can usually be
seen as it adjoins the nasal septum. Collapse of the internal nasal valve suggests
dysfunction. Other means of assessing the adequacy of the internal nasal valve
include the Cottle test and the use of nasal strips.
RELEVANT ANATOMY AND CLINICAL APPLICATIONS
e anatomy of the nose and nasal cavity inuences nasal airow. Structures
including the external nasal valve, internal nasal valve, nasal septum, and turbinates and nasal mucosa are common sites of abnormalities contributing to nasal
airway obstruction.
External Nasal Valve
Nasal airow begins with movement through the nostril and into the vestibule.
e nasal vestibule houses the external nasal valve formed by the alar rim, nasal
sill, caudal septum and medial crus. A weakened or malpositioned lateral crus
can lead to external valve collapse and subsequent obstruction. e external
nasal valve is responsible for approximately one third of total airway resistance.
If there is insucient cartilaginous support, low pressures in the vestibule during
inspiration can collapse the nostril and vestibule, increasing resistance and decreasing ow. is is frequently seen in secondary rhinoplasty patients in whom
the lateral crura have been overresected or weakened by manipulation. Correcting deformities of the external nasal valve hinges on correcting the existing weak
ness of the lateral crus. If the lateral crus is weak, then cartilage gras such as lateral crural strut gras, alar batten gras, or alar contour gras may be required.
-
Internal valve
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Internal
valve
External
valve
External
valve
Alternatively, severe caudal septal deviation can decrease the cross-sectional area
of the external valve and contribute to obstruction at this site. In addition to
strengthening the lateral crus, midline repositioning of the caudal septum is of
equal importance to reestablishing the structural integrity of the external nasal
valve.
Internal Nasal Valve
Resistance to nasal inow is greatest at the internal nasal valve, because this
structure accounts for the majority of total airway resistance. As previously mentioned, the abutment of the caudal edge of the upper lateral cartilage with the
dorsal septum creates the internal valve angle. Airow through the internal nasal
valve may be limited when the angle formed by the junction of the upper lateral
cartilage and the nasal septum is less than the typical to degrees.
ally, the anterior head of the inferior turbinate forms the posterior border, and
hypertrophy will also negatively eect nasal airow at the internal nasal valve.
When the inferior turbinates are a contributing factor, it must be determined
whether bony hypertrophy or mucosal disease is the cause.
,
Addition-
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Airow through the internal nasal valve may be limited when the angle formed
by the junction of the upper lateral cartilage and the nasal septum is less than
the typical 10 to 15 degrees.
Deviation of the dorsal septum can also compromise the internal nasal valve.
is area must be addressed surgically to straighten the septum and restore the
angle of the internal nasal valve. When the upper lateral cartilages lack the rigidity necessary to withstand inspiration pressure, there can be a dynamic collapse
of the valve.
Preserving the structural integrity of the internal nasal valve is crucial when performing rhinoplasty. Disruption may occur during dorsal hump reduction em
phasizing the importance of maintaining or rebuilding the internal valve. Patients
with short nasal bones and long, poorly supported upper lateral cartilages are at
highest risk. Employing component dorsal hump reduction with upper lateral
crura tension-spanning sutures maximizes preservation of the existing anatomy.
Autospreader aps also function to preserve or increase the internal nasal valve
angle. Reconstructing the internal nasal valve is required in primary rhinoplasty
when internal nasal valve dysfunction is present or if the internal nasal valve is
not preserved during treatment of the dorsum.
-
Secondary rhinoplasty more frequently requires reconstruction of the internal
nasal valve when the midvault was not preserved or reconstructed during the
previous nasal surgery. ese patients may have an inverted-V deformity, lateral
wall weakness, and/or distortion of the dorsal aesthetic lines.
Preserving the structural integrity of the internal nasal valve is crucial when
performing rhinoplasty.
Nasal Septum
In addition to comprising the external or internal nasal valve, the septum may
cause obstruction of airow throughout the nasal cavity. Although deviation of
the anterior cartilaginous or posterior bony septum may lead to nasal airway
obstruction, the anterior septum tends to be more clinically relevant. e crosssectional area of the posterior airway is much greater in comparison with that of
the nasal valves of the anterior airway.
Furthermore, the septum provides overall support for the external nose. Seated
on the nasal spine of the maxilla, the nasal septum serves as the scaolding for
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a mucosa and the upper and lower lateral cartilages. Anomalies of the septum
such as deviations, spurs, and perforations may adversely aect nasal airow.
ese changes may be developmental in nature and worsen over time, or septal
abnormalities may occur as a result of trauma. Deviation of the septum may also
inuence turbinate size and mucosal behavior. Typically, the inferior turbinate
on the side opposite the septal deviation will undergo mucosal and/or bony hypertrophy.
,
Surgical correction of the septum in general may be accomplished through septoplasty or septal reconstruction. Septal reconstruction involves midline repositioning of the septum or resection of deviated portions or spurs.
Septal perforations may be asymptomatic or can cause problems ranging from a
whistling sound during respiration to chronic bleeding, crusting, and malodor.
Repair of septal perforations may be achieved using local mucosal aps or the
placement of interposition gras, such as temporal fascia.
Turbinates and Nasal Mucosa
Nasal turbinates are paired extensions of the lateral nasal wall; they increase
overall mucosal surface area. Turbinates contribute a signicant role in air transport and conditioning as well as olfaction. e inferior turbinate is commonly
implicated in nasal airway obstruction because of its proximity to the internal
nasal valve. However, the middle turbinates may also contribute to nasal airway
obstruction.
e inferior turbinate is commonly implicated in nasal airway obstruction because of its proximity to the internal nasal valve.
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Middle
turbinate
Superior
turbinate
Inferior
turbinate
Parasagittal view of lateral nasal wall
Composition of septal and turbinate mucosa includes erectile tissue with capacitance vessels responsible for volume changes. e dynamic nature of mucosal
tissue is essential to normal nasal function. Regulation occurs by expansion (congestion) or contraction (decongestion). Blood supply distributed to the erectile
tissue is responsible for expansion or contraction and is regulated by the autonomic nervous system. Nasal mucosa is also inuenced by such nonanatomic factors as stress and emotion, external temperature, allergies, inammatory conditions, infection, smoking, medications, trauma, pregnancy, and the aging process.
Dierentiating between mucosal disease and bony hypertrophy will help the
surgeon formulate the appropriate treatment plan. Surgical maneuvers such as
inferior turbinate outfracture or submucosal resection will help to lateralize or
decrease turbinate volume, respectively.
,
Mucosal hypertrophy resulting from
rhinitis may be better managed medically.
KEY POINTS
■
ere is an intimate relationship between the anatomy of the nose and its associated physiology. An understanding of this relationship provides the basis
for both nasal analysis and treatment planning.
■
e nasal airway is responsible for % of overall airway resistance underscoring its importance in nasal airow.
■
Nasal airway obstruction may have a structural and/or functional cause, and
accurate diagnosis will guide appropriate treatment planning.
■
In addition to respiration, the nose and nasal cavity perform an additional six
functions: particle ltration, air humidication, temperature modication, olfaction, phonation, and chemoreception.
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■
Most of the air continues over the inferior turbinate, passing through the middle meatus; the greatest resistance occurs at the internal nasal valve.
■
e internal nasal valve is a bottleneck to airow in the nasal cavity and is commonly a site for nasal airway obstruction.
■
Airow through the internal nasal valve may be limited when the angle formed
by the junction of the upper lateral cartilage and the nasal septum is less than
the typical to degrees.
■
Preserving the structural integrity of the internal nasal valve is crucial when
performing rhinoplasty.
■
e inferior turbinate is commonly implicated in nasal airway obstruction because of its proximity to the internal nasal valve.
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