American Journal of Respiratory and Critical Care Medicine

Obesity hypoventilation syndrome describes the association between obesity and the development of chronic daytime alveolar hypoventilation. This syndrome arises from a complex interaction between sleep-disordered breathing, diminished respiratory drive, and obesity-related respiratory impairment, and is associated with significant morbidity and mortality. Therapy directed toward reversing these abnormalities leads to improved daytime breathing, with available treatment options including positive pressure therapy, weight loss, and pharmacological management. However, a lack of large-scale, well-designed studies evaluating these various therapies has limited the development of evidence-based treatment recommendations. Although treatment directed toward improving sleep-disordered breathing is usually effective, not all patients tolerate mask ventilation and awake hypercapnia may persist despite effective use. In the longer term, weight loss is desirable, but data on the success and sustainability of this approach in obesity hypoventilation are lacking. The review outlines the major mechanisms believed to underlie the development of hypoventilation in this subgroup of obese patients, their clinical presentation, and current therapy options.

Obesity is a major public health issue, and despite the increasing attention this disorder has received over the past decade, not only are obesity rates continuing to increase but also we are seeing the emergence of an increasing number of individuals who can be categorized as super obese (body mass index [BMI] > 50 kg/m2) (1). As weight increases, so do the health consequences, including those related to the respiratory system. Obesity hypoventilation syndrome (OHS) refers to the appearance of awake hypercapnia (PaCO2 > 45 mm Hg) in the obese patient (BMI > 30 kg/m2) after other causes that could account for awake hypoventilation, such as lung or neuromuscular disease, have been excluded. Although sleep-disordered breathing is not currently part of the basic definition of OHS, breathing during sleep in these individuals typically encompasses a number of polysomnographic defined phenotypes, including repetitive frank obstructive sleep apnea (OSAS) with hypercapnia, flow limitation causing obstructive hypoventilation, through to hypoventilation in all sleep stages (2). Sleep hypoventilation alone does not define OHS unless daytime hypercapnia is also present. However, it is possible that obese patients with hypoventilation during sleep without awake hypercapnia have a “prodromal” form of OHS and will later develop chronic hypercapnia. This may be similar to the clinical scenario observed in patients with sleep-disordered breathing and neuromuscular disease (3).

The incidence of OHS increases significantly as obesity increases, with a reported prevalence of around 10 to 20% in outpatients presenting to sleep clinics (46) to almost 50% of hospitalized patients with a BMI greater than 50 kg/m2 (7). Current estimates suggest that around 0.3 to 0.4% of the population may have OHS (8, 9). However, the accuracy of prevalence data has been limited by inclusion of multiple small studies, failure to exclude some patients with chronic obstructive pulmonary disease, and variable definitions of obstructive sleep apnea (8). There are no prospective observational cohort studies investigating which patients with OSA will develop OHS with weight gain. Nevertheless, it is reasonable to assume there are several hundred thousand individuals in the United States with OHS. In this review, we highlight the pathophysiology that determines which patients with obesity develop awake hypoventilation and the clinical impact of OHS. Recent data on management approaches are also discussed. The review emphasizes the importance of early detection and intervention in patients with OHS, considering the substantial morbidity and mortality associated with this disorder.

One of the more intriguing aspects of the interaction of respiratory function and obesity is that only some morbidly obese patients develop awake hypoventilation. There are clearly specific differences between obese individuals who maintain normal ventilation awake and asleep, those who develop sleep-disordered breathing with normal daytime ventilation, and those who exhibit awake hypercapnia in the absence of lung or neuromuscular disease. These interindividual differences are complex, reflecting the diverse impact of obesity on breathing (10).

Respiratory Mechanics

Obesity, particularly when it is severe, is associated with significant changes in pulmonary mechanics and respiratory muscle performance. However, many of these effects are magnified in those obese patients who develop OHS compared with equally obese individuals either with or without sleep-disordered breathing (Table 1) (11). Breathing at abnormally low lung volumes reduces chest wall and respiratory system compliance (12, 13) and increases airway resistance (14). When expiratory reserve volume is low, small airway closure and air trapping occur, resulting in expiratory flow limitation and the development of intrinsic positive end-expiratory pressure (15). These changes are more marked in the supine position and contribute to the increased work of breathing by imposing a threshold load on the inspiratory muscles (15, 16). It is not known whether the development of intrinsic positive end-expiratory pressure is greater in patients with OHS compared with equally obese eucapnic individuals. However, it has been shown that the work of breathing in sitting and supine (awake or in stage II sleep) positions is significantly higher in patients with OHS compared with similarly obese eucapnic patients (17).


Obesity Hypoventilation

Eucapnic Morbid Obesity

FEV1/FVCNormalNormal4, 7, 8, 10, 11, 18, 22, 62
Total lung capacitySlightly reducedNormal4, 8, 11, 30, 31, 59, 62
Functional residual capacityReducedReduced10, 13, 15, 31
Vital capacityMarkedly reducedNormal/reduced4, 5, 8, 16, 17, 30
Expiratory reserve volumeMarkedly reducedReduced15, 30, 62
Respiratory system complianceMarkedly reducedReduced12, 16
Work of breathingSignificantly increasedIncreased16, 17
Respiratory driveNormalIncreased16, 29–31
Inspiratory muscle strengthReducedNormal16, 30
Ventilatory response to CO2Normal/reducedNormal/increased18, 30, 32, 33, 35
PaCO2Increased/markedly increasedNormal6, 7, 11, 16, 17, 31–33, 52
Serum bicarbonateIncreasedNormal6, 7, 16, 31, 33
Markedly increased

Adapted from Reference 81.

There are several mechanisms to potentially explain the greater impairment of respiratory function in OHS. First, in those with more severe OHS, respiratory muscle performance may be affected by the biochemical disturbance associated with hypoventilation (acidosis and hypoxemia). Whether a primary myopathic process also exists is currently unknown, as detailed muscle structural analysis has not been performed in individuals with OHS. Second, patients with OHS have a higher degree of central fat distribution, as demonstrated by larger neck circumferences and higher waist:hip ratios compared with eucapnic obese patients or patients with OSAS (11, 18). Central adiposity results in cephalic displacement of the diaphragm. This produces inefficient mechanical performance (19), which worsens when the individual is lying supine. In addition, centrally distributed weight reduces lung volumes to a much greater degree than weight deposited peripherally (20). This would contribute to greater reductions in lung volumes and more marked mechanical ventilatory constraints, which would increase the work of breathing while reducing respiratory muscle efficiency in patients with OHS. The distribution of excess weight is also important in gas exchange with centrally obese individuals, regardless of sex or BMI, exhibiting poorer gas exchange (21).

The Upper Airway

Despite the clear differences between OHS and eucapnic obese patients with respect to respiratory function, there are obviously other factors that influence the emergence of awake hypoventilation, including sleep-disordered breathing.

There is a strong consensus that most patients with OHS have underlying upper airway obstruction (2, 4, 10, 22). Patients typically report a past history of snoring and witnessed apneas, and approximately 90% demonstrate upper airway obstruction on polysomnogram (4). Daytime symptomatology in OHS, such as daytime sleepiness and poor concentration, is similar to that reported in OSAS. In many instances, OHS will respond to relief of upper airway obstruction by nasal continuous positive airway pressure (CPAP). A significant proportion of patients with OHS diagnosed initially with sleep hypoventilation alone will later exhibit OSA if withdrawn from noninvasive ventilation (23). However, there are no large longitudinal studies that inform on the development of OHS over time, and therefore it is difficult to exclude the possibility that some patients can develop OHS in the absence of upper airway dysfunction at some time in the evolution of the disorder. The apnea-hypopnea index has been shown in some studies to be an independent risk factor for the development of hypercapnia (6, 8). However, the issue is likely to be more complex than the frequency of events occurring during sleep. Other authors have highlighted the importance of the duration of disordered breathing events relative to the interapnea period in the development of hypercapnia in patients with obesity and OSA. When periods of abnormal breathing are long and the time to restore ventilation is short and/or respiratory efforts are reduced during the recovery period, acute increases in CO2 can occur (24).

Recently, a model has been proposed that links this transient retention of CO2 during apneic events in sleep with the development of chronic awake hypercapnia (25). A key feature of this model includes the eventual increase in renal bicarbonate concentration to buffer these acute overnight increases in CO2. In turn, this results in depression of ventilatory responsiveness to CO2 during wakefulness, eventually promoting daytime hypoventilation (Figure 1). The greater ventilatory limitations imposed by more extreme central obesity and/or the development of decreased central respiratory drive may initiate the acute failure to compensate for sleep-disordered breathing. However, for awake CO2 to emerge, the renal compensatory mechanism must also be compromised (25). Because the presence of these risk factors can vary considerably between similarly obese patients with OSAS, this model helps to explain why only a subgroup of these individuals develop awake hypercapnia.

Sustained hypoxemia in the absence of obvious upper airway dysfunction will also occur, and the proportion of sleep time spent less than 90% (% total sleep time oxygen saturation as measured by pulse oximetry [SpO2] < 90%) has been shown to be strongly associated with the development of awake hypercapnia (8). Although it is not clear from individual studies whether hypoxia was the cause or consequence of hypercapnia, it is possible that hypoxia could interfere with the synthesis of a number of neurotransmitters involved in central respiratory control (26, 27). Although not yet studied in OHS, sustained hypoxia during sleep delays arousal in response to resistive loading in healthy nonobese males (28) and could contribute to worsening sleep hypoventilation and CO2 retention.

Ventilatory Drive

Alterations in central respiratory drive also underpin which obese patient with or without OSAS will develop daytime hypercapnia. Normally, severe obesity is associated with increased respiratory drive, which assists in maintaining eucapnia despite abnormal chest wall mechanics and high work of breathing (16, 29, 30). Those with OHS do not display this augmented drive (30, 31). In addition, ventilatory responsiveness to hypoxia and hypercapnia is diminished (3234) and does not appear to have a familial basis (35, 36). Rather, this seems to be an acquired phenomenon, with recent work highlighting the potential role of hormones or adipokines in this process.

Leptin is a circulating protein produced mainly by adipose tissue (adipokine) that interacts with hypothalamic receptors to inhibit eating. The leptin-deficient ob/ob mouse model has provided some insight into potential mechanisms of OHS. Compared with wild-type mice and other obese mouse models, ob/ob mice do not produce functioning leptin and exhibit features of OHS with impaired respiratory mechanics, depressed ventilatory responsiveness, and awake hypercapnia. They also have marked reductions in lung volumes, pulmonary compliance, and abnormal respiratory muscle function (37). Leptin replacement in these mice reverses their OHS, and in this way, leptin can be considered a respiratory stimulant.

In humans, leptin deficiency in obesity is extremely rare, and instead, similar to obesity in wild-type mice, there is a variable elevation in circulating leptin. Clearly, if these high leptin levels were biologically active there would be reduced eating and subsequently weight loss. This is obviously not the case in human obesity, and the problem instead is “leptin resistance.” This may play a role in human obesity-related breathing disorders, such as OHS or OSA. Therefore, the development of a resistance to leptin, or perhaps more correctly a “central leptin deficiency,” could contribute to the development of awake hypoventilation by altering respiratory drive output as well, affecting the mechanical properties of the lungs and chest wall, attenuating the normal compensatory mechanisms used by individuals to cope with obesity-related respiratory loads.

Plasma leptin levels are higher in OSAS or OHS compared with weight-matched control subjects without sleep-disordered breathing (38, 39). Importantly, leptin levels are a better predictor of hypercapnia than degree of adiposity (38). Higher serum leptin concentrations are also associated with both a reduced respiratory drive and a reduced response to hypercapnia in severely obese individuals (40). Until recently, the concept of using leptin to treat human obesity (and possibly OHS) has been limited by leptin resistance. New data on possible ways of overcoming leptin resistance and emerging clinical trials suggest that in the future using recombinant leptin to reverse OHS may be possible (41).

Other hormonal mechanisms may also be operative in OHS. Growth hormone increases ventilatory responsiveness (42). Lower 24-hour growth hormone levels, as indicated by lower insulin-like growth factor I (IGF-I) levels, have been observed in OHS, with a strong negative correlation between IGF-I and both PaCO2 and bicarbonate reported (33). However, the effect of recombinant growth hormone on OHS is unknown.

Patients with OHS exhibit lower daytime PaO2 and higher PaCO2 compared with those with morbid obesity or OSAS and thus spend a greater proportion of sleep time with SpO2 < 90% (4, 11, 43). In turn, this will result in more frequent presentation with peripheral edema, pulmonary hypertension, and cor pulmonale (4, 44). Patients with OHS are also more likely to report moderate to severe dyspnea compared with eucapnic patients with OSAS (45). However, clinical presentation in OHS may be identical to patients with OSAS. Consequently, the diagnosis can be overlooked not only in a sleep clinic population of patients with chronic stable disease but even during hospitalization for an acute illness (7, 46). Hospitalization rates and ongoing use of health care service requirements are significantly higher among patients with OHS compared with morbidly obese eucapnic patients (47). Despite this high contact with health care providers, there appears to be a significant delay in the diagnosis of this disorder and the institution of definitive therapy unless referral to a respiratory or sleep physician is made (7, 45, 46).

Although large-scale longitudinal observation data are not available in untreated OHS, patients with this condition are more likely than eucapnic obese patients to require admission to intensive care if hospitalized and are more likely to need invasive ventilation (7). Furthermore, 18 months after hospital discharge, mortality was 23% in a group of patients with OHS compared with 9% in those with uncomplicated obesity (7). In another study evaluating long term noninvasive ventilation, almost half those who refused therapy (7/15) had died within the follow-up period of 50 ± 25 months compared with 3 of 54 patients managed with positive pressure therapy (44).

In addition to morbidity and mortality related to respiratory complications, patients with OHS also exhibit greater cardiometabolic morbidity compared with those with OSAS or simple severe obesity. Rates of systemic hypertension, congestive heart failure, cor pulmonale, and angina are higher among patients with OHS compared with those with eucapnic obesity (47, 48). Patients with OHS are also characterized by higher insulin resistance and more frequently are treated by glucose-lowering medications (18). Given the known interaction of OSA, obesity, and cardiometabolic disease, these findings in OHS are not surprising (49, 50). In a recent prospective controlled study, an increase in the proatherogenic chemokine regulated on activation, normal T-cell expressed and secreted, lower levels of the antiatherogenic adipokine adiponectin, and impaired endothelial function were seen in patients with OHS compared with age- and BMI-matched eucapnic control subjects (18). The presence of high inflammatory markers before and after treatment for sleep-disordered breathing in OHS has also been shown to be a factor associated with poor prognosis in this population (51).

Symptoms and lifestyle limitations caused by the respiratory and obesity aspects of OHS have a significant effect on quality of life (22, 52, 53). These patients experience significant dyspnea (44, 45) and daytime sleepiness (52), which can impact on both the physical and mental/social aspects of quality of life (53). Despite improving gas exchange during sleep and wakefulness with treatment, many individuals still report low quality-of-life scores, which may reflect the ongoing impact of comorbidities present in severe obesity (22). In contrast, in a group of less obese Japanese patients with OHS (mean BMI 36 ± 9 kg/m2), improvements in all domains of the SF-36, apart from bodily pain, occurred after 3 to 6 months of CPAP therapy, with post-treatment scores similar to those reported by nonobese and healthy subjects (52). Improving quality of life is an important aspect of intervention in patients with chronic hypoventilation, including OHS, as there appears to be an association between health-related quality of life and mortality (54).

At present no classification system for severity of OHS has been established and generally accepted. However, various characteristics of the disorder have been suggested by which severity could be graded, including the degree of awake respiratory failure, BMI, complications related to OHS, or even the response to initial CPAP therapy (55). The clinical heterogeneity of OHS highlights the need for specific characterization of patient cohorts to better understand the mechanisms underlying this disorder and how this may influence the response to different therapies (56).

To diagnose OHS, an increased awake PaCO2 is required. However, because arterial blood gases are not routinely performed in sleep clinics and laboratories, and chronic presentation may be similar to uncomplicated OSAS, the disorder can be overlooked unless clinicians have a high index of suspicion. Recently published medical guidelines for patients undergoing bariatric surgery have recognized this issue and recommend that formal respiratory evaluation, including arterial blood gases, be performed as part of the preoperative assessment in those individuals with disordered sleep as well as those with super (BMI > 50) obesity (57). Despite such guidelines, there are no data on the most appropriate cost-effective clinical screening pathway for detection of OHS in patient populations.

Simple measures, such as pulse oximetry and serum bicarbonate, can be readily performed in clinics and serve as useful screening tools to identify potential candidates with OHS. A serum bicarbonate 27 mEq/L or greater has been shown to be highly sensitive (92%) although not specific (50%) for the presence of an increased CO2, and therefore clinically a helpful procedure for screening (6). Similarly, PaO2 is generally less than 70 mm Hg in OHS, particularly in those with more severe disease (11, 43). Therefore, a pulse oximetry reading of less than 94% would suggest the need for an arterial blood gas measurement to clarify the diagnosis (6).

Given the cost and limited facilities for performing full polysomnography in many countries, there is increasing use of simplified respiratory monitoring and autotitrating pressure devices for the diagnosis and treatment of OSAS. Although current clinical guidelines advise against the unattended titration of CPAP in OHS (58), it is likely that a proportion of individuals will be managed in this way simply because the disorder is not recognized. Although CPAP may be effective in some patients (2, 22, 43), sustained nocturnal desaturation and hypoventilation may persist in others, resulting in an incomplete or worsening response to therapy (43, 59). Consequently, it is important to identify patients with OHS before a CPAP titration study.

Although monitoring CO2 during sleep using transcutaneous CO2 is not widely deployed in most clinical centers, the technique can be of value in documenting the degree to which CO2 is retained, especially during REM sleep. In this way it may be a useful technique to detect prodromal OHS in obese patients. It has also been used in severely obese patients during positive airway pressure (PAP) therapy, demonstrating good agreement with PaCO2 measured from arterial blood sampling (60), although careful calibration is required to ensure accuracy of measurements. Nevertheless, there is a lack of evidence from randomized controlled trials to determine whether the diagnosis and management of patients with OHS using continuous transcutaneous monitoring of CO2 offers any clinical benefits over oxygen saturation and awake arterial blood gases measurements alone.

Current treatment approaches for OHS can be broadly classed into two areas: medical therapies, including PAP therapy designed to treat sleep-disordered breathing and improve nocturnal gas exchange, and surgical intervention to promote and maintain weight loss.

Positive Pressure Therapy

Reversal of daytime CO2 can be achieved with CPAP therapy alone when chronic hypercapnia is largely a consequence of upper airway obstruction (2, 22, 43). Flow limitation producing sustained periods of hypoventilation can also occur and again will respond to CPAP therapy if sufficient pressure levels are applied (2). In the majority of cases, titration of CPAP will be the first approach to treatment (22, 61), with the goal of normalizing oxygen saturation and preventing increases in nocturnal CO2 levels. Pressure is titrated upward to eliminate apneas, hypopneas, and snoring and to stabilize oxygen saturation, with a switch to bilevel therapy if sustained desaturation and elevated CO2 levels persist. During bilevel therapy, the level of airway pressure delivered during inspiration (IPAP) and expiration (EPAP) can be adjusted separately, with EPAP increased to eliminate obstructive events while IPAP is increased above the EPAP level to improve alveolar ventilation. A pressure difference between IPAP and EPAP of at least 6 to 7cm H2O is generally required (44, 48, 62). In many published studies, the move to bilevel therapy is undertaken within the first night of PAP titration if SpO2 of 90% or greater is not achieved despite the elimination of obstructive events. Based on this approach, previous reports have suggested that around 20 to 50% of patients with OHS will fail CPAP and require longer-term management with bilevel therapy (2, 43, 61). However, in a small study of patients with OHS randomized to either CPAP or bilevel therapy for a 3-month period, no between-group differences in awake CO2, compliance with therapy, or daytime sleepiness were found. This lack of difference was observed even though 11 of the 18 patients randomized to CPAP therapy continued to exhibit sustained oxygen desaturation between 80 and 88% during the first night of titration (22). Furthermore, after 3 months of therapy, only four of these patients continued to desaturate during sleep despite control of the upper airway, suggesting that a complete initial response to CPAP therapy may not be necessary for longer-term resolution of sleep and daytime hypoventilation. Other work has shown that better adherence with PAP therapy plays an important role in the improvement in CO2 achieved in hypercapnic patients with OSAS, irrespective of whether CPAP or bilevel therapy is used (61). After an initial period of bilevel support, some individuals can be effectively managed with CPAP alone (22, 23, 44, 63). Longer-term data will be required to provide treatment algorithms that explain which patients can be initially managed with CPAP from the outset or switched to this therapy after a period of bilevel ventilation. Longer-term outcomes would need to include quality of life, compliance, relapse, or even mortality.

Longer-term observational studies of bilevel therapy have consistently reported improved awake blood gases, daytime sleepiness, and ventilatory responsiveness to CO2, fewer hospitalizations, and better quality of life compared with baseline measures (47, 53). However, there is only limited evidence from randomized trials comparing clinical outcomes between different treatment modalities (22, 59, 64). Limitations of these trials include the small number of patients studied (1036 patients), the short-term nature of follow-up (single night to 3 mo), and comparisons only performed between different types of PAP (CPAP and bilevel, or bilevel with and without average volume–assured pressure support ventilation), in groups with relatively mild OHS based on daytime CO2 levels. The extent to which the findings of these small randomized trials can be applied to the broader OHS population requires confirmation in larger multicenter studies. Several studies have reported higher discontinuation rates among females compared with males with OHS (44, 48), although the reasons for this are unclear. Given the significantly higher mortality rate reported for patients with untreated OHS compared with those continuing on therapy (7, 44), identifying causes and ensuring alternate therapy options are offered to noncompliant PAP patients is important.

A substantial number of patients may require oxygen therapy in addition to PAP initially to maintain SpO2 greater than 90% (22, 44, 61), but once effective nocturnal ventilation is established, this need decreases significantly. However, suboptimal oxygenation (defined by percentage of recording time with SpO2 < 90%) both during wakefulness and sleep can be present in some individuals despite improved daytime blood gases, and although this did not translate into greater dyspnea or daytime sleepiness in one study (63), daytime hypoxemia both before and persisting after the initiation of PAP therapy has been shown to be an independent predictor of poor survival in OHS (51). To date, outcomes research in OHS has been limited by lack of agreement on endpoints of therapy. We do not really know what constitutes a normal awake PCO2 level in these patients or what represents an acceptable or safe level of oxygen desaturation level during sleep. One important future research agenda is to define and provide evidence supporting treatment goals in patients with OHS.

Even when patients are adherent to PAP therapy, up to 25% may remain hypercapnic with an awake CO2 greater than 45 mm Hg. This needs to be qualified, because adherence is frequently defined as more than 4 or 4.5 hours of PAP therapy during sleep (61). Given that the average sleep length in most individuals is at least 6 to 7 hours per night, some adherent patients will be exposed to residual hypoventilation, and therefore incomplete reversal of awake hypercapnia may occur. In cases in which severe daytime gas exchange abnormalities persist, or the patient is intolerant of mask PAP therapy, alternate treatment options need to be considered.


A reduction in nocturnal obstructive events and normalization of awake CO2 can be achieved in some patients with severe OHS after undergoing a tracheostomy (65, 66). However, daytime hypoventilation may persist (65, 66), whereas in others external blockage of the tracheostomy by excess adipose tissue around the neck can also occur (67). Therefore, although a tracheostomy may be needed in a small subset of patients with OHS who are intolerant of mask PAP therapy and who exhibit life-threatening complications, it may not be a viable long-term option for therapy.

Bariatric Surgery

Early case studies reported significant improvements in lung function, central respiratory drive, and daytime CO2 with weight loss achieved through dietary management (68, 69). Although weight loss is an important long-term goal in the management of OHS, it is often difficult to achieve and maintain by medical management, and although sleep-disordered breathing is often improved it is rarely cured.

Bariatric surgery is the most effective approach to achieving more substantial degrees of weight loss and maintaining this loss over longer periods (70). Although a number of studies have shown significant improvements in lung volumes, especially expiratory reserve volume (71, 72), and gas exchange (73) can be achieved with surgical weight loss, few of these studies have specifically looked at outcomes in patients with OHS (71, 74). Most commonly, bariatric surgery is performed in morbidly obese women, in whom the prevalence of OHS is lower than in men (45). In a study looking at the prevalence of sleep-disordered breathing in consecutive premenopausal women presenting for bariatric surgery, 8% of the study group were found to have OHS (75). However, details regarding postoperative complications and longer-term outcomes were not provided. Although significant improvements in sleep-disordered breathing occur after surgical weight loss, a recent metaanalysis found that the apnea-hypopnea index remained high in many individuals (76). The presence of persisting sleep-disordered breathing is often not recognized, and patients may be reluctant to undergo a postsurgical sleep breathing evaluation if they perceive their sleep symptoms have resolved (77). Consequently, this could lead to a substantial number of patients ceasing PAP therapy prematurely after surgery (78).

The surgical approach to weight loss is being increasingly used in super obese (BMI > 50 kg/m2) and super-super obese individuals (> 60 kg/m2) (79, 80). Although significant and sustained weight loss in these groups was reported, only one in three patients achieved a reduction in BMI below 40 kg/m2 (79). The need for and compliance with PAP therapy in these super obese patients postsurgery has not been addressed, nor have longer-term cardiovascular outcomes been adequately monitored. Given general data on the relatively low level of complications of bariatric surgery in experienced centers (80), consideration should be given to more detailed research involving this approach in the long-term management of OHS.

Although we are developing a better understanding of the complex interactions involved in the development of OHS, there is still the need for earlier diagnosis and more effective management and follow-up strategies. This will require better understanding of the epidemiology of OHS and particularly which patients with obesity and sleep-disordered breathing develop this condition over time. The emergence of an increasing number of super obese individuals will pose management challenges in the near future. Although improved breathing during sleep and wakefulness can generally be achieved with PAP therapy, the lack of large-scale, well-designed studies evaluating long-term outcomes with various therapies has limited the development of evidence-based treatment recommendations. Endpoints of therapy need to be defined by appropriate clinical trials. This will require a coordinated multicenter approach. Given the significant morbidity and mortality associated with untreated or poorly managed disease, more information is needed regarding barriers to using therapy and outcomes for patients in whom response to therapy is incomplete. Finally, the extent to which surgical and new nonsurgical weight-loss approaches impact on longer-term outcomes in OHS compared with PAP therapy warrants further evaluation.

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Correspondence and requests for reprints should be addressed to Amanda Piper, Ph.D., Sleep Unit, Level 11, Building 75, Royal Prince, Alfred Hospital, Missenden Road, Camperdown, NSW 2050 Australia. E-mail:


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