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The treatment of chronic hypoventilation usually requires noninvasive ventilation. However, upper airway obstruction can lead to hypoventilation in conditions such as obesity-hypoventilation syndrome, or chronic obstructive lung diseases (overlap syndrome). In these situations, continuous positive airway pressure can be an effective therapeutic option. This article reviews the pathophysiology of sleep-related hypoventilation, discusses situations where treatment with continuous positive airway pressure is feasible and briefly outlines noninvasive ventilation modes and settings for the treatment of common sleep-related hypoventilation disorders.

Because of the prevalence of extreme obesity in the United States, there has been an increase in prevalence of obesity hypoventilation syndrome (OHS). There is limited information on the characteristics and pattern of positive airway pressure (PAP) adherence in patients with OHS compared with eucapnic patients with obstructive sleep apnea (OSA). This article discusses in detail the impact of PAP therapy on outcomes in patients with OHS, compares adherence between continuous PAP and noninvasive ventilation in OHS, and compares PAP adherence in patients with OHS to patients with moderate to severe OSA enrolled in clinical trials designed to improve CPAP adherence.

Obesity hypoventilation syndrome is the most frequent cause of chronic hypoventilation and is increasingly more common with rising obesity rates. It leads to considerable morbidity and mortality, particularly when not recognized and treated adequately. Long-term nocturnal noninvasive ventilation is the mainstay of treatment but evidence suggests that CPAP may be effective in stable patients. Specific perioperative management is required to reduce complications. Some unique syndromes associated with obesity and hypoventilation include rapid-onset obesity with hypoventilation, hypothalamic, autonomic dysregulation (ROHHAD), and Prader-Willi syndrome. Congenital central hypoventilation syndrome (early or late-onset) is a genetic disorder resulting in hypoventilation. Several acquired causes of chronic central hypoventilation also exist. A high level of clinical suspicion is required to appropriately diagnose and manage affected patients.

Cumulative evidence supports the association of adverse postoperative outcomes with obstructive sleep apnea (OSA) and obesity hypoventilation syndrome (OHS). Although current guidelines recommend preoperative screening for OSA and OHS, the best perioperative management pathways remain unknown. Interventions attempting to prevent complications in the postoperative period largely are consensus based and focused on enhanced monitoring, conservative measures, and specific OSA therapies, such as positive airway pressure. Until further research is available to improve the quality and strength of these recommendations, patients with known or suspected OSA and OHS should be considered at higher risk for perioperative cardiopulmonary complications.

Obesity hypoventilation syndrome is the most frequent cause of chronic hypoventilation and is increasingly more common with rising obesity rates. It leads to considerable morbidity and mortality, particularly when not recognized and treated adequately. Long-term nocturnal noninvasive ventilation is the mainstay of treatment but evidence suggests that CPAP may be effective in stable patients. Specific perioperative management is required to reduce complications. Some unique syndromes associated with obesity and hypoventilation include rapid-onset obesity with hypoventilation, hypothalamic, autonomic dysregulation (ROHHAD), and Prader-Willi syndrome. Congenital central hypoventilation syndrome (early or late-onset) is a genetic disorder resulting in hypoventilation. Several acquired causes of chronic central hypoventilation also exist. A high level of clinical suspicion is required to appropriately diagnose and manage affected patients.

Weaning to noninvasive ventilation in intensive care unit and bridging the patients to home with respiratory support is evolving as the technology of noninvasive ventilation is improving. In patients with chronic obstructive pulmonary disease exacerbation, timing of initiation of noninvasive ventilation is the key, as persistently hypercapnic patients show benefits. High-intensity pressure support seems to do better in comparison to low-intensity pressure support. In patients with obesity and hypercapnia, obesity hypoventilation cannot be ruled out especially in an inpatient setting, and it is crucial that these patients are discharged with noninvasive ventilation.

The obesity hypoventilation syndrome (OHS) is associated with significant morbidity and increased mortality compared with simple obesity and eucapnic obstructive sleep apnea. Accurate diagnosis and commencement of early and appropriate management is fundamental in reducing the significant personal and societal burdens this disorder poses. Sleep disordered breathing is a major contributor to the developmental of sleep and awake hypercapnia, which characterizes OHS, and is effectively addressed through the use of positive airway pressure (PAP) therapy. This article reviews the current evidence supporting different modes of PAP currently used in managing these individuals.

OHSisoftenundiagnosedorun-dertreated before elective noncardiac surgery andis usually associated with many medical comor-bidities. This article describes and examinesmany important steps and strategies that canimprove perioperative outcomes in this high-riskpopulation.IMPORTANT PATHOPHYSIOLOGICCONSIDERATIONSEffect of Obesity, Supine Posture, andAnesthesia on Lung FunctionFunctional residual capacity (FRC) can be expo-nentially reduced in the supine position and Tren-delenburg positions as a function of increasingBMI. Reductions as low as 51% have beendescribed in morbidly obese patients undergoingjejunoileal bypass surgery.

OHS prevalence is expected to increase, following global trends in obesity, and because significant morbidity is associated with this disease, timely diagnosis and appropriate therapy are imperative. The traditional criteria for OHS diagnosis include the presence of daytime alveolar hypoventilation (awake, sea-level, arterial PCO2>45 mm Hg) among patients with BMI ≥30 kg/m2 in the absence of other causes of hypoventilation, and incorporating finger pulse oximetry and serum bicarbonate screening will likely aid in improving diagnosis. The major risk factors for OHS include obesity and OSA; therefore, a high index of suspicion is needed in these patients, particularly in the inpatient setting and before bariatric surgery.

Noninvasive positive airway pressure (NPPV) via mask during sleep is increasingly used to treat adults and children with chronic diurnal alveolar hypoventilation (CAH) syndromes. In addition, patients who have significant hypoventilation only during sleep may also benefit from nocturnal NPPV. The benefits of NPPV include improved sleep quality, improved diurnal and nocturnal arterial PCO2 and PO2, reduced nocturnal dyspnea, and improved quality of life. NPPV treatment has been used in CAH syndromes (Box 1) secondary to restrictive thoracic cage disorders (RTCD), neuromuscular diseases (NMD), central respiratory control disturbances (CRCD), and the obesity hypoventilation syndrome (OHS). NPPV during sleep has also been used with mixed success in patients with chronic obstructive pulmonary disease (COPD) and to manage patients with acute respiratory failure. The

Obesity hypoventilation syndrome is a combination of obesity, unexplained daytime hypercapnia, and sleep-disordered breathing. It is associated with such symptoms as dyspnea, excessive daytime sleepiness, and impaired cognition. The disorder can result in pulmonary hypertension, severe hypoxemia, cor pulmonale, respiratory failure, and increased mortality. While the etiology is still incompletely understood, the combined effects of obesity, sleep-disordered breathing, and altered leptin levels all likely interact to result in a decreased ventilatory drive. Standard treatment consists of positive airway pressure (continuous positive airway pressure or bi-level positive airway pressure) combined with long-term weight management.

Noninvasive ventilation (NIV) has become a proven and accepted treatment for respiratory failure from a variety of underlying causes. NIV during sleep can improve gas exchange in both acute and chronic respiratory failure. During sleep, numerous physiologic changes take place that increase the work of breathing and can therefore worsen sleep quality. These include decrements in tidal volume, worsening of gas exchange, and altered lung mechanics. While institution of NIV has been shown to improve nocturnal Paco 2 and Pao 2 levels and reduce the work of breathing, less is known regarding its effects on sleep and sleep quality. This article reviews data from case series, uncontrolled studies, randomized clinical trials, and meta-analyses. While there is some controversy, the predominance of data suggests that NIV can significantly improve several measures of sleep quality and duration in patients with chronic respiratory failure from numerous underlying causes. Studies have demonstrated improvements in total sleep time, sleep efficiency, subjective sleep quality, and overall quality of life in patients using NIV. In patients with the obesity hypoventilation syndrome, NIV is a cornerstone of therapy, improving daytime hypersomnolence as well as gas exchange. The data for NIV in patients with chronic obstructive pulmonary disease are conflicting and it is unclear if these patients have overall improvements with NIV use. Implementation of chronic NIV is discussed, along with indications and contraindications to therapy.

The association between obesity and hypoventilation has long been recognized. Of historical interest, obesity hypoventilation syndrome was described well before obstructive sleep apnea was recognized in 1969. Since then, we have made great strides to advance our understanding of sleep hypoventilation syndromes. Sleep hypoventilation, particularly obesity hypoventilation syndrome, should no longer be considered a rare condition in the current global epidemic of obesity. Despite the increased recognition of this condition by health care providers, a significant proportion of the population afflicted by sleep hypoventilation remains undiagnosed and untreated, and the potential societal, health-related, and economic consequences remain poorly defined. It therefore gives me great pleasure to see the outstanding contributions from a cast of highly qualified experts and researchers who discuss in great detail the recent advances in epidemiology, pathophysiology, diagnosis, and treatment of adult and pediatric sleep hypoventilation syndromes. While extensive research spanning several decades has increased our understanding of sleep hypoventilation syndromes, the specific purpose of this issue of Sleep Medicine Clinics is to provide a forum that allows a group of internationally recognized experts in the field to thoroughly and critically review the current state of knowledge and to provide insight and guidance on important questions that require further investigation. My most fervent hope is that the state-of-the-art discussions in this issue will stimulate the readership and provide a focused assessment of the current state of knowledge on sleep hypoventilation. I am confident that the outstanding contributions by my colleagues will help current and future investigators and clinicians to formulate many probing questions waiting to be asked and help us find the answers in this exciting and important area of respiratory and sleep medicine. The issue begins with an article providing a historical perspective on sleep hypoventilation syndromes. The following article provides an overview of the pathophysiology of sleep hypoventilation and how it can progress from hypoventilation during sleep to hypoventilation during wakefulness. This is followed by an in-depth discussion regarding diagnostic considerations and technological limitations related to capnometry. The next article provides an update of advances in various modalities of positive airway pressure therapy. Scoring respiratory events during noninvasive ventilation can be challenging, and a systematic description of these events has been summarized in the following article. The next three articles provide a detailed and up-to-date summary of diagnosis, epidemiology, and treatment of obesity hypoventilation syndrome. These three articles are followed by a series of outstanding reviews covering the spectrum of hypoventilation, such as the overlap syndrome, neuromuscular disorders, hypoventilation due to opioids and sedatives, as well as the interaction between anesthesia and sleep hypoventilation syndromes. The last two articles provide a comprehensive review of pediatric hypoventilation syndrome and treatment with noninvasive ventilation in children. I am indebted to my colleagues who graciously accepted my invitation to make exceptional contributions to this issue of Sleep Medicine Clinics. I am also grateful to Dr Teofilo Lee-Chiong for providing a forum that allows an in-depth analysis and discussion of the topic. I hope you will enjoy reading these outstanding articles.