Chronic hypercapnia in obstructive sleep apnea-hypopnea syndrome

Factors Associated with the Efficiency of Home Non-Invasive Ventilation in Patients with Obesity-Hypoventilation Syndrome in Martinique

Obesity-hypoventilation syndrome (OHS) is a respiratory complication of obesity characterized by chronic hypercapnic respiratory failure. It is often associated with several comorbidities and is treated by positive airway pressure (PAP) therapy. This study aimed to identify factors associated with persistent hypercapnia in patients receiving home non-invasive ventilation (NIV). We performed a retrospective study including patients with documented OHS. In total, 143 patients were included (79.7% women, age 67 ± 15.5 years, body mass index 41.6 ± 8.3 kg/m²). After 4.6 ± 4.0 years of follow-up, 72 patients (50.3%) remained hypercapnic. In bivariable analysis, clinical data showed no difference in follow-up duration, number of comorbidities, comorbidities, or circumstance of discovery. Patients with persistent hypercapnia on NIV were generally older, with lower BMI and more comorbidities. (5.5 ± 1.8 versus 4.4 ± 2.1, p = 0.001), female sex (87.5% versus 71.8%), was treated by NIV (100% versus 90.1%, p < 0.01), had lower FVC (56.7 ± 17.2 versus 63.6 ± 18% of theoretical value, p = 0.04), lower TLC (69.1 ± 15.3 versus 74.5 ± 14.6% of theoretical value, p = 0.07), lower RV (88.4 ± 27.1 versus 102.5 ± 29.4% of theoretical value, p = 0.02), higher pCO2 at diagnosis (59.7 ± 11.7 versus 54.6 ± 10.1 mmHg, p = 0.01) and lower pH (7.38 ± 0.03 versus 7.40 ± 0.04, p = 0.007), higher pressure support (12.6 ± 2.6 versus 11.5 ± 2.4 cmH₂O, p = 0.04) and lower EPAP (8.2 ± 1.9 versus 9 ± 2.0 cmH₂O, p = 0.06). There was no difference in non-intentional leaks and daily use between patients between both groups. By multivariable analysis, sex, BMI, pCO2 at diagnosis, and TLC were independent risk factors for persistent hypercapnia on home NIV. In individuals with OHS, persistent hypercapnia on home NIV therapy is frequent. Sex, BMI, pCO2 at diagnosis, and TLC were all associated with an increased risk of persistent hypercapnia in persons treated with home NIV.

Autonomic and respiratory consequences of altered chemoreflex function: clinical and therapeutic implications in cardiovascular diseases

The importance of chemoreflex function for cardiovascular health is increasingly recognized in clinical practice. The physiological function of the chemoreflex is to constantly adjust ventilation and circulatory control to match respiratory gases to metabolism. This is achieved in a highly integrated fashion with the baroreflex and the ergoreflex. The functionality of chemoreceptors is altered in cardiovascular diseases, causing unstable ventilation and apnoeas and promoting sympathovagal imbalance, and it is associated with arrhythmias and fatal cardiorespiratory events. In the last few years, opportunities to desensitize hyperactive chemoreceptors have emerged as potential options for treatment of hypertension and heart failure. This review summarizes up to date evidence of chemoreflex physiology/pathophysiology, highlighting the clinical significance of chemoreflex dysfunction, and lists the latest proof of concept studies based on modulation of the chemoreflex as a novel target in cardiovascular diseases.

Respiratory complications of obesity: from early changes to respiratory failure

Obesity is a significant and increasingly common cause of respiratory compromise. It causes a decrease in static and dynamic pulmonary volumes. The expiratory reserve volume is one of the first to be affected. Obesity is associated with reduced airflow, increased airway hyperresponsiveness, and an increased risk of developing pulmonary hypertension, pulmonary embolism, respiratory tract infections, obstructive sleep apnoea and obesity hypoventilation syndrome. The physiological changes caused by obesity will eventually lead to hypoxic or hypercapnic respiratory failure. The pathophysiology of these changes includes a physical load of adipose tissue on the respiratory system and a systemic inflammatory state. Weight loss has clear, well-defined benefits in improving respiratory and airway physiology in obese individuals.

Elucidating Predictors of Obesity Hypoventilation Syndrome in a Large Bariatric Surgery Cohort

Rationale: Although understanding predictors of obesity hypoventilation syndrome (OHS), a condition associated with increased morbidity and mortality, is of key importance for risk prediction, existing characterization is limited.Objectives: We hypothesize that OHS patients referred for bariatric surgery have more severe obstructive sleep apnea and metabolic derangements compared with their eucapnic counterparts.Methods: A total of 1,718 patients undergoing polysomnography with end-tidal CO₂ monitoring prior to bariatric surgery at Cleveland Clinic from September 2011 to September 2018 were included. OHS was defined by body mass index (BMI) ≥ 30 kg/m² and either polysomnography-based end-tidal CO₂ ≥ 45 mm Hg or serum bicarbonate levels ≥ 27 mEq/L based on the updated European Respiratory Society guidelines. Unadjusted and multivariable logistic regression models (odds ratio; 95% confidence interval) were used to examine OHS predictors consisting of factors in domains of patient characteristics, polysomnography (cardiorespiratory and sleep architecture), laboratory, and metabolic parameters.Results: The analytic sample comprised 1,718 patients with the following characteristics: age of 45.3 ± 12.1 years, 20.7% were male, BMI = 48.6 ± 9 kg/m², and 63.6% were white individuals. OHS prevalence was 68.4%. Unadjusted analyses revealed a 1.5% increased odds of OHS (1.01; 1.00-1.03) per 1-unit BMI increase, 1.7% (1.02; 1.01-1.02) per 1% increase in sleep time Sa_O2 < 90%, 12% increase (1.12; 1.03-1.22) per 1-U increase in hemoglobin A1c, and 3.4% increased odds (1.03; 1.02-1.05) per 5-U increase in apnea-hypopnea index. The association of apnea-hypopnea index with OHS persisted after adjustment for age, sex, race, and BMI and its comorbidities (1.02; 1.01-1.04).Conclusions: OHS was highly prevalent in patients referred for bariatric surgery by more than two-thirds. Even after consideration of confounders including obesity, obstructive sleep apnea remained a strong OHS predictor, as were increasing age, male sex, nocturnal hypoxia, and impaired long-term glucose control. These findings can inform OHS risk stratification in bariatric surgery and set the stage for experimental studies to examine sleep-related respiratory and metabolic contributions to hypoventilation.

Vertical sleeve gastrectomy improves ventilatory drive through a leptin-dependent mechanism

Obesity hypoventilation syndrome (OHS) is a serious disorder characterized by daytime hypercapnia, disordered breathing, and a reduction in chemosensitivity. Vertical sleeve gastrectomy (VSG), a bariatric surgical procedure resulting in weight loss and weight-independent improvements in glucose metabolism, has been observed to substantially improve sleep-disordered breathing. However, it is unclear if the ventilatory effects of VSG are secondary to weight loss or the marked change in metabolic physiology. Using preclinical mouse models, we found that VSG leads to an improvement in the hypercapnic ventilatory response (HCVR) and reductions in circulating leptin levels independent of reductions in body mass, fat mass, and caloric intake. In the absence of leptin, VSG continues to improve body mass, fat mass, and glucose tolerance in ob/ob mice but no longer affects HCVR. However, the HCVR of ob/ob mice can be returned to wild-type levels with leptin treatment. These data demonstrate that VSG improves chemosensitivity and ventilatory drive via a leptin-dependent mechanism. Clinically, these data downgrade the relative contribution of physical, mechanical load in the pathogenesis of OHS, and instead point to physiological components of obesity, including alterations in leptin signaling, as key drivers in OHS.

Ventilatory responses to acute hypoxia and hypercapnia in humans with a patent foramen ovale

Subjects with a patent foramen ovale (PFO) have blunted ventilatory acclimatization to high altitude compared with subjects without PFO. The blunted response observed could be because of differences in central and/or peripheral respiratory chemoreflexes. We hypothesized that compared with subjects without a PFO (PFO-), subjects with a PFO (PFO+) would have blunted ventilatory responses to acute hypoxia and hypercapnia. Sixteen PFO+ subjects (9 female) and 15 PFO- subjects (8 female) completed four 20-min trials on the same day: 1) normoxic hypercapnia (NH), 2) hyperoxic hypercapnia (HH), 3) isocapnic hypoxia (IH), and 4) poikilocapnic hypoxia (PH). Hypercapnic trials were completed before the hypoxic trials, the order of the hypercapnic (NH & HH) and hypoxic (IH & PH) trials were randomized, and trials were separated by ≥40 min. During the NH trials but not the HH trials subjects who were PFO+ had a blunted hypercapnic ventilatory response compared with subjects who were PFO- (1.41 ± 0.46 l·min^-1·mmHg^-1 vs. 1.98 ± 0.71 l·min^-1·mmHg^-1, P = 0.02). There were no differences between the PFO+ and PFO- subjects with respect to the acute hypoxic ventilatory response during IH and PH trials. Hypoxic ventilatory depression was similar between subjects who were PFO+ and PFO- during IH. These data suggest that compared with subjects who were PFO-, subjects who were PFO+ have normal ventilatory chemosensitivity to acute hypoxia but blunted ventilatory chemosensitivity to carbon dioxide, possibly because of reduced carbon dioxide sensitivity of either the central and/or the peripheral chemoreceptors. NEW & NOTEWORTHY Patent foramen ovale (PFO) is found in ~25%-40% of the population. The presence of a PFO appears to be associated with blunted ventilatory responses during acute exposure to normoxic hypercapnia. The reason for this blunted ventilatory response during acute exposure to normoxic hypercapnia is unknown but may suggest differences in either central and/or peripheral chemoreflex contribution to hypercapnia.

The Bidirectional Relationship Between Obstructive Sleep Apnea and Metabolic Disease

Obstructive sleep apnea (OSA) is a common sleep disorder, effecting 17% of the total population and 40-70% of the obese population (1, 2). Multiple studies have identified OSA as a critical risk factor for the development of obesity, diabetes, and cardiovascular diseases (3-5). Moreover, emerging evidence indicates that metabolic disorders can exacerbate OSA, creating a bidirectional relationship between OSA and metabolic physiology. In this review, we explore the relationship between glycemic control, insulin, and leptin as both contributing factors and products of OSA. We conclude that while insulin and leptin action may contribute to the development of OSA, further research is required to determine the mechanistic actions and relative contributions independent of body weight. In addition to increasing our understanding of the etiology, further research into the physiological mechanisms underlying OSA can lead to the development of improved treatment options for individuals with OSA.

Obesity hypoventilation syndrome in the critically ill

This article summarizes available data on the obesity hypoventilation syndrome and its pertinence to intensivists, outlines clinical and pathophysiologic aspects of the disease, discusses multidisciplinary treatments, and reviews the available literature on outcomes specific to the critically ill patient.

Obstructive sleep apnea in the perioperative setting: complications and management strategies

Obstructive sleep apnea (OSA) is frequently encountered in an undiagnosed, untreated state in perioperative patients. It increases the risk of respiratory, cardiac, and infectious complications following surgical procedures. Patients with OSA may require additional monitoring, unplanned escalations in care, and prolonged hospitalization. It is important to identify patients at risk for OSA during the preoperative assessment so that appropriate anesthesia, postsurgical monitoring, and pain control can be planned. Herein, we discuss data regarding perioperative outcomes in patients with OSA, methods to quickly identify patients at high risk for OSA, and implementation of clinical safeguards to minimize OSA-associated complications. An algorithm is provided to guide the perioperative management of patients with OSA.

Sleep disorders in morbid obesity

The increasing prevalence of obesity has lead to an increase in the prevalence of sleep disordered breathing in the general population. The disproportionate structural characteristics of the pharyngeal airway and the diminished neural regulation of the pharyngeal dilating muscles during sleep predispose the obese patients to pharyngeal airway collapsibility. A subgroup of obese apneic patients is unable to compensate for the added load of obesity on the respiratory system, with resultant daytime hypercapnia. Weight loss using dietary modification and life style changes is the safest approach to reducing the severity of sleep apnea, but its efficacy is limited on the long run. Although it has inherent risks, bariatric surgery provides the most immediate result in alleviating sleep apnea. Obesity has been linked also to narcolepsy. The loss of neuropeptides co-localized in hypocretin neurons is suggested as the potential mechanism. Poor sleep quality, which leads to overall sleep loss and excessive daytime sleepiness has also become a frequent complaint in this population. Identifying abnormal nocturnal eating is critically important for patient care. Both sleep related eating disorder and night eating syndrome are treatable and represent potentially reversible forms of obesity.

Obesity hypoventilation syndrome

Obesity is becoming a major medical concern in several parts of the world, with huge economic impacts on health- care systems, resulting mainly from increased cardiovascular risks. At the same time, obesity leads to a number of sleep-disordered breathing patterns like obstructive sleep apnea and obesity hypoventilation syndrome (OHS), leading to increased morbidity and mortality with reduced quality of life. OHS is distinct from other sleep- related breathing disorders although overlap may exist. OHS patients may have obstructive sleep apnea/hypopnea with hypercapnia and sleep hypoventilation, or an isolated sleep hypoventilation. Despite its major impact on health, this disorder is under-recognized and under-diagnosed. Available management options include aggressive weight reduction, oxygen therapy and using positive airway pressure techniques. In this review, we will go over the epidemiology, pathophysiology, presentation and diagnosis and management of OHS.

Assessment and management of patients with obesity hypoventilation syndrome

Obesity hypoventilation syndrome (OHS) is characterized by obesity, daytime hypercapnia, and sleep-disordered breathing in the absence of significant lung or respiratory muscle disease. Compared with eucapnic morbidly obese patients and eucapnic patients with sleep-disordered breathing, patients with OHS have increased health care expenses and are at higher risk of developing serious cardiovascular disease leading to early mortality. Despite the significant morbidity and mortality associated with this syndrome, diagnosis and institution of effective treatment occur late in the course of the syndrome. Given that the prevalence of extreme obesity has increased considerably, it is likely that clinicians will encounter patients with OHS in their clinical practice. Therefore maintaining a high index of suspicion can lead to early recognition and treatment reducing the high burden of morbidity and mortality and related health care expenditure associated with undiagnosed and untreated OHS. In this review we define the clinical characteristics of the syndrome and review the pathophysiology, morbidity, and mortality associated with it. Last, we discuss currently available treatment modalities.

Obesity hypoventilation syndrome: prevalence and predictors in patients with obstructive sleep apnea

Patients with obesity hypoventilation syndrome (OHS) have a lower quality of life, more healthcare expenses, a greater risk of pulmonary hypertension, and a higher mortality compared to eucapnic patients with obstructive sleep apnea (OSA). Despite significant morbidity and mortality associated with OHS, it is often unrecognized and treatment is frequently delayed. The objective of this observational study was to determine the prevalence of OHS in patients with OSA seen at the sleep disorders clinic of a large public urban hospital serving predominantly minority population and to identify clinical--not mechanistic--predictors that should prompt clinicians to measure arterial blood gases. In the first stage, we randomly selected 180 patients referred to our sleep disorders clinic between 2000 and 2004 for suspicion of OSA. From this retrospective random sample we calculated the prevalence of OHS in patients with OSA and identified independent clinical predictors using logistic regression. In the second stage, we prospectively validated these predictors in a sample of 410 consecutive patients referred to the sleep disorders clinic for suspicion of OSA between 2005 and 2006. The prevalence of OHS in patients with OSA was 30% in the retrospective random sample and 20% in the prospective sample. Three variables independently predicted OHS in both samples: serum bicarbonate level (p < 0.001), apnea-hypopnea index (p = 0.006), and lowest oxygen saturation during sleep (p < 0.001). Due to the serious morbidity associated with OHS, we selected a highly sensitive threshold of serum bicarbonate level. A threshold of 27 mEq/l had a sensitivity of 92% and a specificity of 50%. Only 3% of patients with a serum bicarbonate level <27 mEq/l had hypercapnia compared to 50% with a serum bicarbonate > or =27 mEq/l. In conclusion, OHS is common in severe OSA. A normal serum bicarbonate level excludes hypercapnia and an elevated serum bicarbonate level should prompt clinicians to measure arterial blood gases.

Acetazolamide Improves Central Sleep Apnea in Heart Failure

RATIONALE

Acetazolamide is a mild diuretic and a respiratory stimulant. It is used to treat periodic breathing at high altitude.

OBJECTIVES

To determine the therapeutic efficacy of acetazolamide on central sleep apnea associated with heart failure.

METHODS

Twelve male patients with stable systolic heart failure whose initial polysomnograms showed more than 15 episodes per hour of apnea and hypopnea participated in the study. The patients were randomized to a double-blind cross-over protocol with acetazolamide or placebo, taken 1 h before bedtime for six nights with 2 wk of washout.

MEASUREMENTS

Polysomnography, pulmonary function tests, arterial blood gases, and left ventricular ejection fraction were obtained initially along with a sleep questionnaire, history, and physical examination. Baseline measurements were repeated at the end of each arm.

MAIN RESULTS

There were no significant differences between parameters at baseline and placebo. In comparing placebo with acetazolamide, the hourly number of episodes of central apnea (49 +/- 28 vs. 23 +/- 21 [mean +/- SD]; p = 0.004) and the percentage of total sleep time spent below an arterial oxyhemoglobin saturation of 90% (19 +/- 32 vs. 6 +/- 13%; p = 0.01) decreased significantly. Acetazolamide improved subjective perception of overall sleep quality (p = 0.003), feeling rested on awakening (p = 0.007), daytime fatigue (p = 0.02), and falling asleep unintentionally during daytime (p = 0.002).

CONCLUSIONS

In patients with heart failure, administration of a single dose of acetazolamide before sleep improves central sleep apnea and related daytime symptoms.

Transition from acute to chronic hypercapnia in patients with periodic breathing: predictions from a computer model

Acute hypercapnia may develop during periodic breathing from an imbalance between abnormal ventilatory patterns during apnea and/or hypopnea and compensatory ventilatory response in the interevent periods. However, transition of this acute hypercapnia into chronic sustained hypercapnia during wakefulness remains unexplained. We hypothesized that respiratory-renal interactions would play a critical role in this transition. Because this transition cannot be readily addressed clinically, we modified a previously published model of whole-body CO2 kinetics by adding respiratory control and renal bicarbonate kinetics. We enforced a pattern of 8 h of periodic breathing (sleep) and 16 h of regular ventilation (wakefulness) repeated for 20 days. Interventions included varying the initial awake respiratory CO2 response and varying the rate of renal bicarbonate excretion within the physiological range. The results showed that acute hypercapnia during periodic breathing could transition into chronic sustained hypercapnia during wakefulness. Although acute hypercapnia could be attributed to periodic breathing alone, transition from acute to chronic hypercapnia required either slowing of renal bicarbonate kinetics, reduction of ventilatory CO2 responsiveness, or both. Thus the model showed that the interaction between the time constant for bicarbonate excretion and respiratory control results in both failure of bicarbonate concentration to fully normalize before the next period of sleep and persistence of hypercapnia through blunting of ventilatory drive. These respiratory-renal interactions create a cumulative effect over subsequent periods of sleep that eventually results in a self-perpetuating state of chronic hypercapnia.

Short-term and Long-term Effects of Nasal Intermittent Positive Pressure Ventilation in Patients With Obesity-Hypoventilation Syndrome

OBJECTIVE

To assess the outcome of 54 patients with obesity-hypoventilation syndrome (OHS) who were treated with nasal intermittent positive-pressure ventilation (NIPPV).

DESIGN

Descriptive analysis of retrospectively collected clinical data.

MATERIALS AND METHODS

From March 1995 to December 2002, OHS (defined as a body mass index [BMI] of > 30 kg/m2, a daytime Pa(CO2) of > or = 50 mm Hg, and a Pa(O2) of < 60 mm Hg in the absence of COPD) was diagnosed in 69 cases. Fifteen patients (21.7%) refused to be treated with NIPPV, and 20 patients were started on NIPPV therapy electively and 34 following an acute exacerbation. We employed daytime arterial blood gas values and overnight pulse oximetry to determine the initial NIPPV modes and settings. The outcome measures were survival, clinical status, and arterial blood gas levels.

RESULTS

Among the 54 study patients (18 women), the mean (+/- SD) age was 56 +/- 13 years. The mean BMI was 44 +/- 8.8 kg/m2. Sleep apnea syndrome (apnea-hypopnea index, > 5) was present in 87% of the patients. At presentation, 22 of the 54 patients had experienced an acute hypercapnic respiratory failure (pH < 7.34). None of these patients required orotracheal intubation after NIPPV treatment. Initially, 2 patients were treated with volume-preset ventilation, 49 patients used pressure-preset equipment, and 3 patients employed continuous positive airway pressure (CPAP). Forty-seven patients required supplemental oxygen. At the end of the follow-up period (mean duration, 50 months), Pa(O2) had increased by 24 mm Hg from baseline (95% confidence interval [CI], 21 to 28 mm Hg; p < 0.0001) and Pa(CO2) had decreased by 17 mm Hg (95% CI, 13 to 20 mm Hg; p < 0.0001). NIPPV therapy improved subjective sleepiness (mean Epworth sleepiness scale score decrease, 16 +/- 5 to 6 +/- 2; p < 0.001), and dyspnea decreased in all but four patients. During follow-up, three patients died (one of them due to the progression of respiratory failure). NIPPV therapy could be withdrawn in 5 patients who had achieved a sufficient weight loss, and the conditions of 16 patients could be maintained without respiratory failure by the use of simple therapy with CPAP.

CONCLUSIONS

NIPPV therapy is effective in the treatment of patients with OHS, providing a significant improvement in clinical status and gas exchange.

Hypocapnia Is Not a Predictor of Central Sleep Apnea in Patients with Cirrhosis

During sleep, maintenance of rhythmic breathing is critically dependent on the level of PCO(2), such that if the prevailing spontaneous PCO(2) decreases below the apneic threshold, central sleep apnea (CSA) occurs. Several studies have shown that in patients with systolic heart failure (SHF), presence of a low, awake arterial PCO(2) (Pa(CO(2))) increases the likelihood of developing CSA during sleep. We therefore sought to determine if a low Pa(CO(2)) is a predictor of CSA in patients with cirrhosis of the liver and with normal left ventricular systolic function. In 13 hypocapnic (Pa(CO(2)) < 36 mm Hg, mean = 33 mm Hg) patients with SHF and a mean left ventricular ejection fraction of 23%, the mean apnea-hypopnea index, was 28/hour. CSA accounted for most of the breathing disorders. In 10 hypocapnic (Pa(CO(2)) < 36 mm Hg, mean = 32 mm Hg) patients with cirrhosis and a normal left ventricular ejection fraction (60%), the mean apnea-hypopnea index was 2/hour. The maximum central apnea index was 0.2/hour. There were no significant differences in age, demographics, pulmonary function tests, Pa(O(2)), Pa(CO(2)), minute and alveolar ventilation, and ventilatory responses to CO(2) between the two groups. We conclude that, in contrast to SHF, presence of hypocapnia does not predict CSA in cirrhosis.

Respiratory Failure

Respiratory failure is defined as a failure in gas exchange due to an impaired respiratory system--either pump or lung failure, or both. The hallmark of respiratory failure is impairment in arterial blood gases. This review describes the mechanisms leading to respiratory failure, the indices that can be used to better describe gas exchange abnormalities and the physiologic and clinical consequences of these abnormalities. The possible causes of respiratory failure are then briefly mentioned and a quick reference to the clinical evaluation of such patients is made. Finally treatment options are briefly outlined for both acute and chronic respiratory failure.

Effect of obesity and/or sleep apnea on chemosensitivity: differences between men and women

The purpose of this study was to identify if obesity or obstructive sleep apnea (OSA) influence hypercapnic response (HCR) and hypoxic response (HR) taking into account differences between gender. Data are expressed in % predicted based on reference values of our laboratory ('Respir. Physiol. 113 (1998) 157'). Obese women without OSA (n=117, body mass index (BMI) 43+/-8 kg/m(2)) demonstrated an increased ventilatory (VE) and occlusion pressure (P(0.1)) HCR and HR slope: VE HCR 113 (NS), VE HCR/vital capacity (VC) 126 (P<0.05), P(0.1) HCR 130 (P<0.05), VE HR 136 (P<0.05), VE HR/VC 154 (P<0.001) and P(0.1) HR 210 (P<0.001) % predicted. Obese women with OSA (n=34, BMI 42+/-9 kg/m(2)) presented similar increased values for HCR and even more increased values for HR than obese women without OSA matched for age, height and BMI: VE HR 155 (P=NS), VE HR/VC 205 (P<0.05) and P(0.1) HR 273 (P<0.05) % predicted. In obese men (without or with OSA) HCR and HR values were similar to our reference values.

Hypercapnia and ventilatory periodicity in obstructive sleep apnea syndrome

Prevention of acute hypercapnia during obstructive events in obstructive sleep apnea requires a balance between carbon dioxide (CO(2)) loading during the event and CO(2) unloading in the interevent period. Earlier studies have demonstrated that acute CO(2) retention may occur despite high interevent ventilation when the interevent duration is short relative to the duration of the preceding event. The present study examines the relationship between apnea and interapnea durations and relates this assessment of ventilatory periodicity to the degree of chronic hypercapnia in subjects with severe sleep apnea. A total of 18 subjects with sleep apnea (> 40 apnea/hour; chronic awake Pa(CO2) 36-62 mm Hg) and without underlying lung disease underwent polysomnography. For each event, apnea duration, interapnea duration, and apnea/interapnea duration ratio were determined. No relationship was observed between chronic Pa(CO2) and mean apnea or interapnea duration (p > 0.1). However, Pa(CO2) was directly related to apnea/interapnea duration ratio (r = 0.48; p < 0.05) such that with increasing chronic hypercapnia the interapnea duration shortens relative to the apnea duration. The present study suggests that control of the interapnea ventilatory duration relative to the duration of the preceding apnea, is an important component of the integrated ventilatory response to CO(2) loading during apnea and may contribute toward the development and/or maintenance of chronic hypercapnia in obstructive sleep apnea/hypopnea syndrome.

Postevent ventilation as a function of CO(2) load during respiratory events in obstructive sleep apnea

Maintenance of eucapnia during sleep in obstructive sleep apnea (OSA) requires a balance between CO(2) loading during apnea and CO(2) elimination. This study examines individual respiratory events and relates magnitude of postevent ventilation to CO(2) load during the preceding respiratory event in 14 patients with OSA (arterial PCO(2) 42-56 Torr). Ventilation and expiratory CO(2) and O(2) fractions were measured on a breath-by-breath basis during daytime sleep. Calculations included CO(2) load during each event (metabolic CO(2) production - exhaled CO(2)) and postevent ventilation in the 10 s after an event. In 12 of 14 patients, a direct relationship existed between postevent ventilation and CO(2) load during the preceding event (P < 0.05); the slope of this relationship varied across subjects. Thus the postevent ventilation is tightly linked to CO(2) loading during each respiratory event and may be an important mechanism that defends against development of acute hypercapnia in OSA. An inverse relationship was noted between this postevent ventilatory response slope and the chronic awake arterial PCO(2) (r = 0.90, P < 0.001), suggesting that this mechanism is impaired in patients with chronic hypercapnia. The link between development of acute hypercapnia during respiratory events asleep and maintenance of chronic awake hypercapnia in OSA remains to be further investigated.

Hypercapnia in overlap syndrome: possible determinant factors

We retrospectively evaluated data from 213 consecutive patients; 152 were affected by obstructive sleep apnea (OSA), 29 had OSA associated with chronic obstructive pulmonary disease (COPD), also known as overlap syndrome, and 32 had COPD. Patients with obesity-hypoventilation syndrome were not included. The aims of the study were to evaluate the anthropometric, pulmonary, and polysomnographic characteristics of patients affected by overlap syndrome compared to "simple" OSA and to COPD subjects and to analyze the determinants of hypercapnia in overlap syndrome. In the comparison between overlap and OSA patients, the overlap group had a significantly higher PaCO2 (44.59 vs. 39.22 mm Hg; p < 0.01), in the presence of a similar AHI (40.46 vs. 41.59/h). Comparing overlap to COPD patients, overlap showed a significantly higher PaCO2 value (44.59 vs. 39.63 mm Hg; p < 0.005) and had significantly less severe obstructive impairment (FEV 162.93 vs. 47.31%; FEV1/FVC ratio 66.71 vs. 59.25%; p < 0.005). Anthropometric, pulmonary function, and polysomnographic data did not differ between normo- and hypercapnic overlap patients. The best model (stepwise multiple regression analysis) for predicting PaCO2 in overlap patients showed r2 value 0.65: PaO2 contributed to 38%, FEV1 to 15%, and weight to 12%. In conclusion, the occurrence of hypercapnia in overlap patients is only partially explained by the combination of overweight and reduced respiratory function, supporting the hypothesis of a multifactorial genesis.

Determinants of chronic hypercapnia in Japanese men with obstructive sleep apnea syndrome

STUDY OBJECTIVE

To identify the determinants of chronic hypercapnia (ie, PaCO(2), > or = 45 mm Hg) in men with obstructive sleep apnea syndrome (OSAS) without airflow obstruction.

DESIGN

An analysis was conducted of 143 male patients with OSAS, which had been diagnosed by polysomnography (PSG), who had been referred to a university hospital. Patients were classified as hypercapnic (ie, PaCO(2), > or = 45 mm Hg) and normocapnic (ie, PaCO(2), < 45 mm Hg), and obese (ie, body mass index [BMI], > or = 30 kg/m(2)) or nonobese (ie, BMI, < 30 kg/m(2)). Patients with airflow obstruction (ie, FEV(1)/FVC ratio, < 70%) were excluded from the study. Baseline clinical characteristics, pulmonary function, PSG data, and blood gas data were compared between hypercapnic and normocapnic patients. Correlations between PaCO(2) and several anthropometric, respiratory, and polysomnographic variables were determined by stepwise multiple regression analysis.

RESULTS

Fifty-five patients (38%) were hypercapnic. Hypercapnic patients were younger and heavier, and had more abnormalities on pulmonary and PSG testing. Stepwise multiple regression analysis revealed that the PaCO(2) level was influenced significantly by the mean level of arterial oxygen saturation (SaO(2)) during sleep and by the percent of vital capacity (%VC) (R(2) = 0.430; p < 0.0001), indicating that 43% of the total variance in the PaCO(2) could be explained by the mean SaO(2) and %VC in hypercapnic patients. In contrast, only 13% of the total variance in the PaCO(2) was accounted for by the mean SaO(2) and BMI in normocapnic patients (R(2) = 0.134; p = 0.0034). The mean SaO(2), %VC, and PaO(2) were selected as independent variables for predicting the PaCO(2) in obese patients. These variables explained 41% of the total variance in the PaCO(2) (R(2) = 0.407; p < 0.0001), whereas the mean SaO(2) only accounted for 13% of the total variance in PaCO(2) levels in nonobese patients (R(2) = 0.134; p = 0.0064).

CONCLUSION

Nocturnal desaturation and restrictive pulmonary impairment play major roles in determining the PaCO(2) in hypercapnic and obese OSAS patients without airflow obstruction.

The obesity-hypoventilation syndrome revisited: a prospective study of 34 consecutive cases

STUDY OBJECTIVES

Obesity has many detrimental effects on the respiratory function and may lead to chronic hypoventilation in some patients, an association known as the obesity-hypoventilation syndrome (OHS). In many cases, patients with OHS also have sleep apneas. Hereafter, we describe several features of a cohort (n = 34) of patients with OHS and show the comparisons with a large cohort (n = 220) of patients with obstructive sleep apnea syndrome (OSAS). We compare also OHS patients with a group of patients with the association of OSAS and COPD, also known as "overlap" patients.

DESIGN

Descriptive analysis of prospectively collected clinical data.

SETTING

Respiratory care unit and sleep laboratory of university hospital.

RESULTS

In OHS patients, OSAS was present in most of the cases (23 of 26 patients). However, in three patients, OHS was not associated with OSAS, showing that obesity per se may lead to chronic hypoventilation. As expected by definition, OHS patients had, on average the worst diurnal arterial blood gas measurements, compared to the other groups. For the OHS patients, the mean diurnal PaO(2) was 59 +/- 7 mm Hg, which was significantly different from the PaO(2) of the OSAS patients (75 +/- 10 mm Hg; p = 0,001) but also from the overlap patients (66 +/- 10 mm Hg; p = 0.015). Pulmonary hypertension (ie, mean pulmonary artery pressure > 20 mm Hg) was more frequent in OHS patients than in "pure" OSAS patients (58% vs 9%; p = 0.001).

CONCLUSION

Patients with OSAS and chronic respiratory insufficiency had in most cases an associated OHS or COPD. Patients with OHS were older than patients with pure OSAS. They had mild-to-moderate degrees of restrictive ventilatory pattern due to obesity. Severe gas exchange impairment and pulmonary hypertension were quite frequent. The association of OHS and OSAS was the rule. However, in three patients, OHS was not associated with OSAS, suggesting that OHS is an autonomous disease.

Hypercapnic ventilatory response in patients with and without obstructive sleep apnea: do age, gender, obesity, and daytime PaCO(2) matter?

STUDY OBJECTIVE

To evaluate the relationship between obstructive sleep apnea (OSA) and ventilatory responsiveness to carbon dioxide in both men and women.

DESIGN

An analysis of 219 patients referred to an university-based sleep center between 1989 to 1994 was conducted (104 with OSA and 115 without OSA; 43 women and 176 men). These patients had spirometry and a daytime hypercapnic ventilatory response (HCVR) test that was corrected to the patient's ability to attain maximal ventilation. Comparisons between OSA and no-OSA groups, as well as between men and women, were made using multivariate modeling techniques.

RESULTS

There was no significant difference in the slope of correlated HCVR (cHCVR) between those with and without OSA (1.57 +/- 0.57 vs 1.63 +/- 0.66; p = 0.48). In men, an inverse correlation between daytime PCO(2) and cHCVR was observed in both crude and multivariate analyses (crude beta-coefficient = - 0.04 +/- 0.02, p = 0.02; adjusted beta-coefficient = 0.07 +/- 0.02, p < 0.01). Although age and cHCVR did not share a significant relationship in the crude analysis (crude beta-coefficient = - 0.01 +/- 0.01, p = 0.10), with adjustments for confounding variables, a significant inverse relationship between age and cHCVR was observed (beta-coefficient = - 0.02 +/- 0.01, p = 0.04). On the other hand, in women, only body mass index (BMI) was positively correlated with cHCVR (crude beta-coefficient = 0.03 +/- 0.01, p = 0.01; adjusted beta-coefficient = 0.04 +/- 0.01, p < 0.01).

CONCLUSION

OSA disorder is not associated with a blunted ventilatory chemoresponsiveness to carbon dioxide. Elevated PaCO(2) and older age are significant correlates for a low cHCVR in men. For women only, BMI was associated with cHCVR. These findings suggest that men and women may have different ventilatory control mechanisms.

Value of clinical, functional, and oximetric data for the prediction of obstructive sleep apnea in obese patients

OBJECTIVE

To evaluate the diagnostic value of clinical features, pulmonary function testing, blood gas tensions, and oximetric data for case finding of obstructive sleep apnea (OSA) before polysomnography (PSG) in a series of consecutive overweight patients.

METHODS

We studied a population of 102 consecutive patients referred by an obesity clinic for suspected OSA, in whom body mass index was > or = 25 kg/m(2). The following tests were performed: clinical score (CS), pulmonary function tests (PFTs), measurement of arterial blood gas tensions, nocturnal oximetry, and full-night PSG.

RESULTS

Six of 34 women and 34 of 68 men had OSA, defined by an apnea-hypopnea index > or = 15. CS and the cumulative time spent below 80% arterial oxygen saturation (SaO(2)) were higher, and PaO(2), minimal SaO(2), and mean nocturnal SaO(2) (mSaO(2)) were lower in OSA patients than in non-OSA patients. Logistic regression showed that sex, CS, and the ratio of FEV(1) over forced expiratory volume in 0.5 s (an index of upper airway obstruction on flow-volume curves) and mSaO(2), expressed as categorical variables, were independent predictors of OSA. None of these individual variables had a satisfactory diagnostic value for the diagnosis of OSA. A logistic regression model including sex and all continuous variables would have allowed us to predict the presence or absence of OSA confidently in 72.5% of the population, in whom the positive predictive value of the model was 94% and the negative predictive value was 90%.

CONCLUSION

In obese patients referred to a respiratory sleep laboratory and evaluated by CS, PFTs, arterial blood gases, and oximetry, no individual sign or symptom may accurately predict the presence or absence of OSA. Provided that it is validated in prospective studies, a logistic regression model using these variables may be useful for the prediction of OSA.

A mechanism of central sleep apnea in patients with heart failure

BACKGROUND

Breathing is controlled by a negative-feedback system in which an increase in the partial pressure of arterial carbon dioxide stimulates breathing and a decrease inhibits it. Although enhanced sensitivity to carbon dioxide helps maintain the partial pressure of arterial carbon dioxide within a narrow range during waking hours, in some persons a large hyperventilatory response during sleep may lower the value below the apneic threshold, thereby resulting in central apnea. I tested the hypothesis that enhanced sensitivity to carbon dioxide contributes to the development of central sleep apnea in some patients with heart failure.

METHODS

This prospective study included 20 men who had treated, stable heart failure with left ventricular systolic dysfunction. Ten had central sleep apnea, and 10 did not. The patients underwent polysomnography and studies of their ventilatory response to carbon dioxide.

RESULTS

Patients who met the criteria for central sleep apnea had significantly more episodes of central apnea per hour than those without central sleep apnea (mean [+/-SD], 35+/-24 vs. 0.5+/-1.0 episodes per hour). Those with sleep apnea also had a significantly larger ventilatory response to carbon dioxide than those without central sleep apnea (5.1+/-3.1 vs. 2.1+/-1.0 liters per minute per millimeter of mercury, P=0.007), and there was a significant positive correlation between ventilatory response and the number of episodes of apnea and hypopnea per hour during sleep (r=0.6, P=0.01).

CONCLUSIONS

Enhanced sensitivity to carbon dioxide may predispose some patients with heart failure to the development of central sleep apnea.

Sleep apnea in 81 ambulatory male patients with stable heart failure. Types and their prevalences, consequences, and presentations

BACKGROUND

Heart failure is a highly prevalent disorder that continues to be associated with repeated hospitalizations, high morbidity, and high mortality. Sleep-related breathing disorders with repetitive episodes of asphyxia may adversely affect heart function. The main aims of this study were to determine the prevalence, consequences, and differences in various sleep-related breathing disorders in ambulatory male patients with stable heart failure.

METHODS AND RESULTS

This article reports the results of a prospective study of 81 of 92 eligible patients with heart failure and a left ventricular ejection fraction < 45%. There were 40 patients without (hourly rate of apnea/hypopnea, 4 +/- 4; group 1) and 41 patients with (51% of all patients; hourly rate of apnea/hypopnea, 44 +/- 19; group 2) sleep apnea. Sleep disruption and arterial oxyhemoglobin desaturation were significantly more severe and the prevalence of atrial fibrillation (22% versus 5%) and ventricular arrhythmias were greater in group 2 than in group 1. Forty percent of all patients had central sleep apnea, and 11% had obstructive sleep apnea. The latter patients had significantly greater mean body weight (112 +/- 30 versus 75 +/- 16 kg) and prevalence of habitual snoring (78% versus 28%). However, the hourly rate of episodes of apnea and hypopnea (36 +/- 10 versus 47 +/- 21), episodes of arousal (20 +/- 14 versus 23 +/- 11), and desaturation (lowest saturation, 72 +/- 11% versus 78 +/- 12%) were similar in patients with these different types of apnea.

CONCLUSIONS

Fifty-one percent of male patients with stable heart failure suffer from sleep-related breathing disorders: 40% from central and 11% from obstructive sleep apnea. Both obstructive and central types of sleep apnea result in sleep disruption and arterial oxyhemoglobin desaturation. Patients with sleep apnea have a high prevalence of atrial fibrillation and ventricular arrhythmias.

Effect of theophylline on sleep-disordered breathing in heart failure

BACKGROUND

Theophylline has been used to treat central apnea associated with Cheyne-Stokes respiration (periodic breathing). We studied the effect of short-term oral theophylline therapy on periodic breathing associated with stable heart failure due to systolic dysfunction.

METHODS

Fifteen men with compensated heart failure (left ventricular ejection fraction, 45 percent or less) participated in the study. Their base-line polysomnograms showed periodic breathing, with more than 10 episodes of apnea and hypopnea per hour. In a double-blind crossover study, the patients received theophylline or placebo orally twice daily for five days, with one week of washout between the two periods.

RESULTS

After five days of treatment, the mean (+/-SD) plasma theophylline concentration was 11 +/- 2 microgram per milliliter. Theophylline therapy resulted in significant decreases in the number of episodes of apnea and hypopnea per hour (18 +/- 17, vs. 37 +/- 23 with placebo and 47 +/- 21 at base line; P<0.001), the number of episodes of central apnea per hour (6 +/- 14, vs. 26 +/- 21 and 26 +/- 20, respectively; P<0.001), and the percentage of total sleep time during which the arterial oxyhemoglobin saturation was less than 90 percent (6 +/- 11 percent, vs., 23 +/- 37 and 14 +/- 14 percent, respectively; P<0.04). There were no significant differences in the characteristics of sleep, the frequency of ventricular arrhythmias, daytime arterial-blood gas values, or the left ventricular ejection fraction during the base-line, placebo, and theophylline phases of the study.

CONCLUSIONS

In patients with stable heart failure, oral theophylline therapy reduced the number of episodes of apnea and hypopnea and the duration of arterial oxyhemoglobin desaturation during sleep.