Effect of mechanical loading on expiratory and inspiratory muscle activity during NREM sleep

Neural ventilatory drive decline as a predominant mechanism of obstructive sleep apnoea events

BACKGROUND

In the classic model of obstructive sleep apnoea (OSA), respiratory events occur with sleep-related dilator muscle hypotonia, precipitating increased neural ventilatory 'drive'. By contrast, a drive-dependent model has been proposed, whereby falling drive promotes dilator muscle hypotonia to precipitate respiratory events. Here we determine the extent to which the classic versus drive-dependent models of OSA are best supported by direct physiological measurements.

METHODS

In 50 OSA patients (5-91 events/hour), we recorded ventilation ('flow', oronasal mask and pneumotach) and ventilatory drive (calibrated intraoesophageal diaphragm electromyography, EMG) overnight. Flow and drive during events were ensemble averaged; patients were classified as drive dependent if flow fell/rose simultaneously with drive. Overnight effects of lower drive on flow, genioglossus muscle activity (EMGgg) and event risk were quantified (mixed models).

RESULTS

On average, ventilatory drive fell (rather than rose) during events (-20 (-42 to 3)%_baseline, median (IQR)) and was strongly correlated with flow (R=0.78 (0.24 to 0.94)). Most patients (30/50, 60%) were classified as exhibiting drive-dependent event pathophysiology. Lower drive during sleep was associated with lower flow (-17 (-20 to -14)%/drive) and EMGgg (-3.5 (-3.8 to -3.3)%_max/drive) and greater event risk (OR: 2.2 (1.8 to 2.5) per drive reduction of 100%_eupnoea); associations were concentrated in patients with drive-dependent OSA (ie, flow: -37 (-40 to -34)%/drive, OR: 6.8 (5.3 to 8.7)). Oesophageal pressure-without tidal volume correction-falsely suggested rising drive during events (classic model).

CONCLUSIONS

In contrast to the prevailing view, patients with OSA predominantly exhibit drive-dependent event pathophysiology, whereby flow is lowest at nadir drive, and lower drive raises event risk. Preventing ventilatory drive decline is therefore considered a target for OSA intervention.

Exaggerated ventilatory drive estimates from epiglottic and esophageal pressure deflections in the presence of airway occlusion

Esophageal and epiglottic pressure deflections are widely used to quantify ventilatory effort during sleep in patients with obstructive sleep apnea (OSA). However, changes in upper airway patency will fundamentally alter pressure gradients across the respiratory system with different airflow and volume-dependent effects on esophageal versus epiglottic pressure. The magnitude of these obstruction effects on ventilatory effort assessed from pressure deflections has not been systematically investigated. This study sought to quantify the direct effect of airway occlusion on esophageal and epiglottic pressure deflections during sleep in patients with OSA compared with predictions based on classic respiratory mechanics. Pneumotachograph airflow and volume, and esophageal, epiglottic, mask, and gastric pressures were measured throughout a nonoccluded breath before and the first occluded breath after repeated external airway occlusions during sleep in 13 patients with OSA on constant positive airway pressure (CPAP). Inspiratory pressure deflections were approximately doubled with epiglottic pressure, and increased by around 40% with esophageal pressure on the occluded compared with the preoccluded breath. Differences in pressure between pre- and occluded breaths showed strong dependence on volume and flow, in line with theoretical models of respiratory mechanics. A relatively simple correction factor could account for these effects to provide more consistent measures of ventilatory effort from pressure, independent from measurement site and changing airflow conditions. These finding have important implications for interpreting ventilatory effort and arousal threshold measurements and for understanding the relationships between underlying ventilatory drive and pressure deflections in the presence of airway obstruction during sleep.NEW & NOTEWORTHY Esophageal and epiglottic pressure deflection measurements are widely used as gold-standard measures of ventilatory effort without consideration of differential obstruction effects between measurement sites. This study is the first to quantify the effect of airway occlusion on pressure recordings during sleep. The findings of substantial acute effects of occlusion itself on pressure deflections are important to consider in the planning, analysis, and interpretation of studies that make inferences regarding inspiratory effort.

Differential modulation of active expiration during hypercapnia by the medullary raphe in unanesthetized rats

Active expiration represents an important mechanism to improve ventilation in conditions of augmented ventilatory demand, such as hypercapnia. While a rostral ventromedullary region, the parafacial respiratory group (pFRG), has been identified as a conditional expiratory oscillator, little is known about how central chemosensitive sites contribute to modulate active expiration under hypercapnia. In this study, we investigated the influence of the medullary raphe in the emergence of phasic expiratory abdominal activity during hypercapnia in unanesthetized adult male rats, in a state-dependent manner. To do so, reverse microdialysis of muscimol (GABA_A receptor agonist, 1 mM) or 8-OH-DPAT (5-HT_1A agonist, 1 mM) was applied in the MR during sleep and wakefulness periods, both in normocapnic (room air) and hypercapnic conditions (7% CO₂). Electromyography (EMG) of diaphragm and abdominal muscles was performed to measure inspiratory and expiratory motor outputs. We found that active expiration did not occur in room air exposure during wakefulness or sleep. However, hypercapnia did recruit active expiration, and differential effects were observed with the drug dialyses in the medullary raphe. Muscimol increased the diaphragm inspiratory motor output and also increased the amplitude and frequency of abdominal expiratory rhythmic activity during hypercapnia in wakefulness periods. On the other hand, the microdialysis of 8-OH-DPAT attenuated hypercapnia-induced active expiration in a state-dependent manner. Our data suggest that the medullary raphe can either inhibit or potentiate respiratory motor activity during hypercapnia, and the balance of these inhibitory or excitatory outputs may determine the expression of active expiration.

Influence of respiratory mechanics and drive on genioglossus movement under ultrasound imaging

METHODS

Twenty healthy subjects (10 males, age 28±5 years [mean ± SD]) lay supine, awake, with the head in a neutral position. Ventilation was monitored with inductance bands. Real-time B-mode ultrasound movies were analysed. We measured genioglossus motion (i) during spontaneous breathing, voluntary targeted breathing (normal tidal volume Vt), and voluntary hyperpnoea (at 1.5Vt and 2 Vt); (ii) during inspiratory flow resistive loading; (iii) with changes in end-expiratory lung volume (EELV).

RESULTS

Average peak inspiratory displacement of the infero-posterior region of genioglossus was 0.89±0.56 mm; 1.02±0.88 mm; 1.27±0.70 mm respectively for voluntary Vt, and during voluntary hyperpnoea at 1.5Vt and 2Vt. A change in genioglossus motion was observed with increased Vt. During increasing inspiratory resistive loading, the genioglossus displaced less anteriorly (p = 0.005) but more inferiorly (p = 0.027). When lung volume was altered, no significant changes in genioglossus movement were observed (p = 0.115).

CONCLUSION

In healthy subjects, we observed non-uniform heterogeneous inspiratory motion within the inferoposterior part of genioglossus during spontaneous quiet breathing with mean peak displacement between 0.5-2 mm, with more displacement in the posterior region than the anterior. This regional heterogeneity disappeared during voluntary targeted breathing. This may be due to different neural drive to genioglossus during voluntary breathing. During inspiratory resistive loading, the observed genioglossus motion may serve to maintain upper airway patency by balancing intraluminal negative pressure with positive pressure generated by upper airway dilatory muscles. In contrast, changes in EELV were not accompanied by major changes in genioglossus motion.

Hypercapnia-induced active expiration increases in sleep and enhances ventilation in unanaesthetized rats

KEY POINTS

Expiratory muscles (abdominal and thoracic) can be recruited when respiratory drive increases under conditions of increased respiratory demand such as hypercapnia. Studying hypercapnia-induced active expiration in unanaesthetized rats importantly contributes to the understanding of how the control system is integrated in vivo in freely moving animals. In unanaesthetized rats, hypercapnia-induced active expiration was not always recruited either in wakefulness or in sleep, suggesting that additional factors influence the recruitment of active expiration. The pattern of abdominal muscle recruitment varied in a state-dependent manner with active expiration being more predominant in the sleep state than in quiet wakefulness. Pulmonary ventilation was enhanced in periods with active expiration compared to periods without it.

ABSTRACT

Expiration is passive at rest but becomes active through recruitment of abdominal muscles under increased respiratory drive. Hypercapnia-induced active expiration has not been well explored in unanaesthetized rats. We hypothesized that (i) CO₂ -evoked active expiration is recruited in a state-dependent manner, i.e. differently in sleep or wakefulness, and (ii) recruitment of active expiration enhances ventilation, hence having an important functional role in meeting metabolic demand. To test these hypotheses, Wistar rats (280-330 g) were implanted with electrodes for EEG and electromyography EMG of the neck, diaphragm (DIA) and abdominal (ABD) muscles. Active expiratory events were considered as rhythmic ABD_EMG activity interposed to DIA_EMG . Animals were exposed to room air followed by hypercapnia (7% CO₂ ) with EEG, EMG and ventilation ( V̇E ) recorded throughout the experimental protocol. No active expiration was observed during room air exposure. During hypercapnia, CO₂ -evoked active expiration was predominantly recruited during non-rapid eye movement sleep. Its increased occurrence during sleep was evidenced by the decreased DIA-to-ADB ratio (1:1 ratio means that each DIA event is followed by an ABD event, indicating a high occurrence of ABD activity). Moreover, V̇E was also enhanced (P < 0.05) in periods with active expiration. V̇E had a positive correlation (P < 0.05) with the peak amplitude of ABD_EMG activity. The data demonstrate strongly that hypercapnia-induced active expiration increases during sleep and provides an important functional role to support V̇E in conditions of increased respiratory demand.

The relationship between partial upper-airway obstruction and inter-breath transition period during sleep

Short pauses or "transition-periods" at the end of expiration and prior to subsequent inspiration are commonly observed during sleep in humans. However, the role of transition periods in regulating ventilation during physiological challenges such as partial airway obstruction (PAO) has not been investigated. Twenty-nine obstructive sleep apnea patients and eight controls underwent overnight polysomnography with an epiglottic catheter. Sustained-PAO segments (increased epiglottic pressure over ≥5 breaths without increased peak inspiratory flow) and unobstructed reference segments were manually scored during apnea-free non-REM sleep. Nasal pressure data was computationally segmented into inspiratory (T_I, shortest period achieving 95% inspiratory volume), expiratory (T_E, shortest period achieving 95% expiratory volume), and inter-breath transition period (T_Trans, period between T_E and subsequent T_I). Compared with reference segments, sustained-PAO segments had a mean relative reduction in T_Trans (-24.7±17.6%, P<0.001), elevated T_I (11.8±10.5%, P<0.001), and a small reduction in T_E (-3.9±8.0, P≤0.05). Compensatory increases in inspiratory period during PAO are primarily explained by reduced transition period and not by reduced expiratory period.

Short-term potentiation in the control of pharyngeal muscles in obstructive apnea patients

STUDY OBJECTIVES

To determine if activation of the genioglossus (GG) muscle during obstructive apnea events involves short-term potentiation (STP) and is followed by sustained activation beyond the obstructive phase (after-discharge).

DESIGN

Physiological study.

SETTING

Sleep laboratory in a tertiary hospital.

PARTICIPANTS

Twenty-one patients with obstructive apnea.

INTERVENTIONS

Polysomnography on continuous positive airway pressure (CPAP) with measurement of genioglossus activity. Brief dial-downs of CPAP to induce obstructive events.

MEASUREMENTS AND RESULTS

Peak, phasic, and tonic genioglossus activities were measured breath-by-breath before, during, and following three-breath obstructions. Tonic but not phasic activity increased immediately following the first obstructed breath (4.9 ± 1.6 versus 3.6 ± 1.2 %GGMAX; P = 0.01) under conditions where stimuli to genioglossus activation were likely constant, strongly implicating STP in mediating recruitment of tonic activity. Both phasic and tonic activities declined slowly after relief of obstruction (after-discharge). Decay time constants were systematically shorter for phasic than for tonic activity (7.5 ± 3.8 versus 18.1 ± 8.4 sec; P < 0.001). Decay time-constant of peak activity correlated with tonic, but not phasic, recruitment. Cortical arousal near the end of obstruction resulted in a lower after-discharge (P < 0.01). Contribution of tonic activity to the increase in peak activity (6-65%Peak), as well as the decay constant (6-30 sec), varied considerably among patients.

CONCLUSIONS

Short-term potentiation contributes to recruitment of the genioglossus during obstructive episodes and results in sustained tonic activity beyond the obstructive phase, thereby potentially preventing recurrence of obstruction. Wide response differences among subjects suggest that this mechanism may contribute to severity of the disorder. The after-discharge is inhibited following cortical arousal, potentially explaining arousals' destabilizing effect.

Upper airway mechanics in chronic spinal cord injury during sleep

Sleep-disordered breathing has been shown to be more prevalent in patients with spinal cord injury (SCI) than the general population. The pathogenesis of increased sleep-disordered breathing in individuals with chronic SCI is unknown. The purpose of this study is to determine whether SCI level affects upper airway (UA) collapsibility and neuromuscular compensatory responses to obstruction. Twenty-four participants (8 cervical SCI, 8 thoracic SCI, and 8 controls) were studied. The ventilation, timing, UA resistance, and pharyngeal collapsibility, defined by critical closing pressure, were determined during non-rapid eye movement sleep. Inspiratory duty cycle and minute ventilation were observed in response to increasing severity of UA obstruction. Compared with controls, both cervical and thoracic SCI participants demonstrated elevated passive critical closing pressure (0.5 ± 2.2 and 0.9 ± 2.7 vs. −2.5 ± 1.0 cmH2O, respectively; P = 0.01). No difference in UA resistance was observed between groups. Cervical and thoracic SCI individuals exhibited a similar degree of hypoventilation and dose-dependent increase in inspiratory duty cycle in response to UA obstruction. Passive UA collapsibility is increased in both cervical and thoracic SCI compared with control. The neuromuscular compensatory responses to UA obstruction during sleep are preserved in chronic SCI and are independent of the level of injury.

[Pathophysiology of sleep-disordered breathing]

Sleep is a phase during which the respiratory system undergoes major changes. These changes lead to greater vulnerability and a greater risk of abnormalities, even in normal individuals. In the transition from wakefulness to sleep, there is commonly an increase in upper airway resistance and impairment of various protective responses and reflexes, which are efficient in promoting and maintaining upper airway patency during wakefulness. In individuals who present risk factors, such as anatomical abnormalities in the upper airway, these sleep-related changes cannot be efficaciously compensated, which increases the chances that sleep-disordered breathing will occur. Sleep-disordered breathing is characterized by a reduction in the size of upper airways, although the degree of the reduction varies. This reduction has multifactorial causes, which include anatomical abnormalities in the upper airway, alterations in the neuromuscular response and impairment of receptors in the upper airway. Upper airway functional and anatomical changes are likely to have genetic components, and, therefore, individuals exposed to certain environmental factors, such as allergies, have a greater chance of developing sleep-disordered breathing.

Upper airway collapsibility during REM sleep in children with the obstructive sleep apnea syndrome

STUDY OBJECTIVES

In children, most obstructive events occur during rapid eye movement (REM) sleep. We hypothesized that children with the obstructive sleep apnea syndrome (OSAS), in contrast to age-matched control subjects, would not maintain airflow in the face of an upper airway inspiratory pressure drop during REM sleep.

DESIGN

During slow wave sleep (SWS) and REM sleep, we measured airflow, inspiratory time, inspiratory time/total respiratory cycle time, respiratory rate, tidal volume, and minute ventilation at a holding pressure at which flow limitation occurred and at 5 cm H2O below the holding pressure in children with OSAS and in control subjects.

SETTING

Sleep laboratory.

PARTICIPANTS

Fourteen children with OSAS and 23 normal control subjects.

RESULTS

In both sleep states, control subjects were able to maintain airflow, whereas subjects with OSAS preserved airflow in SWS but had a significant decrease in airflow during REM sleep (change in airflow of 18.58 +/- 12.41 mL/s for control subjects vs -44.33 +/- 14.09 mL/s for children with OSAS, P = 0.002). Although tidal volume decreased, patients with OSAS were able to maintain minute ventilation by increasing the respiratory rate and also had an increase in inspiratory time and inspiratory time per total respiratory cycle time

CONCLUSION

Children with OSAS do not maintain airflow in the face of upper-airway inspiratory-pressure drops during REM sleep, indicating a more collapsible upper airway, compared with that of control subjects during REM sleep. However, compensatory mechanisms exist to maintain minute ventilation. Local reflexes, central control mechanisms, or both reflexes and control mechanisms need to be further explored to better understand the pathophysiology of this abnormality and the compensation mechanism.

Effect of a high-flow open nasal cannula system on obstructive sleep apnea in children

OBJECTIVE

Obstructive sleep apnea syndrome in children is associated with significant morbidity. Continuous positive airway pressure (CPAP) treats obstructive apnea in children, but is impeded by low adherence. We, therefore, sought to assess the effect of warm humidified air delivered through an open nasal cannula (treatment with nasal insufflation [TNI]) on obstructive sleep apnea in children with and without adenotonsillectomy.

METHODS

Twelve participants (age: 10 +/- 1 years; BMI: 35 +/- 14 kg/m(2)), with obstructive apnea-hypopnea syndrome ranging from mild to severe (2-36 events per hour) were administered 20 L/min of air through a nasal cannula. Standard sleep architecture, sleep-disordered breathing, and arousal indexes were assessed at baseline, on TNI, and on CPAP. Additional measures of the percentage of time with inspiratory flow limitation, respiratory rate, and inspiratory duty cycle were assessed at baseline and on TNI.

RESULTS

TNI reduced the amount of inspiratory flow limitation, which led to a decrease in respiratory rate and inspiratory duty cycle. TNI improved oxygen stores and decreased arousals, which decreased the occurrence of obstructive apnea from 11 +/- 3 to 5 +/- 2 events per hour (P < .01). In the majority of children, the reduction in the apnea-hypopnea index on TNI was comparable to that on CPAP.

CONCLUSIONS

TNI offers an alternative to therapy to CPAP in children with mild-to-severe sleep apnea. Additional studies will be needed to determine the efficacy of this novel form of therapy.

Systemic vs. central administration of common hypnotics reveals opposing effects on genioglossus muscle activity in rats

STUDY OBJECTIVES

To determine if systemic administration of selected sedative-hypnotics that modulate the function of the y-amino-butyric acid-A (GABAA) receptor can: (i) delay arousal thereby allowing genioglossus (GG) activity to increase more in response to respiratory stimulation during sleep, (ii) also cause the robust increase in GG activity during undisturbed sleep recently observed with barbiturates. We also determined effects on GG activity with local application to the hypoglossal motor nucleus (HMN).

DESIGN, PARTICIPANTS, AND INTERVENTIONS

Sleep-wake states, GG and diaphragm activities were recorded in freely-behaving rats after systemic administration of lorazepam (0.5 mg/kg and 1 mg/kg, n = 9 and 5 mg/kg, n = 7), zolpidem (5 mg/kg and 10 mg/kg, n = 6) and the antihistamine diphenhydramine (20 mg/kg, n = 9). Rats were also exposed to ramp increases in inspired CO2 in NREM sleep. The effects of lorazepam and zolpidem applied directly to the HMN were also determined in 37 anesthetized rats.

MEASUREMENTS AND RESULTS

Lorazepam, zolpidem and diphenhydramine all increased arousal threshold, consistent with their sedative action. GG activity before arousal in response to hypercapnia was increased with lorazepam and zolpidem only, an effect mainly due to increased baseline activity before CO2 stimulation. Lorazepam and zolpidem applied directly to the HMN, however, decreased GG activity.

CONCLUSIONS

Lorazepam and zolpidem have an inhibitory effect on GG activity via local effects at the HMN. Following systemic administration, however, this inhibitory effect can be outweighed both by a delay in arousal (allowing greater CO2-mediated respiratory stimulation in sleep) and excitatory influences on baseline GG activity via mechanisms operating outside the HMN.

Relationship between REM density, duty cycle, and obstructive sleep apnea in children

STUDY OBJECTIVES

The pattern and distribution of rapid eye movement (REM) sleep changes during development, yet there have been few studies of REM density in children. Although children with obstructive apnea syndrome (OSAS) obstruct primarily during REM sleep, the relationship between REM density and obstructive apnea has not been established for this population. We hypothesized that (i) REM density and REM cycle duration increases over the course of the night in children, (ii) the duty cycle (inspiratory time divided by respiratory cycle time) increases over the course of the night in children with suspected OSAS, and (iii) the increase in REM density over the course of the night is associated with increased severity of obstructive apnea.

DESIGN

REM density and respiratory parameters were measured during polysomnography.

SETTING

Sleep laboratory

PATIENTS

76 children with suspected OSAS.

INTERVENTIONS

NA MEASUREMENTS AND RESULTS: REM density and the duration of REM cycles increased over the course of the night until the fifth REM cycle, and then stabilized. The duty cycle increased across the first 6 REM cycles. However, the apnea hypopnea index (AHI) did not increase across REM cycles, and was not affected by the changes in REM density or duty cycle. We speculate that the increase in the duty cycle is a compensatory response to increased upper airway loads during sleep, and that this may lead to ventilatory or upper airway muscle fatigue.

Areas of the brain concerned with ventilatory load compensation in awake man

There is broad agreement that the awake human ventilatory response to a moderate inspiratory load consists of a prolongation of inspiratory time (T(I)) with a maintenance of tidal volume (V(T)) and end-tidal P(C)(O(2)) (P(ET,C)(O(2))), the response being severely blunted in sleep. There is no agreement on the mechanisms underlying this ventilatory response. Six naive healthy males (aged 39-44) were studied supine with their heads in a positron emission tomography (PET) scanner to allow relative regional cerebral blood flow (rCBF) to be measured with H(2)(15)O given intravenously. A linearised resistive load (24 cmH(2)O (l s(-1))(-1)) could be added to the inspiratory limb of a breathing valve inserted into a tightly fitting facemask; inspiratory flow was measured with a pneumotachograph. The load was applied, without alerting the subject, when the radioactivity first reached the head. Six scans were performed with and without the load, in each subject. Relative rCBF contrasts between the loaded and unloaded breathing states showed significant activations in inferior parietal cortex, prefrontal cortex, midbrain, basal ganglia and multiple cerebellar sites. No activations were found in the primary sensorimotor cortex. The findings suggest that there is a pattern of motor behavioural response to the uncomfortable sensation that inspiration is impeded. This results in a prolongation of T(I), the maintenance of V(T) and a reduction in the degree of discomfort, presumably because of the reduction of mean negative pressure in the airways.

Influence of gender on upper airway mechanics: upper airway resistance and Pcrit

It has been proposed that the difference in sleep apnea prevalence is related to gender differences in upper airway anatomy and physiology. To explain the prevalence difference, we hypothesized that men would have an increased upper airway resistance and increased critical closing pressure (Pcrit) compared with women. In protocol 1, resistance at two points, fixed flow of 0.2 l/s (RL) and peak flow (Rpk), was measured in 33 men and 27 women without significant sleep-disordered breathing. We found no difference in either RL (-6.9 +/- 5.9 vs. -8.6 +/- 8.2 cmH2O) or Rpk (-9.3 +/- 6.8 vs. -10.0 +/- 11.9 cmH2O) between the men and women. A multiple linear regression to correct for the effects of age and body mass index confirmed that gender had no effect on resistance. In protocol 2, Pcrit was measured in eight men and eight women without sleep-disordered breathing. We found no difference in Pcrit (-10.4 +/- 3.1 vs. -8.8 +/- 2.7 cmH2O) between men and women. We conclude that there are no significant differences in collapsibility between men and women. We present an unifying hypothesis to explain the divergent findings of gender differences in upper airway physiology.

Chemical control stability in patients with obstructive sleep apnea

The role of chemical control instability in the pathogenesis of obstructive sleep apnea (OSA) is not clear. We studied 32 patients with OSA during sleep while their upper airway was stabilized with continuous positive airway pressure. Twelve patients had repetitive OSA whenever they were asleep, regardless of body position or sleep stage, and were classified as having severe OSA (apnea-hypopnea index [AHI] = 88 +/- 19). The remaining 20 patients had sporadic OSA or repetitive OSA for only part of the time (mild/moderate OSA; AHI = 27 +/- 16). Susceptibility to periodic breathing (PB) was assessed by gradually increasing controller gain, using proportional assist ventilation. The increase in loop gain (LG) at each assist level was quantified from the ratio of assisted tidal volume (VT) to the VT obtained during single-breath reloading tests (VT amplification factor [VTAF]). Nine of 12 patients with severe OSA developed PB, with recurrent central apneas, whereas only six of 20 patients in the mild/moderate group developed PB (p < 0.02). This difference was observed despite the subjection of the mild/moderate group to greater amplification of LG; the highest values of VTAF in the mild/moderate and severe groups were 2.7 +/- 1.0 and 1.9 +/- 0.7, respectively (p < 0.01). We conclude that the chemical control system is more unstable in patients with severe OSA than in patients with milder OSA. We speculate that this may contribute to the severity of OSA, at least in some patients.

Anaesthesia and sleep apnoea

Sleep disordered breathing is a common problem affecting all age groups, particularly in association with certain other medical conditions and syndromes. The pathological consequences of the disorder may be severe, with significant implications for the perioperative management of sufferers. Research into the effects of surgery and anaesthesia on sleep is very much in its infancy. Understanding of the implications of sleep disturbance and sleep disordered breathing for perioperative morbidity and mortality is limited. While several observations have led to considerable speculation in the literature, evidence of a causal relationship is still largely lacking. Anaesthetists are ideally placed to screen large numbers of people for sleep disordered breathing, a source of considerable community morbidity. Recognizing the symptoms, signs and associations of the condition during the preoperative visit is important in planning management, as is recognition of the likelihood of OSA in patients who present difficulty with tracheal intubation or airway maintenance. Particular care is required in the perioperative management of patients with diagnosed or suspected sleep apnoea.

Airway mechanics and ventilation in response to resistive loading during sleep: influence of gender

The male predominance in obstructive sleep apnea (OSA) is currently poorly understood although differences in pharyngeal airway anatomy and physiology have been proposed. As the response to inspiratory resistive loading (IRL) provides important information on both airway collapsibility (mechanics) and ventilatory control, we compared this respiratory response in eight normal women and eight age and body mass index (BMI)-matched men, during stable nonrapid eye movement (NREM) sleep. Upper airway mechanics, ventilation, plus activation of two dilator muscles (genioglossus [GG] and tensor palatini [TP]) were monitored during basal breathing (BL), followed by four sequentially applied loads (5, 10, 15, 25 cm H(2)O/L/s) for three breaths each. Men developed more severe hypopnea in response to identical applied external loads than did women. At a resistance of 25 cm H(2)O/L/s, VT decreased by 26 +/- 1% in women compared with 44 +/- 1% in men (differences between sexes p < 0.05). Pharyngeal resistance (Rpha) in response to IRL increased significantly more in men than women (37.3 +/- 11.2 cm H(2)O/L/s in men at maximal load, compared with an increase of 6.6 +/- 3.9 cm H(2)O/L/s in women, p < 0.05). Men and women had near identical minute ventilation responses to total load (applied extrinsic plus measured intrinsic), implying no differences in central drive or load response. There were no significant increases in GG or TP activation in response to IRL in either sex. We conclude that normal men are more vulnerable to load-induced hypoventilation than women, due to increased upper airway collapse, which could not be explained by differences in dilator muscle activation. This implies a fundamental difference in the upper airway anatomy and/or tissue characteristics between the two sexes.

The respiratory response to inspiratory resistive loading during rapid eye movement sleep in humans

We investigated the respiratory response to an added inspiratory resistive load (IRL) during rapid eye movement (REM) sleep in humans and compared this with those in non-REM (NREM) sleep and wakefulness. Results were obtained from 7 out of 15 healthy subjects (n = 7; 32 +/- 9 years, mean +/- s.d.). Linearised IRLs (4 and 12 cmH(2)O l(-1) s(-1)) were applied for five breaths during NREM sleep (4-10 trials per subject; total 101), REM sleep (2-5 trials; total 46) and wakefulness (2-3 trials; total 40). Respiratory variables were compared, between unloaded breathing (UL: mean of 5 breaths preceding IRL) and the 1st (B1) and 5th (B5) loaded breaths in each state. During wakefulness, 12 cmH(2)O l(-1) s(-1) IRL produced an immediate respiratory compensation with prolongation of inspiratory time (T(I); UL: 2.0 +/- 0.6; B1: 2.6 +/- 0.7 s) and an increase in tidal volume (V(T); UL: 0.49 +/- 0.12; B1: 0.52 +/- 0.12 l). During REM sleep, T(I) was prolonged (UL: 2.0 +/- 0.3; B1: 2.2 +/- 0.5 s), although V(T) fell (UL: 0.27 +/- 0.15; B1: 0.22 +/- 0.10 l). For both wakefulness and REM sleep the TI response was significantly greater than seen in NREM sleep (UL: 1.9 +/- 0.3; B1: 1.9 +/- 0.3 s.). For VT, only the wakefulness response was significantly different from NREM sleep (UL: 0.31 +/- 0.14; B1: 0.21 +/- 0.10 l). The B5 responses were not significantly different between states for any of the variables. REM sleep is associated with partial respiratory load compensation suggesting that exacerbation of sleep disordered breathing in REM (compared to NREM) sleep is unlikely to be secondary to an inability to overcome increases in upper airway resistance.

Chronic recordings of hypoglossal nerve activity in a dog model of upper airway obstruction

The activity of the hypoglossal nerve was recorded during pharyngeal loading in sleeping dogs with chronically implanted cuff electrodes. Three self-coiling spiral-cuff electrodes were implanted in two beagles for durations of 17, 7, and 6 mo. During quiet wakefulness and sleep, phasic hypoglossal activity was either very small or not observable above the baseline noise. Applying a perpendicular force on the submental region by using a mechanical device to narrow the pharyngeal airway passage increased the phasic hypoglossal activity, the phasic esophageal pressure, and the inspiratory time in the next breath during non-rapid-eye-movement sleep. The phasic hypoglossal activity sustained at the elevated level while the force was present and increased with increasing amounts of loading. The hypoglossal nerve was very active in rapid-eye-movement sleep, especially when the submental force was present. The data demonstrate the feasibility of chronic recordings of the hypoglossal nerve with cuff electrodes and show that hypoglossal activity has a fast and sustained response to the internal loading of the pharynx induced by applying a submental force during non-rapid-eye-movement sleep.

Response to inspiratory resistive loading during sleep in normal children and children with obstructive apnea

The response to inspiratory resistance loading (IRL) of the upper airway during sleep in children is not known. We, therefore, evaluated the arousal responses to IRL during sleep in children with the obstructive sleep apnea syndrome (OSAS) compared with controls. Children with OSAS aroused at a higher load than did controls (23 +/- 8 vs. 15 +/- 7 cmH(2)O. l(-1). s; P < 0.05). Patients with OSAS had higher arousal thresholds during rapid eye movement (REM) vs. non-REM sleep (P < 0.001), whereas normal subjects had lower arousal thresholds during REM (P < 0.005). Ventilatory responses to IRL were evaluated in the controls. There was a marked decrease in tidal volume both immediately (56 +/- 17% of baseline at an IRL of 15 cmH(2)O. l(-1). min; P < 0.001) and after 3 min of IRL (67 +/- 23%, P < 0.005). The duty cycle increased. We conclude that children with OSAS have impaired arousal responses to IRL. Despite compensatory changes in respiratory timing, normal children have a decrease in minute ventilation in response to IRL during sleep. However, arousal occurs before gas-exchange abnormalities.

Entrainment of respiration in humans by periodic lung inflations. Effect of state and CO(2)

Lack of synchrony between a patient and the mechanical ventilator occurs when the respiratory rhythm of the patient fails to entrain to machine inflations. Entrainment implies a resetting of the respiratory rhythm such that a fixed temporal relationship exists between the onset of inspiratory activity and the onset of a mechanical breath. We examined the entrainment response to mechanical ventilation of normal humans over a range of machine rates during wakefulness and during isocapnic and hypercapnic NREM sleep. Wakefulness facilitated 1:1 entrainment of the respiratory rhythm to the mechanical ventilator over a wider range of machine frequencies than during NREM sleep (p < 0.001); isocapnic and hypercapnic conditions did not differ (p = 0.95). To evaluate the Hering-Breuer reflexes in the resetting of the respiratory rhythm during sleep, we examined changes in neural inspiratory time (TI) as the relationship between inspiratory efforts and onset of machine inflations changed. As inspiratory efforts extended into the machine inflation cycle, neural TI shortened. We conclude that entrainment responses of normal humans to mechanical ventilation differ depending on state, but mild increases in respiratory drive caused by CO(2) stimulation do not affect these entrainment responses. Furthermore, the changes in neural TI are consistent with observations in animal studies in which Hering-Breuer reflexes mediated entrainment.

Pathogenesis of obstructive sleep apnea

Obstructive sleep apnea is a fairly common disorder with significant adverse health consequences. However, the pathogenetic mechanisms remain incompletely understood. Upper airway (UA) patency is determined by several neuromuscular and nonneuromuscular factors including (1) UA dilating muscle activity, (2) the collapsing transmural pressure generated during inspiration, (3) changes in caudal traction, (4) vasomotor tone, and (5) mucosal adhesive forces. This review addresses the effect of sleep on UA function and how these factors conspire to cause UA obstruction.

Susceptibility to periodic breathing with assisted ventilation during sleep in normal subjects

Assisted ventilation with pressure support (PSV) or proportional assist (PAV) ventilation has the potential to produce periodic breathing (PB) during sleep. We hypothesized that PB will develop when PSV level exceeds the product of spontaneous tidal volume (VT) and elastance (VTsp . E) but that the actual level at which PB will develop [PSV(PB)] will be influenced by the DeltaPCO2 (difference between eupneic PCO2 and CO2 apneic threshold) and by DeltaRR [response of respiratory rate (RR) to PSV]. We also wished to determine the PAV level at which PB develops to assess inherent ventilatory stability in normal subjects. Twelve normal subjects underwent polysomnography while connected to a PSV/PAV ventilator prototype. Level of assist with either mode was increased in small steps (2-5 min each) until PB developed or the subject awakened. End-tidal PCO2, VT, RR, and airway pressure (Paw) were continuously monitored, and the pressure generated by respiratory muscle (Pmus) was calculated. The pressure amplification factor (PAF) at the highest PAV level was calculated from [(DeltaPaw + Pmus)/Pmus], where DeltaPaw is peak Paw - continuous positive airway pressure. PB with central apneas developed in 11 of 12 subjects on PSV. DeltaPCO2 ranged from 1.5 to 5.8 Torr. Changes in RR with PSV were small and bidirectional (+1.1 to -3.5 min-1). With use of stepwise regression, PSV(PB) was significantly correlated with VTsp (P = 0.001), E (P = 0.00009), DeltaPCO2 (P = 0.007), and DeltaRR (P = 0.006). The final regression model was as follows: PSV(PB) = 11.1 VTsp + 0.3E - 0.4 DeltaPCO2 - 0.34 DeltaRR - 3.4 (r = 0.98). PB developed in five subjects on PAV at amplification factors of 1.5-3.4. It failed to occur in seven subjects, despite PAF of up to 7.6. We conclude that 1) a PCO2 apneic threshold exists during sleep at 1.5-5.8 Torr below eupneic PCO2, 2) the development of PB during PSV is entirely predictable during sleep, and 3) the inherent susceptibility to PB varies considerably among normal subjects.

Effect of resistive loading on variational activity of breathing

To examine the effect of resistive loading on variational activity of breathing, we studied 18 healthy subjects breathing at rest and with inspiratory resistive loads of 3 and 6 cm H2O/L/s, applied randomly for 1 h each. Compared with resting breathing, a resistive load of 3 cm H2O/L/s decreased the total variational activity of expiratory time (TE) and minute ventilation (V I), whereas a load of 6 cm H2O/L/s increased the total variational activity of inspiratory time (TI). Compared with the load of 3 cm H2O/L/s, the load of 6 cm H2O/L/s increased total variational activity of tidal volume (VT), TI, TE, and V I. Partitioning of the total variational activity revealed that these alterations were due to changes in the random uncorrelated fraction. Compared with rest, both the resistive loads of 3 and 6 cm H2O/L/s increased the number of breath lags displaying significant serial correlations ("short-term memory") of TI. Compared with rest, the load of 3 cm H2O/L/s increased the autocorrelation coefficient at a lag of one breath for VT and the load of 6 cm H2O/L/s increased the correlated fraction of variational activity of VT. Thus, three measures of correlated behavior-autocorrelation coefficient at a lag of 1 breath, "short-term memory," and the correlated fraction of total variational activity- increased with loading. In conclusion, resistive loading changed total variational activity according to the size of the load: the random fraction decreased with the smaller load but increased with the larger load; in contrast, correlated behavior increased with both loads. The different behaviors of random and correlated variability with loading may reflect different physiologic influences on respiratory control.

Pathophysiology of upper airway obstruction during sleep

Obstructive sleep apnea is a common medical disorder with significant adverse health consequences. The pathogenesis of pharyngeal obstruction during sleep, however, remains elusive. This article addresses the key mechanisms of upper airway (UA) obstruction including the role of transmural pressure, pharyngeal compliance, pharyngeal dilating muscle activity and non-neuromuscular factors. A proposed scheme of the pathophysiology of UA obstruction is outlined.

Control of breathing during sleep assessed by proportional assist ventilation

We used proportional assist ventilation (PAV) to evaluate the sources of respiratory drive during sleep. PAV increases the slope of the relation between tidal volume (VT) and respiratory muscle pressure output (Pmus). We reasoned that if respiratory drive is dominated by chemical factors, progressive increase of PAV gain should result in only a small increase in VT because Pmus would be downregulated substantially as a result of small decreases in PCO2. In the presence of substantial nonchemical sources of drive [believed to be the case in rapid-eye-movement (REM) sleep] PAV should result in a substantial increase in minute ventilation and reduction in PCO2 as the output related to the chemically insensitive drive source is amplified severalfold. Twelve normal subjects underwent polysomnography while connected to a PAV ventilator. Continuous positive air pressure (5.2 +/- 2.0 cmH2O) was administered to stabilize the upper airway. PAV was increased in 2-min steps from 0 to 20, 40, 60, 80, and 90% of the subject's elastance and resistance. VT, respiratory rate, minute ventilation, and end-tidal CO2 pressure were measured at the different levels, and Pmus was calculated. Observations were obtained in stage 2 sleep (n = 12), slow-wave sleep (n = 11), and REM sleep (n = 7). In all cases, Pmus was substantially downregulated with increase in assist so that the increase in VT, although significant (P < 0.05), was small 0.08 liter at the highest assist). There was no difference in response between REM and non-REM sleep. We conclude that respiratory drive during sleep is dominated by chemical control and that there is no fundamental difference between REM and non-REM sleep in this regard. REM sleep appears to simply add bidirectional noise to what is basically a chemically controlled respiratory output.

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.

Control of the respiratory cycle in conscious humans

We studied in conscious humans the relative strength of mechanisms controlling timing and drive components of the respiratory cycle around their resting set points. A system of auditory feedback with end-tidal PCO2 held constant in mild hyperoxia via an open circuit was used to induce subjects independently to change inspiratory time (TI) and tidal volume (VTI) over a wide range above and below the resting values for every breath for up to 1 h. Four protocols were studied in various levels of hypercapnia (1-5% inspired CO2). We found that TI (and expiratory time) could be changed over a wide range (1.17 - 2.86 s, P < 0.01 for TI) and VTI increased by > or = 500 ml (P < 0.01) without difficulty. However, in no protocol was it possible to decrease VTI below the free-breathing resting value in response to reduction of auditory feedback thresholds by up to 600 ml. This applied at all levels of chemical drive studied, with resting VTI values varying from 1.06 to 1.74 liters. When reduction in VTI was forced by the more "programmed" procedure of isocapnic panting, end-expiratory of volume was sacrificed to ensure that peak tidal volume reached a fixed absolute lung volume. These results suggest that the imperative for control of resting breathing is to prevent reduction of VTI below the level dictated by the prevailing chemical drive, presumably to sustain metabolic requirements of the body, whereas respiratory timing is weakly controlled consistent with the needs for speech and other nonmetabolic functions of breathing.

Changes in diaphragmatic EMG activity during sleep in space

It remains unknown how increased upper airway resistance (UAR) during sleep could be a function of gravity. We therefore conducted quantitative evaluation of the gravitational influence on diaphragmatic EMG activity (EMGdi) in an astronaut to estimate the effect of UAR in space. EMGdi was recorded by paired surface electrodes on the ground (control, C) and abroad a short-term space mission (space, S) for 30 consecutive h. Mean EMGdi recorded during quiet breathing in wakefulness was assigned the value of 100. EMGdi in C was significantly enhanced in all sleep stages compared with that while awake in the supine position (mean +/- SD, 230 +/- 23.2% in non-rapid eye movement (non-REM) Stage II, 233 +/- 13.8 in slow-wave sleep, and 233 +/- 40.0 in REM sleep versus 100 +/- 17.3 in wakefulness, p < 0.001). In contrast, there was no statistical difference in EMGdi in S between awake and any non-REM sleep stage (mean +/- SD, 100 +/- 20.5% in wakefulness versus 103 +/- 16.9 in non-REM Stage II and 100 +/- 14.8 in slow-wave sleep; NS). However, EMGdi in REM sleep in S was statistically greater (132 +/- 28.3%) than that during wakefulness or any other sleep stage in space (p < 0.001). Therefore, gravity may play a much more significant role in the normal healthy human in the increased upper airway resistance during sleep than the relative atonia of the upper airway muscles.

Influence of non-REM sleep on inspiratory muscle activity and lung volume in asthmatic patients

To determine the effect of non-REM (NREM) sleep on inspiratory muscle electromyographic (EMG) activity in asthmatic patients and the subsequent effect of such changes on FRC, 12 asthmatic patients with nocturnal worsening were monitored overnight in a horizontal volume-displacement body plethysmograph. During studies FRC was monitored using the single inspiratory occlusion technique, whereas EMG activities of the diaphragm (DI), intercostal (IC), and sternocleidomastoid (SCM) muscles were monitored using surface electrodes. FRC decreased progressively, from 3.63 +/- 0.27 L while awake prior to "lights out" to 3.15 +/- 0.27 L after 60 min of NREM sleep (p < 0.005). With awakening at the end of the study FRC increased to 4.69 +/- 0.48 L (p < 0.01). Onset of NREM sleep was also associated with significant (p < 0.05) reductions in EMG tonic activities of all three inspiratory muscles. These levels of tonic activity returned and were further augmented (p < 0.001) with awakening at the conclusion of the study. Linear relationships were demonstrated between sleep-state dependent changes in FRC and EMG tonic activity for each of the three inspiratory muscles (r = 0.98, 0.93, and 0.97 for DI, IC, and SCM, respectively, p < 0.01). NREM sleep did not significantly alter mean inspiratory phasic activities of these muscles, although there was considerable heterogeneity between individual subjects in the effects of NREM sleep.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of sleep on respiratory muscle activity during mechanical ventilation

The purpose of this study was to determine whether consciousness was critical for the expression of neuromechanical inhibition of breathing during mechanical ventilation. This same mechanical ventilation model also was used to evaluate the relative importance of sleep state in causing CO2 retention during sleep. Positive pressure ventilation was used to suppress respiratory muscle activity; CO2 was then added until a reappearance of inspiratory effort, which defined the recruitment threshold (PCO2RT). Keeping the mechanics of the respiratory system constant through the use of passive mechanical ventilation allowed us to measure the output of the respiratory controller, independent of these parameters. Eight normal subjects were mechanically hyperventilated with a nasal mask during wakefulness and sleep with matched flow rates, frequencies, and tidal volumes. When inspiratory muscle activity was undetectable and end-tidal PCO2 (PETCO2) fell below 30 mm Hg, inspired CO2 was added in stepped increments until inspiration reoccurred. The sleeping state increased both eupneic PETCO2 (42 +/- 4 versus 38 +/- 3 mm Hg) and PCO2RT (48 +/- 3 versus 46 +/- 2 mm Hg) compared with that during wakefulness. Neuromechanical inhibition of inspiratory muscle activity during mechanical ventilation was present during both wakefulness and sleep, as evidenced by the mean difference between PCO2RT and eupneic PETCO2 of 8 and 6 mm Hg, respectively. Recruitment thresholds during wakefulness and sleep were compared to evaluate the effect of sleep on respiratory motor output independent of changes in load, i.e., respiratory mechanics held constant.(ABSTRACT TRUNCATED AT 250 WORDS)