Flow-dependent positive airway pressure to maintain airway patency in sleep apnea-hypopnea syndrome

Mechanical mechanism to induce inspiratory flow limitation in obstructive sleep apnea patients revealed from in-vitro studies

Inspiratory flow limitation means that when the flowrate reaches a certain value, it no longer increases, or even decreases, which is called negative effort dependence flow limitation, even if the inspiration effort is increased. This occurs often in obstructive sleep apnea patients, but its mechanism remains unclear. To reveal the mechanism of inspiratory flow limitation, we constructed a unique partially collapsible in-vitro upper airway model of obstructive sleep apnea patients to observe the change of airway resistance with inspiratory driving pressure. The important findings demonstrate that with the increase of inspiratory effort, the driving pressure increases faster than the airway resistance in the early stages, and then the reverse occurs as the airway becomes narrower. The airway collapse caused by the transmural pressure can lead to a rapid increase in downstream resistance with the increase of inspiratory effort, which is the key reason causing the flow reduction and the formation of typical negative effort dependence flow limitation. The mechanical mechanism revealed in this study will lead to fully new insights into the study and treatment of obstructive sleep apnea.

COMPUTATIONAL FLUID DYNAMICS MODELING OF PHARYNGEAL AIRWAY RESISTANCE BASED ON CONE-BEAM COMPUTED TOMOGRAPHY

Background: Oral appliances for treating patients with sleep related disorders such as snoring and disturbed sleep could alter the morphology of the pharyngeal airway and thus its flow dynamics as well as airway patency. Splint therapy, i.e., positioning the mandibular condyles in centric relation (CR) with maxillary anterior guided orthotics (MAGO), with regard to pharyngeal airway patency, is causing increased attention. The goal of this work is to quantitatively examine the fluid dynamic changes within the pharyngeal airway when the mandible is positioned in CR and its association with airway patency. Materials and Methods: Patient-specific computational fluid dynamics models were reconstructed from pre- and post- treatment high-resolution cone-beam computed tomography images for two patients who had undergone MAGO therapy. The minimal cross-sectional area and total volume of the pharyngeal airway were measured in the pre- and post-treatment for a total of four models. These two subjects were chosen out of 18 patients based on the airway volume changes in pre- and post-treatment. Subject 1 had a dramatic increase in the airway volume, whereas subject 2 kept a similar airway volume following the therapy. Both subjects anecdotally reported improvement in snoring, breathing and overall sleep quality. Results: We examined three parameters, i.e., the classically defined resistance of airway in terms of pressure drop, the minimal cross-sectional area, and the pharyngeal airway volume. We also proposed a new fluid dynamic parameter: the percentage area of higher vorticity, to be correlated with the clinical efficacy of the appliance for airway patency. All four parameters could be used as index factors for subject 1 to explain the measurable clinical outcomes and the subjective report regarding quality of breathing, snoring cessation, and improvement in sleep of the patient. But only the minimal cross-sectional area and percentage area of higher vorticity could explain the clinical observations and subjective outcome in subject 2 who reported similar effects after the treatment. Conclusion: Splint therapy expanded the minimal cross-sectional area in both subjects, but did not necessarily influence the pharyngeal airway volume. The airway shape changed in terms of the percentage area of higher vorticity and the minimal cross-sectional area. Further studies are needed to assess the clinical efficacy of MAGO in reducing or eliminating the sleep-related breathing difficulties and snoring.

Mimicking a flow-limited human upper airway using a collapsible tube: relationships between flow patterns and pressures in a respiratory model

The upper airway (UA) in humans is commonly modeled as a Starling resistor. However, negative effort dependence (NED) observed in some patients with obstructive sleep apnea (OSA) contradicts predictions based on the Starling resistor model in which inspiratory flow is independent of inspiratory driving pressure when flow is limited. In a respiratory bench model consisting of a collapsible tube and an active lung model (ASL5000), inspiratory flow characteristics were investigated in relation to upstream, downstream, and extra-luminal pressures (denoted as P_us, P_ds, and P_out, respectively) by varying inspiratory effort (muscle pressure) from -1 to -20 cmH₂O in the active lung. P_us was provided by a constant airway pressure device and varied from 4 to 20 cmH₂O, and P_out was set at 10 and 15 cmH₂O. Upstream resistance at onset of flow limitation and critical transmural pressure (P_tm) corresponding to opening of the UA were found to be independent of P_us, P_ds, and P_out. With fixed P_tm, when P_ds fell below a specific value (P_ds'), inspiratory peak flow became constant and independent of P_ds. NED plateau flow patterns at mid-inspiration (V̇_n) were produced within the current bench setting when P_ds fell below P_ds'. V̇_n was proportional to P_ds, and the slope (ΔV̇_n/ΔP_ds) increased linearly with P_tm. P_tm and P_ds were the two final independent determinants of inspiratory flow. Our bench model closely mimics a flow-limited human UA, and the findings have implications for OSA treatment and research, especially for bench-testing auto-titrating devices in a more physiological way. NEW & NOTEWORTHY A respiratory model consisting of a collapsible tube was used to mimic a flow-limited human upper airway. Flow-limited breathing patterns including negative effort dependence were produced. Transmural and downstream pressures acting on the tube are the two independent determinants of the resulting inspiratory flow during flow limitation. The findings have implications for obstructive sleep apnea treatment and research, especially for bench-testing auto-titrating devices in a more physiological way.

The Efficacy of Low-Level Continuous Positive Airway Pressure for the Treatment of Snoring

STUDY OBJECTIVES

To assess effects of low-level continuous positive airway pressure (CPAP) on snoring in habitual snorers without obstructive sleep apnea (OSA).

METHODS

A multicenter prospective in-laboratory reversal crossover intervention trial was conducted between September 2013 and August 2014. Habitual snorers were included if they snored (inspiratory sound pressure level ≥ 40 dBA) for ≥ 30% all sleep breaths on a baseline sleep study (Night 1), and if significant OSA and daytime somnolence were absent. Included participants then underwent a CPAP titration study at 2, 4, or 6 cm H₂O (Night 2) to examine snoring responses to step-increases in nasal pressure, a treatment night at optimal pressure (Night 3), followed by baseline night (Night 4). At each pressure, snoring intensity was measured on each breath. Snoring frequency was quantified as a percentage of sleep breaths at thresholds of 40, 45, 50, and 55 dBA. Sleep architecture and OSA severity were characterized using standard measurements.

RESULTS

On baseline sleep studies, participants demonstrated snoring at ≥ 40 dBA on 53 ± 3% and ≥ 45 dBA on 35 ± 4% of breaths. Snoring frequency decreased progressively as nasal pressure increased from 0 to 4 cm H₂O at each threshold, and plateaued thereafter. CPAP decreased snoring frequency by 67% and 85% at 40 and 45 dBA, respectively. Intervention did not alter sleep architecture and sleep apnea decreased minimally.

CONCLUSIONS

Low-level CPAP below the range required to treat OSA diminished nocturnal snoring, and produced uniform reduction in nightly noise production below the World Health Organization's limit of 45 dBA.

CLINICAL TRIAL REGISTRATION

ClinicalTrials.gov, identifier: NCT01949584.

Sleep.7: positive airway pressure therapy for obstructive sleep apnoea/hypopnoea syndrome

The use of continuous positive airway pressure (CPAP) in treating symptoms associated with OSAHS is reviewed. Although it is an imperfect intervention, it continues to evolve and improve in such a way that patients who would not have been able to use this treatment even in the recent past can benefit from it today.

Static and dynamic upper airway obstruction in sleep apnea: role of the breathing gas properties

Increased upper airway collapsibility in the sleep apnea/hypopnea syndrome (SAHS) is usually interpreted by a collapsible resistor model characterized by a critical pressure (Pcrit) and an upstream resistance (Rup). To investigate the role played by the upstream segment of the upper airway, we tested the hypothesis that breathing different gases would modify Rup but not Pcrit. The study was performed on 10 patients with severe SAHS (apnea-hypopnea index: 59 +/- 14 events/hour) when breathing air and helium-oxygen (He-O2) during non-REM sleep. The continuous positive airway pressure that normalized flow (CPAPopt) was measured. Rup and Pcrit were determined from the linear relationship between maximal inspiratory flow VImax and nasal pressure (PN):VImax = (PN - Pcrit)/Rup. Changing the breathing gas selectively modified the severity of dynamic (CPAPopt, Rup) and static (Pcrit) obstructions. CPAPopt was significantly (p = 0.0013) lower when breathing He-O2 (8.44 +/- 1.66 cm H2O; mean +/- SD) than air (10.18 +/- 2.34 cm H2O). Rup was markedly lower (p = 0.0001) when breathing He-O2 (9.21 +/- 3.93 cm H2O x s/L) than air (15.92 +/- 6.27 cm H2O x s/L). Pcrit was similar (p = 0.039) when breathing He-O2 (4.89 +/- 2.37 cm H2O) and air (4.19 +/- 2.93 cm H2O). The data demonstrate the role played by the upstream segment of the upper airway and suggest that different mechanisms determine static (Pcrit) and dynamic (Rup) upper airway obstructions in SAHS.

Collapsible upper airway segment to study the obstructive sleep apnea/hypopnea syndrome in rats

Animal models have been used to study the pathophysiology of the obstructive sleep apnea/hypopnea syndrome (SAHS). Nevertheless, in none of the models described to date have the animals been subjected to the different patterns of upper airway obstructive events (apneas, hypopneas, and inspiratory flow limitation) characterizing SAHS. Our aim was to devise and test a computer-controlled collapsible upper airway segment applicable to rats and able to realistically mimic obstructive SAHS events. The collapsible segment (total volume <2 cm(3) and a dead space of approximately 0.25 cm(3)) consisted of a Starling resistor based on a latex membrane subjected to an external pressure applied by a computer-controlled pressure source. The collapsible segment was tested in eight anaesthetized and tracheostomized rats. The upper airway segment allowed us to induce obstructive apneas and hypopneas with flow and inspiratory effort waveforms similar to the ones observed in patients with SAHS. This collapsible upper airway segment may be a useful tool to implement a rat model of SAHS.

Continuous positive airway pressure: new generations

Automatic positive airway pressure devices are the most technologically advanced positive airway pressure devices available for use in OSA. Although heterogeneous, they have in common the ability to detect and respond to changes in upper airway resistance. Data cannot necessarily be extrapolated from one device to another, and the field is rapidly advancing. Most studies of APAP have been performed in a supervised setting, or patients have been carefully selected to have a high likelihood of OSA uncomplicated by disorders such as alveolar hypoventilation or central apnea or technical problems such as mask leaks. Studies of APAP for the diagnosis of OSA have shown that APAP can diagnose severe OSA effectively, but the diagnosis of mild-moderate OSA is less reliable. APAP devices also can be effective therapy for selected patients with OSA, with overall similar results to conventional fixed CPAP in terms of respiratory disturbances, sleep quality, nocturnal oxygenation, and daytime sleepiness and performance, with less known or other long-term outcomes. In most studies, mean treatment pressures are lower, without change in side effect profile. Compliance and preference with APAP are similar to or somewhat better than CPAP in most studies. APAP also can be used in an attended setting to titrate an effective pressure for use in long-term conventional CPAP therapy, also with similar results to CPAP in many patients. APAP devices are more expensive than CPAP devices, but the cost may be outweighed if a group of patients who can be diagnosed, treated, or titrated safely in the unattended setting can be identified. Although diagnostic and therapeutic algorithms for APAP have been proposed, the best candidates for this modality must be defined better.

Response of automatic continuous positive airway pressure devices to different sleep breathing patterns: a bench study

Evaluating the usefulness of automatic continuous positive airway pressure (CPAP) in treating the sleep apnea-hypopnea syndrome (SAHS) is not easy because the algorithms for automatic CPAP implemented in the devices available are not well known and are probably dependent on the device. In addition, at present it is not possible to test the behavior of automatic CPAP devices in response to well-defined breathing patterns. Our aim was to implement a bench test to characterize the responses of automatic CPAP devices by subjecting them to breathing patterns of patients with SAHS. To this end, a variety of typical breathing patterns (normal, apneas, hypopneas, flow limitation, snoring) previously recorded in patients with SAHS during sleep were reproduced by a breathing waveform generator. Five commercially available automatic CPAP devices were tested. The responses of the devices to apneas, hypopneas, flow limitation, and snoring were considerably different. In some devices, the response was modified by air leaks similar to the ones found in patients. Consequently, the effectiveness of automatic CPAP assessed in clinical tests performed by using particular devices has no general validity. Testing automatic CPAP devices in a bench study is a useful first step in evaluating the performance of this new type of device in adjusting nasal pressure for each patient.

Oscillatory resistance measured during noninvasive proportional assist ventilation

Setting proportional assist ventilation (PAV) requires the measurement of patient resistance and elastance. To avoid patient sedation/paralysis or the use of an esophageal balloon, noninvasive PAV is indirectly set by the "runaway" method or in accordance with patient comfort. The aim of this study was to ascertain whether the forced oscillation technique (FOT) applied by the ventilator during noninvasive PAV is useful in assessing patient respiratory resistance. Nasal PAV was applied to 14 patients with severe chronic obstructive pulmonary disease. During PAV a modified ventilator applied a 5-Hz pressure oscillation to noninvasively assess FOT resistance (Rrs). Lung resistance (RL) was measured in seven of the patients by using an esophageal balloon. Moreover, measurements were also performed in five of the patients when PAV was applied through the mouth. Rrs was close to RL both during nasal (Rrs = 8.9 +/- 3.1, RL = 9.0 +/- 2.6; cm H(2)O x s/L; n = 7, p > 0.05) and mouth (Rrs = 5.6 +/- 2.1, RL = 5.8 +/- 1.4; cm H(2)O x s/L; n = 5, p > 0.05) breathing. Rrs was slightly greater than the maximum value of flow assistance applied during the setting of PAV (FAmax): 11.1 +/- 5.4 and 9.5 +/- 2.9 cm H(2)O x s/L, respectively (n = 14, p > 0.05), both variables being significantly correlated (r = 0.72, p < 0.05). FOT applied by the PAV ventilator allowed the assessment of patient resistance. These results suggest that FOT could be useful in setting PAV flow assistance and in automatically and continuously updating this setting in accordance with patient resistance.

Proportional positive airway pressure: a new concept to treat obstructive sleep apnoea

Proportional positive airway pressure (PPAP) was designed to optimize airway pressure for the therapy of obstructive sleep apnoea (OSA). In a randomized crossover prospective study, the clinical feasibility of PPAP and its immediate effects on the breathing disorder and sleep in comparison with continuous positive airway pressure (CPAP) was evaluated. Twelve patients requiring CPAP therapy underwent CPAP and PPAP titration in a random order. Obstructive and mixed respiratory events could be completely abolished with both forms of treatment. This efficacy could be achieved at a significantly lower mean mask pressure during PPAP titration (8.45+/-2.42 cmH2O) compared to CPAP (9.96+/-2.7 cmH2O) (p=0.002). The mean minimal arterial oxygen saturation (Sa,O2) (82.8+/-6.5%) on the diagnostic night increased significantly (p<0.001) to an average Sa,O2 of 93.35+/-1.71% and 93.19+/-2.9% during CPAP and PPAP titration. Total sleep time, slow wave sleep and rapid eye movement (REM) sleep increased significantly by the same amount during both CPAP and PPAP titration (p<0.001), while sleep stage nonrapid eye movement (NREM) 1 and 2 decreased. Six patients preferred the PPAP titration night, four patients did not have a preference, and two patients preferred CPAP. The present data show that proportional positive airway pressure is as effective as continuous positive airway pressure in eliminating obstructive events and has the same immediate effect on sleep. The lower average mask pressure during proportional positive airway pressure implies potential advantages compared to continuous positive airway pressure. Proportional positive airway pressure presents a new effective therapeutic approach to obstructive sleep apnoea.