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).