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Volume 12 No. 10
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Scientific Investigations

Upper Airway Collapsibility Assessed by Negative Expiratory Pressure while Awake is Associated with Upper Airway Anatomy

Raquel P. Hirata, MSc1; Fabiola Schorr, MD1; Fabiane Kayamori, SLP1; Henrique Takachi Moriya, PhD2; Salvatore Romano, MSc3; Giuseppe Insalaco, MD3; Eloisa M. Gebrim, MD4; Luis Vicente Franco de Oliveira, PhD5; Pedro R. Genta, MD1; Geraldo Lorenzi-Filho, MD1
1Sleep Laboratory, Pulmonary Division, Heart Institute (InCor), Hospital das Clínicas da Faculdade de Medicina da Universidade de São Paulo, Brazil; 2Biomedical Engineering Laboratory, Escola Politécnica, Universidade de São Paulo, Brazil; 3Institute of Biomedicine and Molecular Immunology A. Monroy, Italian National Research Council, Palermo, Italy; 4Radiology Institute (InRad), Hospital das Clínicas da Faculdade de Medicina da Universidade de São Paulo, Brazil; 5Rehabilitation Sciences Master and Doctoral Program, Universidade Nove de Julho, São Paulo, Brazil

ABSTRACT

Study Objectives:

There is a growing interest to develop a simple method to characterize the mechanisms leading to upper airway collapse in order to guide treatment options in patients with obstructive sleep apnea (OSA). Critical closing pressure (Pcrit) during sleep is able to predict the anatomical component of OSA. However, Pcrit is a laborious method that is only used for research purposes. The application of negative expiratory pressure (NEP) is a simple method to assess upper airway collapsibility that can be easily performed during wakefulness. We hypothesized that NEP will be, similarly to Pcrit, associated with upper airway anatomy assessed by computed tomography (CT) scan.

Methods:

Patients under investigation for OSA underwent polysomnography, CT of the upper airway, NEP while awake, and Pcrit during sleep. NEP was performed with −5 cm H2O in supine position using a nasal mask. Pcrit was measured during sleep induced by low doses of midazolam.

Results:

Twenty-eight male subjects were studied (age 45 ± 13 y, body mass index 29.4 ± 4.9 kg/m2, apnea-hypopnea index (AHI) 30 ± 26, range 2 to 86 events/h). NEP and Pcrit were similarly associated with tongue area (r = 0.646 and r = 0.585), tongue volume (r = 0.565 and r = 0.613) and pharyngeal length (r = 0.580 and r = 0.611), respectively (p < 0.05 for all comparisons). NEP and Pcrit were also significantly correlated with AHI (r = 0.490 and r = 0.531). NEP and Pcrit were significantly higher in patients with severe OSA than the remaining population.

Conclusions:

NEP is a simple and promising method that is associated with the anatomical component of upper airway collapsibility. NEP may be valuable to select patients for noncontinuous positive airway pressure alternative therapies for OSA.

Citation:

Hirata RP, Schorr F, Kayamori F, Moriya HT, Romano S, Insalaco G, Gebrim EM, de Oliveira LV, Genta PR, Lorenzi-Filho G. Upper airway collapsibility assessed by negative expiratory pressure while awake is associated with upper airway anatomy. J Clin Sleep Med 2016;12(10):1339–1346.


INTRODUCTION

Obstructive sleep apnea (OSA) is characterized by recurrent collapse of the pharynx during sleep. OSA is considered a public health problem and affects up to one- third of the adult population.1,2 Continuous positive airway pressure (CPAP) is the gold standard treatment for moderate to severe OSA. Adherence to CPAP is far from ideal even in patients with severe OSA.3 However, the response to non-CPAP alternatives such as mandibular advancement device, sedatives, weight loss and oropharyngeal exercises is variable and not easily predictable. Therefore, there is a growing interest to characterize OSA phenotypes in order to individualize OSA therapy. Four main phenotypical traits have been proposed to be involved in OSA pathogenesis: upper airway anatomy, ventilatory control instability, upper airway neuromuscular function, and arousal threshold. However, sophisticated methods are necessary to determine the contribution of each trait. Upper airway anatomy is the most important among all phenotypical traits. Multiple studies using computed tomography (CT) and magnetic resonance imaging have shown that patients with OSA have a smaller airway, larger soft-tissue volume, and an increased upper airway length than proper controls while awake.47 Pharyngeal critical closing pressure (Pcrit) evaluates upper airway collapsibility during nonrapid eye movement (NREM) sleep and determines the anatomical contribution to OSA. Pcrit is associated with anatomic features such as pharyngeal length and tongue volume.812 However, Pcrit is a laborious method that requires a complex setup and experienced investigators capable of recognizing sleep stages and flow limitation during CPAP application.

BRIEF SUMMARY

Current Knowledge/Study Rationale: There is a growing interest to develop simple methods to characterize the mechanisms leading to upper airway collapse in order to guide treatment options in patients with obstructive sleep apnea (OSA). Negative expiratory pressure (NEP) evaluates upper airway collapsibility and can be easily performed during wakefulness.

Study Impact: We showed that NEP applied with a nasal mask in the supine position is associated with the anatomical component of upper airway collapsibility and performs similarly to the laborious method of Pcrit in patients under investigation for OSA. This tool may be valuable to select patients for noncontinuous positive airway pressure alternative therapies for OSA.

Negative expiratory pressure (NEP) applied noninvasively during spontaneous expiration is a simple measurement performed during wakefulness used to evaluate expiratory flow limitation.13 NEP is expected to increase expiratory flow in normal individuals. Expiratory flow fails to increase during NEP application among individuals with intrathoracic or extrathoracic obstruction. NEP was first described to evaluate intrathoracic expiratory flow limitation in patients with chronic obstructive pulmonary disease.13,14 NEP has been more recently used to assess upper airway collapsibility in patients with suspected OSA. NEP is able to differentiate patients with severe OSA and snorers, as well as normal subjects.1518 Despite all this promising evidence, NEP has never been used to predict upper airway anatomy. In this study we hypothesized that NEP will be, similarly to Pcrit, associated with upper airway anatomy evaluated objectively by CT scan.

METHODS

Subjects

Male subjects aged 18–65 y referred to the Heart Institute Sleep Clinic for OSA investigation were considered eligible for the study. Patients with severe nasal obstruction, craniofacial malformations, and intrathoracic obstruction as defined by the ratio between forced expiratory volume in one second over the forced vital capacity (FEV1/FVC) < 0.7 were excluded. The study was approved by the local ethical committee (CAPPesq 80484/2012). Written informed consent was obtained from all subjects before study entry. Most patients enrolled in this study took part in another investigation that assessed the anatomical characteristics associated with upper airway collapsibility.12

Evaluations

Pulmonary Function Test

A pulmonary function test (KoKo Version 4.14, nSpire Health, Louisville, KY, USA) was performed by a well-trained technician according to the guidelines of the Brazilian Society of Pneumology19 and the European Respiratory Society.20

Negative Expiratory Pressure

NEP was performed during wakefulness, with subjects using a nasal mask while supine, with eyes open and the neck in neutral position. Negative pressure was generated by a Super Air Amplifier (Exair model 120021, Cincinnati, OH, USA) attached to a tank of compressed air connected to a solenoid valve (Norgren Ltd model 9500400, Vimercate, Italy). The solenoid valve was operated by software control, automatically activated in early expiration and remained open for 2 sec. A pneumotachograph (Hans Rudolph, model 3830B, Shawnee, KS, USA) was connected to the Air Amplifier and to the nasal mask for measurements of airflow, using a differential pressure transducer (Sensortechnics GmbH, model PCLA02X5, Puchheim, Germany). Mask pressure was measured by a pressure transducer (Sensortechnics GmbH, model PCLA0050, Puchheim, Germany). Flow and pressure signals were filtered through a low-pass filter and sampled at 100 Hz. Both signals were displayed in real time on the computer screen and recorded for subsequent analysis. Ten NEP maneuvers with at least four preceding regular breaths for normalization of breathing pattern were performed in each subject. NEP was analyzed by the ratio between V0.2 during stable breathing (three expirations preceding NEP application) and V0.2 during NEP application (V0.2SB/V0.2NEP), as illustrated in Figure 1. Automatic control of the solenoid valve and data acquisition were performed using custom-designed software written in LabView 8.2 (National Instruments; Austin, TX, USA) developed in the Palermo laboratory. Analysis of flow and volume were performed using a custom-designed software (MatLab, The Math Works, Natick, MA, USA).

Negative expiratory pressure (NEP) application and pharyngeal critical closing pressure (Pcrit) determination of two representative patients.

From top to bottom: flow and pressure from curves during three stable breathings and during NEP application (−5 cm H2O) triggered at onset of expiration. The bottom graph represents the regression line obtained to determination of Pcrit during sleep induction in a separate experiment of patients A and B, respectively. Each point represents the peak inspiratory flow for breaths three to five with flow limitation and the correspondent nasal pressure. The Pcrit is the resultant from the extrapolation of the regression line and represents the Pressure at zero-flow. Patient A is a 52-y-old male (weight = 71.9 kg, height = 1.61 m, BMI = 27.7 kg/m2, and AHI = 15.4 events/h) with NEP = 0.32 and Pcrit = −5.53 cm H2O. Patient B is a 47-y-old male (weight = 139.6 kg, height = 1.88 m, BMI = 39.5 kg/m2, and AHI = 80.2 events/h) with NEP = 0.94 and Pcrit = +4.32. V0.2SB/V0.2NEP, ratio between mean exhaled volume at 0.2 sec during stable breathing and the exhaled volume at 0.2 sec during NEP; ViMax, peak inpiratory flow.

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Figure 1

Negative expiratory pressure (NEP) application and pharyngeal critical closing pressure (Pcrit) determination of two representative patients.

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Polysomnography

Standard overnight polysomnography (PSG) was performed using Alice 5 (Philips Respironics, Murrysville, PA, USA). Monitoring included electroencephalogram, electrooculogram, chin and tibialis electromyography, electrocardiogram, oxymetry, oronasal airflow (thermistor and pressure cannula), measurements of rib cage and abdominal movements, snoring, and body position. Apnea was defined as a reduction of ≥ 90% of thermistor signal amplitude for at least 10 sec. Hypopnea was defined as a reduction of ≥ 30% of nasal pressure amplitude for at least 10 sec followed by oxygen desaturation ≥ 3% or arousal. The apnea-hypopnea index (AHI) was calculated as the total number of respiratory events (apneas + hypopneas) per hour of sleep.21 Subjects with AHI < 5 were considered normal. Mild, moderate, and severe OSA were defined as 5 ≤ AHI < 14.9, 15 ≤ AHI < 29.9, and AHI ≥ 30, respectively.1

Pharyngeal Critical Closing Pressure

Pcrit was performed in the morning, after sleep induction with low doses of midazolam (diluted in a saline solution with a concentration of 1 mg/10 mL). A peripheral vein was cannulated and 5 mL of the solution was administered slowly until a sleeplike state was detected through PSG monitoring. The infusion was restarted if the patient arose and was not able to fall sleep again in 10 min.22 Patients slept in the supine position with all PSG channels (except for nasal pressure and thermistor), a well-fitted nasal mask attached to a heated pneumotachograph (Hans Rudolph, model 3700A), and a differential pressure transducer (Validyne, Northbridge, CA) for measurements of flow and pressure. The mask was connected to a modified CPAP device (Philips Respironics) that could deliver both positive and negative pressures. After sleep onset, airway pressure was increased in order to abolish flow limitation (holding pressure). When stable NREM sleep was reached, CPAP pressure was abruptly reduced 1–2 cm H2O during expiration, held for five respiratory cycles, and then returned to the holding pressure. If the patient remained asleep, the reductions were repeated after 1 min with a further pressure decrease of 1–2 cm H2O. If an arousal occurred, at least 2 min of stable NREM sleep should occur at holding pressure before any further CPAP reduction. This process was repeated progressively until an apnea occurred. The entire procedure was performed two to four times for each subject. Peak inspiratory flow from breaths three to five were plotted against nasal pressure if breaths were considered flow-limited and Pcrit was determined as the zero-flow intercept from a linear regression.8 This analysis was performed using a custom-designed software (MatLab, The Math Works, Inc).

Upper Airway CT Scan

CT scan of the upper airway (Discovery CT 750 HD, GE HealthCare, Milwaukee, WI, USA) was performed during quiet breathing with patients in the supine position with the head in neutral position while awake. The scans were acquired at 2.5 mm collimation/interval and reconstructed at 0.625/0.625 mm thickness/interval, with 120 kV, 100mA, and a rotation time of 0.8 sec. Axial and sagittal image reconstructions were performed to allow linear and volumetric measurements using an Advantage Workstation, version 4.5 (GE Healthcare, Milwaukee, WI, USA). The landmarks identified at the midline sagittal reconstruction were posterior nasal spine, hyoid, mandibular plane, and epiglottis base. The distance between mandibular plane to hyoid (MPH) was determined. Pharyngeal length was defined as the distance of a horizontal lines passing at the level of the hard palate and the base of the epiglottis. Tongue area was measured by tracing the contours on the sagittal plane. Three-dimensional reconstructions were made for volumetric measurements. Tongue volume measurement was performed by identifying tongue limits and manually tracing the contours at each axial image obtained. Airway volume was determined by segmentation technique based on a fixed threshold characteristic of air (Figure 2). A single investigator performed all measurements.

Sagittal CT scan of the upper airway.

MP, mandibular plane; MPH, mandibular plane to hyoid distance; PNS, posterior nasal spine; pharyngeal length, distance between the posterior nasal spine to epiglottis.

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Figure 2

Sagittal CT scan of the upper airway.

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Statistical Analysis

Variables were described as mean ± SD or absolute number [percentage] when appropriate. We used the Kolmogorov-Smirnov test to evaluate the normality of the variables and Pearson correlation coefficient to analyze the associations between upper airway collapsibility variables (Pcrit and NEP) and upper airway anatomy. We also performed a linear regression model using the upper airway anatomy measurements as dependent variables and NEP or Pcrit and body mass index (BMI) as independent variables in order to control results for BMI. The sample was divided according to the median value of V0.2SB/V0.2NEP and anthropometric and anatomical variables were compared using independent Student t-test. One-way analysis of variance and Bonferroni post hoc analyses were used to compare subjects according to OSA severity.

RESULTS

Thirty male subjects were invited to the study. Two subjects were excluded because of intrathoracic airway obstruction (FEV1/FVC < 0.7). The included sample consisted of middle-aged overweight males with a wide range of AHI (Table 1). The duration of Pcrit determination was 3.1 ± 0.9 h and the dose of midazolam was 3.7 ± 1.7 mg. Holding pressure was 9.5 ± 2.7 cm H2O, the mean number of series of pressure drops was 3.6 ± 1.2 and the number of pressure drops was 29.8 ± 7.2. NEP was obtained in approximately 10 min.

Demographic, clinical, and sleep characteristics of the subjects (n = 28).

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Table 1

Demographic, clinical, and sleep characteristics of the subjects (n = 28).

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Anthropometric, upper airway anatomy, and collapsibility characteristics according to OSA severity are presented in Table 2. Patients with severe OSA were older and presented with a higher BMI, a higher Pcrit, and NEP (as defined by V0.2SB/V0.2NEP ratio). Anatomical parameters were also different between different AHI categories.

Anthropometric, upper airway anatomy and collapsibility characteristics according to obstructive sleep apnea severity.

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Table 2

Anthropometric, upper airway anatomy and collapsibility characteristics according to obstructive sleep apnea severity.

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Table 3 shows anthropometric, sleep, and anatomical variables divided in two groups according to the median value of V0.2SB/V0.2NEP. Patients with higher upper airway collapsibility measured by NEP had higher BMI and also had increased tongue dimensions (area and volume), pharyngeal length, and soft palate length. Figure 1 represents the flow tracing from a patient with a low V0.2SB/V0.2NEP and one with a high V0.2SB/V0.2NEP, as well as the respective Pcrit regression lines. The associations between pharyngeal length and tongue volume with NEP and Pcrit are illustrated in Figure 3. Correlations between upper airway anatomy variables and upper airway collapsibility are presented in Table 4. NEP and Pcrit were still associated with variables such as pharyngeal length (β coefficient = 0.64; p = 0.002 for NEP and β coefficient = 0.64; p = 0.001 for Pcrit) and tongue area (β coefficient = 0.38; p = 0.012 for NEP and β coefficient = 0.30; p = 0.044 for Pcrit), independently of the BMI. NEP was correlated to Pcrit (r = 0.39). NEP and Pcrit were also significantly correlated with AHI (r = 0.490 and r = 0.531, respectively).

Anthropometric, sleep and anatomical variables according to the median value of negative expiratory pressure.

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Table 3

Anthropometric, sleep and anatomical variables according to the median value of negative expiratory pressure.

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Correlation analyses among upper airway collapsibility measured while awake (NEP - left side) and during sleep (Pcrit - right side) with pharyngeal length (distance between posterior nasal spine to epiglottis) and tongue dimension (represented by tongue volume).

NEP, negative expiratory pressure; Pcrit, pharyngeal critical closing pressure.

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Figure 3

Correlation analyses among upper airway collapsibility measured while awake (NEP - left side) and during sleep (Pcrit - right side) with pharyngeal length (distance between posterior nasal spine to epiglottis) and tongue dimension (represented by tongue volume).

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Associations between upper airway collapsibility evaluated by negative expiratory pressure while awake and Pcrit during sleep with anthropometric variables and upper airway anatomy (n = 28).

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Table 4

Associations between upper airway collapsibility evaluated by negative expiratory pressure while awake and Pcrit during sleep with anthropometric variables and upper airway anatomy (n = 28).

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DISCUSSION

In the current study we showed that NEP is correlated with the anatomical component of upper airway collapsibility in patients under investigation for OSA. First, among patients investigated for OSA with a wide range of AHI, both NEP and Pcrit were significantly worse in patients with severe OSA than in the remaining population (Table 2). Second, when the patients were divided according to the median values of NEP, those with worse collapsibility were more obese, had larger neck circumference, and had worse anatomical characteristics including increased tongue area and volume, and increased soft palate and pharyngeal length (Table 3). Finally, the correlations between NEP and upper airway anatomy were similar to Pcrit measured during sleep (r ranging from 0.5 to 0.7) (Table 4).

There are anatomical and nonanatomical phenotypical traits that contribute to the genesis of OSA.23,24 Patients with severely compromised upper airway anatomy are less likely to benefit from non-CPAP therapies because nonanatomical traits (ventilatory control instability, upper airway neuromuscular function, and arousal threshold) have less influence on upper airway collapse. In our study, patients with severe OSA had a higher tongue area and volume, a longer pharyngeal length, and a lower positioned hyoid bone than patients with no OSA (Table 2). These results are in line with studies showing that tongue dimensions and pharyngeal length also contribute to increase the risk of upper airway collapse during sleep.4,7,2527 In addition, a lower positioned hyoid bone is the most commonly reported cephalometric abnormality among patients with OSA. Hyoid bone position also has a close relationship with OSA severity.5,6 Pcrit is a well-established method that is able to predict the anatomical component of OSA.912 However, Pcrit is a laborious examination performed during sleep that requires experienced investigators and is only used for research purposes. In contrast, NEP is a simple noninvasive technique performed while awake, does not require patient collaboration, and can be determined in approximately 10 min. One study showed an association between NEP and pharyngeal area estimated by acoustic pharyngometry.28 However, NEP was expressed in absolute values of expired volume with no corrections for lung volumes and upper airway anatomy was not determined.28 Despite all methodological differences that help to explain the relatively low correlation between NEP and Pcrit (r = 0.39), we have shown that NEP and Pcrit were similarly associated with the main components of upper airway anatomy relevant to OSA including tongue area (r = 0.646 and r = 0.585), tongue volume (r = 0.565 and r = 0.613), and pharyngeal length (r = 0.580 and r = 0.611), respectively.

NEP application should elicit an expiratory flow increase; therefore, the volume exhaled during NEP application should be greater than during stable breathing. In subjects with OSA, enlargement of soft tissues surrounding the oropharynx increases the resistance to flow during NEP application, causing a drop in flow and consequently a reduced exhaled volume. The ratio (V0.2SB/V0.2NEP) represents the volume exhaled at 0.2s during spontaneous breathing over the volume exhaled at 0.2s during NEP application. The higher this ratio, the more collapsible is the upper airway. The method of NEP determination varies among published studies. The majority of studies applied NEP using a mouthpiece either while sitting or supine.15,16,18,2832 This technique was adapted from earlier studies that aimed to evaluate intrathoracic obstruction.13,14,33 However, the use of a mouthpiece excludes the retropalatal region that is a common site of obstruction during sleep in patients with OSA.34 In addition, a partial opening of the mouth is necessary for mouthpiece use. Mouth opening has been shown to increase upper airway collapsibility.35 In contrast to previous reports, in our study NEP was applied through a nasal mask in the supine position. Our method therefore may convey more relevant information for patients under investigation for OSA. However, future studies are necessary to evaluate the effect of the interface and body position on the evaluation of upper airway collapsibility. The timing of NEP application has not been standardized and may vary from the onset up to end of expiration.13,28,31,36 In our study, NEP was automatically applied at the exact transition from inspiration to expiration and we analyzed the exhaled volume during the first 0.2 sec. Tantucci et al. showed that there was no reflex-mediated activation of genioglossus when NEP was applied in early expiration.37 Therefore, in our study NEP can be considered a passive upper airway collapsibility evaluation, independent of voluntary or reflex muscle recruitment.

This study has several potential limitations. Pcrit was assessed during induced sleep with midazolam. However, we have previously demonstrated that Pcrit determined during induced sleep by low doses of midazolam was similar to Pcrit during natural sleep.22 NEP and Pcrit were performed in different conditions. NEP was assessed during wakefulness while Pcrit was assessed during sleep. The number of patients was relatively small; future studies are necessary to validate NEP and V0.2SB/V0.2NEP values in large clinical cohorts. We studied only male subjects in order to avoid the inherent differences between sexes in upper airway collapsibility and respiratory responses to upper airway obstruction.38,39 Therefore, our results cannot be extrapolated to women. Finally, the method of NEP varies widely as previously discussed. Therefore, our results cannot be extrapolated to other methods of NEP application and analysis.

In conclusion, NEP application while awake in the supine position with a nasal mask is a simple and promising method that correlates with the anatomical component of upper airway collapsibility in patients being assessed for OSA. NEP may be valuable to select patients for non-CPAP alternative therapies for OSA.

DISCLOSURE STATEMENT

This study was supported by Sao Paulo Research Foundation (FAPESP), grant numbers: 2012/20743-5 and 2011/12120-5. Philips Respironics provided the modified CPAP device used in the study. The authors have indicated no financial conflicts of interest. The work was peformed at the Sleep Laboratory, Pulmonary Division, Heart Institute (InCor), Hospital das Clínicas da Faculdade de Medicina da Universidade de São Paulo, Brazil.

ABBREVIATIONS

AHI

apnea-hypopnea index

BMI

body mass index

CPAP

continous positive airway pressure

CT

computed tomography

FEV1/FVC

ratio between forced expiratory volume in one second and forced vital capacity

MPH

mandibular plane to hyoid distance

NEP

negative expiratory pressure

OSA

obstructive sleep apnea

Pcrit

pharyngeal critical closing pressure

PNS

posterior nasal spine

PSG

polysomnography

REM

rapid eye movements

SpO2

peripheral oxygen saturation

V0.2SB/V0.2NEP

ratio between mean exhaled volume at 0.2s during stable breathing (3 expirations preceding NEP application) and the volume exhaled at 0.2s during NEP application

ACKNOWLEDGMENTS

The authors thank Philips Respironics for providing the modified CPAP device. Author contributions: Raquel P. Hirata contributed to study design, data collection, analysis and interpretation of the data, and manuscript draft; Fabiane Kayamori, Fabiola Schorr and Eloisa M. Gebrim contributed to data collection and analysis, and critical review of the manuscript; Henrique Takachi Moriya contributed to data analysis and interpretation, and critical review of the manuscript; Pedro R. Genta contributed to study design, data analysis and interpretation, and critical review of the manuscript; Salvatore Romano, Giuseppe Insalaco, and Luis Vicente Franco de Oliveira contributed to data collection and analysis, and critical review of the manuscript, Geraldo Lorenzi-Filho contributed to study design, data analysis and interpretation, drafting and critical review of the manuscript.

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