Abstract
Nasal breathing (NB) and oral breathing (OB) are two modes of respiration, and the extent to which they affect respiratory muscles and brain function. The primary objective of this study was to explore the impact of NB versus OB on respiratory muscle and brain function. A literature review was conducted by searching the National Library of Medicine (PubMed) and Scopus databases from January 2000 and May 2024. One hundred twenty-six articles were retrieved from the databases searched, and at the end of the selection process, 11 articles were included in the present review. Most studies (91%) were experimental and had adult healthy volunteers; 64% of the included studies focused on the effects of NB and OB on brain function, while the remaining 36% focused on respiratory muscles. A total of 313 participants comprised the population, most of whom were women (63%). Although most studies were conducted on adults, a percentage of participants (15%) were children. NB and OB elicit different brain areas and heterogeneously influence respiratory muscle function. Knowledge of the underlying mechanisms could be beneficial for, for example, personalizing respiratory and manual techniques when rehabilitating individuals with neurological or respiratory impairments.
INTRODUCTION
Respiration plays a crucial role in metabolism, providing oxygen for efficient physical and mental function. Nasal breathing (NB) is the commonly used mode of respiration and plays a significant role in the development of facial, oral, and respiratory muscles, as well as in the formation and physiological activity of facial bones.1, 2 Increased nasal resistance and conditions such as cold, nasal allergies, persistent rhinitis, and adenoid hypertrophy (producing airway blockage) disrupt the posterior oral seal with the soft palate and tongue, allowing air to flow into the oral cavity and causing the lips to open. It is possible for the NB to be replaced by the oral breathing (OB) in the absence of factors preventing air passage through the nasopharynx. In such cases, individuals frequently exhibit OB patterns that can result in a number of negative consequences, including headache, alterations in head position, fatigue, drowsiness, mouth-opening during sleep, snoring, nasal itching, saliva dripping onto the pillow, nocturnal dyspnea and nasal obstruction.2-6 If the air breathed through the mouth is not filtered, humidified, and heated, this can result in decreased lung function and electromyography (EMG) activation of the respiratory muscles.7 Previous evidence suggests that OB may be associated with an increased risk of impaired brain function related to low oxygen saturation in the human brain.8, 9 A study utilizing functional magnetic resonance imaging (fMRI) discovered that, in addition to impairments in working memory, olfactory memory, arithmetic abilities, and learning skills, individuals with OB exhibited a diminished blood oxygenation level-dependent signal in the hippocampus, brainstem, and cerebellum. Studies have found the achievements of academic skills in children using OB to be lower than those breathing via NB.9-11 The impact of breathing mode on EMG activity of the respiratory muscles remains a topic of contention among researchers, with no consensus yet reached. Current evidence suggests that OB produces different brain activity than NB, but there is a lack of electroencephalographic (EEG) research on the relationship between cognitive ability and the breathing mode used.10
Aim
The present study aimed to investigate the effects of OB and NB on respiratory muscle and brain function.
Search Process
A literature review12 was conducted by searching the National Library of Medicine (PubMed) and Scopus databases from January 2000 to May 2024. Two search strings, ‘oral breathing’ AND ‘nasal breathing’ AND ‘EMG’ and ‘oral breathing’ AND ‘nasal breathing’ AND ‘EEG’ were built, and an additional search was conducted on Google.
Two authors independently searched databases and assessed citations for inclusion, while a third author contributed to resolve disagreements over the appropriateness of the articles.
Duplicates were removed from the retrieved citations, and abstracts were evaluated for eligibility. The PRISMA guidelines were used as a guide.13
Inclusion and Exclusion Criteria
To be included in the study, research must focus on comparing NB and OB with EMG of respiratory muscles [upper trapezius (UT), sternocleidomastoid, and diaphragm] and reporting EEG activity and brain function.
There were no limits on age or gender, and only studies conducted in the English language were included.
Letters to the editor, conference proceedings, abstracts, and studies that did not describe NB or OB were excluded.
Data Analysis
The included citations were categorized based on their descriptive and experimental methodology and then analyzed. From the included articles, the first author’s name, publication year, country where the study was conducted, study design, demographic characteristics of participants and their number, type of assessments, and main findings were retrieved and tabulated.
One hundred twenty-six articles were retrieved from the searched databases, and after removing duplicates (n = 24), 102 citations were screened for eligibility, with 86 being excluded for not meeting the inclusion criteria. At the end of the selection process, 11 articles were included in this review (Figure 1). The majority of studies (91%) were experimental and had adult healthy volunteers (Table 1); 64% of the included studies focused on the effects of NB and OB on brain function (Table 2), while the remaining 36% focused on respiratory muscles (Table 3). A total of 313 participants comprised the population, most of whom were women (63%). Although most studies were conducted on adults, a minority of participants (15%) were children (Table 1).
DISCUSSION
Effects of Nasal and Oral Breathing on Brain Function
NB enhances the activity and connectivity of brain regions associated with the default mode network (DMN) in healthy subjects.14 This effect is not limited to the DMN, but may also spread to a broader brain area, as DMN connectivity indicates proper attention and self-cognitive skills. NB can affect different olfactory cortical and subcortical regions, which may be essential in transitioning from unconsciousness to wakefulness.14
In a study on the effects of music and breathing mode on emotions, listening to various types of music during NB increased the participants’ arousal levels and perceived relaxation.15
A study was conducted to compare the psychophysiological and phenomenological impacts of slow nasal breathing (SNB) and slow oral breathing (SOB) among meditation practitioners. The cardiorespiratory parameters were not significantly different between the SNB and SOB groups.16 After SNB and SOB, an elevation in slow rhythms (delta and theta rhythms) in EEG activity compared with baseline was observed. The increase after SNB was related to the prefrontal and central posterior areas associated with the intrinsic network17 and/or the DMN,18 whereas the enhancement after SOB was limited to posterior areas. Compared with SOB, SNB led to a significant amplification of slow rhythms in the medial prefrontal region. The results suggest that there is an increase in theta-high-beta coupling following SNB, potentially contributing to the regulation of brain functions supported by frontoparietal networks,17 thus facilitating large-scale integration processes associated with self-awareness19 and consciousness.20
This study aimed to investigate how cognitive function is affected during OB using fMRI. The authors performed a 2-back working memory task on a group of healthy participants during NB and OB and measured changes in neural activity.21 Specifically, the study found a significant association between working memory and functional connections among the left cerebellum and the left and right inferior parietal gyri, which were more activated in nasal breathers.
Another study investigated the effects of oxygen deprivation during OB on brain function in different working memory tasks with varying oxygen demand levels.22 The analysis of the EEG signals revealed that the difference in oxygenation was one of the main factors differentiating the influence of OB and NB on brain function.
The oxygen supply was more effective in reducing the characteristic changes between the EEG signals during NB and OB during the more complex tasks that required more oxygen.22
A previous study investigated alterations in brain activity during OB while simultaneously performing a cognitive task, utilizing EEG to measure brain waves at rest and during the n-back tasks (0-back and 2-back), alongside physiological variables, including SpO2, ETCO2, and respiratory rate.10 Theta and alpha powers exhibited decreased levels during OB compared to NB while at rest, and alpha power demonstrated reduced levels during the 0-back and 2-back tasks. Beta and gamma waves exhibit diminished power, specifically during the 2-back task. Additionally, SpO2 and respiratory rate significantly decreased during OB compared with NB, whereas ETCO2 levels were substantially elevated during OB. Although behavioral outcomes, including accuracy and reaction time, did not differ significantly between the two groups, the observed pattern of cerebral activity in the OB group was distinct from that of the NB group. This pattern of activity was linked to brain regions involved in cognitive processes. The observed alterations seem connected to the reduced oxygen saturation during OB, suggesting that OB could be a factor leading to different brain activity patterns when cognitive skills are involved.
To explore the hypothesis suggesting a connection between cortical oscillatory activity and the human respiratory cycle, albeit at a considerably slower rhythm of approximately 0.16-0.33 Hz, researchers gathered intracranial EEG data from a limited sample of patients with medically refractory epilepsy.23 They found that high-frequency oscillations were entrained not only in the piriform cortex but also in the amygdala and hippocampus, and dysregulation of limbic oscillatory synchrony occurred in all three brain regions, suggesting that variations in low-frequency (delta) power might act like a carrying rhythm within the lower rate of NB, with higher frequency oscillations embedded or entangled within the limbic system.24 It has been demonstrated that OB has a detrimental effect on cognitive performance, whereas NB has been shown to have a beneficial effect, including improvement in reaction time to fearful stimuli and accuracy in visual object recognition.
Effects of Nasal and Oral Breathing on Respiratory Muscle Activity
The EMG activities of the UT and sternocleidomastoideus (SCM) muscles and DA were evaluated in adults during NB and OB.6 The EMG activity of the SCM muscle was significantly lower in the OB group during sniffing, peak nasal inspiratory flow, and maximum inspiratory pressure, whereas no changes were found in the resting state and total lung capacity (TLC). For the UT muscle during rest and TLC, EMG activity did not differ between the OB and NB groups, but for the left part of the UT muscle, EMG activity was significantly higher in the OB group. The SCM muscle had greater activation in both groups during fast and short inspiratory workloads, with lower activation in the OB group. DA was markedly lower in the OB group at TLC, but no change was observed during sniffing.6 The forward head posture commonly seen in OB causes the chest to rise due to overuse of the SCM, which reduces the effectiveness of the diaphragm. In addition, OB can lead to hypertrophy of the accessory inspiratory muscles, which impede diaphragmatic movement because of their reduced mobility and lack of coordination with the abdominal muscles.25
A further study will examine the SCM and UT EMG activities during OB and NB in children. The results indicated that children who breathe through their mouths exhibited increased EMG activity during rest and decreased EMG activity during maximal voluntary contraction compared to children who breathe through their noses.26 In children with OB, increased SCM and UT activity indicates a change in head posture due to nasal obstruction, which requires more effort for inspiration and consequently heightens the EMG activity of the accessory inspiratory muscles.
One study examined the effects of NB and OB on genioglossus-EMG (GG-EMG) activity in response to hypoxia and found that OB resulted in significantly less increased GG-EMG activity compared with NB.27 Additionally, older subjects exhibited decreased GG response to hypoxia, which was most prominent during OB. In both examined groups, there was no difference in minute ventilation (MV) and tidal volume/inspiratory time (VT/TI). All subjects displayed a linear increase in MV, VT/TI, and GG-EMG activity in response to progressively induced isocapnic hypoxia. The authors have investigated the EMG activity of the GG and geniohyoid (GH) muscles and whether differences in breathing mode, as well as changes in posture, affect GG and GH activity.28 During maximal jaw opening, GH-EMG activity was higher than GG activity; moreover, GG activity varied significantly in terms of breathing mode and posture (the OB group had higher EMG activity than the NB group). In human studies, the GH has been identified as an accessory respiratory muscle. However, the GH muscle EMG activity remained unaffected by changes in breathing mode and posture, whereas the GG muscle was affected, as previously noted. Although the GH muscle may have a lesser role as a respiratory muscle compared with the GG muscle, it still plays a significant role in respiratory function by virtue of its direct attachment to the hyoid bone, which is crucial for maintaining upper airway patency. Despite the absence of detectable alterations in GH muscle activity in response to changes in respiratory mode and posture, the authors observed that under more challenging conditions, such as severe hypoxia, the capacity of the GH muscle to maintain force output during high activation levels may be negatively impacted.28 In summary, the EMG activity of the GG muscle was more efficient than that of the GH muscle in maintaining proper upper airway function.
In a related study, the impact of the respiratory pathway on the GG and NDM muscles during cycling exercise in an upright position was investigated.29 The findings revealed that NDM EMG activity was markedly elevated during NB, whereas GG-EMG activity was not influenced by NB or OB. Moreover, the EMG activity of the NDM muscle exhibited significantly greater sensitivity to nasal ventilation than to oral or total ventilation during both upright and supine exercise.
The primary limitation of this review is that the material retrieved was heterogeneous because it included studies conducted both in healthy individuals and patients and with high variance in age because of the presence of children and adults. Therefore, the results reported here should be considered with caution because they cannot be extended to a wider context. Furthermore, given that most participants were healthy individuals, extending the findings of the present review to a clinical context is difficult.
CONCLUSION
The present review confirms that the nasal and OB elicit different brain areas and heterogeneously influence respiratory muscle function. Knowledge of the underlying mechanisms could be beneficial, for example, in personalizing respiratory and manual techniques when rehabilitating individuals with neurological or respiratory impairments. Additionally, changes in posture, respiratory muscle EMG activity, and respiratory function resulting from different breathing modes are clinically significant, as they can affect rehabilitation components in individuals with respiratory impairments. To enhance the generalizability of the findings, future research should conduct randomized controlled trials involving a broader range of pathologies and patients from various age groups. This approach would increase the applicability of the results in clinical contexts.
