Postural effects on pharyngeal protective reflex mechanisms

Effects of hypoxia on genioglossus and scalene reflex responses to brief pulses of negative upper-airway pressure during wakefulness and sleep in healthy men

Hypoxia can depress ventilation, respiratory load sensation, and the cough reflex, and potentially other protective respiratory reflexes such as respiratory muscle responses to increased respiratory load. In sleep-disordered breathing, increased respiratory load and hypoxia frequently coexist. This study aimed to examine the effects of hypoxia on the reflex responses of 1) the genioglossus (the largest upper airway dilator muscle) and 2) the scalene muscle (an obligatory inspiratory muscle) to negative-pressure pulse stimuli during wakefulness and sleep. We hypothesized that hypoxia would impair these reflex responses. Fourteen healthy men, 19-42 yr old, were studied on two separate occasions, approximately 1 wk apart. Bipolar fine-wire electrodes were inserted orally into the genioglossus muscle, and surface electrodes were placed overlying the left scalene muscle to record EMG activity. In random order, participants were exposed to mild overnight hypoxia (arterial oxygen saturation approximately 85%) or medical air. Respiratory muscle reflex responses were elicited via negative-pressure pulse stimuli (approximately -10 cmH(2)O at the mask, 250-ms duration) delivered in early inspiration during wakefulness and sleep. Negative-pressure pulse stimuli resulted in a short-latency activation followed by a suppression of the genioglossus EMG that did not alter with hypoxia. Conversely, the predominant response of the scalene EMG to negative-pressure pulse stimuli was suppression followed by activation with more pronounced suppression during hypoxia compared with normoxia (mean +/- SE suppression duration 64 +/- 6 vs. 38 +/- 6 ms, P = 0.006). These results indicate differential sensitivity to the depressive effects of hypoxia in the reflex responsiveness to sudden respiratory loads to breathing between these two respiratory muscles.

Current trends in the treatment of obstructive sleep apnea

Obstructive sleep apnea (OSA) is a condition of partial or complete upper airway obstruction leading to increased resistance to airflow and potential cessation of breathing during sleep. Effective treatment of OSA is challenging and there has been greater recognition by the medical and dental disciplines. By understanding the rationale, indications, benefits, risks and success of the various treatment options available, clinicians will be able to make more informed treatment recommendations in patient management.

Drive to the human respiratory muscles

The motor control of the respiratory muscles differs in some ways from that of the limb muscles. Effectively, the respiratory muscles are controlled by at least two descending pathways: from the medulla during normal quiet breathing and from the motor cortex during behavioural or voluntary breathing. Neurophysiological studies of single motor unit activity in human subjects during normal and voluntary breathing indicate that the neural drive is not uniform to all muscles. The distribution of neural drive depends on a principle of neuromechanical matching. Those motoneurones that innervate intercostal muscles with greater mechanical advantage are active earlier in the breath and to a greater extent. Inspiratory drive is also distributed differently across different inspiratory muscles, possibly also according to their mechanical effectiveness in developing airway negative pressure. Genioglossus, a muscle of the upper airway, receives various types of neural drive (inspiratory, expiratory and tonic) distributed differentially across the hypoglossal motoneurone pool. The integration of the different inputs results in the overall activity in the muscle to keep the upper airway patent throughout respiration. Integration of respiratory and non-respiratory postural drive can be demonstrated in respiratory muscles, and respiratory drive can even be observed in limb muscles under certain circumstances. Recordings of motor unit activity from the human diaphragm during voluntary respiratory tasks have shown that depending on the task there can be large changes in recruitment threshold and recruitment order of motor units. This suggests that descending drive across the phrenic motoneurone pool is not necessarily consistent. Understanding the integration and distribution of drive to respiratory muscles in automatic breathing and voluntary tasks may have implications for limb motor control.

Pharyngeal motor control and the pathogenesis of obstructive sleep apnea

The upper airway in patients with obstructive sleep apnea (OSA) is thought to collapse during sleep at least in part, because of a sleep related reduction in upper airway dilator muscle activity. Therefore, a comprehensive understanding of the neural regulation of these muscles is warranted. The dilator muscles can be classified in two broad categories; those that have respiratory related activity and those that fire constantly throughout the respiratory cycle. The motor control of these two groups likely differs with the former receiving input from respiratory neurons and negative pressure reflex circuits. The activity of both muscle groups is reduced shortly after sleep onset, indicating that both receive input from brainstem neurons involved in sleep regulation. In the apnea patient, this may lead to pharyngeal airway collapse. This review briefly describes the currently proposed sleep and respiratory neural pathways and how these circuits interact with the upper airway dilator muscle motorneurones, including recent evidence from animal studies.

Genioglossus reflex inhibition to upper-airway negative-pressure stimuli during wakefulness and sleep in healthy males

During wakefulness, obstructive sleep apnoea patients appear to compensate for an anatomically narrow upper airway by increasing upper airway dilator muscle activity, e.g. genioglossus, at least partly via a negative-pressure reflex that may be diminished in sleep. Previous studies have assessed the negative-pressure reflex using multi-unit, rectified, moving-time-average EMG recordings during brief pulses of negative upper-airway pressure. However, moving-time averaging probably obscures the true time-related reflex morphology, potentially masking transient excitatory and inhibitory components. This study aimed to re-examine the genioglossus negative-pressure reflex in detail, without moving-time averaging. Bipolar fine-wire electrodes were inserted per orally into the genioglossus muscle in 17 healthy subjects. Two upper airway pressure catheters were inserted per nasally. Genioglossus EMG reflex responses were generated via negative-pressure stimuli (approximately -10 cmH2O at the choanae, 250 ms duration) delivered during wakefulness and sleep. Ensemble-averaged, rectified, genioglossus EMG recordings demonstrated reflex activation (onset latency 26+/-1 ms; peak amplitude 231+/-29% of baseline) followed by a previously unreported suppression (peak latency 71+/-4 ms; 67+/-8% of baseline). Single-motor-unit activity, clearly identifiable in approximately 10% of trials in six subjects, showed a concomitant increase in the interspike interval from baseline (26+/-9 ms, P=0.01). Genioglossus negative-pressure reflex morphology and amplitude of the initial peak were maintained in non-rapid eye movement (NREM) sleep but suppression amplitude was more pronounced during NREM and declined further during REM sleep compared to wakefulness. These data indicate there are both excitatory and inhibitory components to the genioglossus negative-pressure reflex which are differentially affected by state.

Mechanisms used to restore ventilation after partial upper airway collapse during sleep in humans

BACKGROUND

Most patients with obstructive sleep apnoea (OSA) can restore airflow after an obstructive respiratory event without arousal at least some of the time. The mechanisms that enable this ventilatory recovery are unclear but probably include increased upper airway dilator muscle activity and/or changes in respiratory timing. The aims of this study were to compare the ability to recover ventilation and the mechanisms of compensation following a sudden reduction of continuous positive airway pressure (CPAP) in subjects with and without OSA.

METHODS

Ten obese patients with OSA (mean (SD) apnoea-hypopnoea index 62.6 (12.4) events/h) and 15 healthy non-obese non-snorers were instrumented with intramuscular genioglossus electrodes and a mask/pneumotachograph which was connected to a modified CPAP device that could deliver either continuous positive or negative pressure. During stable non-rapid eye movement sleep the CPAP was repeatedly reduced 2-10 cm H2O below the level required to eliminate flow limitation and was held at this level for 5 min or until arousal from sleep occurred.

RESULTS

During reduced CPAP the increases in genioglossus activity (311.5 (49.4)% of baseline in subjects with OSA and 315.4 (76.2)% of baseline in non-snorers, p = 0.9) and duty cycle (123.8 (3.9)% of baseline in subjects with OSA and 118.2 (2.8)% of baseline in non-snorers, p = 0.4) were similar in both groups, yet patients with OSA could restore ventilation without cortical arousal less often than non-snorers (54.1% vs 65.7% of pressure drops, p = 0.04). When ventilatory recovery did not occur, genioglossus muscle and respiratory timing changes still occurred but these did not yield adequate pharyngeal patency/ventilation.

CONCLUSIONS

Compensatory mechanisms (increased genioglossus muscle activity and/or duty cycle) often restore ventilation during sleep but may be less effective in obese patients with OSA than in non-snorers.

Genioglossal muscle response to CO2 stimulation during NREM sleep

STUDY OBJECTIVES

The objective was to evaluate the responsiveness of upper airway muscles to hypercapnia with and without intrapharyngeal negative pressure during non-rapid eye movement (NREM) sleep and wakefulness.

DESIGN

We assessed the genioglossal muscle response to CO2 off and on continuous positive airway pressure (CPAP) (to attenuate negative pressure) during stable NREM sleep and wakefulness in the supine position.

SETTING

Laboratory of the Sleep Medicine Division, Brigham and Women's Hospital.

PATIENTS OR PARTICIPANTS

Eleven normal healthy subjects.

INTERVENTIONS

During wakefulness and NREM sleep, we measured genioglossal electromyography (EMG) on and off CPAP at the normal eupneic level and at levels 5 and 10 mm Hg above the awake eupneic level.

MEASUREMENTS AND RESULTS

We observed that CO2 could increase upper-airway muscle activity during NREM sleep and wakefulness in the supine position with and without intrapharyngeal negative pressure. The application of nasal CPAP significantly decreased genioglossal EMG at all 3 levels of PETCO2 during NREM sleep (13.0 +/- 4.9% vs. 4.6 +/- 1.6% of maximal EMG, 14.6 +/- 5.6% vs. 7.1 +/- 2.3% of maximal EMG, and 17.3 +/- 6.3% vs. 10.2 +/- 3.1% of maximal EMG, respectively). However, the absence of negative pressure in the upper airway did not significantly affect the slope of the pharyngeal airway dilator muscle response to hypercapnia during NREM sleep (0.72 +/- 0.30% vs. 0.79 +/- 0.27% of maximal EMG per mm Hg PCO2, respectively, off and on CPAP).

CONCLUSIONS

We conclude that both chemoreceptive and negative pressure reflex inputs to this upper airway dilator muscle are still active during stable NREM sleep.

Pathogenesis of Obstructive and Central Sleep Apnea

Considerable progress has been made over the last several decades in our understanding of the pathophysiology of both central and obstructive sleep apnea. Central sleep apnea, in its various forms, is generally the product of an unstable ventilatory control system (high loop gain) with increased controller gain (high hypercapnic responsiveness) generally being the cause. High plant gain can contribute under certain circumstances (hypercapnic patients). On the other hand, obstructive sleep apnea can develop as the result of a variety of physiologic characteristics. The combinations of these may vary considerably between patients. Most obstructive apnea patients have an anatomically small upper airway with augmented pharyngeal dilator muscle activation maintaining airway patency awake, but not asleep. However, individual variability in several phenotypic characteristics may ultimately determine who develops apnea and how severe the apnea will be. These include: (1) upper airway anatomy, (2) the ability of upper airway dilator muscles to respond to rising intrapharyngeal negative pressure and increasing Co(2) during sleep, (3) arousal threshold in response to respiratory stimulation, and (4) loop gain (ventilatory control instability). As a result, patients may respond to different therapeutic approaches based on the predominant abnormality leading to the sleep-disordered breathing.

Effects of topical anesthesia on upper airway resistance during wake-sleep transitions

Deformation of the upper airway (UA) by negative transmural pressure alters the activity of UA mechanoreceptors, causing a reflex increase in UA muscle activity. Topical anesthesia of the UA mucosa, which greatly reduces this reflex response, causes an increase in UA resistance during stage 2 sleep. We hypothesized that topical anesthesia of the UA mucosa would predispose to UA instability at sleep onset and, therefore, examined the effect of UA anesthesia on pharyngeal resistance (Rph) in stage 1 sleep. Eleven normal, healthy volunteers were instrumented to record standard polysomnographic variables, respiratory airflow, and UA pressure at the nasal choanae and the epiglottis. Subjects were permitted to sleep until stable stage 2 sleep was reached and were then awoken. This procedure was repeated three times to obtain reproducible wake-sleep transitions. The UA mucosa was then anesthetized with 10% lidocaine to the oropharynx and laryngopharynx, and the pharyngeal mechanics were studied during the subsequent wake-sleep transition. Three subjects were excluded because of failure to resume sleep postanesthesia. Rph was significantly higher after anesthesia during stage 1 sleep [2.88 ± 0.77 cmH2O·l−1·s (mean ± SE)] compared with control (0.95 ± 0.35 cmH2O·l−1·s; P < 0.05), but there was no difference during wakefulness. Furthermore, there was a significant rise in Rph at wake-to-sleep transitions and a significant fall in Rph at sleep-to-wake transitions after anesthesia ( P < 0.05) but not in the control condition. We conclude that sensory receptors in the UA mucosa contribute to the maintenance of UA patency at wake-sleep transition in normal humans.