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. Author manuscript; available in PMC: 2024 Apr 1.
Published in final edited form as: Otolaryngol Head Neck Surg. 2023 Jan 29;168(4):868–875. doi: 10.1177/01945998221120793

The Accuracy and Reliability of Visually Assessed Pharyngeal Opening Pressures During Drug-Induced Sleep Endoscopy

Jason L Yu 1,2, Eric Thuler 1, Everett G Seay 1, Alan R Schwartz 1, Raj C Dedhia 1,3
PMCID: PMC10125901  NIHMSID: NIHMS1892464  PMID: 36040822

Abstract

Objective.

To examine the accuracy and interrater reliability of a visually assessed vs airflow-based measure of pharyngeal collapsibility obtained in patients with obstructive sleep apnea undergoing drug-induced sleep endoscopy (DISE).

Study Design.

Prospective observational study.

Setting.

Academic tertiary care practice.

Methods.

Patients underwent DISE with airflow monitoring and nasal positive airway pressure titration to determine visual and airflow-based levels of pharyngeal opening pressure (PhOP). Visual DISE-PhOP was assessed by 2 blinded independent raters and defined as the pressure at which visual confirmation of airway collapse, including snoring, was abolished. Airflow-based DISE-PhOP was defined as the minimally effective positive airway pressure that abolished inspiratory flow limitation. Equivalence testing between visual and airflow DISE-PhOP of each rater was performed with the two one sided T-test (TOST) with an a priori equivalence bound of ±1 cm H2O. Interrater reliability was evaluated with the intraclass correlation coefficient.

Results.

One hundred patients were enrolled in the study and 77 completed the full evaluation. The population was predominantly male (74%) with an average age of 54.8 years, body mass index of 30.1 kg/m2, and apnea-hypopnea index of 30.7 events/h. Equivalence testing showed that both raters were within ± 1 cm H2O of airflow-based DISE-PhOP (−0.43 to 0.09 cm H2O and −0.32 to 0.48 cm H2O). Interrater reliability of visual DISE-PhOP between the raters was also good to excellent with an intraclass correlation coefficient of 0.895 (95% CI, 0.84-0.932).

Conclusion.

DISE-PhOP, a measure of upper airway collapsibility, was equivalent between airflow-based and visual assessments with strong interrater reliability, supporting its adoption as a standardized objective parameter in clinical DISE.

Keywords: obstructive sleep apnea, sleep endoscopy, sleep surgery

Drug-induced sleep endoscopy (DISE) is a clinical procedure utilized by sleep surgeons to visually characterize upper airway collapse for surgical evaluation of patients with diagnosed obstructive sleep apnea (OSA).1 Collapse patterns during DISE are often described by the VOTE criteria, a standard descriptor of airway collapse patterns.2 Previous studies have determined that specific upper airway collapse patterns are associated with improved success rates in several OSA surgical procedures.3,4 However, the visual description of airway collapse can be subjective, with studies showing only moderate interrater reliability among evaluators.4,5 The addition of reliable quantitative measures of upper airway collapse during surgical evaluation may improve OSA surgical decision making.

Upper airway collapsibility can be quantified by 2 measures of airway pressure. First, the pressure at which the airway completely collapses and airflow ceases is defined as the pharyngeal critical closing pressure (Pcrit).6 Pcrit is positively associated with OSA severity, and reductions in Pcrit after uvulopalatopharyngoplasty have been associated with improved response rates.7,8 Second, the minimally effective pressure required to open the airway for nonflow-limited breathing is defined as the pharyngeal opening pressure (PhOP), which represents the passive mechanical load of airway structures contributing to pharyngeal collapse.9 In this way, continuous positive airway pressure (CPAP) machine settings have been associated with improved response rates to oral appliance therapy and hypoglossal nerve stimulation surgery.10,11 However, the availability and reliability of these data are limited due to patient noncompliance, variation in titration modality (eg, auto-PAP vs in laboratory), and differences in mask interfaces.12,13 Additionally, conventional CPAP titration does not allow for direct visualization of the airway, which can help clinicians explore anatomic causes for elevated pressures, such as persistent CPAP-associated epiglottic obstruction.14 Recently, PhOP visualized during CPAP titration in DISE was predictive of surgical response in hypoglossal nerve stimulation surgery.15 With the potential prognostic value of DISE collapsibility measures in surgical outcomes, there exists a growing need for rigorous study of such measurement techniques.

The gold standard for measuring pharyngeal collapsibility (Pcrit and PhOP) is based on changes in airflow in response to increasing pressure during CPAP titration. Measuring collapsibility by this method requires a calibrated pneumotachometer to quantify airflow, adding significant additional resources to the DISE procedure. Additionally, Pcrit values are often negative, requiring computational methods to extrapolate values. However, PhOP can be assessed visually—without additional instrumentation or calculations—as the pressure at which visual airway collapse is abolished as CPAP is increased progressively.9,15 A visually assessed PhOP during DISE (DISE-PhOP) thereby allows the sleep surgeon to measure upper airway collapsibility using only a CPAP machine, nasal mask, and tubing.

Previous studies have not compared visually assessed vs airflow-based DISE-PhOP. In addition, it remains unclear whether visual DISE-PhOP is reliable among evaluators. The purpose of this study was to compare visual and airflow-based DISE-PhOP and determine the interrater reliability of the visual method. We hypothesized that visual DISE-PhOP will be equivalent to measured DISE-PhOP and that there will be good-excellent interrater reliability. The results of the study validate the visual assessment of PhOP as an accurate and reliable metric for assessing airway collapsibility that can be used in surgical decision making during DISE.

Methods

This single-institution prospective observational study of subjects undergoing an expanded DISE protocol was approved by a University of Pennsylvania Institutional Review Board (IRB833511). Patients were consecutively enrolled from the Sleep Surgery Clinic in the Department of Otorhinolaryngology at the University of Pennsylvania. All subjects ≥18 years old with a diagnosis of OSA who consented to clinical DISE were included in the study. Written consent for the study was obtained from all participants.

Characterizing Airway Dynamics During DISE (DISE-CAD)

Equipment.

The procedure for characterizing airway dynamics during DISE (DISE-CAD) was performed as previously described.9 In short, patients presented to the operating room and underwent propofol sedation. The following instruments were used during this enhanced DISE protocol:

Electroencephalogram pads: placed on the patient’s forehead to monitor sedation via the BIS Brain Monitoring System for Critical Care (Medtronic).

Blood pressure cuff: to monitor blood pressure during the procedure.

Pulse oximeter: to monitor oxygenation and desaturation during the procedure.

Nasal mask and pneumotachometer: a nasal CPAP mask is connected to a CPAP machine as well as a pneumotachometer that translates changes in air pressure during respiration into airflow. The mask is modified with a hole at the elbow assembly to allow for a fiber optic camera (Figure 1). A piece of film dressing (Tegaderm; 3M) was placed over the hold to help prevent leakage around the inserted camera.

Pressure catheters: 2 Mikro-Cath pressure catheters (Millar Inc) are placed through the anesthetized nostril and positioned at different sites within the pharynx to measure pressure at different levels of the airway, reflecting respiratory effort and collapse sites.

Flexible nasopharyngoscope: a fiberoptic endoscope is placed through a port in the nasal mask into the nose to visualize the upper airway.

CPAP: CPAP titration was performed with a ResMed S9 VPAP titration device, and pressure settings were adjusted with EasyCare Tx software version 7.00 (ResMed, Inc). The ResMed S9 VPAP is a standard in-laboratory titration device used by sleep laboratories to obtain therapeutic CPAP in patients with OSA (Figure 1).

Figure 1.

Figure 1.

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(A) Example of customized nasal CPAP mask with port for endoscope placement. Flow sensors and CPAP were attached to the tubing connector. (B) The ResMed S9 VPAP was used as our CPAP titration device. CPAP, continuous positive airway pressure.

Airflow, pressure catheters, video endoscopy, and CPAP titration were integrated with the Nox C1 access point and recorded in Noxturnal Software System (version 1.0.0.27200; Nox Medical).

Procedure.

At the start of the procedure, the nasopharyngoscope was first passed through a single, topically medicated (4% lidocaine, 0.05% oxymetazoline) nostril to visualize the soft palate. Propofol was then administered with the rate calculated with a probability ramp infusion system.16 When the patient demonstrated obstructive hypopnea or apnea (based on nasal airflow and epiglottic catheter inputs), the clinical end point was reached, and the propofol infusion rate was maintained. At that time, a baseline examination was performed with the nasopharyngoscope, alternating between the nasopharynx and oropharynx to visualize all levels of the upper airway. The titration was performed starting at 4 cm H2O and raised by 1 to 2 cm H2O until the inspiratory flow limitation (IFL) on pneumotachometry was abolished. IFL is characterized by a plateau effect where respiratory airflow prematurely peaks and no longer increases despite further increases in inspiratory effort.17 Additional findings associated with IFL include high-frequency snoring and prolonged inspiratory duty cycle.17 Airflow during nonflow-limited respiration continues to increase until peak respiratory effort.

The pressure at which IFL transitions to nonflow-limited respiration and pressure equilibrates throughout the pharynx is defined as DISE-PhOP. Figure 2 depicts the transition from IFL to DISE-PhOP. Throughout the CPAP titration, the nasopharyngoscope is moved between the nasopharynx and oropharynx every 2 or 3 breaths to visualize collapse along the entire length of the pharynx at each pressure level.

Figure 2.

Figure 2.

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(A) Inspiratory flow limitation where airflow (Flow) peaks prematurely and does not increase despite continued increase in respiratory effort (Pds). (B) Transition to nonflow-limited breathing where airflow corresponds to respiratory effort.

Airflow-Based DISE-PhOP

Airflow-based DISE-PhOP was measured from airflow tracings recorded during the DISE evaluation. At baseline under sedation during DISE, subjects exhibited IFL. Measured DISE-PhOP was defined as the minimally effective CPAP level that resulted in nonflow-limited breathing for ≥50% of all breaths at each pressure level. Calculations were performed with LabChart 8 (version 8.12.16; ADInstruments).

Visual DISE-PhOP

To determine visual DISE-PhOP, 2 raters trained in evaluating DISE (J.L.Y., E.T.) reviewed the video portion of the DISE recording while blinded to other measures of airway dynamics, including airflow and pressure tracings. Raters were also blinded to each other’s assessments. The recordings were reviewed via the Noxturnal Software System, which was controlled by a third investigator (E.G.S.), who indicated when the CPAP trial started and ended on the video but provided no information from the physiologic signals that were recorded during the procedure. Visual DISE-PhOP was defined as the pressure at which airway collapse was abolished at all levels of the airway during inspiration visualized on nasopharyngoscopy, including snoring, for a minimum of 4 breaths. The raters were able to replay the recording as needed to precisely identify the breath at which collapse was abolished. The third investigator recorded the mask pressure (cm H2O) at the onset of inspiration for the identified breath as the visual DISE-PhOP measurement.

Statistical Analysis

Equivalence testing between the visual DISE-PhOP of each rater and the airflow-based DISE-PhOP was performed with the two one-sided t test (TOST), a validated statistical method to determine whether 2 methods are statistically equivalent.18,19 An a priori equivalence bound of ±1 cm H2O was used for this calculation, as we agreed that a 1–cm H2O difference was the minimal clinically significant difference between the DISE-PhOP values. Interrater reliability was evaluated with the intraclass correlation coefficient (ICC). Interpretation of ICC values was based on Koo and Li where an ICC <0.5 indicated poor reliability; 0.5 to 0.75, moderate reliability; 0.75 to 0.90, good reliability; and >0.90, excellent reliability.20 To assess bias and provide a measure of agreement between visual and airflow-based DISE-PhOP, a Bland-Altman analysis was used, which included calculations of the mean difference (95% CI) and limits of agreement (equal to ±2 SD of the mean difference).21

A sample size calculation was performed for equivalence testing between visually assessed DISE-PhOP and airflow-based DISE-PhOP. To determine equivalence via a TOST for equivalence with an alpha of 0.05, power (1 – beta) of 0.8, and an acceptable mean difference between groups of ±1 cm H2O, 50 subjects were required. Statistical analysis was performed in RStudio (version 1.3.1073; RStudio).

Results

Between June 3, 2020, and June 11, 2021, 100 subjects were enrolled in the study, of which 77 completed the full evaluation. Sixteen subjects were excluded for missing or poor-quality recording data; 4 did not complete the entire CPAP titration; 2 were known to a rater (compromised blinding); and 1 subject’s DISE was terminated early due to profound oxygen desaturations.

Table 1 shows the baseline demographics of the study population. The cohort was predominantly male (74%) with an average age of 54.8 years, body mass index of 30.7kg/m2, and apnea-hypopnea index of 30.7 events/h. Among the subjects, 42.8% had a history of airway surgery, the most common being tonsillectomy and septoplasty. Equivalence testing between visual and airflow-based DISE-PhOP showed that both raters were within ± 1 cm H2O of measured DISE-PhOP (−0.43 to 0.09 cm H2O and −0.32 to 0.48 cm H2O; Figure 3).

Table 1.

Baseline Demographic Data of Participants (N = 77).

Characteristic Mean ± SD or No. (%)
Age, y 54.8 ± 14.0
Sex
 Female 20 (26.0)
 Male 57 (74.0)
Body mass index, kg/m2 30.1 ± 4.73
Apnea-hypopnea index, events/h 30.7 ± 20.5
Prior airway surgerya 33 (42.8)
 Adenotonsillectomy/tonsillectomy 19
 Septoplasty/inferior turbinate reduction 13
 Functional endoscopic sinus surgery 5
 Uvulopalatopharyngoplasty 3
 Maxillomandibular advancement 3
 Other 7
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a

Many patients had >1 airway procedure, so the tally is >33.

Figure 3.

Figure 3.

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TOST equivalence testing of the mean difference between visual DISE-PhOP obtained by two raters and airflow based DISE-PhOP. The dashed and dotted line represent the ±1 cmH2O and ±0.5cmH2O equivalence bounds.

The ICC of visual DISE-PhOP between the raters was 0.895 (95% CI, 0.84-0.932; Figure 4). There was strong agreement between visual and airflow-based DISE-PhOP when raters 1 and 2 were compared individually on Bland-Altman analysis (Figure 5), with a mean difference of −0.17 cm H2O (95% CI,−0.48 to 0.14) and 0.08 cm H2O (95% CI, −0.39 to 0.55), respectively. Bland-Altman limits of agreement between visual and airflow-based DISE-PhOP were −2.86 to 2.52 cm H2O for rater 1 and −4.00 to 4.16 cm H2O for rater 2, suggesting that 95% of measurements fell within these limits.

Figure 4.

Figure 4.

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Scatterplot shows the correlation of visual DISE-PhOP (cm H2O) between rater 1 and rater 2. Interrater reliability with the intraclass correlation coefficient showed good to excellent reliability with a value of 0.895 (95% CI, 0.84-0.932). DISE, drug-induced sleep endoscopy; PhOP, pharyngeal opening pressure.

Figure 5.

Figure 5.

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Bland-Altman analysis of rater 1 and rater 2 to airflow-based DISE-PhOP shows minimal mean differences of −0.17 cm H2O (95% CI, −0.48 to 0.14) and 0.08 cm H2O (95% CI, −0.39 to 0.55), respectively. DISE, drug-induced sleep endoscopy; PhOP, pharyngeal opening pressure.

Discussion

This study is the first to compare visually assessed and airflow-based DISE-PhOP. The results demonstrate that the visually assessed DISE-PhOP was equivalent to the airflow-based DISE-PhOP, with TOST showing both raters well within the 11 cm H2O equivalence bounds. In fact, the 95% CI for both raters fell within ±0.5 cm H2O. Interrater reliability was good to excellent. There was minimal bias on Bland-Altman analysis between both raters and airflow-based DISE-PhOP, with a small mean difference between the methods. The findings suggest that visual assessment of pharyngeal collapse during CPAP titration under DISE can yield reliable measurements of PhOP while obviating the need for complex pressure-flow monitoring.

The VOTE criteria are currently the gold standard in describing upper airway collapse during DISE, yet their prognostic capacity remains mixed and limited.4,22,23 Specific VOTE findings are associated with surgical success, such as nonconcentric circumferential collapse in predicting hypoglossal nerve stimulation response.3 Meraj et al reported that VOTE scoring in transoral robotic surgery tongue base reduction was not predictive of success in surgery.24 Green et al studied DISE in pharyngeal surgery and reported that oropharyngeal lateral wall collapse on DISE was associated with a 50% reduction in odds of success in pharyngeal surgery; however, no other VOTE findings from that study were associated with surgical outcomes.4 Interrater agreement of VOTE in that study was only moderate, suggesting that the evaluation can be subjective. The subjective nature of this evaluation represents an ostensible limitation. Additionally, because collapse patterns are described at 4 specific subsites of the airway, there are multiple combinations of collapse patterns that are generically described as “multilevel” collapse. The subjective nature and complex variation in the findings make the VOTE classification ultimately difficult to interpret and study. The addition of DISE-PhOP represents a significant advancement in the diagnostic assessment for OSA surgery whereby a quantitative measure of airway collapsibility can be obtained under direct visualization of airway anatomy.

The visual DISE-PhOP represents an overall assessment of the collapsibility of the upper airway. The effectiveness of surgical therapies can be interpreted by their ability to “drop the PhOP” to near atmospheric pressure (0 cm H2O), which would allow patients to breathe without airflow limitation at room air and thus resolve OSA. The effectiveness of surgery will also depend on the initial severity of upper airway collapse. Lower DISE-PhOP values signify patients with lighter “anatomic loads” who stand to have a greater chance of surgical response as compared with those with higher DISE-PhOP values. Seay et al reported that responders to hypoglossal nerve stimulation had an average DISE-PhOP of 5.0 cm H2O vs 9.2 cm H2O in nonresponders. This is the only known study examining DISE-PhOP and OSA surgical outcomes.15 Future research studies are needed to examine DISE-PhOP for the array of sleep surgery procedures, including skeletal neurostimulation, soft tissue, and nasal procedures. Taken together, determining visual DISE-PhOP values will add a dimension of personalized care, facilitating procedure selection and anticipatory guidance during the post-DISE clinic visit.

Addition of a CPAP titration to the clinical DISE workflow is also practically achievable. Although we performed our titration with a CPAP device typically used for in-laboratory sleep study titration, any home CPAP device will work and can be inexpensively purchased or brought by the patient. Our nasal CPAP mask was customized for our research purposes, but there are commercially available nasal CPAP masks with bronchoscopy port adapters to allow for nasopharyngoscope insertion that are already being used by clinical sleep surgeons during DISE.25

The study had several strengths. First, a comprehensive physiology workstation was used to capture airflow during the DISE, including a pneumotachometer integrated into a polysomnography workstation. Second, 2 independent raters blinded to the results of the measured DISE-PhOP were used to demonstrate equivalence testing. Finally, we exceeded our sample size target, adequately powering the primary aim of our study.

There were also some important limitations of our study. Overall, visual and airflow-based DISE-PhOP was similar, but there were a few samples with large differences between the measures. This is reflected by the wide confidence interval with a few outliers seen on Bland-Altman analysis. Among others, 3 factors are likely to have contributed to these outliers. First, visually assessed opening of the airway can be missed if the camera was focused on a different part of the airway when collapse was abolished, thereby overestimating visual DISE-PhOP. Second, the CPAP device had a lower pressure limit of 4 cm H2O, thereby creating a “floor effect,” rendering our data set unable to generate PhOP values between 0 and 4 cm H2O. Two subjects were reported by both raters to have a visual DISE-PhOP of 0 cm H2O (open airway at baseline), but the airflow-based DISE-PhOP was recorded at 4 cm H2O, the lowest setting of the CPAP machine. Iatrogenic epiglottic collapse occurred in 4 patients whereby epiglottic collapse was not visualized at baseline but occurred during the CPAP titration. Visual DISE-PhOP can be difficult to determine in this sample as the epiglottis may appear persistently partially collapsed despite normalization of airflow. Surgeons should be aware of these limiting factors when trying to obtain a visual PhOP during DISE. Despite these limitations, the results of the study still showed that visual DISE-PhOP was equivalent to airflow-based DISE-PhOP on TOST equivalence testing and had a good to excellent ICC.

When CPAP is applied, there are inherent airflow leaks due to mask fit, patient movement, high pressure, or other factors. The addition of a camera port into the mask can add to the leak, and the pressure set by the CPAP machine is not the pressure felt by the patient, leading to an overestimation of the true pressure. As part of our study, we obtained a pressure sampled at the mask using a pressure sensor from the Nox C1 platform in 54 of the 77 included patients to evaluate the pressure being felt by the patient. The absolute mean difference among the pressures from the 54 samples by raters 1 and 2 (108 values total) was 0.65 cm H2O, which we determined not to be clinically significant.

Both raters (J.L.Y. and E.T.) participate in DISE physiology research and were experienced in observing upper airway collapsibility along with airflow and pressure measures. Therefore, it is possible that this prior knowledge had influenced their assessment of DISE-PhOP. There may be greater variability in DISE-PhOP among raters unfamiliar with evaluating pressure flow characteristics during DISE. However, additional training and experience as the procedure becomes more widely adopted are likely to address this concern. Finally, although recent findings provide evidence that PhOPs can predict surgical success, the relationship of DISE-PhOP to other OSA characteristics and its utility in determining surgical options has not yet been fully elucidated. Future research can compare the DISE-PhOP with other factors known to influence OSA to better understand contributory factors that lead to airway collapsibility.

Conclusion

DISE-PhOP is a significant advancement in the diagnostic assessment for OSA surgery whereby a quantitative measure of airway collapsibility can be obtained in a controlled setting with direct visualization of airway anatomy. Visual DISE-PhOP is equivalent to airflow-based DISE-PhOP and showed good to excellent interrater reliability. This evidence supports that DISE-PhOP can be visually assessed by surgeons with the simplified setup of a CPAP machine and tubing, facilitating the adoption of quantifiable physiology into routine DISE evaluations.

Acknowledgments

We thank Kendra Troske, BA, for assisting with data collection and analysis.

Funding source:

Jason L. Yu receives relevant support for this project from the National Institutes of Health (P01 HL094307). Alan R. Schwartz receives support relevant for this project from the National Institutes of Health (1R01HL144859-03). Raj C. Dedhia receives support relevant for this project from the National Institutes of Health (1R01HL144859-03)

Footnotes

This article is accepted for presentation at the 2022 AAO-HNSF Annual Meeting & OTO Experience; September 10-14, 2022; Philadelphia, Pennsylvania.

Competing interests: Everett G. Seay—consultant for Inspire Medical Systems. Eric Thuler—study investigator for Inspire Medical Systems. Alan R. Schwartz—study investigator for Apnimed, Pulmodyne, Philips Respironics, Respimetrix, and Signifier Medical; advisor to AE Mann Foundation, Deerfield Catalyst, Invicta Medical, LivaNova, Nyxoah, Respicardia, dayZz, and Itamar Medical. Raj C. Dedhia—study investigator for Inspire Medical Systems and Nyxoah Medical Systems

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