Discussion
This study found a statistically significant but weak correlation
between patient-reported evaluation of nasal breathing and objective
measurement of nasal airflow in patients not selected according to
symptoms of nasal obstruction or to the level of nasal airflow. A weak
or very weak correlation between patient-reported evaluation of nasal
breathing and objective measurement of nasal airflow was found in groups
of patients selected according to symptoms of nasal obstruction.
The current investigation supports that a moderate to high nasal airflow
does not necessarily lead to a subjective sensation of a good nasal
breathing, according to the widespread NOSE scores amongst patients with
moderate to high values of PNIF. Moreover, our results also suggest that
a low nasal airflow prevents the sensation of a good nasal breathing to
happen.
It is well established, according to Poiseuille‘s Law, that airway
resistance is inversely proportional to the 4th power
of the radius of the space passed through. Yet, several reasons may
explain why a wide nasal airway, as measured by acoustic rhinometry or
by the cross-section dimension on imaging techniques of the nasal
airway, may not necessarily correspond to a good nasal airflow. Firstly,
a nasal airway too wide can prevent the negative pressure necessary to
inhale air to be generated. Secondly, an overly wide nasal cavity may
decrease laminar airflow and significantly increase turbulent airflow,
which may disturb nasal breathing (33-35). And, thirdly, the dimensions
of the nasal airway have no relation to the resilience of the nasal
cavity walls, which is necessary to withstand the negative pressure
generated during inspiration.
Therefore, a wide nasal airway is not the sole factor for obtaining a
suitable and pleasant airflow. Objective measurements of the nasal
airflow do not always correlate with the subjective sensation of nasal
breathing, as reported in our study as well as in others‘ (1-10),
suggesting that factors other than an appropriate airflow are important
in determining the subjective sensation of a suitable nasal breathing.
Several reports have found that the sensation of nasal breathing is more
related to variations in nasal mucosa temperature due to the cooling
effect of the airstream than to the level of nasal airflow (8,21-23).
The sensation of nasal breathing is delivered to the brain by
non-myelinated trigeminal afferent fibers originating in receptors
located in the nasal mucosa (36-38). In this system, the target
thermoreceptor in the nasal mucosa has been identified as the
non-selective voltage-dependent cation channel transient receptor
potential melastin family member 8 (TRPM8) (39,40). These
thermoreceptors, which are distributed throughout the mucosa of the
nasal cavity (41,42) and mainly in the anterior part of the nasal airway
(37), are sensitive to temperature changes in the nasal mucosa caused by
the airstream. Mucosal cooling directly excites these receptors, as the
airstream passes through the nasal cavity, triggering trigeminal
afferent stimulation and providing perception of nasal breathing
(21-23,38,43,44).
Therefore, for an appropriate sensation of nasal breathing a nasal
cavity wide enough to allow the airstream to cool the nasal mucosa is
necessary. Bailey et al (44) found a
negative correlation between nasal resistance and the degree of mucosal
cooling. But also an adequate nasal airflow is necessary for providing
temperature changes in the nasal mucosa. Lindemann et al (45)
found that the increased nasal airflow during deep breathing was
associated with greater oscillations in nasal mucosal temperature and
greater sensation of airflow than in quiet resting breathing.
The mean value of PNIF in our series was noticeably lower than the mean
values that have previously been published in series with healthy
populations (26,46-49). This probably reflects the fact that our study
was performed in rhinoplasty-seeking patients, some complaining of nasal
obstruction, and not in a healthy population, as the mean NOSE score of
48.4 (SD 24.4) in our series indicates.
Previous researchers (11-16) have found correlation between
patient-reported assessment of the nasal airway and objective
measurements of nasal airway resistance or of nasal airflow to be
dependent on the severity of symptoms of nasal obstruction. Our
investigation focused on groups of patients that were created in
accordance to the degree of nasal obstruction as reflected by the NOSE
score, and found that this correlation was weak or very weak in each of
these subgroups (r =-0.250, r =-0.007, r =-0.104).
These findings are in line with the results of other publications
(17-20). Our study shows that patients with symptoms of nasal
obstruction have different degrees of nasal airflow, which favors that
symptoms of nasal obstruction are not solely determined by nasal
airflow.
Based on the mean value of PNIF of our study, we split our series of 79
patients into three groups of patients, with low, moderate or high PNIF
values, and tried to find if correlation between patient-reported
assessment of the nasal airway and objective measurements of nasal
airflow was dependent on the degree of nasal airflow. This correlation
was very weak for patients with moderate or high nasal airflow value
(r =-0.190, r =-0.014). For patients with low PNIF value,
however, there was a moderate correlation between nasal airflow and the
NOSE score (r =-0.404), suggesting that a low nasal airflow
prevents a good sensation of nasal breathing. These results are in favor
that an adequate nasal airflow may or may not be associated with
sensation of good nasal breathing, consistent with the assumption that
factors other than airflow play an important role on providing sensation
of nasal breathing. Our findings also suggest that an inadequate nasal
airflow will negatively act on the patient´s sensation of a suitable
nasal breathing, probably due to insufficient nasal mucosal cooling.
This finding has important outcomes in the way patients with nasal
obstruction should be addressed. If patients with symptoms of nasal
obstruction have inadequate nasal airflow, then improving the airflow is
an essential step towards improving nasal breathing sensation.
This study was accomplished in a non-homogeneous group of patients, with
predominance of female patients. Also, the groups of patients created
based on the NOSE score and on the PNIF level were relatively small in
number, which may have influenced the low coefficients of correlation in
some of these groups. Likewise, all the patients included in the series
were rhinoplasty-seeking patients, and not randomly selected
individuals. This may have interfered on the difference between the mean
value of PNIF of this series when compared to previously published
series in healthy populations. Moreover, some of the patients included
in the study had already been submitted to nasal surgery, which may have
changed the perception of nasal breathing reflected by the NOSE score
and nasal airflow reflected by PNIF value. Nevertheless, none of the
patients undergoing revision rhinoplasty had complaints of nasal
obstruction. Lastly, the NOSE score is currently the most frequently
used patient-reported assessment of symptoms of nasal obstruction,
though having initially been developed as a patient reported outcome
measurement of the effect of septoplasty on nasal obstruction (24).