Effect of episodic hypoxia on the susceptibility to hypocapnic central apnea during NREM sleep

Incidence of postoperative opioid-induced respiratory depression episodes in patients on room air or supplemental oxygen: a post-hoc analysis of the PRODIGY trial

BACKGROUND

Supplemental oxygen (SO) potentiates opioid-induced respiratory depression (OIRD) in experiments on healthy volunteers. Our objective was to examine the relationship between SO and OIRD in patients on surgical units.

METHODS

This post-hoc analysis utilized a portion of the observational PRediction of Opioid-induced respiratory Depression In patients monitored by capnoGraphY (PRODIGY) trial dataset (202 patients, two trial sites), which involved blinded continuous pulse oximetry and capnography monitoring of postsurgical patients on surgical units. OIRD incidence was determined for patients receiving room air (RA), intermittent SO, or continuous SO. Generalized estimating equation (GEE) models, with a Poisson distribution, a log-link function and time of exposure as offset, were used to compare the incidence of OIRD when patients were receiving SO vs RA.

RESULTS

Within the analysis cohort, 74 patients were always on RA, 88 on intermittent and 40 on continuous SO. Compared with when on RA, when receiving SO patients had a higher risk for all OIRD episodes (incidence rate ratio [IRR] 2.7, 95% confidence interval [CI] 1.4-5.1), apnea episodes (IRR 2.8, 95% CI 1.5-5.2), and bradypnea episodes (IRR 3.0, 95% CI 1.2-7.9). Patients with high or intermediate PRODIGY scores had higher IRRs of OIRD episodes when receiving SO, compared with RA (IRR 4.5, 95% CI 2.2-9.6 and IRR 2.3, 95% CI 1.1-4.9, for high and intermediate scores, respectively).

CONCLUSIONS

Despite oxygen desaturation events not differing between SO and RA, SO may clinically promote OIRD. Clinicians should be aware that postoperative patients receiving SO therapy remain at increased risk for apnea and bradypnea.

TRIAL REGISTRATION

Clinicaltrials.gov: NCT02811302, registered June 23, 2016.

The hypoxic ventilatory response and hypoxic burden are predictors of the magnitude of ventilatory long-term facilitation in humans

Mild intermittent hypoxia initiates progressive augmentation (PA) and ventilatory long-term facilitation (vLTF) in humans. The magnitude of these forms of plasticity might be influenced by anthropometric and physiological variables, as well as protocol elements. However, the impact of many of these variables on the magnitude of respiratory plasticity has not been established in humans. A meta-analysis was completed using anthropometric and physiological variables obtained from 124 participants that completed one of three intermittent hypoxia protocols. Simple correlations between the aggregate variables and the magnitude of PA and vLTF standardized to baseline was completed. Thereafter, the variables correlated to PA or vLTF were input into a multilinear regression equation. Baseline measures of the hypoxic ventilatory response was the sole predictor of PA (R = 0.370, P = 0.012). Similarly, this variable along with the hypoxic burden predicted the magnitude of vLTF (R = 0.546, P < 0.006 for both variables). In addition, the magnitude of PA was strongly correlated to vLTF (R = 0.617, P < 0.001). Anthropometric measures do not predict the magnitude of PA and vLTF in humans. Alternatively, the hypoxic ventilatory response was the sole predictor of PA, and in combination with the hypoxic burden, predicted the magnitude of vLTF. These influences should be considered in the design of mild intermittent hypoxia protocol studies in humans. Moreover, the strong correlation between PA and vLTF suggests that a common mechanistic pathway may have a role in the initiation of these forms of plasticity. KEY POINTS: Mild intermittent hypoxia initiates progressive augmentation (PA) and ventilatory long-term facilitation (vLTF) in humans. Many of the anthropometric and physiological variables that could impact the magnitude of these forms of plasticity are unknown. Anthropometric and physiological variables were measured from a total of 124 participants that completed one of three distinct intermittent hypoxia protocols. The variables correlated to PA or vLTF were input into a multilinear regression analysis. The hypoxic ventilatory response was the sole predictor of PA, while this variable in addition to the average hypoxic burden predicted the magnitude of vLTF. A strong correlation between PA and vLTF was also revealed. These influences should be considered in the design of mild intermittent hypoxia protocol studies in humans. Moreover, the strong correlation between PA and vLTF suggests that a common mechanistic pathway may have a role in the initiation of these forms of plasticity.

Volume of parapharyngeal fat pad in obstructive sleep apnea syndrome: prognostic role for multilevel sleep surgery

STUDY OBJECTIVES

To evaluate the prognostic role of volume of parapharyngeal fat pad (VPPFP) after multilevel sleep surgery in patients with obstructive sleep apnea syndrome.

METHODS

This retrospective cohort study was conducted in 50 patients with moderate to severe obstructive sleep apnea syndrome who underwent polysomnography (preoperative and postoperative 6 months) and preoperative facial computed tomography with multilevel sleep surgery between May 2010 and February 2019. All patients had failed or refused positive airway pressure treatment.

RESULTS

Of the 50 patients with moderate to severe obstructive sleep apnea syndrome who underwent multilevel sleep surgery, 46 were male (92.0%) with mean ± standard deviation age of 41.2 ± 12.5 years. On the preoperative polysomnography, mean ± standard deviation of apnea-hypopnea index and CT90 (cumulative percentage of time spent at oxygen saturation less than 90%) were 43.4 ± 19.3 events/h and 5.6 ± 9.6%, respectively. The average VPPFP measured by facial computed tomography scan was 4.9 ± 1.9 cm3. Multiple linear regression analysis showed that VPPFP was significantly correlated (R2 = 0.38) with age (β = 0.05; 95% confidence interval [CI], 0.01-0.09) and body mass index (β = 0.31; 95% CI, 0.16-0.45). Surgical success rate was 38%, and VPPFP higher than 5.1 cm3 was significantly associated with surgical failure after covariate adjustment (P = .01; odds ratio = 0.09; 95% CI, 0.02-0.48). Postoperative apnea-hypopnea index was positively correlated (R2 = 0.40) with CT90 (β = 1.33; 95% CI, 0.74-1.92) and VPPFP (β = 3.52; 95% CI, 0.30-6.74).

CONCLUSIONS

VPPFP correlated with age and body mass index, and high VPPFP and CT90 were associated with high postoperative apnea-hypopnea index. VPPFP larger than 5.1 cm3 was a possible risk factor for surgical failure, which may inform a decision on multilevel sleep surgery as salvage therapy for positive airway pressure treatment.

CITATION

Kim BK, Park SI, Hong SD, Jung YG, Kim HY. Volume of parapharyngeal fat pad in obstructive sleep apnea syndrome: prognostic role for multilevel sleep surgery. J Clin Sleep Med. 2022;18(12):2819-2828.

Practice recommendations for the role of physiotherapy in the management of sleep disorders: the 2022 Brazilian Sleep Association Guidelines

This clinical guideline supported by the Brazilian Sleep Association comprises a brief history of the development of Brazilian sleep physiotherapy, outlines the role of the physiotherapist as part of a sleep health team, and describes the clinical guidelines in respect of the management of some sleep disorders by the physiotherapist (including sleep breathing disorders, i.e., obstructive sleep apnea, central sleep apnea, upper airway resistance syndrome, hypoventilation syndromes and overlap syndrome, and pediatric sleep breathing disorders; sleep bruxism; circadian rhythms disturbances; insomnia; and Willis-Ekbom disease/periodic limb movement disorder. This clinical practice guideline reflects the state of the art at the time of publication and will be reviewed and updated as new information becomes available.

Tetraplegia is associated with increased hypoxic ventilatory response during nonrapid eye movement sleep

People with cervical spinal cord injury (SCI) are likely to experience chronic intermittent hypoxia while sleeping. The physiological effects of intermittent hypoxia on the respiratory system during spontaneous sleep in individuals with chronic cervical SCI are unknown. We hypothesized that individuals with cervical SCI would demonstrate higher short- and long-term ventilatory responses to acute intermittent hypoxia (AIH) exposure than individuals with thoracic SCI during sleep. Twenty participants (10 with cervical SCI [9 male] and 10 with thoracic SCI [6 male]) underwent an AIH and sham protocol during sleep. During the AIH protocol, each participant experienced 15 episodes of isocapnic hypoxia using mixed gases of 100% nitrogen (N2 ) and 40% carbon dioxide (CO2 ) to achieve an oxygen saturation of less than 90%. This was followed by two breaths of 100% oxygen (O2 ). Measurements were collected before, during, and 40 min after the AIH protocol to obtain ventilatory data. During the sham protocol, participants breathed room air for the same amount of time that elapsed during the AIH protocol and at approximately the same time of night. Hypoxic ventilatory response (HVR) during the AIH protocol was significantly higher in participants with cervical SCI than those with thoracic SCI. There was no significant difference in minute ventilation (V.E. ), tidal volume (V.T. ), or respiratory frequency (f) during the recovery period after AIH in cervical SCI compared to thoracic SCI groups. Individuals with cervical SCI demonstrated a significant short-term increase in HVR compared to thoracic SCI. However, there was no evidence of ventilatory long-term facilitation following AIH in either group.

Central sleep apnea

Central apnea syndrome is a disorder with protean manifestations and concomitant conditions. It can occur as a distinct clinical entity or as part of another clinical syndrome. The pathogenesis of central sleep apnea (CSA) varies depending on the clinical condition. Sleep-related withdrawal of the ventilatory drive to breathe is the common denominator among all cases of central apnea, whereas hypocapnia is the final common pathway leading to apnea in the majority of central apnea. Medical conditions most closely associated with CSA include heart failure, stroke, spinal cord injury, and opioid use, among others. Nocturnal polysomnography is the standard diagnostic method, including measurement of sleep and respiration. The latter includes detection of flow, measurement of oxyhemoglobin saturation and detection of respiratory effort. Management strategy incorporates clinical presentation, associated conditions, and the polysomnographic findings in an individualized manner. The pathophysiologic heterogeneity may explain the protean clinical manifestations and the lack of a single effective therapy for all patients. While research has enhanced our understanding of the pathogenesis of central apnea, treatment options are extrapolated from treatment of obstructive sleep apnea. Co-morbid conditions and concomitant obstructive sleep apnea influence therapeutic approach significantly. Therapeutic options include positive pressure therapy, pharmacologic therapy, and supplemental Oxygen. Continuous positive airway pressure (CPAP) is the initial standard of care, although the utility of other modes of positive pressure therapy, as well as pharmacotherapy and device-based therapies, are currently being investigated.

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.

Treatment-emergent central sleep apnea: a unique sleep-disordered breathing

Treatment-emergent central sleep apnea (TECSA) is a specific form of sleep-disordered breathing, characterized by the emergence or persistence of central apneas during treatment for obstructive sleep apnea. The purpose of this review was to summarize the definition, epidemiology, potential mechanisms, clinical characteristics, and treatment of TECSA. We searched for relevant articles up to January 31, 2020, in the PubMed database. The prevalence of TECSA varied widely in different studies. The potential mechanisms leading to TECSA included ventilatory control instability, low arousal threshold, activation of lung stretch receptors, and prolonged circulation time. TECSA may be a self-limited disorder in some patients and could be resolved spontaneously over time with ongoing treatment of continuous positive airway pressure (CPAP). However, central apneas persist even with the regular CPAP therapy in some patients, and new treatment approaches such as adaptive servo-ventilation may be necessary. We concluded that several questions regarding TECSA remain, despite the findings of many studies, and it is necessary to carry out large surveys with basic scientific design and clinical trials for TECSA to clarify these irregularities. Further, it will be vital to evaluate the baseline demographic and polysomnographic data of TECSA patients more carefully and comprehensively.

Research Priorities for Patients with Heart Failure and Central Sleep Apnea. An Official American Thoracic Society Research Statement

Background: Central sleep apnea (CSA) is common among patients with heart failure and has been strongly linked to adverse outcomes. However, progress toward improving outcomes for such patients has been limited. The purpose of this official statement from the American Thoracic Society is to identify key areas to prioritize for future research regarding CSA in heart failure.

Methods: An international multidisciplinary group with expertise in sleep medicine, pulmonary medicine, heart failure, clinical research, and health outcomes was convened. The group met at the American Thoracic Society 2019 International Conference to determine research priority areas. A statement summarizing the findings of the group was subsequently authored using input from all members.

Results: The workgroup identified 11 specific research priorities in several key areas: 1) control of breathing and pathophysiology leading to CSA, 2) variability across individuals and over time, 3) techniques to examine CSA pathogenesis and outcomes, 4) impact of device and pharmacological treatment, and 5) implementing CSA treatment for all individuals

Conclusions: Advancing care for patients with CSA in the context of heart failure will require progress in the arenas of translational (basic through clinical), epidemiological, and patient-centered outcome research. Given the increasing prevalence of heart failure and its associated substantial burden to individuals, society, and the healthcare system, targeted research to improve knowledge of CSA pathogenesis and treatment is a priority.

A comprehensive review of respiratory, autonomic and cardiovascular responses to intermittent hypoxia in humans

This review explores forms of respiratory and autonomic plasticity, and associated outcome measures, that are initiated by exposure to intermittent hypoxia. The review focuses primarily on studies that have been completed in humans and primarily explores the impact of mild intermittent hypoxia on outcome measures. Studies that have explored two forms of respiratory plasticity, progressive augmentation of the hypoxic ventilatory response and long-term facilitation of ventilation and upper airway muscle activity, are initially reviewed. The role these forms of plasticity might have in sleep disordered breathing are also explored. Thereafter, the role of intermittent hypoxia in the initiation of autonomic plasticity is reviewed and the role this form of plasticity has in cardiovascular and hemodynamic responses during and following intermittent hypoxia is addressed. The role of these responses in individuals with sleep disordered breathing and spinal cord injury are subsequently addressed. Ultimately an integrated picture of the respiratory, autonomic and cardiovascular responses to intermittent hypoxia is presented. The goal of the integrated picture is to address the types of responses that one might expect in humans exposed to one-time and repeated daily exposure to mild intermittent hypoxia. This form of intermittent hypoxia is highlighted because of its potential therapeutic impact in promoting functional improvement and recovery in several physiological systems.

Treatment-Emergent Central Apnea: Physiologic Mechanisms Informing Clinical Practice

The purpose of this review was to describe our management approach to patients with treatment-emergent central sleep apnea (TECSA). The emergence of central sleep apnea during positive airway pressure therapy occurs in approximately 8% of titration studies for OSA, and it has been associated with several demographic, clinical, and polysomnographic factors, as well as factors related to the titration study itself. TECSA shares similar pathophysiology with central sleep apnea. In fact, central and OSA pathophysiologic mechanisms are inextricably intertwined, with ventilatory instability and upper airway narrowing occurring in both entities. TECSA is a "dynamic" process, with spontaneous resolution with ongoing positive airway pressure therapy in most patients, persistence in some, or appearing de novo in a minority of patients. Management strategy for TECSA aims to eliminate abnormal respiratory events, stabilize sleep architecture, and improve the underlying contributing medical comorbidities. CPAP therapy remains a standard therapy for TECSA. Expectant management is appropriate given its transient nature in most cases, whereas select patients would benefit from an early switch to an alternative positive airway pressure modality. Other treatment options include supplemental oxygen and pharmacologic therapy.

Amelioration of sleep-disordered breathing with supplemental oxygen in older adults

Elderly adults demonstrate increased propensity for breathing instability during sleep compared with younger adults, and this may contribute to increased prevalence of sleep-disordered breathing (SDB) in this population. Hence, in older adults with SDB, we examined whether addition of supplemental oxygen (O₂) will stabilize breathing during sleep and alleviate SDB. We hypothesized that exposure to supplemental O₂ during non-rapid eye movement (NREM) sleep will stabilize breathing and will alleviate SDB by reducing ventilatory chemoresponsiveness and by widening the carbon dioxide (CO₂) reserve. We studied 10 older adults with mild-to-moderate SDB who were randomized to undergo noninvasive bilevel mechanical ventilation with exposure to room air or supplemental O₂ (Oxy) to determine the CO₂ reserve, apneic threshold (AT), and controller and plant gains. Supplemental O₂ was introduced during sleep to achieve a steady-state O₂ saturation ≥95% and fraction of inspired O₂ at 40%-50%. The CO₂ reserve increased significantly during Oxy versus room air (-4.2 ± 0.5 mmHg vs. -3.2 ± 0.5 mmHg, P = 0.03). Compared with room air, Oxy was associated with a significant decline in the controller gain (1.9 ± 0.4 L/min/mmHg vs. 2.5 ± 0.5 L/min/mmHg, P = 0.04), with reductions in the apnea-hypopnea index (11.8 ± 2.0/h vs. 24.4 ± 5.6/h, P = 0.006) and central apnea-hypopnea index (1.7 ± 0.6/h vs. 6.9 ± 3.9/h, P = 0.03). The AT and plant gain were unchanged. Thus, a reduced slope of CO₂ response resulted in an increased CO₂ reserve. In conclusion, supplemental O₂ reduced SDB in older adults during NREM sleep via reduction in chemoresponsiveness and central respiratory events.NEW & NOTEWORTHY This study demonstrates for the first time in elderly adults without heart disease that intervention with supplemental oxygen in the clinical range will ameliorate central apneas and hypopneas by decreasing the propensity to central apnea through decreased chemoreflex sensitivity, even in the absence of a reduction in the plant gain. Thus, the study provides physiological evidence for use of supplemental oxygen as therapy for mild-to-moderate SDB in this vulnerable population.

Variations in loop gain and arousal threshold during NREM sleep are affected by time of day over a 24-hour period in participants with obstructive sleep apnea

We investigated whether time of day affects loop gain (LG) and the arousal threshold (AT) during non-rapid eye movement (NREM) sleep. Eleven men with obstructive sleep apnea (apnea-hypopnea index > 5 events/h) completed a constant-routine protocol that comprised 3-h sleep sessions in the evening [10 PM (1) to 1 AM], morning (6 AM to 9 AM), afternoon (2 PM to 5 PM), and subsequent evening [10 PM (2) to 1 AM]. During each sleep session LG and the AT were measured during NREM sleep with a model-based approach. Our results showed the presence of a rhythmicity in both LG (P < 0.0001) and the AT (P < 0.001) over a 24-h period. In addition, LG and the AT were greater in the morning compared with both evening sessions [6 AM vs. 10 PM (1) vs. 10 PM (2): LG (1 cycle/min): 0.71 ± 0.23 vs. 0.60 ± 0.22 (P = 0.01) vs. 0.56 ± 0.10 (P < 0.001), AT (fraction of eupneic breathing): 1.45 ± 0.47 vs. 1.28 ± 0.36 (P = 0.02) vs. 1.20 ± 0.18 (P = 0.001)]. No difference in LG and the AT existed between the evening sessions (LG: P = 0.27; AT: P = 0.24). LG was correlated to measures of the hypocapnic ventilatory response (i.e., a measure of chemoreflex sensitivity) (r = 0.72 and P = 0.045) and the critical closing pressure (i.e., a measure of airway collapsibility) (r = 0.77 and P = 0.02) that we previously published. We conclude that time of day, independent of hallmarks of sleep apnea, affects LG and the AT during NREM sleep. These modifications may contribute to increases in breathing instability in the morning compared with other periods throughout the day/night cycle in individuals with obstructive sleep apnea. In addition, efficaciousness of treatments for obstructive sleep apnea that target LG and the AT may be modified by a rhythmicity in these variables.NEW & NOTEWORTHY Loop gain and the arousal threshold during non-rapid eye movement (NREM) sleep are greater in the morning compared with the afternoon and evening. Loop gain measures are correlated to chemoreflex sensitivity and the critical closing pressure measured during NREM sleep in the evening, morning, and afternoon. Breathing (in)stability and efficaciousness of treatments for obstructive sleep apnea may be modulated by a circadian rhythmicity in loop gain and the arousal threshold.

Efficacy of Velopharyngeal Surgery for Positional Obstructive Sleep Apnea Hypopnea Syndrome

BACKGROUND

Positional obstructive sleep apnea hypopnea syndrome (P-OSAHS) is a distinct OSAHS type. Whether velopharyngeal surgery is efficacious for patients with P-OSAHS remains unclear.

AIM/OBJECTIVE

To investigate the efficacy and factors influencing velopharyngeal surgery for treatment of patients with P-OSAHS, defined as the apnea hypopnea index (AHI) in different body postures (supine AHI ≥2*nonsupine AHI).

MATERIALS AND METHODS

A total of 44 patients with P-OSAHS who underwent velopharyngeal surgery were retrospectively studied. The clinical data of these patients, including polysomnography (PSG), physical examination, and surgical information, were collected for analysis. All patients underwent a PSG about 6 months after surgery to determine the treatment outcomes.

RESULTS

The overall AHI of the 44 patients decreased from 40.2 ± 18.7 events/h to 18.5 ± 17.5 events/h after surgery (P < .001). There were 29 responders (65.9%) according to the classical definition of surgical success. The percentage of sleep time with oxygen saturation below 90% (CT90) was the only predictive parameter for surgical success (P = .014, odds ratio value = 0.894). There was no significant difference between the change in supine AHI (-55.9 ± 35.2%) and the change in nonsupine AHI (-43.4 ± 74.1%; P = .167), and these 2 parameters were significantly correlated (r = 0.616, P < .001). Among the 38 patients with residual OSAHS (residual AHI ≥5), 28 had persistent P-OSAHS, and the percentage was as high as 82.4%.

CONCLUSIONS AND SIGNIFICANCE

Patients with P-OSAHS with a lower CT90 value are more likely to benefit from velopharyngeal surgery. Positional therapy could be indicated for most of the patients who are not cured by such surgery.

Short-term postoperative CPAP may improve the outcomes of velopharyngeal surgery for obstructive sleep apnea

PURPOSE

To evaluate the effects of short-term postoperative continuous positive airway pressure (CPAP) on the outcomes of velopharyngeal surgery for obstructive sleep apnea (OSA).

MATERIALS AND METHODS

This study included 119 OSA patients who underwent velopharyngeal surgery. Based on the results of postoperative pulse oximetry, the patients were divided into 3 groups: intervention, control, and observation. Patients with oxygen desaturation index (ODI) > 10 and lowest SpO₂ < 90% were randomly assigned to the CPAP intervention group and non-CPAP control. Patients with ODI ≤10 or lowest SpO₂ ≥ 90% were assigned to the non-CPAP observation group. Patients in the intervention group completed at least 3 months of CPAP treatment. Postoperative polysomnography data were compared to assess the difference of prognosis between the three groups.

RESULTS

Baseline data showed no significant differences between the three groups except the observational group showed a significantly larger tonsil size relative to the intervention and control groups. However, there was no significant difference in terms of tonsil size between the control and intervention groups. The surgical success rate of the intervention group was 80.65%, whereas it was 55.17% in the control group, with significant difference. The success rate of the observation group was 85.71% which was significantly different from that of the control group, but not the intervention group.

CONCLUSION

Short-term postoperative CPAP treatment may improve the outcomes of velopharyngeal surgery for OSA in patients who have respiratory events related hypoxia after surgery. Further studies are necessary for the underlying mechanisms.

In patients with heart failure the burden of central sleep apnea increases in the late sleep hours

Study Objectives

Periodic breathing with central sleep apnea (CSA) is common in patients with left ventricular systolic dysfunction. Based on the pathophysiological mechanisms underlying CSA, we hypothesized that the frequency of CSA episodes would increase in the late hours of non-rapid eye movement (NREM) of sleep.

Methods

Forty-one patients with left ventricular ejection fraction <40% underwent full-night-attended polysomnography scored by a central core lab. Because central apneas occur primarily in NREM sleep, total NREM sleep time for each patient was divided into 8 equal duration segments. Segment event counts were normalized to an events/hour index based on sleep segment duration.

Results

Central apnea index (CAI) varied among sleep segments (p = 0.001). As expected CAI was higher in segment 1 compared to segments 2 and 3, increasing during later segments. The minimum CAI occurred in segment 2 with mean ± SD of 21 ± 3 events/hour and maximum CAI was in segment 8 with 37 ± 4 events/hour. We also determined central apnea duration which varied among segments (p = 0.005), with longer durations later in the night (segment 1: 22 ± 1 seconds; segment 8: 26 ± 1 seconds, p < 0.001). Data were also analyzed including rapid eye movement (REM) sleep, with similar results. Further, comparison of CAI between the first and second half of the night showed a significant increase in the index. Circulation time did not change across the segments (p = 0.073).

Conclusions

In patients with left ventricular dysfunction and CSA, central apnea burden (number and duration) increases during later hours of sleep. These findings have pathophysiological and therapeutic implications.

Clinical Trial Registration

NCT01124370.

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.

Correlates and consequences of central sleep apnea in a national sample of US veterans

The prevalence and consequences of central sleep apnea (CSA) in adults are not well described. By utilizing the large Veterans Health Administration (VHA) national administrative databases, we sought to determine the incidence, clinical correlates, and impact of CSA on healthcare utilization in Veterans. Analysis of a retrospective cohort of patients with sleep disorders was performed from outpatient visits and inpatient admissions from fiscal years 2006 through 2012. The CSA group, defined by International Classification of Diseases-9, was compared with a comparison group. The number of newly diagnosed CSA cases increased fivefold during this timeframe; however, the prevalence was highly variable depending on the VHA site. The important predictors of CSA were male gender (odds ratio [OR] = 2.31, 95% confidence interval [CI]: 1.94-2.76, p < 0.0001), heart failure (HF) (OR = 1.78, 95% CI: 1.64-1.92, p < 0.0001), atrial fibrillation (OR = 1.83, 95% CI: 1.69-2.00, p < 0.0001), pulmonary hypertension (OR = 1.38, 95% CI:1.19-1.59, p < 0.0001), stroke (OR = 1.65, 95% CI: 1.50-1.82, p < 0.0001), and chronic prescription opioid use (OR = 1.99, 95% CI: 1.87-2.13, p < 0.0001). Veterans with CSA were at an increased risk for hospital admissions related to cardiovascular disorders compared with the comparison group (incidence rate ratio [IRR] = 1.50, 95% CI: 1.16-1.95, p = 0.002). Additionally, the effect of prior HF on future admissions was greater in the CSA group (IRR: 4.78, 95% CI: 3.87-5.91, p < 0.0001) compared with the comparison group (IRR = 3.32, 95% CI: 3.18-3.47, p < 0.0001). Thus, CSA in veterans is associated with cardiovascular disorders, chronic prescription opioid use, and increased admissions related to the comorbid cardiovascular disorders. Furthermore, there is a need for standardization of diagnostics methods across the VHA to accurately diagnose CSA in high-risk populations.

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.

Inherent vs. Induced Loop Gain Abnormalities in Obstructive Sleep Apnea

Unstable ventilatory chemoreflex control, quantified as loop gain, is recognized as one of four key pathophysiological traits that contribute to cause obstructive sleep apnea (OSA). Novel treatments aimed at reducing loop gain are being investigated, with the intention that future OSA treatment may be tailored to the individual's specific cause of apnea. However, few studies have evaluated loop gain in OSA and non-OSA controls and those that have provide little evidence to support an inherent abnormality in either overall chemical loop gain in OSA patients vs. non-OSA controls, or its components (controller and plant gain). However, intermittent hypoxia may induce high controller gain through neuroplastic changes to chemoreflex control, and may also decrease plant gain via oxidative stress induced inflammation and reduced lung function. The inherent difficulties and limitations with loop gain measurements are discussed and areas where further research are required are highlighted, as only by understanding the mechanisms underlying OSA are new therapeutic approaches likely to emerge in OSA.

Daytime loop gain is elevated in obstructive sleep apnea but not reduced by CPAP treatment

Reduced ventilatory control stability (elevated loop gain) is a key nonanatomical, pathological trait contributing to obstructive sleep apnea (OSA), yet the mechanisms responsible remain unclear. We sought to identify the key factors contributing to elevated loop gain in OSA (controller vs. plant contributions) and to examine whether abnormalities in these factors persist after OSA treatment. In 15 males (8 OSA, 7 height, weight- and age -matched controls), we measured loop gain, controller gain, and plant gain using a pseudorandom binary CO₂ stimulation method during wakefulness. Factors potentially influencing plant gain were also assessed (supine lung volume via helium dilution and spirometry). Measures were repeated 2 and 6 wk after initiating continuous positive airway pressure treatment. Loop gain (LG) was higher in OSA versus controls (LG at 1 cycle/min 0.28 ± 0.04 vs. 0.16 ± 0.04, P = 0.046, respectively), and the controller exhibited a greater peak response to CO₂ and faster roll-off in OSA. OSA patients also exhibited reduced forced expiratory volume in the first second and forced vital capacity compared with controls (92.2 ± 1.7 vs. 102.9 ± 3.5% predicted, P = 0.021; 93.4 ± 3.1 vs. 106.6 ± 3.6% predicted, P = 0.015, respectively). There was no effect of treatment on any variable. These findings confirm loop gain is higher in untreated OSA patients than in matched controls; however, this was not affected by treatment. NEW & NOTEWORTHY Elevated loop gain contributes to obstructive sleep apnea (OSA) pathophysiology. However, whether loop gain is inherently elevated in OSA or induced by OSA itself, whether it is elevated due to increased chemoreflex sensitivity or obesity-dependent reduced lung volume, and whether it is treatment reversible, are all currently uncertain. This study found loop gain was elevated in OSA versus age-, sex-, height-, and weight-matched controls. However, this was not altered by 6-wk continuous positive airway pressure treatment.

Persistent High Residual AHI After CPAP Use

ABSTRACT

Treatment-emergent central sleep apnea has recently been noted after various treatment modalities for obstructive sleep apnea. It often remits spontaneously or can be treated with continuous positive airway pressure. However, we encountered a pediatric patient with obstructive sleep apnea who presented with severe complications, including growth failure, attention-deficit hyperactivity disorder, poor school performance, daytime sleepiness, and urinary difficulty that required permanent cystostomy. His obstructive sleep apnea resolved after adenotonsillectomy. However, treatment-emergent central sleep apnea developed after adenotonsillectomy and was further aggravated after continuous positive airway pressure and bilevel positive airway pressure without a backup respiratory rate use. After bilevel positive airway pressure with a backup respiratory rate treatment for 3 months initially, all his symptoms improved, except growth failure. Later, after adaptive servoventilation was used for 10 months, the patient's growth began to improve.

Aging is associated with increased propensity for central apnea during NREM sleep

The reason for increased sleep-disordered breathing with predominance of central apneas in the elderly is unknown. We hypothesized that the propensity to central apneas is increased in older adults, manifested by a reduced carbon-dioxide (CO₂) reserve in older compared with young adults during non-rapid eye movement sleep. Ten elderly and 15 young healthy adults underwent multiple brief trials of nasal noninvasive positive pressure ventilation during stable NREM sleep. Cessation of mechanical ventilation (MV) resulted in hypocapnic central apnea or hypopnea. The CO₂ reserve was defined as the difference in end-tidal CO₂ ([Formula: see text]) between eupnea and the apneic threshold, where the apneic threshold was [Formula: see text] that demarcated the central apnea closest to the eupneic [Formula: see text]. For each MV trial, the hypocapnic ventilatory response (controller gain) was measured as the change in minute ventilation (V̇e) during the MV trial for a corresponding change in [Formula: see text]. The eupneic [Formula: see text] was significantly lower in elderly vs. young adults. Compared with young adults, the elderly had a significantly reduced CO₂ reserve (-2.6 ± 0.4 vs. -4.1 ± 0.4 mmHg, P = 0.01) and a higher controller gain (2.3 ± 0.2 vs. 1.4 ± 0.2 l·min^-1·mmHg^-1, P = 0.007), indicating increased chemoresponsiveness in the elderly. Thus elderly adults are more prone to hypocapnic central apneas owing to increased hypocapnic chemoresponsiveness during NREM sleep. NEW & NOTEWORTHY The study describes an original finding where healthy older adults compared with healthy young adults demonstrated increased breathing instability during non-rapid eye movement sleep, as suggested by a smaller carbon dioxide reserve and a higher controller gain. The findings may explain the increased propensity for central apneas in elderly adults during sleep and potentially guide the development of pathophysiology-defined personalized therapies for sleep apnea in the elderly.

Intermittent hypercapnic hypoxia during sleep does not induce ventilatory long-term facilitation in healthy males

Intermittent hypoxia-induced ventilatory neuroplasticity is likely important in obstructive sleep apnea pathophysiology. Although concomitant CO2levels and arousal state critically influence neuroplastic effects of intermittent hypoxia, no studies have investigated intermittent hypercapnic hypoxia effects during sleep in humans. Thus the purpose of this study was to investigate if intermittent hypercapnic hypoxia during sleep induces neuroplasticity (ventilatory long-term facilitation and increased chemoreflex responsiveness) in humans. Twelve healthy males were exposed to intermittent hypercapnic hypoxia (24 × 30 s episodes of 3% CO2and 3.0 ± 0.2% O2) and intermittent medical air during sleep after 2 wk washout period in a randomized crossover study design. Minute ventilation, end-tidal CO2, O2saturation, breath timing, upper airway resistance, and genioglossal and diaphragm electromyograms were examined during 10 min of stable stage 2 sleep preceding gas exposure, during gas and intervening room air periods, and throughout 1 h of room air recovery. There were no significant differences between conditions across time to indicate long-term facilitation of ventilation, genioglossal or diaphragm electromyogram activity, and no change in ventilatory response from the first to last gas exposure to suggest any change in chemoreflex responsiveness. These findings contrast with previous intermittent hypoxia studies without intermittent hypercapnia and suggest that the more relevant gas disturbance stimulus of concomitant intermittent hypercapnia frequently occurring in sleep apnea influences acute neuroplastic effects of intermittent hypoxia. These findings highlight the need for further studies of intermittent hypercapnic hypoxia during sleep to clarify the role of ventilatory neuroplasticity in the pathophysiology of sleep apnea.

NEW & NOTEWORTHY Both arousal state and concomitant CO2levels are known modulators of the effects of intermittent hypoxia on ventilatory neuroplasticity. This is the first study to investigate the effects of combined intermittent hypercapnic hypoxia during sleep in humans. The lack of neuroplastic effects suggests a need for further studies more closely replicating obstructive sleep apnea to determine the pathophysiological relevance of intermittent hypoxia-induced ventilatory neuroplasticity.

Control of Ventilation in Health and Disease

Control of ventilation occurs at different levels of the respiratory system through a negative feedback system that allows precise regulation of levels of arterial carbon dioxide and oxygen. Mechanisms for ventilatory instability leading to sleep-disordered breathing include changes in the genesis of respiratory rhythm and chemoresponsiveness to hypoxia and hypercapnia, cerebrovascular reactivity, abnormal chest wall and airway reflexes, and sleep state oscillations. One can potentially stabilize breathing during sleep and treat sleep-disordered breathing by identifying one or more of these pathophysiological mechanisms. This review describes the current concepts in ventilatory control that pertain to breathing instability during wakefulness and sleep, delineates potential avenues for alternative therapies to stabilize breathing during sleep, and proposes recommendations for future research.

Heme oxygenase-1-dependent central cardiorespiratory adaptations to chronic intermittent hypoxia in mice

Chronic intermittent hypoxia (CIH) increases sympathetic tone and respiratory instability. Our previous work showed that chronic hypoxia induces the oxygen-sensing enzyme heme oxygenase-1 (HO-1) within the C1 sympathoexcitatory region and the pre-Bötzinger complex (pre-BötC). We therefore examined the effect of CIH on time course of induced expression of HO-1 within these regions and determined whether the induction of HO-1 correlated with changes in respiratory, sigh frequency, and sympathetic responses (spectral analysis of heart rate) to acute hypoxia (10% O₂) during 10 days of exposure to CIH in chronically instrumented awake wild-type (WT) and HO-1 null mice (HO-1^-/-). HO-1 was induced within the C1 and pre-BötC regions after 1 day of CIH. There were no significant differences in the baseline respiratory parameters between WT and HO-1^-/- Prior to CIH, acute hypoxia increased respiratory frequency in both WT and HO-1^-/-; however, minute diaphragm electromyogram activity increased in WT but not HO-1^-/- The hypoxic respiratory response after 1 and 10 days of CIH was restored in HO-1^-/- CIH resulted in an initial significant decline in 1) the hypoxic sigh frequency response, which was restored in WT but not HO-1^-/-, and 2) the baseline sympathetic activity in WT and HO-1^-/-, which remained stable subsequently in WT but not in HO-1^-/- We conclude that 1) CIH induces expression of HO-1 in the C1 and pre-BötC regions within 1 day and 2) HO-1 is necessary for hypoxia respiratory response and contributes to the maintenance of the hypoxic sigh responses and baseline sympathetic activity during CIH.

Reduced respiratory neural activity elicits a long-lasting decrease in the CO2 threshold for apnea in anesthetized rats

Two critical parameters that influence breathing stability are the levels of arterial pCO₂ at which breathing ceases and subsequently resumes - termed the apneic and recruitment thresholds (AT and RT, respectively). Reduced respiratory neural activity elicits a chemoreflex-independent, long-lasting increase in phrenic burst amplitude, a form of plasticity known as inactivity-induced phrenic motor facilitation (iPMF). The physiological significance of iPMF is unknown. To determine if iPMF and neural apnea have long-lasting physiological effects on breathing, we tested the hypothesis that patterns of neural apnea that induce iPMF also elicit changes in the AT and RT. Phrenic nerve activity and end-tidal CO₂ were recorded in urethane-anesthetized, ventilated rats to quantify phrenic nerve burst amplitude and the AT and RT before and after three patterns of neural apnea that differed in their duration and ability to elicit iPMF: brief intermittent neural apneas, a single brief "massed" neural apnea, or a prolonged neural apnea. Consistent with our hypothesis, we found that patterns of neural apnea that elicited iPMF also resulted in changes in the AT and RT. Specifically, intermittent neural apneas progressively decreased the AT with each subsequent neural apnea, which persisted for at least 60min. Similarly, a prolonged neural apnea elicited a long-lasting decrease in the AT. In both cases, the magnitude of the AT decrease was proportional to iPMF. In contrast, the RT was transiently decreased following prolonged neural apnea, and was not proportional to iPMF. No changes in the AT or RT were observed following a single brief neural apnea. Our results indicate that the AT and RT are differentially altered by neural apnea and suggest that specific patterns of neural apnea that elicit plasticity may stabilize breathing via a decrease in the AT.

Sleep Disordered Breathing in Four Resource-Limited Settings in Peru: Prevalence, Risk Factors, and Association with Chronic Diseases

STUDY OBJECTIVES

Sleep disordered breathing (SDB) is a highly prevalent condition in high-income countries, with major consequences for cardiopulmonary health, public safety, healthcare utilization, and mortality. However, its prevalence and effect in low- and middle-income countries are less well known. We sought to determine the prevalence, risk factors, and comorbidities of SDB symptoms in four resource-limited settings.

DESIGN

Cross-sectional analysis of the CRONICAS Cohort, a population-based age- and sex-stratified sample.

SETTING

Four resource-limited settings in Peru varying in altitude, urbanization, and air pollution.

PARTICIPANTS

There were 2,682 adults aged 35 to 92 y.

MEASUREMENTS AND RESULTS

Self-reported SDB symptoms (habitual snoring, observed apneas, Epworth Sleepiness Scale), sociodemographics, medical history, anthropometrics, spirometry, blood biomarkers were reported. We found a high prevalence of habitual snoring (30.2%, 95% confidence interval [CI] 28.5-32.0%), observed apneas (20.9%, 95% CI 19.4-22.5%) and excessive daytime sleepiness (18.6%, 95% CI 17.1-20.1%). SDB symptoms varied across sites; prevalence and adjusted odds for habitual snoring were greatest at sea level, whereas those for observed apneas were greatest at high altitude. In multivariable analysis, habitual snoring was associated with older age, male sex, body mass index (BMI), and higher socioeconomic status; observed apneas were associated with BMI; and excessive daytime sleepiness was associated with older age, female sex, and medium socioeconomic status. Adjusted odds of cardiovascular disease, depression, and hypertension and total chronic disease burden increased progressively with the number of SDB symptoms. A threefold increase in the odds of having an additional chronic comorbid disease (adjusted odds ratio 3.57, 95% CI 2.18-5.84) was observed in those with all three versus no SDB symptoms.

CONCLUSIONS

Sleep disordered breathing symptoms were highly prevalent, varied widely across four resource-limited settings in Peru, and exhibited strong independent associations with chronic diseases.

Effect of age on long-term facilitation and chemosensitivity during NREM sleep

The reason for increased sleep-disordered breathing with a predominance of central apneas in the elderly is unknown. We speculate that ventilatory control instability may provide a link between aging and the onset of unstable breathing during sleep. We sought to investigate potential underlying mechanisms in healthy, elderly adults during sleep. We hypothesized that there is 1) a decline in respiratory plasticity or long-term facilitation (LTF) of ventilation and/or 2) increased ventilatory chemosensitivity in older adults during non-, this should be hyphenated, non-rapid rapid eye movement (NREM) sleep. Fourteen elderly adults underwent 15, 1-min episodes of isocapnic hypoxia (EH), nadir O2 saturation: 87.0 ± 0.8%. Measurements were obtained during control, hypoxia, and up to 20 min of recovery following the EH protocol, respectively, for minute ventilation (VI), timing, and inspiratory upper-airway resistances (RUA). The results showed the following. 1) Compared with baseline, there was a significant increase in VI (158 ± 11%, P < 0.05) during EH, but this was not accompanied by augmentation of VI during the successive hypoxia trials nor in VI during the recovery period (94.4 ± 3.5%, P = not significant), indicating an absence of LTF. There was no change in inspiratory RUA during the trials. This is in contrast to our previous findings of respiratory plasticity in young adults during sleep. Sham studies did not show a change in any of the measured parameters. 2) We observed increased chemosensitivity with increased isocapnic hypoxic ventilatory response and hyperoxic suppression of VI in older vs. young adults during NREM sleep. Thus increased chemosensitivity, unconstrained by respiratory plasticity, may explain increased periodic breathing and central apneas in elderly adults during NREM sleep.

Obstructive Sleep Apnea and Kidney Disease: A Potential Bidirectional Relationship?

Chronic kidney disease (CKD) is associated with high mortality rates and heavy economic and social burdens. Nearly 10% of the United States population suffer from CKD, with fatal outcomes increased by 16-40 times even before reaching end-stage renal disease. The prevalence of obstructive sleep apnea (OSA) is between 3% and 7% in the general population, and has increased dramatically during the last 2 decades along with increased rates of obesity. However, the prevalence of OSA is much greater in patients with CKD. In addition, aggressive dialysis improves OSA. The current literature suggests a bidirectional association between CKD and OSA through a number of potential pathological mechanisms, which increase the possibility of both diseases being possible risk factors for each other. CKD may lead to OSA through a variety of mechanisms, including alterations in chemoreflex responsiveness, pharyngeal narrowing due to fluid overload, and accumulation of uremic toxins. It is also being increasingly recognized that OSA can also accelerate loss of kidney function. Moreover, animals exposed to intermittent hypoxia suffer histopathological renal damage. Potential mechanisms of OSA-associated renal dysfunction include renal hypoxia, hypertension, endothelial dysfunction, activation of the sympathetic nervous system, and increased oxidative stress.

Tetraplegia is associated with enhanced peripheral chemoreflex sensitivity and ventilatory long-term facilitation

Cardiorespiratory plasticity induced by acute intermittent hypoxia (AIH) may contribute to recovery following spinal cord injury (SCI). We hypothesized that patients with cervical SCI would demonstrate higher minute ventilation (V̇e) following AIH compared with subjects with thoracic SCI and able-bodied subjects who served as controls. Twenty-four volunteers (8 with cervical SCI, 8 with thoracic SCI, and 8 able-bodied) underwent an AIH protocol during wakefulness. Each subject experienced 15 episodes of isocapnic hypoxia using mixed gases of 100% nitrogen (N2), 8% O2, and 40% CO2 to achieve oxygen saturation ≤90% followed by room air (RA). Measurements were obtained before, during, and 40 min after AIH to obtain ventilation and heart rate variability data [R-R interval (RRI) and low-frequency/high-frequency power (LF/HF)]. AIH results were compared with those of sham studies conducted in RA during the same time period. Individuals with cervical SCI had higher V̇e after AIH compared with able-bodied controls (117.9 ± 23.2% vs. 97.9 ± 11.2%, P < 0.05). RRI decreased during hypoxia in all individuals (those with cervical SCI, from 1,009.3 ± 65.0 ms to 750.2 ± 65.0 ms; those with thoracic SCI, from 945.2 ± 65.0 ms to 674.9 ± 65.0 ms; and those who were able-bodied, from 949 ± 75.0 to 682.2 ± 69.5 ms; P < 0.05). LH/HF increased during recovery in individuals with thoracic SCI and those who were able-bodied (0.54 ± 0.22 vs. 1.34 ± 0.22 and 0.67 ± 0.23 vs. 1.82 ± 0.23, respectively; P < 0.05) but remained unchanged in the group with cervical SCI. Our conclusion is that patients with cervical SCI demonstrate ventilatory long-term facilitation following AIH compared with able-bodied controls. Heart rate responses to hypoxia are acutely present in patients with cervical SCI but are absent during posthypoxic recovery.

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.

Therapeutic potential of intermittent hypoxia: a matter of dose

Intermittent hypoxia (IH) has been the subject of considerable research in recent years, and triggers a bewildering array of both detrimental and beneficial effects in multiple physiological systems. Here, we review the extensive literature concerning IH and its impact on the respiratory, cardiovascular, immune, metabolic, bone, and nervous systems. One major goal is to define relevant IH characteristics leading to safe, protective, and/or therapeutic effects vs. pathogenesis. To understand the impact of IH, it is essential to define critical characteristics of the IH protocol under investigation, including potentially the severity of hypoxia within episodes, the duration of hypoxic episodes, the number of hypoxic episodes per day, the pattern of presentation across time (e.g., within vs. consecutive vs. alternating days), and the cumulative time of exposure. Not surprisingly, severe/chronic IH protocols tend to be pathogenic, whereas any beneficial effects are more likely to arise from modest/acute IH exposures. Features of the IH protocol most highly associated with beneficial vs. pathogenic outcomes include the level of hypoxemia within episodes and the number of episodes per day. Modest hypoxia (9-16% inspired O2) and low cycle numbers (3-15 episodes per day) most often lead to beneficial effects without pathology, whereas severe hypoxia (2-8% inspired O2) and more episodes per day (48-2,400 episodes/day) elicit progressively greater pathology. Accumulating evidence suggests that "low dose" IH (modest hypoxia, few episodes) may be a simple, safe, and effective treatment with considerable therapeutic potential for multiple clinical disorders.

Effects of hyperoxia and hypoxia on the physiological traits responsible for obstructive sleep apnoea

Oxygen therapy is known to reduce loop gain (LG) in patients with obstructive sleep apnoea (OSA), yet its effects on the other traits responsible for OSA remain unknown. Therefore, we assessed how hyperoxia and hypoxia alter four physiological traits in OSA patients. Eleven OSA subjects underwent a night of polysomnography during which the physiological traits were measured using multiple 3-min 'drops' from therapeutic continuous positive airway pressure (CPAP) levels. LG was defined as the ratio of the ventilatory overshoot to the preceding reduction in ventilation. Pharyngeal collapsibility was quantified as the ventilation at CPAP of 0 cmH2O. Upper airway responsiveness was defined as the ratio of the increase in ventilation to the increase in ventilatory drive across the drop. Arousal threshold was estimated as the level of ventilatory drive associated with arousal. On separate nights, subjects were submitted to hyperoxia (n = 9; FiO2 ∼0.5) or hypoxia (n = 10; FiO2 ∼0.15) and the four traits were reassessed. Hyperoxia lowered LG from a median of 3.4 [interquartile range (IQR): 2.6-4.1] to 2.1 (IQR: 1.3-2.5) (P < 0.01), but did not alter the remaining traits. By contrast, hypoxia increased LG [median: 3.3 (IQR: 2.3-4.0) vs. 6.4 (IQR: 4.5-9.7); P < 0.005]. Hypoxia additionally increased the arousal threshold (mean ± s.d. 10.9 ± 2.1 l min(-1) vs. 13.3 ± 4.3 l min(-1); P < 0.05) and improved pharyngeal collapsibility (mean ± s.d. 3.4 ± 1.4 l min(-1) vs. 4.9 ± 1.3 l min(-1); P < 0.05), but did not alter upper airway responsiveness (P = 0.7). This study demonstrates that the beneficial effect of hyperoxia on the severity of OSA is primarily based on its ability to reduce LG. The effects of hypoxia described above may explain the disappearance of OSA and the emergence of central sleep apnoea in conditions such as high altitude.

Sleep disordered breathing in chronic spinal cord injury

STUDY OBJECTIVES

Spinal cord injury (SCI) is associated with 2-5 times greater prevalence of sleep disordered breathing (SDB) than the general population. The contribution of SCI on sleep and breathing at different levels of injury using two scoring methods has not been assessed. The objectives of this study were to characterize the sleep disturbances in the SCI population and the associated physiological abnormalities using quantitative polysomnography and to determine the contribution of SCI level on the SDB mechanism.

METHODS

We studied 26 consecutive patients with SCI (8 females; age 42.5 ± 15.5 years; BMI 25.9 ± 4.9 kg/m2; 15 cervical and 11 thoracic levels) by spirometry, a battery of questionnaires and by attended polysomnography with flow and pharyngeal pressure measurements. Inclusion criteria for SCI: chronic SCI (> 6 months post injury), level T6 and above and not on mechanical ventilation. Ventilation, end-tidal CO2 (PETCO2), variability in minute ventilation (VI-CV) and upper airway resistance (RUA) were monitored during wakefulness and NREM sleep in all subjects. Each subject completed brief history and exam, Epworth Sleepiness Scale (ESS), Pittsburgh Sleep Quality Index (PSQI), Berlin questionnaire (BQ) and fatigue severity scale (FSS). Sleep studies were scored twice, first using standard 2007 American Academy of Sleep Medicine (AASM) criteria and second using new 2012 recommended AASM criteria.

RESULTS

Mean PSQI was increased to 10.3 ± 3.7 in SCI patients and 92% had poor sleep quality. Mean ESS was increased 10.4 ± 4.4 in SCI patients and excessive daytime sleepiness (ESS ≥ 10) was present in 59% of the patients. Daytime fatigue (FSS > 20) was reported in 96% of SCI, while only 46% had high-risk score of SDB on BQ. Forced vital capacity (FVC) in SCI was reduced to 70.5% predicted in supine compared to 78.5% predicted in upright positions (p < 0.05). Likewise forced expiratory volume in first second (FEV1) was 64.9% predicted in supine compared to 74.7% predicted in upright positions (p < 0.05). Mean AHI in SCI patients was 29.3 ± 25.0 vs. 20.0 ± 22.8 events/h using the new and conventional AASM scoring criteria, respectively (p < 0.001). SCI patients had SDB (AHI > 5 events/h) in 77% of the cases using the new AASM scoring criteria compared to 65% using standard conventional criteria (p < 0.05). In cervical SCI, VI decreased from 7.2 ± 1.6 to 5.5 ± 1.3 L/min, whereas PETCO2 and VI-CV, increased during sleep compared to thoracic SCI.

CONCLUSION

The majority of SCI survivors have symptomatic SDB and poor sleep that may be missed if not carefully assessed. Decreased VI and increased PETCO2 during sleep in patients with cervical SCI relative to thoracic SCI suggests that sleep related hypoventilation may contribute to the pathogenesis SDB in patients with chronic cervical SCI.

Ventilatory long-term facilitation is evident after initial and repeated exposure to intermittent hypoxia in mice genetically depleted of brain serotonin

Our study was designed to determine if central nervous system (CNS) serotonin is required for the induction of ventilatory long-term facilitation (LTF) in intact, spontaneously breathing mice. Nineteen tryptophan hydroxylase 2-deficient (Tph2(-/-)) mice, devoid of serotonin in the CNS, and their wild-type counterparts (Tph2(+/+)) were exposed to intermittent hypoxia each day for 10 consecutive days. The ventilatory response to intermittent hypoxia was greater in the Tph2(+/+) compared with the Tph2(-/-) mice (1.10 ± 0.10 vs. 0.77 ± 0.01 ml min(-1)·percent(-1) oxygen; P ≤ 0.04). Ventilatory LTF, caused by increases in breathing frequency, was evident in Tph2(+/+) and Tph2(-/-) mice following exposure to intermittent hypoxia each day; however, the magnitude of the response was greater in the Tph2(+/+) compared with the Tph2(-/-) mice (1.11 ± 0.02 vs. 1.05 ± 0.01 normalized to baseline on each day; P ≤ 0.01). The magnitude of ventilatory LTF increased significantly from the initial to the finals days of the protocol in the Tph2(-/-) (1.06 ± 0.02 vs. 1.11 ± 0.03 normalized to baseline on the initial days; P ≤ 0.004) but not in the Tph2(+/+) mice. This enhanced response was mediated by increases in tidal volume. Body temperature and metabolic rate did not account for differences in the magnitude of ventilatory LTF observed between groups after acute and repeated daily exposure to intermittent hypoxia. We conclude that ventilatory LTF, after acute exposure to intermittent hypoxia, is mediated by increases in breathing frequency and occurs in the absence of serotonin, although the magnitude of the response is diminished. This weakened response is enhanced following repeated daily exposure to intermittent hypoxia, via increases in tidal volume, to a similar magnitude evident in Tph2(+/+) mice. Thus the magnitude of ventilatory LTF following repeated daily exposure to intermittent hypoxia is not dependent on the presence of CNS serotonin.

Testosterone conversion blockade increases breathing stability in healthy men during NREM sleep

STUDY OBJECTIVES

Gender differences in the prevalence of sleep apnea/hypopnea syndrome may be mediated via male sex hormones. Our objective was to determine the exact pathway for a testosterone-mediated increased propensity for central sleep apnea via blockade of the 5α-reductase pathway of testosterone conversion by finasteride.

DESIGN

Randomization to oral finasteride vs. sham, single-center study.

SETTING

Sleep research laboratory.

PARTICIPANTS

Fourteen healthy young males without sleep apnea.

INTERVENTION

Hypocapnia was induced via brief nasal noninvasive positive pressure ventilation during stable NREM sleep. Cessation of mechanical ventilation resulted in hypocapnic central apnea or hypopnea.

MEASUREMENTS AND RESULTS

The apnea threshold (AT) was defined as the end-tidal CO₂(P(ET)CO₂) that demarcated the central apnea closest to the eupneic P(ET)CO₂. The CO₂ reserve was defined as the difference in P(ET)CO₂ between eupnea and AT. The apneic threshold and CO₂ reserve were measured at baseline and repeated after at a minimum of 1 month. Administration of finasteride resulted in decreased serum dihydrotestosterone. In the finasteride group, the eupneic ventilatory parameters were unchanged; however, the AT was decreased (38.9 ± 0.6 mm Hg vs.37.7 ± 0.9 mm Hg, P = 0.02) and the CO₂ reserve was increased (-2.5 ± 0.3 mm Hg vs. -3.8 ± 0.5 mm Hg, P = 0.003) at follow-up, with a significantly lower hypocapnic ventilatory response, thus indicating increased breathing stability during sleep. No significant changes were noted in the sham group on follow-up study.

CONCLUSIONS

Inhibition of testosterone action via the 5α-reductase pathway may be effective in alleviating breathing instability during sleep, presenting an opportunity for novel therapy for central sleep apnea in selected populations.

Tetraplegia is a risk factor for central sleep apnea

Sleep-disordered breathing (SDB) is highly prevalent in patients with spinal cord injury (SCI); the exact mechanism(s) or the predictors of disease are unknown. We hypothesized that patients with cervical SCI (C-SCI) are more susceptible to central apnea than patients with thoracic SCI (T-SCI) or able-bodied controls. Sixteen patients with chronic SCI, level T6 or above (8 C-SCI, 8 T-SCI; age 42.5 ± 15.5 years; body mass index 25.9 ± 4.9 kg/m(2)) and 16 matched controls were studied. The hypocapnic apneic threshold and CO2 reserve were determined using noninvasive ventilation. For participants with spontaneous central apnea, CO2 was administered until central apnea was abolished, and CO2 reserve was measured as the difference in end-tidal CO2 (PetCO2) before and after. Steady-state plant gain (PG) was calculated from PetCO2 and VE ratio during stable sleep. Controller gain (CG) was defined as the ratio of change in VE between control and hypopnea or apnea to the ΔPetCO2. Central SDB was more common in C-SCI than T-SCI (63% vs. 13%, respectively; P < 0.05). Mean CO2 reserve for all participants was narrower in C-SCI than in T-SCI or control group (-0.4 ± 2.9 vs.-2.9 ± 3.3 vs. -3.0 ± 1.2 l·min(-1)·mmHg(-1), respectively; P < 0.05). PG was higher in C-SCI than in T-SCI or control groups (10.5 ± 2.4 vs. 5.9 ± 2.4 vs. 6.3 ± 1.6 mmHg·l(-1)·min(-1), respectively; P < 0.05) and CG was not significantly different. The CO2 reserve was an independent predictor of apnea-hypopnea index. In conclusion, C-SCI had higher rates of central SDB, indicating that tetraplegia is a risk factor for central sleep apnea. Sleep-related hypoventilation may play a significant role in the mechanism of SDB in higher SCI levels.

Drug effects on ventilatory control and upper airway physiology related to sleep apnea

Understanding the inter-relationship between pharmacological agents, ventilatory control, upper airway physiology and their consequent effects on sleep-disordered breathing may provide new directions for targeted drug therapy. Where available, this review focuses on human studies that contain both drug effects on sleep-disordered breathing and measures of ventilatory control or upper airway physiology. Many of the existing studies are limited in sample size or comprehensive methodology. At times, the presence of paradoxical findings highlights the complexity of drug therapy for OSA. The existing studies also highlight the importance of considering inter-individual pharmacokinetics and underlying causes of sleep apnea in interpreting drug effects on sleep-disordered breathing. Practical ways to assess an individual's ventilatory control and how it interacts with upper airway physiology is required for future targeted pharmacotherapy in sleep apnea.

Intermittent hypoxia, respiratory plasticity and sleep apnea in humans: present knowledge and future investigations

This review examines the role that respiratory plasticity has in the maintenance of breathing stability during sleep in individuals with sleep apnea. The initial portion of the review considers the manner in which repetitive breathing events may be initiated in individuals with sleep apnea. Thereafter, the role that two forms of respiratory plasticity, progressive augmentation of the hypoxic ventilatory response and long-term facilitation of upper airway and respiratory muscle activity, might have in modifying breathing events in humans is examined. In this context, present knowledge regarding the initiation of respiratory plasticity in humans during wakefulness and sleep is addressed. Also, published findings which reveal that exposure to intermittent hypoxia promotes breathing instability, at least in part, because of progressive augmentation of the hypoxic ventilatory response and the absence of long-term facilitation, are considered. Next, future directions are presented and are focused on the manner in which forms of plasticity that stabilize breathing might be promoted while diminishing destabilizing forms, concurrently. These future directions will consider the potential role of circadian rhythms in the promotion of respiratory plasticity and the role of respiratory plasticity in enhancing established treatments for sleep apnea.

The impact of arousal state, sex, and sleep apnea on the magnitude of progressive augmentation and ventilatory long-term facilitation

We examined the impact of arousal state, sex, and obstructive sleep apnea (OSA) on the magnitude of progressive augmentation of the hypoxic ventilatory response and ventilatory long-term facilitation (vLTF). We also examined whether exposure to intermittent hypoxia during sleep has an impact on the apnea-hypopnea index (AHI) in individuals with OSA. Ten men and seven women with OSA, along with ten healthy men and ten healthy women, were exposed to twelve 2-min episodes of hypoxia (end-tidal PO(2): 50 Torr) in the presence of sustained hypercapnia (end-tidal PCO(2): 3 Torr above baseline), followed by a 30-min recovery period during wakefulness and sleep. The OSA participants completed an additional sham study during sleep. The AHI during the first hour of sleep following the intermittent hypoxia and sham protocols were compared. Progressive augmentation was only evident during wakefulness and was enhanced in the OSA participants. vLTF was evident during wakefulness and sleep. When standardized to baseline, vLTF was greater during wakefulness and was enhanced in the OSA group (men: wakefulness 1.39 ± 0.08 vs. sleep 1.14 ± 0.03; women: wakefulness 1.35 ± 0.03 vs. sleep 1.16 ± 0.05 fraction of baseline; P ≤ 0.001) compared with control (men: wakefulness 1.19 ± 0.03 vs. sleep 1.09 ± 0.03; women: wakefulness 1.26 ± 0.05 vs. sleep 1.08 ± 0.04 fraction of baseline; P ≤ 0.001). The AHI following exposure to intermittent hypoxia was increased (intermittent hypoxia 72.8 ± 7.3 vs. sham 56.5 ± 7.0 events/h; P ≤ 0.01). Sex-related differences were not observed for the primary measures. We conclude that progressive augmentation is not evident, and the magnitude of vLTF is diminished during sleep compared with wakefulness in men and women. However, when present, the phenomena are enhanced in individuals with OSA. The AHI data indicate that, under the prevailing experimental conditions, vLTF did not serve to mitigate apnea severity.

The treatment of central sleep apnea syndromes in adults: practice parameters with an evidence-based literature review and meta-analyses

The International Classification of Sleep Disorders, Second Edition (ICSD-2) distinguishes 5 subtypes of central sleep apnea syndromes (CSAS) in adults. Review of the literature suggests that there are two basic mechanisms that trigger central respiratory events: (1) post-hyperventilation central apnea, which may be triggered by a variety of clinical conditions, and (2) central apnea secondary to hypoventilation, which has been described with opioid use. The preponderance of evidence on the treatment of CSAS supports the use of continuous positive airway pressure (CPAP). Much of the evidence comes from investigations on CSAS related to congestive heart failure (CHF), but other subtypes of CSAS appear to respond to CPAP as well. Limited evidence is available to support alternative therapies in CSAS subtypes. The recommendations for treatment of CSAS are summarized as follows: CPAP therapy targeted to normalize the apnea-hypopnea index (AHI) is indicated for the initial treatment of CSAS related to CHF. (STANDARD)Nocturnal oxygen therapy is indicated for the treatment of CSAS related to CHF. (STANDARD)Adaptive Servo-Ventilation (ASV) targeted to normalize the apnea-hypopnea index (AHI) is indicated for the treatment of CSAS related to CHF. (STANDARD)BPAP therapy in a spontaneous timed (ST) mode targeted to normalize the apnea-hypopnea index (AHI) may be considered for the treatment of CSAS related to CHF only if there is no response to adequate trials of CPAP, ASV, and oxygen therapies. (OPTION)The following therapies have limited supporting evidence but may be considered for the treatment of CSAS related to CHF after optimization of standard medical therapy, if PAP therapy is not tolerated, and if accompanied by close clinical follow-up: acetazolamide and theophylline. (OPTION)Positive airway pressure therapy may be considered for the treatment of primary CSAS. (OPTION)Acetazolamide has limited supporting evidence but may be considered for the treatment of primary CSAS. (OPTION)The use of zolpidem and triazolam may be considered for the treatment of primary CSAS only if the patient does not have underlying risk factors for respiratory depression. (OPTION)The following possible treatment options for CSAS related to end-stage renal disease may be considered: CPAP, supplemental oxygen, bicarbonate buffer use during dialysis, and nocturnal dialysis. (OPTION) .

Impact of repeated daily exposure to intermittent hypoxia and mild sustained hypercapnia on apnea severity

We examined whether exposure to intermittent hypoxia (IH) during wakefulness impacted on the apnea/hypopnea index (AHI) during sleep in individuals with sleep apnea. Participants were exposed to twelve 4-min episodes of hypoxia in the presence of sustained mild hypercapnia each day for 10 days. A control group was exposed to sustained mild hypercapnia for a similar duration. The intermittent hypoxia protocol was completed in the evening on day 1 and 10 and was followed by a sleep study. During all sleep studies, the change in esophageal pressure (ΔPes) from the beginning to the end of an apnea and the tidal volume immediately following apneic events were used to measure respiratory drive. Following exposure to IH on day 1 and 10, the AHI increased above baseline measures (day 1: 1.95 ± 0.42 fraction of baseline, P ≤ 0.01, vs. day 10: 1.53 ± 0.24 fraction of baseline, P < 0.06). The indexes were correlated to the hypoxic ventilatory response (HVR) measured during the IH protocol but were not correlated to the magnitude of ventilatory long-term facilitation (vLTF). Likewise, ΔPes and tidal volume measures were greater on day 1 and 10 compared with baseline (ΔPes: -8.37 ± 0.84 vs. -5.90 ± 1.30 cmH(2)0, P ≤ 0.04; tidal volume: 1,193.36 ± 101.85 vs. 1,015.14 ± 119.83 ml, P ≤ 0.01). This was not the case in the control group. Interestingly, the AHI on day 10 (0.78 ± 0.13 fraction of baseline, P ≤ 0.01) was significantly less than measures obtained during baseline and day 1 in the mild hypercapnia control group. We conclude that enhancement of the HVR initiated by exposure to IH may lead to increases in the AHI during sleep and that initiation of vLTF did not appear to impact on breathing stability. Lastly, our results suggest that repeated daily exposure to mild sustained hypercapnia may lead to a decrease in breathing events.

Tempol ameliorates pharyngeal dilator muscle dysfunction in a rodent model of chronic intermittent hypoxia

Respiratory muscle dysfunction is implicated in the pathophysiology of obstructive sleep apnea syndrome (OSAS), an oxidative stress disorder prevalent in men. Pharmacotherapy for OSAS is an attractive option, and antioxidant treatments may prove beneficial. We examined the effects of chronic intermittent hypoxia (CIH) on breathing and pharyngeal dilator muscle structure and function in male and female rats. Additionally, we tested the efficacy of antioxidant treatment in preventing (chronic administration) or reversing (acute administration) CIH-induced effects in male rats. Adult male and female Wistar rats were exposed to alternating cycles of normoxia and hypoxia (90 s each; Fi(O(2)) = 5% O(2) at nadir; Sa(O(2)) ∼ 80%) or sham treatment for 8 h/d for 9 days. Tempol (1 mM, superoxide dismutase mimetic) was administered to subgroups of sham- and CIH-treated animals. Breathing was assessed by whole-body plethysmography. Sternohyoid muscle contractile and endurance properties were examined in vitro. Muscle fiber type and cross-sectional area and the activity of key metabolic enzymes were determined. CIH decreased sternohyoid muscle force in male rats only. This was not attributable to fiber transitions or alterations in oxidative or glycolytic enzyme activity. Muscle weakness after CIH was prevented by chronic Tempol supplementation and was reversed by acute antioxidant treatment in vitro. CIH increased normoxic ventilation in male rats only. Sex differences exist in the effects of CIH on the respiratory system, which may contribute to the higher prevalence of OSAS in male subjects. Antioxidant treatment may be beneficial as an adjunct OSAS therapy.