Elimination of central sleep apnoea by mitral valvuloplasty: the role of feedback delay in periodic breathing

Central sleep apnoea is a form of periodic breathing which resembles Cheyne-Stokes respiration but occurs only during sleep. One mechanism in the pathogenesis is a delay in chemical feedback from the lungs to the medullary respiratory centre. We explored the relationship between circulatory feedback delay in a patient with central sleep apnoea and Cheyne-Stokes respiration before and after mitral valve repair. Preoperatively the patient had severe central sleep apnoea and an increased circulation time. Following mitral valvuloplasty the circulation time was decreased with resolution of central sleep apnoea. This case demonstrates the role of feedback delay in central sleep apnoea and suggests that similar haemodynamic mechanisms may lead to central sleep apnoea and Cheyne-Stokes respiration.

Hypercapnic duty cycle is an intermediate physiological phenotype linked to mouse chromosome 5

We hypothesized that upper airway obstruction (UAO) leads to a compensatory increase in the duty cycle [ratio of inspiratory time to respiratory cycle length (Ti/Tt)], which is determined by genetic factors. We examined the compensatory Ti/Tt responses to 1). UAO and hypercapnia among normal individuals and 2). hypercapnia in different inbred strains, C3H/HeJ (C3) and C57BL/6J (B6), and their first- and second-generation (F2) offspring. 3). We then used the compensatory Ti/Tt response in the F2 to determine genetic linkage to the mouse genome. First, normal individuals exhibited a similar increase in the Ti/Tt during periods of hypercapnia (0.11 +/- 0.07) and UAO (0.09 +/- 0.06) compared with unobstructed breathing (P < 0.01). Second, the F2 offspring of C3 and B6 progenitors showed an average Ti/Tt response to 3% CO2 (0.42 +/- 0.005%) that was significantly (P < 0.01) greater than that of the two progenitors. Third, with a peak log of the odds ratio score of 4.4, Ti/Tt responses of F2 offspring are genetically linked to an interval between 58 and 64 centimorgans (cM) on mouse chromosome 5. One gene in the interval, Dagk4 at 57 cM, is polymorphic for C3 and B6 mice. Two other genes, Adrbk2 at 60 cM and Nos1 at 65 cM, have biological plausibility in mechanisms of upper airway patency and chemosensitivity, respectively. In summary, Ti/Tt may serve as an intermediate physiological phenotype for compensatory neuromuscular response mechanisms for maintaining ventilation in the face of UAO and hypoventilation and to help target specific candidate genes that may play a role in the expression of sleep-disordered breathing.

Toward a causal model of cardiovascular responses to stress and the development of cardiovascular disease

OBJECTIVE

Cardiovascular reactivity is hypothesized to mediate the relationship between stress and cardiovascular disease. We describe three considerations that are crucial for a causal model of cardiovascular responses to stress: the need for laboratory-life generalizability, the role of interactions between environmental exposures and individual response predispositions, and the importance of the duration of both stressor exposure and cardiovascular responding.

METHODS

We illustrate current understanding of stress-cardiovascular disease relationships with examples from the human and animal psychophysiology, epidemiology, and genetics literature.

RESULTS

In a causal model of reactivity, the usefulness of laboratory assessment rests on the assumption that laboratory-based cardiovascular reactivity predicts responses in the natural environment. We find only limited generalizability and suggest that cardiovascular responses to stress can be better understood when examined in the natural environment. The interaction of individual response predispositions and stressor exposures contributes to the development and progression of cardiovascular disease; stress-disease relationships could therefore be better understood if predispositions and exposures were assessed simultaneously in interactive models. Cardiovascular responses to stress are likely to be most deleterious when responses are prolonged. Responses may vary in their magnitude, frequency, and duration; however, reactivity captures only response magnitude. The assessment of anticipatory and recovery measures, with response magnitude, may therefore lead to a more useful model of the stress-disease relationship.

CONCLUSIONS

A causal model of cardiovascular responses to stress should generalize to the real world, assess interactions between individual predispositions and environmental exposures, and focus on sustained pathogenic exposures and responses.

Upper airway collapsibility: measurement techniques and therapeutic implications

Several techniques are currently available that aim to characterize upper airway function/mechanics during wakefulness or sleep. Based on the concept of a Starling resistor, we propose a standardized protocol to measure the critical pressure (Pcrit) (an indicator of upper airway collapsibility) during sleep. The effect of therapeutic interventions such as weight loss, positional changes or uvulopalatopharyngoplasty on Pcrit is illustrated by data from the literature. We propose that measurement of Pcrit become implemented in the diagnostic work-up of selected patients with sleep-related breathing disorder to help making a correct therapeutic decision.

Limitations of current validation protocols for home blood pressure monitors for individual patients

BACKGROUND

Automatic blood pressure monitoring conducted at home is increasingly used in the diagnosis and management of hypertension. We assessed the adequacy of existing British Hypertension Society (BHS) and Association for the Advancement of Medical Instrumentation (AAMI) validation standards for automatic blood pressure monitoring devices.

SUBJECT AND METHODS

A theoretical study and an empirical test are presented to estimate the proportion of persons for whom a blood pressure monitor validated according to existing BHS and AAMI standards would be inaccurate.

RESULTS

The results suggest that a major limitation of both protocols is the lack of attention given to the number of individual patients for whom a monitor may be inaccurate. A blood pressure monitor that meets the AAMI and BHS validation criteria may report blood pressures in error by more than 5 mmHg for more than half of the people.

CONCLUSIONS

A validation standard that does not take account of the person-effects on error will lead to a substantial proportion of persons using self-monitors that are systematically inaccurate for that person.

What is the white-coat effect and how should it be measured?

The white coat effect is conceived as a measure of the blood pressure response to a clinic visit, but there is no agreement as to exactly how it should be defined. The most widely used definition is the difference between the average clinic and daytime ambulatory blood pressures, but other methods that have been used include the difference between clinic and home pressures, measurements using ambulatory blood pressures only, clinic measurements only, and laboratory (reactivity) testing. Few studies have compared the different methods, but the reactivity method has reported bigger changes of blood pressure and heart rate than the others. The effect tends to be greater in older than younger patients, in women than in men, but is present to a greater or lesser degree in almost all hypertensive patients. It is diminished but not obliterated by drug treatment. It is not closely related to overall blood pressure variability, and does not predict cardiovascular risk. The white coat effect appears to be idiosyncratic to the clinic setting.

A model of sleep-disordered breathing in the C57BL/6J mouse

To investigate the pathophysiological sequelae of sleep-disordered breathing (SDB), we have developed a mouse model in which hypoxia was induced during periods of sleep and was removed in response to arousal or wakefulness. An on-line sleep-wake detection system, based on the frequency and amplitude of electroencephalograph and electromyograph recordings, served to trigger intermittent hypoxia during periods of sleep. In adult male C57BL/6J mice (n = 5), the sleep-wake detection system accurately assessed wakefulness (97.2 +/- 1.1%), non-rapid eye movement (NREM) sleep (96.0 +/- 0.9%) and rapid eye movement (REM) sleep (85.6 +/- 5.0%). After 5 consecutive days of SDB, 554 +/- 29 (SE) hypoxic events were recorded over a 24-h period at a rate of 63.6 +/- 2.6 events/h of sleep and with a duration of 28.2 +/- 0.7 s. The mean nadir of fraction of inspired O(2) (FI(O(2))) on day 5 was 13.2 +/- 0.1%, and 137.1 +/- 13.2 of the events had a nadir FI(O(2)) <10% O(2). Arterial blood gases confirmed that hypoxia of this magnitude lead to a significant degree of hypoxemia. Furthermore, 5 days of SDB were associated with decreases in both NREM and REM sleep during the light phase compared with the 24-h postintervention period. We conclude that our murine model of SDB mimics the rate and magnitude of sleep-induced hypoxia, sleep fragmentation, and reduction in total sleep time found in patients with moderate to severe SDB in the clinical setting.

Female gender exacerbates respiratory depression in leptin-deficient obesity

Obese females are less predisposed to sleep-disordered breathing and have higher serum leptin levels than males of comparable body weight. Because leptin is a powerful respiratory stimulant, especially during sleep, we hypothesized that the elevated leptin level is necessary to maintain normal ventilatory control in obese females. We examined ventilatory control during sleep and wakefulness in male and female leptin-deficient obese C57BL/6J-Lep(ob) mice, wild-type C57BL/6J mice with dietary-induced obesity and high serum leptin levels, and normal weight wild-type C57BL/6J mice. Both male and female C57BL/6J-Lep(ob) mice had depressed hypercapnic ventilatory response (HCVR) in comparison with wild-type animals. In comparison with male C57BL/6J-Lep(ob) mice, female C57BL/6J-Lep(ob) mice had reduced HCVR and respiratory drive (a ratio of tidal volume to inspiratory time) both during non-rapid eye movement (NREM) sleep and wakefulness. In contrast, the HCVR did not differ between sexes in wild-type mice during NREM sleep and wakefulness, but was lower in females during REM sleep. Thus, leptin deficiency in female obesity is even more detrimental to hypercapnic ventilatory control during wakefulness and NREM sleep than in obese, leptin-deficient males.

Therapeutic electrical stimulation of the hypoglossal nerve in obstructive sleep apnea

BACKGROUND

Hypoglossal nerve stimulation has been demonstrated to relieve upper airway obstruction acutely, but its effect on obstructive sleep apnea is not known.

OBJECTIVE

To determine the response in obstructive sleep apnea to electrical stimulation of the hypoglossal nerve.

METHODS

Eight patients with obstructive sleep apnea were implanted with a device that stimulated the hypoglossal nerve unilaterally during inspiration. Sleep and breathing patterns were examined at baseline before implantation and after implantation at 1, 3, and 6 months and last follow-up.

RESULTS

Unilateral hypoglossal nerve stimulation decreased the severity of obstructive sleep apnea throughout the entire study period. Specifically, stimulation significantly reduced the mean apnea-hypopnea indices in non-rapid eye movement (mean +/- SD episodes per hour, 52.0 +/- 20.4 for baseline nights and 22.6 +/- 12.1 for stimulation nights; P<.001) and rapid eye movement (48.2 +/- 30.5 and 16.6 +/- 17.1, respectively; P<.001) sleep and reduced the severity of oxyhemoglobin desaturations. With improvement in sleep apnea, a trend toward deeper stages of non-rapid eye movement sleep was observed. Moreover, all patients tolerated long-term stimulation at night and did not experience any adverse effects from stimulation. Even after completing the study protocol, the 3 patients who remained free from stimulator malfunction continued to use this device as primary treatment.

CONCLUSION

The findings demonstrate the feasibility and therapeutic potential for hypoglossal nerve stimulation in obstructive sleep apnea.

The impact of insulin-dependent diabetes on ventilatory control in the mouse

Insulin-dependent diabetes mellitus (IDDM) can lead to ventilatory depression and decreased sensitivity to hypercapnia. We examined relationships between ventilation, plasma insulin, leptin, ketones, and blood glucose levels in two mouse models of IDDM: (1) streptozotocin-induced diabetes in C57BL/6J mice on a regular diet or with induced obesity from a high fat diet; and (2) spontaneous diabetes mellitus in NOD-Ltj mice. In both mouse models, IDDM resulted in depression of the hypercapnic ventilatory response (HCVR). This ventilatory depression was not associated with decreases in plasma insulin or leptin levels. There was, however, a strong association between the duration of hyperglycemia, the decline in HCVR, and increased glycosylation of the diaphragm. Hyperventilation was observed in only six of 14 C57BL/6J obese wild-type mice, despite a significant degree of diabetic ketoacidosis (DKA) in all 14 animals. In mice with DKA, there was a significant correlation between the increase in baseline minute ventilation (V E) and hyperleptinemia (r = 0.77, p < 0.01). In leptin-deficient C57BL/6J-Lep(ob) mice, low levels of both V E and ketones were observed. These results suggest that: (1) depression of the HCVR in IDDM is associated with hyperglycemia and glycosylation of the diaphragm; and (2) the hyperventilation of DKA is leptin dependent.

Long-term monitoring of respiration with a mediastinal pressure sensor in dogs

The ability of an implanted mediastinal pressure sensor to produce a stable respiratory signal that could be used to trigger stimulation of upper airway muscles was examined. In 6 dogs, a pressure sensor was secured to the manubrium (4 by wires and 2 by transmanubrial placement). In 6 other dogs, the pressure sensor was placed in the upper anterior mediastinum. The animals were monitored for a minimum of 8 weeks (2 transmanubrial sensors for 12 months). Sensors that were able to maintain a midline position, high in the mediastinum, had the best signals. A caudal sensor position or abutment against an intrathoracic structure caused signal inversion (unusable signals). Transmanubrial placement resulted in a stable signal for 1 year. We conclude that long-term monitoring of respiration with a mediastinal pressure sensor can be successfully performed in dogs, providing an adequate signal for nerve-muscle stimulation. Separation from cardiovascular structures improves signal quality.

Abbreviated method for assessing upper airway function in obstructive sleep apnea

STUDY OBJECTIVES

Previous studies have shown that the level of flow through the upper airway in patients with obstructive sleep apnea (OSA) is determined by the critical closing pressure (Pcrit) and the upstream resistance (RN). We developed a standardized protocol for delineating quasisteady-state pressure-flow relationships for the upper airway from which these variables could be derived. In addition, we investigated the effect of body position and sleep stage on these variables by determining Pcrit and RN, and their confidence intervals (CIs), for each condition.

DESIGN

Pressure-flow relationships were constructed in the supine and lateral recumbent positions (nonrapid eye movement [NREM] sleep, n = 10) and in the supine position (rapid eye movement [REM] sleep, n = 5).

SETTING

University Hospital Antwerp, Belgium.

PATIENTS

Ten obese patients (body mass index, 32.0+/-5.6 kg/m(2)) with severe OSA (respiratory disturbance index, 63.0+/-14.6 events/h) were studied.

INTERVENTIONS

Pressure-flow relationships were constructed from breaths obtained during a series of step decreases in nasal pressure (34.1+/-6.5 runs over 3.6+/-1.2 h) in NREM sleep and during 7.8+/-2.2 runs over 0.8+/-0.6 h in REM sleep.

RESULTS

Maximal inspiratory airflow reached a steady state in the third through fifth breaths following a decrease in nasal pressure. Analysis of pressure-flow relationships derived from these breaths showed that Pcrit fell from 1.8 (95% CI, -0.1 to 2.7) cm H(2)O in the supine position to -1.1 cm H(2)O (95% CI, -1.8 to 0.4 cm H(2)O; p = 0.009) in the lateral recumbent position, whereas RN did not change significantly. In contrast, no significant effect of sleep stage was found on either Pcrit or RN.

CONCLUSIONS

Our methods for delineating upper airway pressure-flow relationships during sleep allow for multiple determinations of Pcrit within a single night from which small yet significant differences can be discerned between study conditions.

Usefulness of continuous oxygen insufflation into trachea for management of upper airway obstruction during anesthesia

BACKGROUND

Severe complications associated with upper airway obstruction often occur during the perioperative period. Development of a simple and reliable technique for reversing the impaired airway patency may improve airway management. The purpose of the current study is to evaluate the usefulness of transtracheal oxygen insufflation (TTI) for management of upper airway obstruction during anesthesia and to explore the mechanisms of TTI in detail.

METHODS

During propofol anesthesia in eight spontaneously breathing patients, the upper airway cross-sectional area and pressure-flow measurements during neck flexion with TTI were compared with those during triple airway maneuvers (TAM) without TTI. Blood gas analyses assessed efficacy of CO2 elimination during TTI in an additional nine patients.

RESULTS

TTI achieved adequate PaCO2 and PaO2 levels equivalent to those during TAM. In addition to a significantly smaller cross-sectional area during TTI, the location and slope of the pressure-flow relation during TTI completely differed from those during TAM, indicating that upper airway resistance was much higher during TTI. Notably, minute ventilation during TTI was significantly smaller than that during TAM, suggesting reduced dead space or other mechanisms for CO2 elimination.

CONCLUSIONS

TTI is capable of maintaining adequate blood gases through mechanisms different from those of conventional airway support in anesthetized subjects with upper airway obstruction.

A model of obstructive sleep apnea in normal humans. Role of the upper airway

We determined whether upper airway obstruction in normal individuals with intact reflexes could produce the syndrome of obstructive sleep apnea. Upper airway obstruction was produced in 12 normal individuals by lowering nasal pressure to -10 cm H(2)O during sleep. Full night polysomnography was performed during two consecutive nights of sleep with subatmospheric nasal pressure and compared with control nights before and after the negative pressure nights. We found that the application of negative pressure was associated with the development of recurrent obstructive apneas (non-REM-disordered breathing rate, 32.6 +/- 34.8 and 37.8 +/- 29.1 events/h during each of two negative pressure nights; p < 0.001) that were associated with oxyhemoglobin desaturation, arousals from sleep, and alterations in sleep stage distribution. Moreover, the median daytime sleep latency after two nights of sleep with subatmospheric pressure fell from 6.9 +/- 1.1 to 3.4 +/- 0.6 min, and rose significantly again to 8.1 +/- 1.5 min (p < 0.03) after the control night following subatmospheric pressure nights. Our findings suggest that a decrease in the pharyngeal transmural pressure alone is a sufficient condition for the production of the sleep apnea syndrome in normal individuals.

High-flow transtracheal insufflation treats obstructive sleep apnea. A pilot study

To determine the effect of transtracheal insufflation (TTI) on obstructive sleep apnea (OSA), we examined breathing patterns in five tracheostomized patients with OSA at varying TTI flow rates when breathing with a closed tracheostomy. The breathing patterns and polysomnographic responses to air insufflation were studied as TTI was increased from 0 to 15 L/min for brief periods of non-rapid eye movement (NREM) sleep (Experiment 1). The frequency of sleep-disordered breathing episodes remained high at 0 and 5 L/min (87.0 +/- 33.7 and 79.4 +/- 24.4 episodes per hour NREM) and decreased significantly to 41.3 +/- 31.5 and 43.4 +/- 31.4 episodes/h NREM sleep at rates of 10 and 15 L/min, respectively (p = 0.003). At high levels of TTI (10 and 15 L/min), obstructive apneas and hypopneas decreased but periodic laryngeal obstructions were induced during stage 1 NREM sleep. To prevent laryngeal obstructions, a servo-control system was used to briefly interrupt TTI during these events. When this system was implemented for more prolonged periods of sleep (Experiment 2, total sleep time 176.6 +/- 12.5 min), high-flow TTI (hf-TTI, 15 L/min) led to an overall reduction in the combined frequency of obstructive apneas and laryngeal obstructions from 63.8 +/- 21.8 to 10.7 +/- 9.1 (p < 0.03) and was associated with a marked reduction in arousal frequency from 60.0 +/- 26.0 to 8. 3 +/- 5.4/h in NREM sleep, and from 67.5 +/- 3.5 to 0 +/- 0/h in rapid eye movement (REM) sleep. Our findings demonstrate that hf-TTI stabilized breathing patterns in apneic patients, and was safe and efficacious for prolonged periods of sleep.

Neural and local effects of hypoxia on cardiovascular responses to obstructive apnea

Obstructive sleep apnea (OSA) acutely increases systemic (Psa) and pulmonary (Ppa) arterial pressures and decreases ventricular stroke volume (SV). In this study, we used a canine model of OSA (n = 6) to examine the role of hypoxia and the autonomic nervous system (ANS) in mediating these cardiovascular responses. Hyperoxia (40% oxygen) completely blocked any increase in Ppa in response to obstructive apnea but only attenuated the increase in Psa. In contrast, after blockade of the ANS (20 mg/kg iv hexamethonium), obstructive apnea produced a decrease in Psa (-5.9 mmHg; P < 0.05) but no change in Ppa, and the fall in SV was abolished. Both the fall in Psa and the rise in Ppa that persisted after ANS blockade were abolished when apneas were induced during hyperoxia. We conclude that 1) hypoxia can account for all of the Ppa and the majority of the Psa response to obstructive apnea, 2) the ANS increases Psa but not Ppa in obstructive apnea, 3) the local effects of hypoxia associated with obstructive apnea cause vasodilation in the systemic vasculature and vasoconstriction in the pulmonary vasculature, and 4) a rise in Psa acts as an afterload to the heart and decreases SV over the course of the apnea.

Effects of arousal and sleep state on systemic and pulmonary hemodynamics in obstructive apnea

During obstructive sleep apnea (OSA), systemic (Psa) and pulmonary (Ppa) arterial pressures acutely increase after apnea termination, whereas left and right ventricular stroke volumes (SV) reach a nadir. In a canine model (n = 6), we examined the effects of arousal, parasympathetic blockade (atropine 1 mg/kg iv), and sleep state on cardiovascular responses to OSA. In the absence of arousal, SV remained constant after apnea termination, compared with a 4.4 +/- 1.7% decrease after apnea with arousal (P < 0.025). The rise in transmural Ppa was independent of arousal (4.5 +/- 1.0 vs. 4.1 +/- 1.2 mmHg with and without arousal, respectively), whereas Psa increased more after apnea termination in apneas with arousal compared with apneas without arousal. Parasympathetic blockade abolished the arousal-induced increase in Psa, indicating that arousal is associated with a vagal withdrawal of the parasympathetic tone to the heart. Rapid-eye-movement (REM) sleep blunted the increase in Psa (pre- to end-apnea: 5.6 +/- 2.3 mmHg vs. 10.3 +/- 1.6 mmHg, REM vs. non-REM, respectively, P < 0.025), but not transmural Ppa, during an obstructive apnea. We conclude that arousal and sleep state both have differential effects on the systemic and pulmonary circulation in OSA, indicating that, in patients with underlying cardiovascular disease, the hemodynamic consequences of OSA may be different for the right or the left side of the circulation.

Leptin, obesity, and respiratory function

Leptin is a protein produced by adipose tissue that circulates to the brain and interacts with receptors in the hypothalamus to inhibit eating. The importance of this single peptide is vividly demonstrated by the profound obesity exhibited by the ob/ob mouse (C57BL/6J-Lep(ob)) which is unable to produce functional leptin. The measurement of respiratory function in the ob/ob mouse shows that the profound obesity is associated with impaired respiratory mechanics and depressed respiratory control, particularly during sleep. Longitudinal studies and leptin replacement studies in the ob/ob mouse indicate that leptin may act as both as a growth factor in the lung and as a neurohumoral modulator of central respiratory control mechanisms. Moreover, wildtype mice with diet-induced obesity have normal respiratory function associated with markedly elevated leptin levels. Human obesity, similar to obesity in wildtype mice, also causes an elevation in circulating leptin. However, unlike the tight relationship between obesity and elevated leptin present in an inbred strain of wildtype mice, human obesity is associated with more variable leptin levels for a given degree of adiposity. Thus, the possibility exists that a relative deficiency in leptin, or a leptin resistance, may play a role in obesity-related breathing disorders such as obesity hypoventilation syndrome (OHS) or obstructive sleep apnea (OSA).

The effect of tensor veli palatini stimulation on upper airway patency

OBJECTIVE

To evaluate the effect of selective electrical stimulation of the tensor veli palatini muscle on upper airway patency.

METHODS

Pressure-flow relationships were evaluated, in a feline isolated upper airway preparation, to determine the role of the soft palate musculature on airflow dynamics. The tensor veli palatini muscles were selectively stimulated while monitoring upper airway collapsibility (critical pressure), maximal inspiratory airflow, and the nasal resistance upstream to the flow-limiting site.

RESULTS

Tensor veli palatini stimulation resulted (mean +/- SEM) in an increase in maximal inspiratory airflow from 74 +/- 13 mL/s to 93 +/- 18 mL/s (P= .04). The increase in maximal inspiratory airflow was associated with a decrease in critical pressure from -2.3 +/- 1.7 cm H2O to -4.7 +/- 2.7 cm H2O (P= .01) and an increase in nasal resistance from 32.4 +/- 24.3 cm H2O x L(-1) s(-1) to 50.8 +/- 29.7 cm H2O x L(-1) s(-1) (P= .02).

CONCLUSIONS

Tensor veli palatini stimulation decreases upper airway collapsibility and is likely an integral component in maintaining airway patency. However, the effects of the isolated tensor veli palatini muscles are less significant than those seen previously with physiologic stimuli such as hypercapnia. These findings suggest that upper airway patency, although contributed to by the tensor veli palatini, requires the coordinated activation of palatopharyngeal muscles to adequately influence upper airway collapsibility.

Modeling hypersomnolence in sleep-disordered breathing. A novel approach using survival analysis

The etiology of excessive daytime sleepiness in patients with sleep-disordered breathing (SDB) is not well defined. In this study, we examined the relationships between several clinical and polysomnographic parameters and the degree of hypersomnolence in 741 patients with SDB (apnea-hypopnea index [AHI] >/= 10 events/h). The study sample was obese (body mass index [BMI]: 35.3 +/- 8.5 kg/m2) and had evidence of moderate SDB (AHI: 47.6 +/- 29.3 events/h). Hypersomnolence was quantified with the multiple sleep latency test (MSLT) and survival analysis was used to assess the risk factors for hypersomnolence. In a multivariate proportional hazards model, AHI and nocturnal hypoxemia were independent predictors of hypersomnolence (MSLT < 10 min). The adjusted relative risks (RR) of hypersomnolence were 1.00, 1.30, and 1.65 for patients with an AHI of 10 to 29.9, 30 to 59.9, and >/= 60 events/h, respectively. A positive association between hypersomnolence and oxyhemoglobin desaturation (DeltaSaO2) was observed with RR of 1.00, 1.18, 1.43, and 1.94 for a DeltaSaO2 of </= 5%, 5.1 to 10%, 10.1 to 15%, and > 15%, respectively. Sleep fragmentation, as assessed by the distribution of sleep stages, was also an independent predictor of hypersomnolence. Using stage 1 sleep as a reference, an increase in stage 2 and slow wave sleep (SWS) were protective from hypersomnolence. For a 10% increase in stage 2 or SWS the adjusted RR for hypersomnolence were 0.93 and 0.79, respectively. REM sleep showed no significant association with the degree of hypersomnolence. These results suggest that AHI, nocturnal hypoxemia, and sleep fragmentation are independent determinants of hypersomnolence in SDB.

Leptin prevents respiratory depression in obesity

Human obesity leads to an increase in respiratory demands. As obesity becomes more pronounced some individuals are unable to compensate, leading to elevated arterial carbon dioxide levels (PaCO2), alveolar hypoventilation, and increased cardiorespiratory morbidity and mortality (Pickwickian syndrome). The mechanisms that link obesity and hypoventilation are unknown, but thought to involve depression of central respiratory control mechanisms. Here we report that obese C57BL/6J-Lepob mice, which lack circulating leptin, also exhibit respiratory depression and elevated PaCO2 (> 10 mm Hg; p < 0. 0001). A role for leptin in restoring ventilation in these obese, mutant mice was investigated. Three days of leptin infusion (30 microg/d) markedly increased minute ventilation (V E) across all sleep/wake states, but particularly during rapid eye movement (REM) sleep when respiration was otherwise profoundly depressed. The effect of leptin was independent of food intake, weight, and CO2 production, indicating a reversal of hypoventilation by stimulation of central respiratory control centers. Furthermore, leptin replacement in mutant mice increased CO2 chemosensitivity during non-rapid eye movement (NREM) (4.0 +/- 0.5 to 5.6 +/- 0.4 ml/min/%CO2; p < 0.01) and REM (-0.1 +/- 0.5 to 3.0 +/- 0.8 ml/min/%CO2; p < 0.01) sleep. We also demonstrate in wild-type mice that ventilation is appropriately compensated when obesity is diet-induced and endogenous leptin levels are raised more than tenfold. These results suggest that leptin can prevent respiratory depression in obesity, but a deficiency in central nervous system (CNS) leptin levels or activity may induce hypoventilation and the Pickwickian syndrome in some obese subjects. O'Donnell CP, Schaub CD, Haines AS, Berkowitz DE, Tankersley CG, Schwartz AR, Smith PL. Leptin prevents respiratory depression in obesity.

Pharyngeal airway obstruction in obstructive sleep apnea: pathophysiology and clinical implications

In obstructive sleep apnea, airflow obstruction is caused by pharyngeal collapse. Current evidence suggests that anatomic alterations predispose to pharyngeal collapse. This article examines the role of structures that elongate and dilate the pharynx in maintaining pharyngeal patency. Their influence on pharyngeal collapsibility is considered. Insights gained are then presented within a conceptual framework that can be used for treating patients.

Evaluation of patients with sleep apnea after tracheotomy

OBJECTIVE

To determine the effect of tracheotomy on polysomnographic and arterial blood gas data in patients with obstructive sleep apnea (OSA).

DESIGN

A retrospective study of all patients who underwent tracheotomy and were studied polysomnographically at the Johns Hopkins Sleep Disorders Center, Baltimore, Md, since 1981.

SETTING

A regional sleep disorders center.

PATIENTS

Twenty-eight patients (8 women and 20 men), aged 22 through 77 years. Patients were categorized into 2 groups on the basis of whether they had already undergone tracheotomy before polysomnography. Group 1 patients all had a polysomnographic diagnosis of OSA before tracheotomy. They were further subdivided on the basis of whether cardiopulmonary decompensation had been absent (group 1a, n=10) or present (group 1b, n=13). Group 2 patients (n=5) had undergone tracheotomy to treat upper airway obstruction that developed after non-apnea-related upper aerodigestive tract surgeries.

INTERVENTION

Tracheotomy.

MAIN OUTCOME MEASURES

Nocturnal non-rapid eye movement, apnea-hypopnea index, percentage oxyhemoglobin saturation, and arterial blood gas data.

RESULTS

Patients with OSA underwent tracheotomy as definitive treatment for the apnea (n=15), to prevent postoperative upper airway compromise after uvulopalatopharyngoplasty (n=7), and to treat upper airway compromise after non-apnea-related upper aerodigestive tract surgeries (n=6). Tracheotomy alleviated apnea in all 10 patients with uncomplicated sleep apnea (group 1a). For patients with OSA complicated by cardiopulmonary decompensation (group 1b), tracheotomy improved but did not eliminate sleep apnea in 7 of the 13 patients, despite overall improvement in arterial blood gas values. For patients whose sleep apnea had not been diagnosed polysomnographically before tracheotomy (group 2), tracheotomy was still required to treat OSA that had previously not been recognized.

CONCLUSIONS

Tracheotomy effectively treated patients with uncomplicated OSA, but was much less effective in treating patients with OSA and cardiopulmonary decompensation. In patients who underwent tracheotomy in conjunction with other upper aerodigestive tract surgeries, concomitant obstructive sleep apnea often required continued use of a tracheotomy to maintain upper airway patency.