Jill K. Edwards

Effect of wake‐sleep transitions and rapid eye movement sleep on pharyngeal muscle response to negative pressure in humans

1 Genioglossus (GG) activation in response to upper airway negative pressure may be an important mechanism in the maintenance of airway patency. This reflex occurs during wakefulness but is diminished during stable non‐rapid eye movement (NREM) sleep. Since obstructive events occur more commonly at wake‐sleep transitions and during rapid eye movement (REM) sleep than during stable NREM sleep, we assessed the GG reflex during these two vulnerable states. 2 Seventeen healthy adults were studied throughout one evening and overnight. Electroencephalograms (EEGs), electro‐oculograms (EOGs), submental electromyogram (EMG), GG EMG (intramuscular electrodes), and choanal plus epiglottic pressures were recorded. The GG reflex response to pulses of −8 cmH2O choanal pressure applied via nose mask during early inspiration was quantified repeatedly during relaxed wakefulness, within five breaths of wake‐sleep transition (EEG alpha‐theta transition) and during REM sleep. Only trials without EEG arousal were analysed, resulting in data from 14 subjects during sleep onset and 10 subjects during REM sleep (overall, 174–491 trials per state). 3 During wakefulness there was brisk GG reflex activation in response to negative pressure (amplitude: +78.5 ± 28.3 % baseline (mean ± s.e.m.); latency to maximal response: 177 ± 16 ms). 4 At sleep onset, although there was marked variability among individuals, there was no significant reduction in the magnitude of the GG reflex for the group as a whole (amplitude: +33.2 ± 8.2 % baseline; latency: 159 ± 15 ms). 5 In contrast, during REM sleep there was a reduction of GG reflex (amplitude: −12.6 ± 8.3 % baseline (P= 0.017vs. awake); latency: 160 ± 10 ms (n.s. vs. awake)) and greater airway collapsibility during the applied pressures (P= 0.043vs. awake). 6 We conclude that there was no systematic reduction in the GG reflex to negative pressure at sleep onset. Nonetheless, it remains possible that sleep‐deprived normal subjects and patients with sleep apnoea could react differently. 7 The apparent inhibition of the GG reflex during REM sleep may help explain why the upper airway is vulnerable to collapse during this state.

Phasic mechanoreceptor stimuli can induce phasic activation of upper airway muscles in humans

1 Upper airway dilator muscles are phasically activated throughout breathing by respiratory pattern generator neurons. Studies have shown that non‐physiological upper airway mechanoreceptive stimuli (e.g. rapidly imposed pulses of negative pressure) also activate these muscles. Such reflexes may become activated during conditions that alter airway resistance in order to stabilise airway patency. 2 To determine the contribution of ongoing mechanoreceptive reflexes to phasic activity of airway dilators, we assessed genioglossal electromyogram (GG EMG: rectified with moving time average of 100 ms) during slow (physiological) oscillations in negative pressure generated spontaneously and passively (negative pressure ventilator). 3 Nineteen healthy adults were studied while awake, during passive mechanical ventilation across normal physiological ranges of breathing rates (13–19 breaths min−1) and volumes (0.5–1.0 l) and during spontaneous breathing across the physiological range of end‐tidal carbon dioxide (PET,CO2; 32–45 mmHg). 4 Within‐breath phasic changes in airway mechanoreceptor stimuli (negative pressure or flow) were highly correlated with within‐breath phasic genioglossal activation, probably representing a robust mechanoreceptive reflex. These reflex relationships were largely unchanged by alterations in central drive to respiratory pump muscles or the rate of mechanical ventilation within the ranges studied. A multivariate model revealed that tonic GG EMG, PET,CO2 and breath duration provided no significant independent information in the prediction of inspiratory peak GG EMG beyond that provided by epiglottic pressure, which alone explained 93 % of the variation in peak GG EMG across all conditions. The overall relationship was: Peak GG EMG = 79.7 − (11.3 x Peak epiglottic pressure), where GG EMG is measured as percentage of baseline, and epiglottic pressure is in cmH2O. 5 These data provide strong evidence that upper airway dilator muscles can be activated throughout inspiration via ongoing mechanoreceptor reflexes. Such a feedback mechanism is likely to be active on a within‐breath basis to protect upper airway patency in awake humans. This mechanism could mediate the increased genioglossal activity observed in patients with obstructive sleep apnoea (i.e. reflex compensation for an anatomically smaller airway).

Control of upper airway muscle activity in younger versus older men during sleep onset

Pharyngeal dilator muscles are clearly important in the pathophysiology of obstructive sleep apnoea syndrome (OSA). We have previously shown that the activity of both the genioglossus (GGEMG) and tensor palatini (TPEMG) are decreased at sleep onset, and that this decrement in muscle activity is greater in the apnoea patient than in healthy controls. We have also previously shown this decrement to be greater in older men when compared with younger ones. In order to explore the mechanisms responsible for this decrement in muscle activity nasal continuous positive airway pressure (CPAP) was applied to reduce negative pressure mediated muscle activation. We then investigated the effect of sleep onset (transition from predominantly α to predominantly θ EEG activity) on ventilation, upper airway muscle activation and upper airway resistance (UAR) in middle‐aged and younger healthy men. We found that both GGEMG and TPEMG were reduced by the application of nasal CPAP during wakefulness, but that CPAP did not alter the decrement in activity in either muscle seen in the first two breaths following an α to θ transition. However, CPAP prevented both the rise in UAR at sleep onset that occurred on the control night, and the recruitment in GGEMG seen in the third to fifth breaths following the α to θ transition. Further, GGEMG was higher in the middle‐aged men than in the younger men during wakefulness and was decreased more in the middle‐aged men with the application of nasal CPAP. No differences were seen in TPEMG between the two age groups. These data suggest that the initial sleep onset reduction in upper airway muscle activity is due to loss of a ‘wakefulness’ stimulus, rather than to loss of responsiveness to negative pressure. In addition, it suggests that in older men, higher wakeful muscle activity is due to an anatomically more collapsible upper airway with more negative pressure driven muscle activation. Sleep onset per se does not appear to have a greater effect on upper airway muscle activity as one ages.

The influence of gender and upper airway resistance on the ventilatory response to arousal in obstructive sleep apnoea in humans

The termination of obstructive respiratory events is typically associated with arousal from sleep. The ventilatory response to arousal may be an important determinant of subsequent respiratory stability/instability and therefore may be involved in perpetuating obstructive respiratory events. In healthy subjects arousal is associated with brief hyperventilation followed by more prolonged hypoventilation on return to sleep. This study was designed to assess whether elevated sleeping upper airway resistance (RUA) alters the ventilatory response to arousal and subsequent breathing on return to sleep in patients with obstructive sleep apnoea (OSA). Inspired minute ventilation (VI), RUA and end‐tidal CO2 pressure (PET,CO2) were measured in 22 patients (11 men, 11 women) with OSA (mean ±s.e.m., apnoea–hypopnoea index (AHI) 48.9 ± 5.9 events h−1) during non‐rapid eye movement (NREM) sleep with low RUA (2.8 ± 0.3 cmH2O l−1 s; optimal continuous positive airway pressure (CPAP) = 11.3 ± 0.7 cmH2O) and with elevated RUA (17.6 ± 2.8 cmH2O l−1 s; sub‐optimal CPAP = 8.4 ± 0.8 cmH2O). A single observer, unaware of respiratory data, identified spontaneous and tone‐induced arousals of 3–15 s duration preceded and followed by stable NREM sleep. VI was compared between CPAP levels before and after spontaneous arousal in 16 subjects with tone‐induced arousals in both conditions. During stable NREM sleep at sub‐optimal CPAP, PET,CO2 was mildly elevated (43.5 ± 0.8 versus 42.5 ± 0.8 Torr). However, baseline VI (7.8 ± 0.3 versus 8.0 ± 0.3 l min−1) was unchanged between CPAP conditions. For the first three breaths following arousal, VI was higher for sub‐optimal than optimal CPAP (first breath: 11.2 ± 0.9 versus 9.3 ± 0.6 l min−1). The magnitude of hypoventilation on return to sleep was not affected by the level of CPAP and both obstructive and central respiratory events were rare following arousal. Similar results occurred after tone‐induced arousals which led to larger responses than spontaneous arousals. VI for the first breath following arousal under optimal CPAP was greater in men than women (11.0 ± 0.4 versus 7.6 ± 0.6 l min−1). These results demonstrate that the ventilatory response to arousal is influenced by pre‐arousal airway resistance and gender. Whether this contributes to the perpetuation of respiratory events and the pathogenesis of OSA is unclear.

Genioglossal inspiratory activation: Central respiratory vs mechanoreceptive influences

Upper airway dilator muscles are phasically activated during respiration. We assessed the interaction between central respiratory drive and local (mechanoreceptive) influences upon genioglossal (GG) activity throughout inspiration. GG(EMG) and airway mechanics were measured in 16 awake subjects during baseline spontaneous breathing, increased central respiratory drive (inspiratory resistive loading; IRL), and decreased respiratory drive (hypocapnic negative pressure ventilation), both prior to and following dense upper airway topical anesthesia. Negative epiglottic pressure (P(epi)) was significantly correlated with GG(EMG) across inspiration (i.e. within breaths). Both passive ventilation and IRL led to significant decreases in the sensitivity of the relationship between GG(EMG) and P(epi) (slope GG(EMG) vs P(epi)), but yielded no change in the relationship (correlation) between GG(EMG) and P(epi). During negative pressure ventilation, pharyngeal resistance increased modestly, but significantly. Anesthesia in all conditions led to decrements in phasic GG(EMG), increases in pharyngeal resistance, and decrease in the relationship between P(epi) and GG(EMG). We conclude that both central output to the GG and local reflex mediated activation are important in maintaining upper airway patency.

Upper airway collapsibility, dilator muscle activation and resistance in sleep apnoea

The calibre of the upper airway is thought to be dependant upon its passive anatomy/collapsibility and the activation of pharyngeal dilator muscles. During awake periods, the more collapsible upper airway in obstructive sleep apnoea (OSA) increases the dilator muscle activity through a negative-pressure reflex. A direct correlation between the critical closing pressure (Pcrit), as a measure of anatomy/collapsability and electromyogram (EMG) activity of genioglossus EMG (GG-EMG) and tensor palatini EMG (TP-EMG), was hypothesised. The relationship between these indices and pharyngeal resistance (Rphar) was also examined. The study involved eight males with a mean age of 48 (interquartile range 46–52) yrs with OSA, and an apnoea/hypopnoea index of 75 (65–101)·hr−1 on two nights breathing normally and on nasal continuous positive airway pressure (nCPAP). The Pcrit was measured during nonrapid eye movement sleep on nCPAP using brief, incremental reductions in mask pressure. GG-EMG and TP-EMG were measured breath-by-breath, awake, during sleep onset and on nCPAP. Rphar was measured using airway pressures and flow. Wakeful GG-EMG, early sleep TP-EMG and the sleep decrement in TP-EMG were directly related to Pcrit. Muscle activation was negatively correlated with Rphar for TP-EMG awake and GG-EMG early in sleep. In conclusion these results confirm that dilator muscle activation is directly related to airway narrowing and reduces resistance across patients with obstructive sleep apnoea.