Diaphragmatic activity during REM sleep in the adult cat

Neuromotor control of spontaneous quiet breathing in awake rats evaluated by assessments of diaphragm EMG stationarity

The neuromotor control of the diaphragm muscle (DIAm) is dynamic. The activity of the DIAm can be recorded via electromyography (EMG), which represents the temporal summation of motor unit action potentials. Our goal in the present study was to investigate DIAm neuromotor control during quiet spontaneous breathing (eupnea) in awake rats by evaluating DIAm EMG at specific temporal locations defined by motor unit recruitment and derecruitment. We evaluated the nonstationarity of DIAm EMG activity to identify DIAm motor unit recruitment and derecruitment durations. Combined with assessments of root mean square (RMS) and sum of squares (SS) EMG, the durations of these phases provide physiological information about the temporal aspects of motor control. During eupnea in awake rats (n = 10), the duration of motor unit recruitment comprised 61 ± 19 ms of the onset-to-peak duration (214 ± 62 ms) of the DIAm RMS EMG. The peak-to-offset duration of DIAm EMG activity was 453 ± 96 ms, with a terminating period of derecruitment of 161 ± 44 ms. The burst duration was 673 ± 128 ms. Both the RMS EMG amplitude and the SS EMG were higher at the completion of motor unit recruitment than at the start of motor unit derecruitment, suggesting that offset discharge rates were lower than onset discharge rates. Our analyses provide novel insights into the time domain aspects of DIAm neuromotor control and allow indirect estimates of the contribution of recruitment and frequency to RMS EMG amplitude during eupnea in awake rats.NEW & NOTEWORTHY We characterized three phases of neuromotor control-motor unit recruitment, sustained activity, and derecruitment-based on statistical assessments of stationarity of the diaphragm muscle (DIAm) EMG activity in awake rats. Our findings may allow indirect estimates of the contribution of motor unit recruitment and frequency coding toward generating force and provide novel insights about the temporal aspects of DIAm neuromotor control and descending respiratory drive in unanesthetized animals.

Obstructive sleep apnea in young infants: Sleep position dependence and spontaneous improvement

OBJECTIVES

The natural evolution of obstructive sleep apnea (OSA) in young infants is not established.

METHODS

We re-evaluated 10-year pediatric sleep center infant polysomnography (PSG) data, excluding infants with syndromes, genetic defects, structural anomalies or periodic breathing > 5% of sleep time.

RESULTS

Obstructive events > 1 h^-1 were evident in 255 infants, of which 91 were eligible for the study. Of the 38 infants in a follow-up study, 30 (79%) were male, 15 (40%) were born prematurely, 25 (66%) had observed apneas, and 13 (33%) had experienced a brief, unexplained event or had a sibling of the infant died suddenly. The first PSG was performed at a median corrected age of 4 weeks (interquartile range [IQR] 2-7) and the second at 11 weeks (IQR 9-14). The obstructive apnea and hypopnea index (OAHI) was greater in the supine compared to side-sleeping position in both recordings (p < 0.001), whereas OAHI dropped from 10 h^-1 (IQR 6-24) in the first PSG to 3 h^-1 (IQR 1-9) in the second PSG (p < 0.001). OSA alleviation was also observable as a decrease in the number of oxygen desaturations (p < 0.001), as a decrease in transcutaneous (p = 0.001) and end-tidal carbon dioxide (p = 0.01) 95th percentile levels, and work of breathing (p = 0.002). Seven infants had a third PSG to verify a satisfactory improvement of OSA.

CONCLUSIONS

OSA in young infants without a clear syndrome or structural anomaly is sleep position dependent and shows improvement during the following few months.

Obstructive sleep apnea is position dependent in young infants

BACKGROUND

Obstructive sleep apnea in infants with Pierre Robin sequence is sleep-position dependent. The influence of sleep position on obstructive events is not established in other infants.

METHODS

We re-evaluated ten-year pediatric sleep center data in infants aged less than six months, with polysomnography performed in different sleep positions. We excluded infants with syndromes, genetic defects, or structural anomalies.

RESULTS

Comparison of breathing between supine and side sleeping positions was performed for 72 infants at the median corrected age of 4 weeks (interquartile range (IQR) 2-8 weeks). Of the infants, 74% were male, 35% were born prematurely, and 35% underwent study because of a life-threatening event or for being a SIDS sibling. Upper airway obstruction was more frequent (obstructive apnea-hypopnea index (OAHI), p < 0.001), 95th-percentile end-tidal carbon dioxide levels were higher (p = 0.004), and the work of breathing was heavier (p = 0.002) in the supine than in the side position. Median OAHI in the supine position was 8 h^-1 (IQR 4-20 h^-1), and in the side position was 4 h^-1 (IQR 0-10 h^-1).

CONCLUSIONS

Obstructive upper airway events in young infants are more frequent when supine than when sleeping on the side.

IMPACT

The effect of sleep position on obstructive sleep apnea is not well established in infants other than in those with Pierre Robin sequence. A tendency for upper airway obstruction is position dependent in most infants aged less than 6 months. Upper airway obstruction is more common, end-tidal carbon dioxide 95th-percentile values higher, and breathing more laborious in the supine than in the side-sleeping position. Upper airway obstruction and obstructive events have high REM sleep predominance. As part of obstructive sleep apnea treatment in young infants, side-sleeping positioning may prove useful.

Breathing during sleep

The depth, rate, and regularity of breathing change following transition from wakefulness to sleep. Interactions between sleep and breathing involve direct effects of the central mechanisms that generate sleep states exerted at multiple respiratory regulatory sites, such as the central respiratory pattern generator, respiratory premotor pathways, and motoneurons that innervate the respiratory pump and upper airway muscles, as well as effects secondary to sleep-related changes in metabolism. This chapter discusses respiratory effects of sleep as they occur under physiologic conditions. Breathing and central respiratory neuronal activities during nonrapid eye movement (NREM) sleep and REM sleep are characterized in relation to activity of central wake-active and sleep-active neurons. Consideration is given to the obstructive sleep apnea syndrome because in this common disorder, state-dependent control of upper airway patency by upper airway muscles attains high significance and recurrent arousals from sleep are triggered by hypercapnic and hypoxic episodes. Selected clinical trials are discussed in which pharmacological interventions targeted transmission in noradrenergic, serotonergic, cholinergic, and other state-dependent pathways identified as mediators of ventilatory changes during sleep. Central pathways for arousals elicited by chemical stimulation of breathing are given special attention for their important role in sleep loss and fragmentation in sleep-related respiratory disorders.

Breathing with neuromuscular disease: Does compensatory plasticity in the motor drive to breathe offer a potential therapeutic target in muscular dystrophy?

Duchenne muscular dystrophy is a fatal neuromuscular disease associated with respiratory-related morbidity and mortality. Herein, we review recent work by our group exploring deficits and compensation in the respiratory control network governing respiratory homeostasis in a pre-clinical model of DMD, the mdx mouse. Deficits at multiple sites of the network provide considerable challenges to respiratory control. However, our work has also revealed evidence of compensatory neuroplasticity in the motor drive to breathe enhancing diaphragm muscle activity during increased chemical drive. The finding may explain the preserved capacity for mdx mice to increase ventilation in response to chemoactivation. Given the profound dysfunction in the primary pump muscle of breathing, we argue that activation of accessory muscles of breathing may be especially important in mdx (and perhaps DMD). Notwithstanding the limitations resulting from respiratory muscle dysfunction, it may be possible to further leverage intrinsic physiological mechanisms serving to compensate for weak muscles in attempts to preserve or restore ventilatory capacity. We discuss current knowledge gaps and the need to better appreciate fundamental aspects of respiratory control in pre-clinical models so as to better inform intervention strategies in human DMD.

Neural Control of the Upper Airway: Respiratory and State-Dependent Mechanisms

Upper airway muscles subserve many essential for survival orofacial behaviors, including their important role as accessory respiratory muscles. In the face of certain predisposition of craniofacial anatomy, both tonic and phasic inspiratory activation of upper airway muscles is necessary to protect the upper airway against collapse. This protective action is adequate during wakefulness, but fails during sleep which results in recurrent episodes of hypopneas and apneas, a condition known as the obstructive sleep apnea syndrome (OSA). Although OSA is almost exclusively a human disorder, animal models help unveil the basic principles governing the impact of sleep on breathing and upper airway muscle activity. This article discusses the neuroanatomy, neurochemistry, and neurophysiology of the different neuronal systems whose activity changes with sleep-wake states, such as the noradrenergic, serotonergic, cholinergic, orexinergic, histaminergic, GABAergic and glycinergic, and their impact on central respiratory neurons and upper airway motoneurons. Observations of the interactions between sleep-wake states and upper airway muscles in healthy humans and OSA patients are related to findings from animal models with normal upper airway, and various animal models of OSA, including the chronic-intermittent hypoxia model. Using a framework of upper airway motoneurons being under concurrent influence of central respiratory, reflex and state-dependent inputs, different neurotransmitters, and neuropeptides are considered as either causing a sleep-dependent withdrawal of excitation from motoneurons or mediating an active, sleep-related inhibition of motoneurons. Information about the neurochemistry of state-dependent control of upper airway muscles accumulated to date reveals fundamental principles and may help understand and treat OSA. © 2016 American Physiological Society. Compr Physiol 6:1801-1850, 2016.

Pontine mechanisms of respiratory control

Pontine respiratory nuclei provide synaptic input to medullary rhythmogenic circuits to shape and adapt the breathing pattern. An understanding of this statement depends on appreciating breathing as a behavior, rather than a stereotypic rhythm. In this review, we focus on the pontine-mediated inspiratory off-switch (IOS) associated with postinspiratory glottal constriction. Further, IOS is examined in the context of pontine regulation of glottal resistance in response to multimodal sensory inputs and higher commands, which in turn rules timing, duration, and patterning of respiratory airflow. In addition, network plasticity in respiratory control emerges during the development of the pons. Synaptic plasticity is required for dynamic and efficient modulation of the expiratory breathing pattern to cope with rapid changes from eupneic to adaptive breathing linked to exploratory (foraging and sniffing) and expulsive (vocalizing, coughing, sneezing, and retching) behaviors, as well as conveyance of basic emotions. The speed and complexity of changes in the breathing pattern of behaving animals implies that "learning to breathe" is necessary to adjust to changing internal and external states to maintain homeostasis and survival.

Neural control of the upper airway: integrative physiological mechanisms and relevance for sleep disordered breathing

The various neural mechanisms affecting the control of the upper airway muscles are discussed in this review, with particular emphasis on structure-function relationships and integrative physiological motor-control processes. Particular foci of attention include the respiratory function of the upper airway muscles, and the various reflex mechanisms underlying their control, specifically the reflex responses to changes in airway pressure, reflexes from pulmonary receptors, chemoreceptor and baroreceptor reflexes, and postural effects on upper airway motor control. This article also addresses the determinants of upper airway collapsibility and the influence of neural drive to the upper airway muscles, and the influence of common drugs such as ethanol, sedative hypnotics, and opioids on upper airway motor control. In addition to an examination of these basic physiological mechanisms, consideration is given throughout this review as to how these mechanisms relate to integrative function in the intact normal upper airway in wakefulness and sleep, and how they may be involved in the pathogenesis of clinical problems such obstructive sleep apnea hypopnea.

The rhythmic, transverse medullary slice preparation in respiratory neurobiology: contributions and caveats

Our understanding of the sites and mechanisms underlying rhythmic breathing as well as the neuromodulatory control of respiratory rhythm, pattern, and respiratory motoneuron excitability during perinatal development has advanced significantly over the last 20 years. A major catalyst was the development in 1991 of the rhythmically-active medullary slice preparation, which provided precise mechanical and chemical control over the network as well as enhanced physical and optical access to key brainstem regions. Insights obtained in vitro have informed multiple mechanistic hypotheses. In vivo tests of these hypotheses, performed under conditions of reduced control and precision but more obvious physiological relevance, have clearly established the significance for respiratory neurobiology of the rhythmic slice preparation. We review the contributions of this preparation to current understanding/concepts in respiratory control, and outline the limitations of this approach in the context of studying rhythm and pattern generation, homeostatic control mechanisms and murine models of human genetic disorders that feature prominent breathing disturbances.

Midbrain and medullary control of postinspiratory activity of the crural and costal diaphragm in vivo

Studies on brain stem respiratory neurons suggest that eupnea consists of three phases: inspiration, postinspiration, and expiration. However, it is not well understood how postinspiration is organized in the diaphragm, i.e., whether postinspiration differs in the crural and costal segments of the diaphragm and what the influence is of postinspiratory neurons on diaphragm function during eupnea. In this in vivo study we investigated the postinspiratory activity of the two diaphragm segments during eupnea and the changes in diaphragm function following modulation of eupnea. Postinspiratory neurons in the medulla were stereotaxically localized extracellularly and neurochemically stimulated. We used three types of preparations: precollicularly decerebrated unanesthetized cats and rats and anesthetized rats. In all preparations, during eupnea, postinspiratory activity was found in the crural but not in the costal diaphragm. When eupnea was discontinued in decerebrate cats in which stimulation in the nucleus retroambiguus induced activation of laryngeal or abdominal muscles, all postinspiratory activity in the crural diaphragm was abolished. In decerebrate rats, stimulation of the midbrain periaqueductal gray abolished postinspiration in the crural diaphragm but induced activation in the costal diaphragm. In anesthetized rats, stimulation of medullary postinspiratory neurons abolished the postinspiratory activity of the crural diaphragm. Vagal nerve stimulation in these rats increased the intensity of postinspiratory neuronal discharge in the solitary nucleus, leading to decreased activity of the crural diaphragm. These data demonstrate that three-phase breathing in the crural diaphragm during eupnea exists in vivo and that postinspiratory neurons have an inhibitory effect on crural diaphragm function.

Phasic motor activity of respiratory and non-respiratory muscles in REM sleep

OBJECTIVES

In this study, we quantified the profiles of phasic activity in respiratory muscles (diaphragm, genioglossus and external intercostal) and non-respiratory muscles (neck and extensor digitorum) across REM sleep. We hypothesized that if there is a unique pontine structure that controls all REM sleep phasic events, the profiles of the phasic twitches of different muscle groups should be identical. Furthermore, we described how respiratory parameters (e.g., frequency, amplitude, and effort) vary across REM sleep to determine if phasic processes affect breathing.

METHODS

Electrodes were implanted in Wistar rats to record brain activity and muscle activity of neck, extensor digitorum, diaphragm, external intercostal, and genioglossal muscles. Ten rats were studied to obtain 313 REM periods over 73 recording days. Data were analyzed offline and REM sleep activity profiles were built for each muscle. In 6 animals, respiratory frequency, effort, amplitude, and inspiratory peak were also analyzed during 192 REM sleep periods.

RESULTS

Respiratory muscle phasic activity increased in the second part of the REM period. For example, genioglossal activity increased in the second part of the REM period by 63.8% compared to the average level during NREM sleep. This profile was consistent between animals and REM periods (η(2)=0.58). This increased activity seen in respiratory muscles appeared as irregular bursts and trains of activity that could affect rythmo-genesis. Indeed, the increased integrated activity seen in the second part of the REM period in the diaphragm was associated with an increase in the number (28.3%) and amplitude (30%) of breaths. Non-respiratory muscle phasic activity in REM sleep did not have a profile like the phasic activity of respiratory muscles. Time in REM sleep did not have an effect on nuchal activity (P=0.59).

CONCLUSION

We conclude that the concept of a common pontine center controlling all REM phasic events is not supported by our data. There is a drive in REM sleep that affects specifically respiratory muscles. The characteristic increase in respiratory frequency during REM sleep is induced by this drive.

REM sleep-like episodes of motoneuronal depression and respiratory rate increase are triggered by pontine carbachol microinjections in in situ perfused rat brainstem preparation

Hypoglossal nerve activity (HNA) controls the position and movements of the tongue. In persons with compromised upper airway anatomy, sleep-related hypotonia of the tongue and other pharyngeal muscles causes increased upper airway resistance, or total upper airway obstructions, thus disrupting both sleep and breathing. Hypoglossal nerve activity reaches its nadir, and obstructive episodes are longest and most severe, during rapid eye movement stage of sleep (REMS). Microinjections of a cholinergic agonist, carbachol, into the pons have been used in vivo to investigate the mechanisms of respiratory control during REMS. Here, we recorded inspiratory-modulated phrenic nerve activity and HNA and microinjected carbachol (25-50 nl, 10 mm) into the pons in an in situ perfused working heart-brainstem rat preparation (WHBP), an ex vivo model previously validated for studies of the chemical and reflex control of breathing. Carbachol microinjections were made into 40 sites in 33 juvenile rat preparations and, at 24 sites, they triggered depression of HNA with increased respiratory rate and little change of phrenic nerve activity, a pattern akin to that during natural REMS in vivo. The REMS-like episodes started 151 ± 73 s (SD) following microinjections, lasted 20.3 ± 4.5 min, were elicited most effectively from the dorsal part of the rostral nucleus pontis oralis, and were prevented by perfusion of the preparation with atropine. The WHBP offers a novel model with which to investigate cellular and neurochemical mechanisms of REMS-related upper airway hypotonia in situ without anaesthesia and with full control over the cellular environment.

Emerging principles and neural substrates underlying tonic sleep-state-dependent influences on respiratory motor activity

Respiratory muscles with dual respiratory and non-respiratory functions (e.g. the pharyngeal and intercostal muscles) show greater suppression of activity in sleep than the diaphragm, a muscle almost entirely devoted to respiratory function. This sleep-related suppression of activity is most apparent in the tonic component of motor activity, which has functional implications of a more collapsible upper airspace in the case of pharyngeal muscles, and decreased functional residual capacity in the case of intercostal muscles. A major source of tonic drive to respiratory motoneurons originates from neurons intimately involved in states of brain arousal, i.e. neurons not classically involved in generating respiratory rhythm and pattern per se. The tonic drive to hypoglossal motoneurons, a respiratory motor pool with both respiratory and non-respiratory functions, is mediated principally by noradrenergic and glutamatergic inputs, these constituting the essential components of the wakefulness stimulus. These tonic excitatory drives are opposed by tonic inhibitory glycinergic and gamma-amino butyric acid (GABA) inputs that constrain the level of respiratory-related motor activity, with the balance determining net motor tone. In sleep, the excitatory inputs are withdrawn and GABA release into the brainstem is increased, thus decreasing respiratory motor tone and predisposing susceptible individuals to hypoventilation and obstructive sleep apnoea.

The midbrain periaqueductal gray control of respiration

The midbrain periaqueductal gray (PAG) organizes basic survival behavior, which includes respiration. How the PAG controls respiration is not known. We studied the PAG control of respiration by injecting D,L-homocysteic acid in the PAG in unanesthetized precollicularly decerebrated cats. Injections in different parts of the PAG caused different respiratory effects. Stimulation in the dorsomedial PAG induced slow and deep breathing and dyspnea. Stimulation in the dorsolateral PAG resulted in active breathing and tachypnea consistent with the respiratory changes during fright and flight. Stimulation in the medial part of lateral PAG caused inspiratory apneusis. Stimulation in lateral parts of the lateral and ventrolateral PAG produced respiratory changes associated with vocalization (mews, alternating mews and hisses, or hisses). D,L-homocysteic acid injections in the caudal ventrolateral PAG induced irregular breathing. These results demonstrate that the PAG exerts a strong influence on respiration, suggesting that it serves as the behavioral modulator of breathing.

Contribution of cholinergic systems to state-dependent modulation of respiratory control

Respiration is altered during different stages of the sleep-wake cycle. We review the contribution of cholinergic systems to this alteration, with particular reference to the role of muscarinic acetylcholine receptors (MAchRs) during rapid eye movement (REM) sleep. Available evidence demonstrates that MAchRs have potent excitatory effects on medullary respiratory neurones and respiratory motoneurones, and are likely to contribute to changes in central chemosensitive drive to the respiratory control system. These effects are likely to be most prominent during REM sleep, when cholinergic brainstem neurones show peak activity levels. It is possible that MAchR dysfunction is involved in sleep-disordered breathing, such as obstructive sleep apnea.

Effects of sleep-wake state on the genioglossus vs.diaphragm muscle response to CO(2) in rats

The effects of sleep on the ventilatory responses to hypercapnia have been well described in animals and in humans. In contrast, there is little information for genioglossus (GG) responses to a range of CO(2) stimuli across all sleep-wake states. Given the notion that sleep, especially rapid eye movement (REM) sleep, may cause greater suppression of muscles with both respiratory and nonrespiratory functions, this study tests the hypothesis that GG activity will be differentially affected by sleep-wake states with major suppression in REM sleep despite excitation by CO(2). Seven rats were chronically implanted with electroencephalogram, neck, GG, and diaphragm electrodes, and responses to 0, 1, 3, 5, 7, and 9% CO(2) were recorded. Diaphragm activity and respiratory rate increased with CO(2) (P < 0.001) across sleep-wake states with significant increases at 3-5% CO(2) compared with 0% CO(2) controls (P < 0.05). Phasic GG activity also increased in hypercapnia but required higher CO(2) (7-9%) for significant activation (P < 0.05). Further studies in 15 urethane-anesthetized rats with the vagi intact (n = 6) and cut (n = 9) showed that intact vagi delayed GG recruitment with hypercapnia but did not affect diaphragm responses. In the naturally sleeping rats, we also showed that GG activity was significantly reduced in non-REM and REM sleep (P < 0.04) and was almost abolished in REM even with stimulation by 9% CO(2) (decrease = 80.4% vs. wakefulness). Such major suppression of GG activity in REM, even with significant respiratory stimulation, may explain why obstructive apneas are more common in REM sleep.

Cholinergic modulation of respiratory brain-stem neurons and its function in sleep-wake state determination

1. Shifts in behavioural state are controlled by reciprocal changes in discharge of cholinergic and aminergic groups of brain-stem/pontine neurons. During rapid eye movement (REM) sleep, cholinergic neurons are most active and aminergic neurons are least active. 2. Significant changes occur in the central control of breathing during REM sleep; respiration rate increases in frequency and variability, brain-stem respiratory neuron discharge is generally enhanced and the outputs of some respiratory motor neuron pools are depressed. 3. Hypoglossal motor neurons (HM) control tongue movement and their depression during REM sleep has been implicated in obstructive sleep apnoea. The cellular basis of HM depression has been investigated in vitro and may be due to enhanced activation of cholinergic receptors or decreased activation of aminergic receptors. 4. In vitro preparations that show respiratory rhythmogenesis possess advantages for the investigation of the neurochemical basis of state-dependent changes in respiration. Cholinergic changes in respiratory modulation of HM recorded in rhythmic brain-stem slices from mice depend on the site of activation of cholinergic receptors.

Augmenting expiratory neuronal activity in sleep and wakefulness and in relation to duration of expiration

Augmenting expiratory cells (n = 23) were recorded in the rostral medulla of five cats in sleep and wakefulness. The objective was to determine the relationship of their activity to the duration of expiration (TE) and, particularly, to TE in rapid-eye-movement (REM) sleep, when expirations are short and may even cause fractionated breathing. Correlation analysis (Kendall's tau) showed no consistent relationship in any state between the breath-by-breath mean activity of augmenting expiratory cells and TE. This result contradicts predications of an inverse relationship between augmenting expiratory activity and TE. Some cells (11 of 23) were more active in REM than in non-REM sleep and were active during fractionated breathing. This suggests that fractionated breathing in REM sleep is caused by short expiratory phases and not by intermittent inhibition of an ongoing inspiration.

Neural-mechanical coupling of breathing in REM sleep

During rapid-eye-movement (REM) sleep the ventilatory response to airway occlusion is reduced. Possible mechanisms are reduced chemosensitivity, mechanical impairment of the chest wall secondary to the atonia of REM sleep, or phasic REM events that interrupt or fractionate ongoing diaphragm electromyogram (EMG) activity. To differentiate between these possibilities, we studied three chronically instrumented dogs before, during, and after 15-20 s of airway occlusion during non-REM (NREM) and phasic REM sleep. We found that 1) for a given inspiratory time the integrated diaphragm EMG (Di) was similar or reduced in REM sleep relative to NREM sleep; 2) for a given Di in response to airway occlusion and the hyperpnea following occlusion, the mechanical output (flow or pressure) was similar or reduced during REM sleep relative to NREM sleep; 3) for comparable durations of airway occlusion the Di and integrated inspiratory tracheal pressure tended to be smaller and more variable in REM than in NREM sleep, and 4) significant fractionations (caused visible changes in tracheal pressure) of the diaphragm EMG during airway occlusion in REM sleep occurred in approximately 40% of breathing efforts. Thus reduced and/or erratic mechanical output during and after airway occlusion in REM sleep in terms of flow rate, tidal volume, and/or pressure generation is attributable largely to reduced neural activity of the diaphragm, which in turn is likely attributable to REM effects, causing reduced chemosensitivity at the level of the peripheral chemoreceptors or, more likely, at the central integrator. Chest wall distortion secondary to the atonia of REM sleep may contribute to the reduced mechanical output following airway occlusion when ventilatory drive is highest.