The role of high loop gain induced by intermittent hypoxia in the pathophysiology of obstructive sleep apnoea

Differences in Physiologic Endotypes Between Nonpositional and Positional OSA: Results From the Shanghai Sleep Health Study Cohort

BACKGROUND

Positional OSA (POSA) is a recognized subtype of OSA that exhibits distinct endotypic characteristics when compared with nonpositional OSA (NPOSA). The basis for the disparity in endotypes between these subtypes remains poorly understood.

RESEARCH QUESTION

(1) Do individuals with NPOSA and POSA have different underlying OSA endotypes? (2) Which endotypic characteristics are critical in determining NPOSA and POSA severity?

STUDY DESIGN AND METHODS

Within the Shanghai Sleep Health Study cohort, individuals with OSA were recruited and classified as having POSA or NPOSA. Endotypes were calculated using polysomnography.

RESULTS

Endotype analysis was conducted in 1,036 individuals with OSA. Compared with individuals with NPOSA, those with POSA had lower loop gain calculated during all sleep stages and all sleep positions (0.55; interquartile range [IQR], 0.46-0.66 vs 0.68, IQR, 0.52-0.90; P < .001), lower arousal threshold calculated during all sleep stages and all sleep positions (ArTHAll) (138.67; IQR, 118.94-180.87 percentage of the eupneic ventilation [%Veupnea] vs 189.00; IQR, 129.71-257.76 %Veupnea; P < .001), lower pharyngeal collapsibility calculated during all sleep stages and all sleep positions (VpassiveAll) (91.85; IQR, 83.13-95.15 %Veupnea vs 76.38; IQR, 23.77-92.08 %Veupnea; P < .001), and higher muscle compensation calculated during all sleep stages and all sleep positions (6.50; IQR, -6.77 to 16.39 %Veupnea vs 3.65; IQR, -10.47 to 12.14 %Veupnea; P = .003). Logistic regression analyses indicated that higher VpassiveAll was associated with increased odds of POSA vs NPOSA. In NPOSA, fully adjusted linear regression analyses indicated that VpassiveAll (β = -0.55; 95% CI, -0.68 to -0.42; P < .001) and lower loop gain calculated during all sleep stages and all sleep positions (β = 0.19; 95% CI, 0.08-0.30; P < .001) were significant independent predictors of the apnea hypopnea index, with VpassiveAll being the most critical factor. In contrast, in POSA, collapsibility appeared to be less influential (β = -0.09; 95% CI, -0.21 to 0.03; P = .138). Nonanatomic endotypic characteristics (LGAll: β = 0.29; 95% CI, 0.18-0.41; P < .001; arousal threshold in all sleep stages and all sleep positions: β = 0.15; 95% CI, 0.01-0.28; P = .031; muscle compensation in all sleep stages and all sleep positions: β = -0.21; 95% CI, -0.29 to -0.12; P < .001) were significant in determining the severity of POSA, with loop gain being the most crucial factor.

INTERPRETATION

This study highlights the differences in endotypes between NPOSA and POSA. In Chinese individuals, anatomic factors were more significant in determining the severity of NPOSA, whereas nonanatomic traits were more likely to determine the severity of POSA. Future research should focus on developing personalized management strategies for individuals with NPOSA and POSA based on their endotypes.

TRIAL REGISTRATION

Chinese Clinical Trial Registry; No.: ChiCTR1900025714; URL: https://www.chictr.org.cn/indexEN.html.

Pathophysiological mechanisms and therapeutic approaches in obstructive sleep apnea syndrome

Obstructive sleep apnea syndrome (OSAS) is a common breathing disorder in sleep in which the airways narrow or collapse during sleep, causing obstructive sleep apnea. The prevalence of OSAS continues to rise worldwide, particularly in middle-aged and elderly individuals. The mechanism of upper airway collapse is incompletely understood but is associated with several factors, including obesity, craniofacial changes, altered muscle function in the upper airway, pharyngeal neuropathy, and fluid shifts to the neck. The main characteristics of OSAS are recurrent pauses in respiration, which lead to intermittent hypoxia (IH) and hypercapnia, accompanied by blood oxygen desaturation and arousal during sleep, which sharply increases the risk of several diseases. This paper first briefly describes the epidemiology, incidence, and pathophysiological mechanisms of OSAS. Next, the alterations in relevant signaling pathways induced by IH are systematically reviewed and discussed. For example, IH can induce gut microbiota (GM) dysbiosis, impair the intestinal barrier, and alter intestinal metabolites. These mechanisms ultimately lead to secondary oxidative stress, systemic inflammation, and sympathetic activation. We then summarize the effects of IH on disease pathogenesis, including cardiocerebrovascular disorders, neurological disorders, metabolic diseases, cancer, reproductive disorders, and COVID-19. Finally, different therapeutic strategies for OSAS caused by different causes are proposed. Multidisciplinary approaches and shared decision-making are necessary for the successful treatment of OSAS in the future, but more randomized controlled trials are needed for further evaluation to define what treatments are best for specific OSAS patients.

Breathing rate variability in obstructive sleep apnea during wakefulness

STUDY OBJECTIVES

Obstructive sleep apnea (OSA) is defined by pauses in breathing during sleep, but daytime breathing dysregulation may also be present. Sleep may unmask breathing instability in OSA that is usually masked by behavioral influences during wakefulness. A breath-hold (BH) challenge has been used to demonstrate breathing instability. One measure of breathing stability is breathing rate variability (BRV). We aimed to assess BRV during rest and in response to BH in OSA.

METHODS

We studied 62 participants (31 with untreated OSA: respiratory event index [mean ± SD] 20 ± 15 events/h, 12 females, age 51 ± 14 years, body mass index [BMI] 32 ± 8 kg/m²; 31 controls: 17 females, age 47 ± 13 years; BMI 26 ± 4 kg/m²). Breathing movements were collected using a chest belt for 5 minutes of rest and during a BH protocol (60 seconds baseline, 30 seconds BH, 90 seconds recovery, 3 repeats). From the breathing movements, we calculated median breathing rate (BR) and interquartile BRV at rest. We calculated change in BRV during BH recovery from baseline. Group comparisons of OSA vs control were conducted using analysis of covariance with age, sex, and BMI as covariates.

RESULTS

We found 10% higher BRV in OSA vs controls (P < .05) during rest. In response to BH, BRV increased 7% in OSA vs 1% in controls (P < .001). Resting BR was not significantly different in OSA and controls, and sex and age did not have any significant interaction effects. BMI was associated with BR at rest (P < .05) and change in BRV with BH (P < .001), but no significant BMI-by-group interaction effect was observed.

CONCLUSIONS

The findings suggest breathing instability as reflected by BRV is high in OSA during wakefulness, both at rest and in response to a stimulus. Breathing instability together with high blood pressure variability in OSA may reflect a compromised cardiorespiratory consequence in OSA during wakefulness.

CITATION

Pal A, Martinez F, Akey MA, et al. Breathing rate variability in obstructive sleep apnea during wakefulness. J Clin Sleep Med. 2022;18(3):825-833.

Respiratory neuroplasticity: Mechanisms and translational implications of phrenic motor plasticity

Widespread appreciation that neuroplasticity is an essential feature of the neural system controlling breathing has emerged only in recent years. In this chapter, we focus on respiratory motor plasticity, with emphasis on the phrenic motor system. First, we define related but distinct concepts: neuromodulation and neuroplasticity. We then focus on mechanisms underlying two well-studied models of phrenic motor plasticity: (1) phrenic long-term facilitation following brief exposure to acute intermittent hypoxia; and (2) phrenic motor facilitation after prolonged or recurrent bouts of diminished respiratory neural activity. Advances in our understanding of these novel and important forms of plasticity have been rapid and have already inspired translation in multiple respects: (1) development of novel therapeutic strategies to preserve/restore breathing function in humans with severe neurological disorders, such as spinal cord injury and amyotrophic lateral sclerosis; and (2) the discovery that similar plasticity also occurs in nonrespiratory motor systems. Indeed, the realization that similar plasticity occurs in respiratory and nonrespiratory motor neurons inspired clinical trials to restore leg/walking and hand/arm function in people living with chronic, incomplete spinal cord injury. Similar application may be possible to other clinical disorders that compromise respiratory and non-respiratory movements.

A longitudinal study of the accuracy of positive pressure therapy machine-detected apnea-hypopnea events

STUDY OBJECTIVES

During positive airway pressure (PAP) therapy for sleep apnea syndromes, the machine detected respiratory event index (REI_FLOW) is an important method for clinicians to evaluate the beneficial effects of PAP. There are concerns about the accuracy of this detection, which also confounds a related question-how common and severe are residual events on PAP.

METHODS

Subjects with OSA who underwent a split night polysomnography were recruited prospectively. Those treated with PAP and tracked by the EncoreAnywhere system were analyzed. The ones who stopped PAP within one month were excluded for this analysis. Compliance, therapy data and waveform data were analyzed. Machine detected versus manually scored events were compared at the 1st, 3rd, 6th and 12th month from PAP initiation. Logistic regression was used to determine factors associated with a high REI_FLOW difference.

RESULTS

One hundred and seventy-nine patients with a mean age 59.06 ± 13.97 years old, median body mass index 33.60 (29.75-38.75) kg/m², and median baseline AHI 46.30 (31.50-65.90) times/hour were included. The difference between the machine detected REI_FLOW and manually scored REI_FLOW was 10.72 ±8.43 in the first month and remained stable for up to 12 months. Male sex and large leak ≥ 1.5% were more frequent in patients who had an REI_FLOW difference of ≥ 5 / hour of use. A titration arousal index ≥ 15/ hour of sleep, and higher ratio of unstable to stable breathing were also associated with an REI_FLOW difference ≥ 5 times/hour of use.

CONCLUSIONS

There is a substantial and sustained difference between manual and automated event estimates during PAP therapy, and some associated factors were identified.

The relationship between three-dimensional craniofacial and upper airway anatomical variables and severity of obstructive sleep apnoea in adults

OBJECTIVE

The aim of this study was to evaluate the relationship between three-dimensional craniofacial and upper airway anatomical variables and severity of obstructive sleep apnoea (OSA) using cone beam computed tomography (CBCT).

METHODS

This was a prospective observational study, consisted of 95 adult OSA whose polysomnography and CBCT were available. Three-dimensional craniofacial and upper airway anatomical analysis were performed using 12 CBCT variables, including sagittal and vertical jaw relationships, maxillary width, the volume, length and minimum axial area of upper airway. The severity of OSA was evaluated through the apnea-hypopnea index (AHI). A hierarchical regression was performed to analyze the relationship between OSA severity and craniofacial and upper airway anatomical variables after controlling patients' demographic characteristics (gender, age, and BMI).

RESULTS

After controlling patients' gender, age, and BMI, individual CBCT variables including MAA, V-RPA, L-RPA and Go-Me were related to AHI. The final hierarchical regression model with demographic variables in Step 1 and CBCT variables (MAA, L-RPA and Go-Me) in Step 2 indicated that CBCT variables added additional explanatory power for AHI (ΔF(3,88) = 5.176, P = 0.002). Among these variables, L-RPA and Go-Me were statistically significant (P < 0.05).

LIMITATION

The OSA severity was expressed by AHI alone.

CONCLUSIONS

Three-dimensional craniofacial and upper airway morphology played an essential role in OSA severity. The most relevant anatomical characteristic with OSA severity were the length of retropalatal airway and mandibular body, which could be used to recognize severe OSA patients and as estimators for selecting the most appropriate treatment modality for OSA patients.

Pharmacologically Induced Ventilatory Depression in the Postoperative Patient: A Sleep-Wake State-Dependent Perspective

Pharmacologically induced ventilatory depression (PIVD) is a common postoperative complication with a spectrum of severity ranging from mild hypoventilation to severe ventilatory depression, potentially leading to anoxic brain injury and death. Recent studies, using continuous monitoring technologies, have revealed alarming rates of previously undetected severe episodes of postoperative ventilatory depression, rendering the recognition of such episodes by the standard intermittent assessment practice, quite problematic. This imprecise description of the epidemiologic landscape of PIVD has thus stymied efforts to understand better its pathophysiology and quantify relevant risk factors for this postoperative complication. The residual effects of various perianesthetic agents on ventilatory control, as well as the multiple interactions of these drugs with patient-related factors and phenotypes, make postoperative recovery of ventilation after surgery and anesthesia a highly complex physiological event. The sleep-wake, state-dependent variation in the control of ventilation seems to play a central role in the mechanisms potentially enhancing the risk for PIVD. Herein, we discuss emerging evidence regarding the epidemiology, risk factors, and potential mechanisms of PIVD.

Algorithm for automatic detection of self-similarity and prediction of residual central respiratory events during continuous positive airway pressure

Study Objectives

Sleep-disordered breathing is a significant risk factor for cardiometabolic and neurodegenerative diseases. High loop gain (HLG) is a driving mechanism of central sleep apnea or periodic breathing. This study presents a computational approach that identifies “expressed/manifest” HLG via a cyclical self-similarity feature in effort-based respiration signals.

Methods

Working under the assumption that HLG increases the risk of residual central respiratory events during continuous positive airway pressure (CPAP), the full night similarity, computed during diagnostic non-CPAP polysomnography (PSG), was used to predict residual central events during CPAP (REC), which we defined as central apnea index (CAI) higher than 10. Central apnea labels are obtained both from manual scoring by sleep technologists and from an automated algorithm developed for this study. The Massachusetts General Hospital sleep database was used, including 2466 PSG pairs of diagnostic and CPAP titration PSG recordings.

Results

Diagnostic CAI based on technologist labels predicted REC with an area under the curve (AUC) of 0.82 ± 0.03. Based on automatically generated labels, the combination of full night similarity and automatically generated CAI resulted in an AUC of 0.85 ± 0.02. A subanalysis was performed on a population with technologist-labeled diagnostic CAI higher than 5. Full night similarity predicted REC with an AUC of 0.57 ± 0.07 for manual and 0.65 ± 0.06 for automated labels.

Conclusions

The proposed self-similarity feature, as a surrogate estimate of expressed respiratory HLG and computed from easily accessible effort signals, can detect periodic breathing regardless of admixed obstructive features such as flow limitation and can aid the prediction of REC.

Orofacial Myofunctional Therapy in Obstructive Sleep Apnea Syndrome: A Pathophysiological Perspective

Obstructive sleep apnea (OSA) syndrome is a multi-factorial disorder. Recently identified pathophysiological contributing factors include airway collapsibility, poor pharyngeal muscle responsiveness, a low arousal threshold, and a high loop gain. Understanding the pathophysiology is of pivotal importance to select the most effective treatment option. It is well documented that conventional treatments (continuous positive airway pressure (CPAP), upper airway surgery, and dental appliance) may not always be successful in the presence of non-anatomical traits, especially in mild to moderate OSA. Orofacial myofunctional therapy (OMT) consists of isotonic and isometric exercises targeted to oral and oropharyngeal structures, with the aim of increasing muscle tone, endurance, and coordinated movements of pharyngeal and peripharyngeal muscles. Recent studies have demonstrated the efficacy of OMT in reducing snoring, apnea-hypopnea index, and daytime sleepiness, and improving oxygen saturations and sleep quality. Myofunctional therapy helps to reposition the tongue, improve nasal breathing, and increase muscle tone in pediatric and adult OSA patients. Studies have shown that OMT prevents residual OSA in children after adenotonsillectomy and helps adherence in CPAP-treated OSA patients. Randomized multi-institutional studies will be necessary in the future to determine the effectiveness of OMT in a single or combined modality targeted approach in the treatment of OSA. In this narrative review, we present up-to-date literature data, focusing on the role of OSA pathophysiology concepts concerning pharyngeal anatomical collapsibility and muscle responsiveness, underlying the response to OMT in OSA patients.

Children with down syndrome and sleep disordered breathing display impairments in ventilatory control

OBJECTIVES

To investigate the role of ventilatory control instability (i.e. loop gain) in children with Down syndrome and sleep disordered breathing.

METHODS

Children (3-19 years) with Down syndrome and sleep disordered breathing (n = 14) were compared with typically developing children (n = 14) matched for age, sex and sleep disordered breathing severity. All children underwent overnight polysomnography. Spontaneous sighs were identified and a 180s analysis window (60s pre-sigh to 120s post-sigh) containing flow measurements and oxygen saturation were created. Loop gain, a measure of the sensitivity of the negative feedback loop that controls ventilation, was estimated by fitting a mathematical model of ventilatory control to the post-sigh ventilatory pattern. Results; Loop gain was significantly higher in children with Down syndrome compared to matched typically developing children (median loop gain [interquartile range]: 0.36 [0.33, 0.55] vs 0.32 [0.24, 0.38]; P = 0.0395). While children with Down syndrome also had significantly lower average oxygen saturation associated within each analysis window compared to typically developing children (mean ± standard deviation: 96.9 ± 1.3% vs 98.0 ± 1.0%; P = 0.0155), loop gain was not related to polysomnographic measures of hypoxia.

CONCLUSIONS

Higher loop gain in children with Down syndrome and sleep disordered breathing indicates that these children have more unstable ventilatory control, compared to age, sex and sleep disordered breathing severity matched typically developing children. This may be due to an inherent impairment in ventilatory control in children with Down syndrome contributing to their increased risk of sleep disordered breathing which may inform alternative treatment options for this population.

Models of anatomically based oropharyngeal rehabilitation with a multilevel approach for patients with obstructive sleep apnea: a meta-synthesis and meta-analysis

OBJECTIVES

Obstructive sleep apnea (OSA) is a sleep-related breathing disorder associated with dysfunction of oropharyngeal muscles to maintain upper airway patency during sleep. Oropharyngeal rehabilitation (OPR) was developed to restore, reconstruct, and reeducate oropharyngeal muscle function, but current protocols and effectiveness of OPR have been inconsistent. The purpose of this study was to review (1) indications of OPR, (2) protocols of OPR, and (3) effectiveness of OPR.

METHODS

We searched MEDLINE, EMBASE, and the Cochrane Library and then conducted both meta-synthesis and meta-analysis according to the statement of Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA).

RESULTS

A total of eight studies with 203 patients were included. By means of meta-synthesis, the patients with middle age, BMI < 40 kg/m², mild-to-moderate OSA, and non-severe upper airway anatomical abnormality were found to benefit from OPR. The protocol of OPR was summarized to be an anatomically based, multilevel approach, including the retropalatal, retroglossal, hypopharyngeal, TMJ, and facial levels. By using meta-analysis, overall outcomes were presented as apnea hypopnea index (AHI) with significant improvement from 25.2 ± 7.8/h to 16.1 ± 6.6/h (mean difference [MD] - 9.8 [95% CI - 11.0 to - 8.6], p < 0.0001); the lowest oxygen saturation (LSAT) improved from 80.2 ± 4.7 to 83.8 ± 2.9% (MD 3.0% [95% CI 2.0 to 4.0], p < 0.0001); Epworth sleepiness scale (ESS) improved from 11.8 ± 1.9 to 6.3 ± 1.6 (MD - 5.9 [95% CI - 7.5 to - 4.2], p < 0.001), neck circumference (NC) from 35.2 ± 1.1 to 34.7 ± 0.9 cm (MD - 0.6 [95% CI - 0.9 to - 0.2], p = 0.002), BMI from 24.8 ± 3.7 to 24.8 ± 4.1 kg/m² (MD - 0.0; 95% CI - 0.5 to 0.5, p = 0.95). All outcomes except BMI demonstrated significant improvement from OPR.

CONCLUSIONS

Meta-analysis of previous OPR reports shows an improvement in AHI of 39%, compared with the usual surgical definition of success at 50%. Only mild and moderate cases of OSA were referred for OPR in the prior studies. In order to improve outcomes with OPR, a comprehensive approach to rehabilitation should be emphasized.

Parapharyngeal fat pad area at the subglosso-supraglottic level is associated with corresponding lateral wall collapse and apnea-hypopnea index in patients with obstructive sleep apnea: a pilot study

The aim of this study was to assess associations between fat pad areas at various anatomic levels and the sites of lateral wall collapse and disease severity in adult patients with obstructive sleep apnea (OSA). Forty-one patients with OSA who prospectively underwent drug-induced sleep computed tomography were included. Areas of parapharyngeal fat pads and degrees of lateral wall collapse at three representative anatomic levels (nasopharynx, oropharynx, and subglosso-supraglottis), and apnea-hypopnea index (AHI) were measured. In the subglosso-supraglottic region, the parapharyngeal fat pad area in 17 (41%) patients with complete lateral wall collapse was significantly larger than that in 24 (59%) patients without complete collapse (median, 236.0 mm² vs 153.0 mm²; P = 0.02). In multivariate regression analysis, the parapharyngeal fat pad area at the subglosso-supraglottic level (β = 0.02; P = 0.01) and body mass index (β = 3.24; P = 0.01) were independently associated with AHI. Our preliminary results supported that parapharyngeal fat pads at the subglosso-supraglottic level may be involved in the development of lateral wall collapse and then determine the severity of OSA. Further studies are warranted to investigate the effect of reducing parapharyngeal fat pads in the treatment of OSA.

The Role of Animal Models in Developing Pharmacotherapy for Obstructive Sleep Apnea

Obstructive sleep apnea (OSA) is a highly prevalent disease characterized by recurrent closure of the upper airway during sleep. It has a complex pathophysiology involving four main phenotypes. An abnormal upper airway anatomy is the key factor that predisposes to sleep-related collapse of the pharynx, but it may not be sufficient for OSA development. Non-anatomical traits, including (1) a compromised neuromuscular response of the upper airway to obstruction, (2) an unstable respiratory control (high loop gain), and (3) a low arousal threshold, predict the development of OSA in association with anatomical abnormalities. Current therapies for OSA, such as continuous positive airway pressure (CPAP) and oral appliances, have poor adherence or variable efficacy among patients. The search for novel therapeutic approaches for OSA, including pharmacological agents, has been pursued over the past years. New insights into OSA pharmacotherapy have been provided by preclinical studies, which highlight the importance of appropriate use of animal models of OSA, their applicability, and limitations. In the present review, we discuss potential pharmacological targets for OSA discovered using animal models.

Competing mechanisms of plasticity impair compensatory responses to repetitive apnoea

KEY POINTS

Intermittent reductions in respiratory neural activity, a characteristic of many ventilatory disorders, leads to inadequate ventilation and arterial hypoxia. Both intermittent reductions in respiratory neural activity and intermittent hypoxia trigger compensatory enhancements in inspiratory output when experienced separately, forms of plasticity called inactivity-induced inspiratory motor facilitation (iMF) and long-term facilitation (LTF), respectively. Reductions in respiratory neural activity that lead to moderate, but not mild, arterial hypoxia occludes plasticity expression, indicating that concurrent induction of iMF and LTF impairs plasticity through cross-talk inhibition of their respective signalling pathways. Moderate hypoxia undermines iMF by enhancing NR2B-containing NMDA receptor signalling, which can be rescued by exogenous retinoic acid, a molecule necessary for iMF. These data suggest that in ventilatory disorders characterized by reduced inspiratory motor output, such as sleep apnoea, endogenous mechanisms of compensatory plasticity may be impaired, and that exogenously activating respiratory plasticity may be a novel strategy to improve breathing.

ABSTRACT

Many forms of sleep apnoea are characterized by recurrent reductions in respiratory neural activity, which leads to inadequate ventilation and arterial hypoxia. Both recurrent reductions in respiratory neural activity and hypoxia activate mechanisms of compensatory plasticity that augment inspiratory output and lower the threshold for apnoea, inactivity-induced inspiratory motor facilitation (iMF) and long-term facilitation (LTF), respectively. However, despite frequent concurrence of reduced respiratory neural activity and hypoxia, mechanisms that induce and regulate iMF and LTF have only been studied separately. Here, we demonstrate that recurrent reductions in respiratory neural activity ('neural apnoea') accompanied by cessations in ventilation that result in moderate (but not mild) hypoxaemia do not elicit increased inspiratory output, suggesting that concurrent induction of iMF and LTF occludes plasticity. A key role for NMDA receptor activation in impairing plasticity following concurrent neural apnoea and hypoxia is indicated since recurrent hypoxic neural apnoeas triggered increased phrenic inspiratory output in rats in which spinal NR2B-containing NMDA receptors were inhibited. Spinal application of retinoic acid, a key molecule necessary for iMF, bypasses NMDA receptor-mediated constraints, thereby rescuing plasticity following hypoxic neural apnoeas. These studies raise the intriguing possibility that endogenous mechanisms of compensatory plasticity may be impaired in some individuals with sleep apnoea, and that exogenously activating pathways giving rise to respiratory plasticity may be a novel pharmacological strategy to improve breathing.

The Effect of Upper Airway Surgery on Loop Gain in Obstructive Sleep Apnea

STUDY OBJECTIVES

Controversy exists as to whether elevated loop gain is a cause or consequence of obstructive sleep apnea (OSA). Upper airway surgery is commonly performed in Asian patients with OSA who have failed positive airway pressure therapy and who are thought to have anatomical predisposition to OSA. We hypothesized that high loop gain would decrease following surgical treatment of OSA due to reduced sleep apnea severity.

METHODS

Polysomnography was performed preoperatively and postoperatively to assess OSA severity in 30 Chinese participants who underwent upper airway surgery. Loop gain was calculated using a validated clinically-applicable method by fitting a feedback control model to airflow.

RESULTS

Patients were followed up for a median (interquartile range) of 130 (62, 224) days after surgery. Apnea-hypopnea index (AHI) changed from 60.8 (33.7, 71.7) to 18.4 (9.9, 42.5) events/h (P < .001). Preoperative and postoperative loop gain was 0.70 (0.58, 0.80) and 0.53 (0.46, 0.63) respectively (P < .001). There was a positive association between the decrease in loop gain and the improvement of AHI (P = .025).

CONCLUSIONS

High loop gain was reduced by surgical treatment of OSA in our cohort. These data suggest that elevated loop gain may be acquired in OSA and may provide mechanistic insight into improvement in OSA with upper airway surgery.

CLINICAL TRIAL REGISTRATION

Registry: ClinicalTrials.gov, Title: The Impact of Sleep Apnea Treatment on Physiology Traits in Chinese Patients With Obstructive Sleep Apnea, Identifier: NCT02696629, URL: https://clinicaltrials.gov/show/NCT02696629.

Ventilatory control sensitivity in patients with obstructive sleep apnea is sleep stage dependent

Study Objectives

The severity of obstructive sleep apnea (OSA) is known to vary according to sleep stage; however, the pathophysiology responsible for this robust observation is incompletely understood. The objective of the present work was to examine how ventilatory control system sensitivity (i.e. loop gain) varies during sleep in patients with OSA.

Methods

Loop gain was estimated using signals collected from standard diagnostic polysomnographic recordings performed in 44 patients with OSA. Loop gain measurements associated with nonrapid eye movement (NREM) stage 2 (N2), stage 3 (N3), and REM sleep were calculated and compared. The sleep period was also split into three equal duration tertiles to investigate how loop gain changes over the course of sleep.

Results

Loop gain was significantly lower (i.e. ventilatory control more stable) in REM (Mean ± SEM: 0.51 ± 0.04) compared with N2 sleep (0.63 ± 0.04; p = 0.001). Differences in loop gain between REM and N3 (p = 0.095), and N2 and N3 (p = 0.247) sleep were not significant. Furthermore, N2 loop gain was significantly lower in the first third (0.57 ± 0.03) of the sleep period compared with later second (0.64 ± 0.03, p = 0.012) and third (0.64 ± 0.03, p = 0.015) tertiles. REM loop gain also tended to increase across the night; however, this trend was not statistically significant [F(2, 12) = 3.49, p = 0.09].

Conclusions

These data suggest that loop gain varies between REM and NREM sleep and modestly increases over the course of sleep. Lower loop gain in REM is unlikely to contribute to the worsened OSA severity typically observed in REM sleep, but may explain the reduced propensity for central sleep apnea in this sleep stage.

Non–steady State Modeling of the Ventilatory Depressant Effect of Remifentanil in Awake Patients Experiencing Moderate-to-severe Obstructive Sleep Apnea

Editor’s Perspective

What We Already Know about This Topic

What This Article Tells Us That Is New

Background

Evidence suggests that obstructive sleep apnea promotes postoperative pulmonary complications by enhancing vulnerability to opioid-induced ventilatory depression. We hypothesized that patients with moderate-to-severe obstructive sleep apnea are more sensitive to remifentanil-induced ventilatory depression than controls.

Methods

After institutional approval and written informed consent, patients received a brief remifentanil infusion during continuous monitoring of ventilation. We compared minute ventilation in 30 patients with moderate-to-severe obstructive sleep apnea diagnosed by polysomnography and 20 controls with no to mild obstructive sleep apnea per polysomnography. Effect site concentrations were estimated by a published pharmacologic model. We modeled minute ventilation as a function of effect site concentration and the estimated carbon dioxide. Obstructive sleep apnea status, body mass index, sex, age, use of continuous positive airway pressure, apnea/hypopnea events per hour of sleep, and minimum nocturnal oxygen saturation measured by pulse oximetry in polysomnography were tested as covariates for remifentanil effect site concentration at half-maximal depression of minute ventilation (Ce50) and included in the model if a threshold of 6.63 (P &lt; 0.01) in the reduction of objective function was reached and improved model fit.

Results

Our model described the observed minute ventilation with reasonable accuracy (22% median absolute error). We estimated a remifentanil Ce50 of 2.20 ng · ml–1 (95% CI, 2.09 to 2.33). The estimated value for Ce50 was 2.1 ng · ml–1 (95% CI, 1.9 to 2.3) in patients without obstructive sleep apnea and 2.3 ng · ml–1 (95% CI, 2.2 to 2.5) in patients with obstructive sleep apnea, a statistically nonsignificant difference (P = 0.081). None of the tested covariates demonstrated a significant effect on Ce50. Likelihood profiling with the model including obstructive sleep apnea suggested that the effect of obstructive sleep apnea on remifentanil Ce50 was less than 5%.

Conclusions

Obstructive sleep apnea status, apnea/hypopnea events per hour of sleep, or minimum nocturnal oxygen saturation measured by pulse oximetry did not influence the sensitivity to remifentanil-induced ventilatory depression in awake patients receiving a remifentanil infusion of 0.2 μg · kg–1 of ideal body weight per minute.

Personalised medicine in sleep respiratory disorders: focus on obstructive sleep apnoea diagnosis and treatment

In all fields of medicine, major efforts are currently dedicated to improve the clinical, physiological and therapeutic understanding of disease, and obstructive sleep apnoea (OSA) is no exception. The personalised medicine approach is relevant for OSA, given its complex pathophysiology and variable clinical presentation, the interactions with comorbid conditions and its possible contribution to poor outcomes. Treatment with continuous positive airway pressure (CPAP) is effective, but CPAP is poorly tolerated or not accepted in a considerable proportion of OSA patients. This review summarises the available studies on the physiological phenotypes of upper airway response to obstruction during sleep, and the clinical presentations of OSA (phenotypes and clusters) with a special focus on our changing attitudes towards approaches to treatment. Such major efforts are likely to change and expand treatment options for OSA beyond the most common current choices (i.e CPAP, mandibular advancement devices, positional treatment, lifestyle changes or upper airway surgery). More importantly, treatment for OSA may become more effective, being tailored to each patient's need.

Chronic Intermittent Hypoxia Differentially Impacts Different States of Inspiratory Activity at the Level of the preBötzinger Complex

The preBötzinger complex (preBötC) is a medullary brainstem network crucially involved in the generation of different inspiratory rhythms. In the isolated brainstem slice, the preBötC reconfigures to produce different rhythms that we refer to as "fictive eupnea" under baseline conditions (i.e., carbogen), and "fictive gasping" in hypoxia. We recently demonstrated that fictive eupnea is irregular following exposure to chronic intermittent hypoxia (CIH). However, it is unknown how CIH impacts fictive gasping. To address this, brain slices containing the preBötC were prepared from control and CIH exposed mice. Electrophysiological recordings of rhythmogenesis were obtained during the perihypoxic interval. We examined how CIH affects various dynamic aspects of the rhythm characterized by: (1) the irregularity score (IrS), to assess burst-to-variability; (2) the fluctuation value (χ), to quantify the gain of oscillations throughout the time series; and (3) Sample Entropy (sENT), to characterize the pattern/structure of oscillations in the time series. In baseline conditions, CIH increased IrS of amplitude (0.21 ± 0.2) and χ of amplitude (0.34 ± 0.02) but did not affect sENT of amplitude. This indicated that CIH increased burst-to-burst irregularity and the gain of amplitude fluctuations but did not affect the overall pattern/structure of amplitude oscillations. During the transition to hypoxia, 33% of control rhythms whereas 64% of CIH-exposed rhythms showed no doubling of period, suggesting that the probability for stable rhythmogenesis during the transition to hypoxia was greater following CIH. While 29% of control rhythms maintained rhythmicity throughout hypoxia, all slices from CIH exposed mice exhibited rhythms throughout the hypoxic interval. During hypoxia, differences in χ for amplitude were no longer observed between groups. To test the contribution of the persistent sodium current, we examined how riluzole influenced rhythmogenesis following CIH. In networks exposed to CIH, riluzole reduced the IrS of amplitude (-24 ± 14%) yet increased IrS of period (+49 ± 17%). Our data indicate that CIH affects the preBötC, in a manner dependent on the state of the oxygenation. Along with known changes that CIH has on peripheral sensory organs, the effects of CIH on the preBötC may have important implications for sleep apnea, a condition characterized by rapid transitions between normoxia and hypoxia.

Analysis of Hypoxic and Hypercapnic Ventilatory Response in Healthy Volunteers

Introduction

A previous study has suggested that the Human Leukocyte Antigen (HLA) allele DQB1*06:02 affects hypoxic ventilatory response (HVR) but not hypercapnic ventilatory response (HCVR) in an Asian population. The current study evaluated the relationship in Caucasians and Asians. In addition we assessed whether gender or polymorphisms in genes participating in the control of breathing affect HVR and HCVR.

Methods

A re-breathing system was used to measure HVR and HCVR in 551 young adults (56.8% Caucasians, 30% Asians). HLA-DQB1*06:02 and tagged polymorphisms and coding variants in genes participating in breathing (PHOX2B, GPR4 and TASK2/KCNK5) were analyzed. The associations between HVR/HCVR and HLA-DQB1*06:02, genetic polymorphisms, and gender were evaluated using ANOVA or frequentist association testing with SNPTEST.

Results

HVR and gender are strongly correlated. HCVR and gender are not. Mean HVR in women was 0.276±0.168 (liter/minute/%SpO2) compared to 0.429±0.266 (liter/minute/%SpO2) in men, p<0.001 (55.4% higher HVR in men). Women had lower baseline minute ventilation (8.08±2.36 l/m vs. 10.00±3.43l/m, p<0.001), higher SpO2 (98.0±1.3% vs. 96.6±1.7%, p<0.001), and lower EtCO2 (4.65±0.68% vs. 4.82±1.02%, p = 0.025). One hundred and two (18.5%) of the participants had HLA-DQB1*06:02. No association was seen between HLA-DQB1*06:02 and HVR or HCVR. Genetic analysis revealed point wise, uncorrected significant associations between two TASK2/KCNK5 variants (rs2815118 and rs150380866) and HCVR.

Conclusions

This is the largest study to date reporting the relationship between gender and HVR/ HCVR and the first study assessing the association between genetic polymorphisms in humans and HVR/HCVR. The data suggest that gender has a large effect on hypoxic breathing response.

Annual review of selected scientific literature: Report of the committee on scientific investigation of the American Academy of Restorative Dentistry

STATEMENT OF PROBLEM

It is clear the contemporary dentist is confronted with a blizzard of information regarding materials and techniques from journal articles, advertisements, newsletters, the internet, and continuing education events. While some of that information is sound and helpful, much of it is misleading at best.

PURPOSE

This review identifies and discusses the most important scientific findings regarding outcomes of dental treatment to assist the practitioner in making evidence-based choices. This review was conducted to assist the busy dentist in keeping abreast of the latest scientific information regarding the clinical practice of dentistry.

MATERIAL AND METHODS

Each of the authors, who are considered experts in their disciplines, was asked to peruse the scientific literature published in 2015 in their discipline and review the articles for important information that may have an impact on treatment decisions. Comments on experimental methodology, statistical evaluation, and overall validity of the conclusions are included in many of the reviews.

RESULTS

The reviews are not meant to stand alone but are intended to inform the interested reader about what has been discovered in the past year. The readers are then invited to go to the source if they wish more detail.

CONCLUSIONS

Analysis of the scientific literature published in 2015 is divided into 7 sections, dental materials, periodontics, prosthodontics, occlusion and temporomandibular disorders, sleep-disordered breathing, cariology, and implant dentistry.

Treatment of Obstructive Sleep Apnea. Prospects for Personalized Combined Modality Therapy

Obstructive sleep apnea (OSA) is a common sleep disorder with serious associated morbidities. Although several treatment options are currently available, variable efficacy and adherence result in many patients either not being treated or receiving inadequate treatment long term. Personalized treatment based on relevant patient characteristics may improve adherence to treatment and long-term clinical outcomes. Four key traits of upper airway anatomy and neuromuscular control interact to varying degrees within individuals to cause OSA. These are: (1) the pharyngeal critical closing pressure, (2) the stability of ventilator chemoreflex feedback control (loop gain), (3) the negative intraesophageal pressure that triggers arousal (arousal threshold), and (4) the level of stimulus required to activated upper airway dilator muscles (upper airway recruitment threshold). Simplified diagnostic methods are being developed to assess these pathophysiological traits, potentially allowing prediction of which treatment would best suit each patient. In contrast to current practice of using various treatment modes alone, model predictions and pilot clinical trials show improved outcomes by combining several treatments targeted to each patient's pathophysiology profile. These developments could theoretically improve efficacy and adherence to treatment and in turn reduce the social and economic health burden of OSA and the associated life-threatening morbidities. This article reviews OSA pathophysiology and identifies currently available and investigational treatments that may be combined in the future to optimize therapy based on individual profiles of key patient pathophysiological traits.

Plasticity in respiratory motor neurons in response to reduced synaptic inputs: A form of homeostatic plasticity in respiratory control?

For most individuals, the respiratory control system produces a remarkably stable and coordinated motor output-recognizable as a breath-from birth until death. Very little is understood regarding the processes by which the respiratory control system maintains network stability in the presence of changing physiological demands and network properties that occur throughout life. An emerging principle of neuroscience is that neural activity is sensed and adjusted locally to assure that neurons continue to operate in an optimal range, yet to date, it is unknown whether such homeostatic plasticity is a feature of the neurons controlling breathing. Here, we review the evidence that local mechanisms sense and respond to perturbations in respiratory neural activity, with a focus on plasticity in respiratory motor neurons. We discuss whether these forms of plasticity represent homeostatic plasticity in respiratory control. We present new analyses demonstrating that reductions in synaptic inputs to phrenic motor neurons elicit a compensatory enhancement of phrenic inspiratory motor output, a form of plasticity termed inactivity-induced phrenic motor facilitation (iPMF), that is proportional to the magnitude of activity deprivation. Although the physiological role of iPMF is not understood, we hypothesize that it has an important role in protecting the drive to breathe during conditions of prolonged or intermittent reductions in respiratory neural activity, such as following spinal cord injury or during central sleep apnea.

Three-dimensional imaging of the upper airway anatomy in obstructive sleep apnea: a systematic review

INTRODUCTION

The pathogenesis of upper airway collapse in people with obstructive sleep apnea (OSA) is not fully understood. The aim of this study was to systematically review the literature in order to assess the most relevant anatomical characteristics of the upper airway related to the pathogenesis of OSA by analyzing the three-dimensional upper airway anatomy.

METHOD

A PICO (population/patient, intervention, comparison, outcome) search strategy, focusing on the upper airway anatomy of people with OSA, was conducted using the following databases: MEDLINE (PubMed), Excerpta Medica database (EMBASE), Web of Science, and Cochrane Library. The studies in which three-dimensional images were made from the participants who were awake and in the supine position during quiet breathing were selected in this systematic review.

RESULTS

Of the 758 unique retrieved studies, eight fulfilled the criteria for this systematic review. The minimum cross-sectional area of the upper airways of people with OSA, which is influenced by many factors such as hard and soft tissues surrounding the upper airway, was significantly smaller than that of those without OSA.

CONCLUSION

Within the limitation of the selected studies, this systematic review suggested that a small minimum cross-sectional area is the most relevant anatomical characteristic of the upper airway related to the pathogenesis of OSA.

Loop gain in severely obese women with obstructive sleep apnoea

Our objective was to assess whether obstructive sleep apnoea (OSA) patients were characterised by a reduced central CO2 controller gain (CG) and an enhanced plant gain (PG). We matched three groups of women (n=10 per group) enrolled in a previous study (Essalhi et al., J. Asthma. 50: 565-572, 2013): obese women with a respiratory disturbance index (RDI)≥15/h and with a RDI<15, and lean women without OSA (RDI<5). Tidal ventilation recordings during wakefulness with end-tidal PCO2 monitoring allowed the assessment of loop gain (LG) and its components (PG and CG). LG were similar for the three groups (p=0.844) while both PG and CG depicted significant differences (p=0.046 and p=0.011, respectively). Obese women with OSA were characterised by an increased PG and a reduced CG as compared to obese women without OSA. A negative relationship between CG and RDI (rho=-0.46, p=0.008) was evidenced. In conclusion, OSA in women is associated with a reduced central CO2 controller gain and an enhanced plant gain.

Time of day affects the frequency and duration of breathing events and the critical closing pressure during NREM sleep in participants with sleep apnea

We investigated if the number and duration of breathing events coupled to upper airway collapsibility were affected by the time of day. Male participants with obstructive sleep apnea completed a constant routine protocol that consisted of sleep sessions in the evening (10 PM to 1 AM), morning (6 AM to 9 AM), and afternoon (2 PM to 5 PM). On one occasion the number and duration of breathing events was ascertained for each sleep session. On a second occasion the critical closing pressure that demarcated upper airway collapsibility was determined. The duration of breathing events was consistently greater in the morning compared with the evening and afternoon during N1 and N2, while an increase in event frequency was evident during N1. The critical closing pressure was increased in the morning (2.68 ± 0.98 cmH2O) compared with the evening (1.29 ± 0.91 cmH2O; P ≤ 0.02) and afternoon (1.25 ± 0.79; P ≤ 0.01). The increase in the critical closing pressure was correlated to the decrease in the baseline partial pressure of carbon dioxide in the morning compared with the afternoon and evening (r = -0.73, P ≤ 0.005). Our findings indicate that time of day affects the duration and frequency of events, coupled with alterations in upper airway collapsibility. We propose that increases in airway collapsibility in the morning may be linked to an endogenous modulation of baseline carbon dioxide levels and chemoreflex sensitivity (12), which are independent of the consequences of sleep apnea.