Influence of upper airway pressure changes on genioglossus muscle respiratory activity

Effect of topical upper airway anesthesia on apnea duration through the night in obstructive sleep apnea

It has previously been reported that the duration of obstructive apneas increases from the beginning to the end of the night (M. Charbonneau, J. M. Marin, A. Olha, R. J. Kimoff, R. D. Levy, and M. Cosio. Chest 106: 1695-1701, 1994). The purpose of this study was to test the hypothesis that stimulation of upper airway (UA) sensory receptors during obstructed inspiratory efforts contributes to arousal and apnea termination and that a progressive attenuation of this mechanism through the night contributes to apnea lengthening. We studied seven patients (six men, one woman) with severe obstructive sleep apnea (apnea-hypopnea index = 93 +/- 26 events/h) during two consecutive nights of polysomnographic monitoring. On one night (random order), we performed topical UA anesthesia with 0.2% tetracaine and on the control night, sham anesthesia. We measured apnea duration, esophageal pressure (Pes) during apneas, and apneic O2 desaturation. Consistent with previous findings, apnea duration, number of efforts per apnea, and peak Pes at end apnea increased from the beginning to the end of the control nights. UA anesthesia produced a significant increase in apnea duration at the beginning of the night but no change in apnea length at the end of the night. Peak Pes and the rate of increase in Pes during the anesthesia nights were greater than during control nights, but the rate of increase in Pes was similar for the beginning and end of the control and anesthesia nights. These findings suggest that UA sensory receptors play a role in mediating apnea termination at the beginning of the night but that the contribution of these receptors diminishes as the night progresses such that greater inspiratory efforts are required to trigger arousal, leading to apnea prolongation.

Control of epiglottic position in dogs: role of negative upper airway pressure

We investigated the influence of negative upper airway pressure (NUAP) on hyoepiglotticus and genioglossus muscle electromyographic (EMG) activity in anaesthetised (sodium pentobarbitone/ chloralose) dogs breathing via a tracheostomy. Changes in pressure were not transmitted through the entire upper airway, thus confirming airway occlusion during NUAP. When NUAP was applied at the larynx, peak inspiratory and tonic EMG activity of the genioglossus and HE both increased significantly (p < 0.05) and reached a plateau at NUAP of -10 to -20 cmH2O. Nasal NUAP at any level failed to influence either genioglossus or HE EMG activity. Following bilateral section of the internal branches of the superior laryngeal nerves (SLNin), resting levels of HE and genioglossus EMG activity decreased to virtually zero. Moreover, NUAP applied at the larynx now failed to recruit EMG activity for either muscle. These findings suggest active control of epiglottic position in dogs during NUAP.

Reduced voluntary drive to the human diaphragm at low lung volumes

Maximal inspiratory and transdiaphragmatic pressures vary with lung volume but the possibility that some of this variability reflects variable voluntary drive to the diaphragm has not been investigated systematically. We assessed the influence of lung volume on the ability to activate the diaphragm with voluntary effort during maximal Mueller manoeuvres. Voluntary activation of the diaphragm was assessed using twitch interpolation with bilateral phrenic nerve stimulation in 6 subjects. Each performed 10 maximal efforts at lung volumes around functional residual capacity (FRC) and additional efforts at volumes above and below FRC. Voluntary activation of the diaphragm was higher at lung volumes above FRC (> 60% TLC; activation 98.3 +/- 2.6%) than at lung volumes around FRC (45-60% TLC; activation 95.5 +/- 3.5%) and below FRC (< 45% TLC; activation 83.3 +/- 15.8%; p < 0.05). Submaximal diaphragm activation at low lung volumes may reflect differences in the length-tension relationships of the various inspiratory muscles and/or reflex inhibition of phrenic motoneurones at low lung volumes.

The effect of the tongue retaining device on awake genioglossus muscle activity in patients with obstructive sleep apnea

Knowledge of how dental appliances alter upper airway muscle activity when they are used for the treatment of snoring and/or obstructive sleep apnea (OSA) is very limited. The purpose of this study was to define the effect of a tongue retaining device (TRD) on awake genioglossus (GG) muscle activity in 10 adult subjects with OSA and in 6 age and body mass index (BMI) matched symptom-free control subjects. The TRD is a custom-made appliance designed to allow the tongue to remain in a forward position between the anterior teeth by holding the tongue in an anterior bulb with negative pressure, during sleep. This pulls the tongue forward to enlarge the volume of the upper airway and to reduce upper airway resistance. In this study, two customized TRDs were used for each subject. The TRD-A did not have an anterior bulb but incorporated lingual surface electrodes to record the GG electromyographic (EMG) activity. The TRD-B contained an anterior bulb and two similar electrodes. The GG EMG activity was also recorded while patients used the TRD-B but were instructed to keep their tongue at rest outside the anterior bulb; this condition is hereafter referred to as TRD-X. The GG EMG activity and nasal airflow were simultaneously recorded while subjects used these customized TRDs during spontaneous awake breathing in both the upright and supine position. The following results were obtained and were consistent whether subjects were in the upright or the supine position. The GG EMG activity was greater with the TRD-B than with the TRD-A in control subjects (p < 0.05), whereas the GG EMG activity was less with the TRD-B than with the TRD-A in subjects with OSA (p < 0.01). Furthermore, there was no significant difference between the GG EMG activity of the TRD-A and the TRD-X in control subjects, whereas there was less activity with the TRD-X than with the TRD-A in subjects with OSA (p < 0.05). On the basis of these findings, it was concluded that the TRD has different effects on the awake GG muscle activity in control subjects and patients with OSA. The resultant change in the anatomic configuration of the upper airway caused by the TRD may be important in the treatment of OSA because such a change may alleviate the impaired upper airway function.

Effects of superior laryngeal nerve section on ventilation in neonatal guinea-pigs

Ventilation was measured by barometric plethysmography in conscious, 10-14 day-old guinea-pigs with superior laryngeal nerves (SLN) intact or sectioned. In SLN-intact animals, hypercapnia caused concentration-dependent increases in respiratory frequency, tidal volume and minute ventilation but hypoxia had no effects. SLN section reduced respiratory frequency and minute ventilation during normoxia and reduced the ventilatory response to 6% CO2. In the same animals under anaesthesia, upper airway (UA) cooling decreased respiratory frequency and increased peak inspiratory flow in SLN-intact but not in SLN-sectioned animals. CO2 in the UA caused a tachypnoea which was also present in SLN-sectioned animals and when the nose was bypassed. These results show that UA afferents participate in ventilatory control in neonatal guinea-pigs. Moderate UA cooling causes a SLN-dependent decrease in respiratory frequency but UA CO2 causes tachypnoea which is not SLN-mediated and contrasts with the inhibitory effect of UA CO2 on breathing described in adults of other species.

The influence of nasal obstruction and its relief on oxygen saturation during sleep and the early postoperative period

Nasal obstruction causes abnormal breathing patterns during sleep which often result in hypoxaemia. We studied the effect of nasal obstruction on arterial oxygen saturation in 20 healthy patients undergoing elective surgery which required bilateral nasal packing. Ten patients were subject to complete nasal obstruction while the remaining half had the obstruction relieved by the insertion of nasal cannulae. We found that nasal obstruction did not have any detrimental effect on the already low incidence of oxygen desaturation. There was no effect of the cannulae on the degree or frequency of hypoxaemia, although they did appear to improve the quality of sleep during the first postoperative night. They were more comfortable by day than simple packing alone and no adverse effects were observed.

Sniff-like aspiration reflex evoked by pressure pulses from the upper airways in cats

Respiratory effects of single positive and negative pressure pulses (PPP, NPP) applied to the functionally isolated upper airways (UA) were studied in 11 anaesthetized cats breathing spontaneously through a tracheal tube. The UA pressure and the changes of tracheal airflow were recorded and the blood pressure and electrocardiogram were occasionally monitored. Sniff-like aspiration reflexes comprising powerful spasmodic inspirations could be elicited by PPP or NPP of 20 to 110 cm H2O or -14 to -140 cm H2O. The responses to NPP but also to PPP characterized by high peak inspiratory flow, mean inspiratory flow and tidal volume (PIF = 312.5 +/- 64.3 and 231.1 +/- 21.7 ml.sec-1; VTI = 178.3 +/- 46.7 and 110.1 +/- 14.4 ml.sec-1; VT = 40.9 +/- 8.3 and 22.5 +/- 1.7 ml) resembled closely the aspiration reflex elicited by mechanical stimulation of the pharyngeal wall. Occasionally, sneezing, minor modifications of breathing pattern and solitary forced inspirations could be induced by lower pressures. The results indicate that sudden pressure stimulation of the UA evokes vigorous respiratory responses including the aspiration reflex. These reflexes and their alterations may contribute to development or release of both UA obstruction and apnoea, at least in cats.

Role of proprioceptors in the mylohyoid muscle

Afferent discharges of the mylohyoid muscle branch during respiration were studied electrophysiologically in the rat. Afferent discharges from the mylohyoid muscle branch of the mylohyoid nerve were found to be synchronized with respiration. Stretching of the mylohyoid muscle elicited afferent discharges of the mylohyoid muscle branch, suggesting that lengthening of the mylohyoid muscle caused electrical activity in the proprioceptors. When the central cut end of the mylohyoid muscle branch was stimulated electrically, reflex discharges were recorded from the EMG lead at the sternohyoid muscle where it is innervated by the cervical nerve. The latency between the electrical stimulation and the action potential in the sternohyoid muscle was 3-4 ms. Therefore, the mylohyoid muscle branch may transmit information to the sternohyoid muscle regarding the stretching actions of the mylohyoid muscle resulting from movements of the hyoid bone.

Flow-regulatory function of upper airway in health and disease: a unified pathogenetic view of sleep-disordered breathing

Although the Starling resistor behavior of the upper airway during sleep has been well established in health and disease, its physiological implications have not been fully appreciated. The purposes of the present communication are to reassess the current state of knowledge within the framework of the Starling resistor concept and to examine the implications of the concept on homeostatic feedback respiratory control and the pathogenesis of the sleep apnea syndrome. The main inferences drawn from the assessment include: (1) Owing to the Starling resistor properties of the upper airway and the well-organized neurochemical control mechanism, the upper airway performs important homeostatic flow regulatory function; it appropriately dampens the potentially unstable breathing during sleep and prevents the PaCO2 from falling below the apneic threshold; (2) Under certain conditions, the upper airway flow regulatory function fails to achieve appropriate dampening, leading to development of a variety of sleep-related breathing disorders that include underdamping due to overly sensitive central chemoresponsiveness and/or excessive lung to chemoreceptor transport lag--central sleep apnea; overdamping due to upper airway obstructive dysfunction--obstructive sleep apnea and/or hypopnea; and, finally, conditions with mixed features of central underdamping with coexisting collapsible upper airway; and (3) Successful treatment of these conditions requires restoration of appropriate damping. The overdamping imposed by the faulty upper airway is effectively reduced by surgical and medical approaches, and by application of nasal continuous positive airway pressure (CPAP). Reduction of PaCO2 by use of acetalzolamide and/or aminophylline reduces the plant gain, thus effectively offsetting the underdamping of central origin. Owing to the dual effect of nasal CPAP on the upper airway and respiratory pump, use of nasal CPAP can also effectively reduce the plant gain, accounting for the therapeutic effect of nasal CPAP on the central sleep apnea.

Effect of posture and breathing route on genioglossal electromyogram activity in normal subjects and in patients with the sleep apnea/hypopnea syndrome

Patients with the sleep apnea/hypopnea syndrome (SAHS) often have more apneas supine that sitting. We have shown radiologically that although the retropalatal airway narrows on lying down, the retroglossal airway widens. We have thus investigated the effect of posture on genioglossal EMG activity in 10 normal subjects and 10 patients with SAHS (58 +/- 29 SD apneas + hypopneas/h) using peroral intramuscular EMG electrodes. Data were analyzed by three-way analysis of variance, with diagnosis, posture, and route as factors. Peak inspiratory and tonic expiratory genioglossal EMG were both than sitting, with no significant difference between normal subjects and SAHS higher (p < 0.001) supine patients, although there was a trend (p < 0.09) toward the supine posture having a greater effect on peak inspiratory EMG in the SAHS patients. There was no significant effect of breathing route on either peak inspiratory (p > 0.9) or tonic expiratory (p > 0.8) genioglossal EMG, but there were significant differences between the groups (p < 0.01), the SAHS patients having higher and the normal subjects lower EMG tone with nasal in comparison with oral breathing on both inspiration and expiration. This study therefore shows that both body posture and breathing route are important determinants of genioglossal EMG tone.

Influence of sleep on genioglossus muscle activation by negative pressure in normal men

An important mechanism controlling genioglossus (GG) muscle activity is the reflex response to negative airway pressure. We hypothesize that this reflex response may be lost during sleep and believe that this loss may be important in the pathogenesis of airway collapse during sleep. Thus, we determined the effect of non-rapid eye movement (NREM) sleep on the GG electromyogram (EMG) response to brief (0.2 to 0.6 s) episodes of negative pressure generation (NPG) in the upper airway of six normal subjects. Up to 100 NPGs (mean 58 +/- 12) were recorded both awake and during stable NREM sleep. During wakefulness, the change in GG moving time average EMG from basal to peak levels (during NPG) was 17.1 +/- 2.5 au (a 154 +/- 22% increase above basal levels). This response was markedly reduced during NREM sleep (2.7 +/- 1.2 au; p < 0.01). The latency of the GG EMG response was 53.8 +/- 11.5 ms during wakefulness (n = 6), but much longer during sleep (132.7 +/- 24.5 ms; n = 3; p < 0.03). We conclude that in normal subjects (1) the GG muscle responds to negative airway pressure by reflex activation during wakefulness, and (2) this reflex activation is reduced or lost during NREM sleep. We speculate that loss of this mechanism during sleep may contribute to pharyngeal collapse in obstructive apnea patients.

Genioglossus muscle activity and inspiratory timing in obstructive sleep apnea

Atypical tongue muscle activity during sleep may contribute to the development of obstructive sleep apnea (OSA). Inspiratory genioglossus (GG) muscle activity was investigated in 10 OSA adults and 4 symptom-free controls. On the basis of overnight monitoring during nonREM sleep, the duration of the inspiratory GG activity and the total GG activity cycle is shorter in patients with OSA. The duration of inspiration and the duration of one total respiratory cycle is also shorter in patients with OSA. The commencement time lag between inspiratory GG activity and the onset of inspiration is shorter in patients with OSA during nonapneic breathing which indicates that inspiratory GG activity is activated relatively later in these patients. Furthermore, the inspiratory GG activity occurs after inspiration during an apnea, but the timing of GG activity onset progressively advances during the apnea. Earlier GG reactivation occurs before inspiration during the first nonoccluded breath at the end of an apnea. During subsequent tidal breathing, the timing of the GG onset progressively decreases after the onset of inspiration until the next obstructive apnea occurs. This observation suggests that the timing relationship between GG inspiratory activity and inspiratory effort is of physiologic importance in the pathogenesis of OSA. Furthermore, it may explain why dental appliances, such as the tongue retaining device, are highly effective in the resolution of OSA in selected patients.

Effects of halothane, enflurane, and isoflurane on laryngeal receptors in dogs

The effects of halothane, enflurane, and isoflurane on laryngeal receptors were investigated in 6 anesthetized dogs breathing spontaneously through a tracheostomy. Single unit action potentials were recorded from the peripheral cut end of the superior laryngeal nerve (SLN) while different concentrations of volatile anesthetics (1.25, 2.5, 5.0%) were administered in the expiratory direction at a constant air-flow (6 l/min) for 1 min through the functionally isolated upper airway. A total of 21 respiratory-modulated mechanoreceptors, 18 "irritant" receptors, and 7 cold receptors were studied. The overall results obtained from the 16 respiratory-modulated mechanoreceptors challenged with the 3 anesthetic gases disclosed a prevalent inhibitory effect and halothane proved to be the most effective of the 3 gases. The activity during both the inspiratory and expiratory phase was significantly reduced only by halothane (inspiratory phase, P < 0.01; expiratory phase, P < 0.05), while neither isoflurane nor enflurane caused significant changes in receptor activity. Of the 18 irritant receptors, 14 receptors increased their activity in a dose-related manner in response to one or more of the anesthetics although the effect of halothane was more pronounced than those of enflurane and isoflurane. All of the 7 cold receptors consistently increased their activity in a dose-related manner in response to halothane whereas 3 of 7 receptors were insensitive to enflurane and 4 of 7 receptors were insensitive to isoflurane. Our results indicate that, while all three commonly used anesthetics can have an effect on different types of laryngeal receptors, the effects of halothane are more pronounced than those of the other two gases in terms of changes in receptor activity.

Role of chemical drive in recruiting upper airway and inspiratory intercostal muscles in patients with obstructive sleep apnea

Upper airway dilating muscle activity increases during apneic episodes in patients with obstructive sleep apnea (OSA). To elucidate the relative contribution of chemical and nonchemical stimuli to augmentation of the upper airway dilating muscle, we measured the response of genioglossus muscle (GG) and inspiratory intercostal muscle (IIM) activities to obstructive apnea during non-REM sleep and compared them with the response to progressive hypoxia and hypercapnia during awake periods in seven male patients with OSA. GG EMG was measured with a wire electrode inserted percutaneously, and IIM EMG was measured with surface electrodes placed in the second intercostal space parasternally. Responses to hypoxia and to hypercapnia were assessed by rebreathing methods in the supine position while awake. Following these measurements, a sleep study was conducted with the EMG electrodes placed in the same locations. The relationship between GG and IIM activities during the cycle of apnea and postapneic ventilation in non-REM sleep was quasi-linear, and the slope of the regression line was significantly greater than those during progressive hypoxia and progressive hypercapnia. The amplitude of GG activity at 70% of maximum IIM activities in the hypoxic test was 140 +/- 20% (mean +/- SEM) during non-REM sleep, which was also significantly greater than that during hypoxia (51 +/- 10%) and that during hypercapnia (59 +/- 15%). These results suggest that nonchemical factors contribute considerably to augmentation of GG activity during obstructive apneic episodes. The nonchemical stimuli may arise from mechanoreceptors activated by upper airway obstruction and behavioral factors associated with change in sleep states.

Response to external obstruction in preterm infants with apnea

A proportion of preterm infants respond to an external airway obstruction by becoming apneic. We have studied 23 infants (median birthweight, 1.14 kg; gestation, 29 weeks) on 80 occasions, to determine the time course of the response and its relationship with spontaneous apnea occurrence. Upper airway flow was measured with a face mask and pneumotachograph, and a tap was turned intermittently to produce an occlusion. A total of 380 occlusions were analyzed. The infants became apneic during the obstruction on 72 occasions (19%), and after the obstruction on 122 occasions (32%). Both of these events were significantly more common than immediately prior to the obstruction, when apnea occurred on 29 occasions (8%). Of the apneas following occlusion 57% were central in type. The point in the respiratory cycle at which obstruction occurred had no effect on the production of apnea. An obstruction score was calculated for each study. This was the mean of the number of apneas during and after each obstruction, expressed as a percentage of the number of obstructions per study. This score was positively correlated with the number of spontaneous apneas recorded. Obstruction score rose from a mean of 20% during days 0-14, to 34% during days 15-28, and thereafter it declined. This pattern may be relevant to the time course of apnea in susceptible infants.

Alteration of ventilatory activity by intralaryngeal CO2 in the cat

1. We investigated the responses of phrenic and hypoglossal nerve activities to the addition of 3, 5 and 10% CO2 to a constant flow of warm, humidified air through the isolated upper airway in decerebrate, paralysed, artificially ventilated cats. 2. In bilaterally vagotomized animals, intralaryngeal CO2 caused a dose-related decrease in peak integrated phrenic activity. This response became attenuated with time, but was still discernible after 3 min of continuous intralaryngeal CO2. In the same experiments, intralaryngeal CO2 caused a gradual increase in peak integrated hypoglossal nerve activity. 3. Intermittent pulsing of intralaryngeal CO2 during neural inspiration or expiration resulted in similar, but smaller decreases in the phrenic activity of some animals. Hypoglossal activity was not influenced appreciably by this procedure. 4. Systemic hypercapnia attenuated the phrenic responses to intralaryngeal CO2. The hypoglossal responses were greatly reduced or abolished. 5. In vagally intact cats, ventilated by a servo-respirator in accordance with phrenic nerve activity, intralaryngeal CO2 resulted in only a trace of reduction in phrenic discharge. After bilateral vagotomy, the same animals showed typical responses, as described above. 6. All responses to intralaryngeal CO2 were abolished after bilateral section of the superior laryngeal nerves (SLNs). 7. We conclude that intralaryngeal CO2 acts by way of receptors with afferents in the SLNs to decrease phrenic and increase hypoglossal nerve activities. The responses are not importantly gated during neural inspiration or expiration. The responses to intralaryngeal CO2 are most clearly demonstrable after bilateral vagotomy, suggesting that vagal mechanisms serve to stabilize respiratory motor neural activity in intact animals.

Effects of topical nasal anaesthesia on shift of breathing route in adults

The position of the soft palate is known to determine the breathing route, but the physiological mechanisms that bring about a shift from nasal to oral breathing are unclear. To test the hypothesis that activation of receptors in the nasal passage may be involved in reflex initiation of oral breathing after nasal obstruction, we investigated respiratory responses to nasal occlusion before and after topical lignocaine anaesthesia of the nasal passages. Eleven volunteers were fitted with custom-made partitioned face masks, which separated nasal and oral passages. Air flow through each passage was detected by changes in airway pressure and carbon dioxide concentration. Nine subjects were habitual nasal breathers both before and after topical anaesthesia with 4% lignocaine. Among these subjects, the time to initiate oral breathing in response to nasal occlusion was significantly shorter before anaesthesia than afterwards (mean 4.4 [SD 2.5] vs 10.8 [7.4] s, p less than 0.01). Similarly, the time to resume nasal breathing after release of nasal occlusion was significantly shorter before topical anaesthesia than afterwards (6.9 [4.9] vs 12.1 [7.8] s, p less than 0.01). Topical anaesthesia did not affect respiration rate, end-tidal carbon dioxide concentration, or arterial oxygen saturation. These findings suggest that in human beings sensory information from receptors in the nasal passage has an important role in controlling the shift of breathing route.

Waking genioglossal electromyogram in sleep apnea patients versus normal controls (a neuromuscular compensatory mechanism)

Pharyngeal collapse in obstructive sleep apnea patients is likely a product of a sleep-related decrement in pharyngeal dilator muscle activity superimposed upon abnormal airway anatomy. We postulate that during wakefulness, increased pharyngeal dilator muscle activity in apnea patients compensates for diminished airway size thus maintaining patency. We studied the waking genioglossus (GG) electromyogram (EMG) activity in 11 OSA patients and 14 age-matched controls to determine if GG activity is higher in the awake state in apnea patients than controls. To make this determination, we developed a reproducible methodology whereby true maximal GG EMG could be defined and thus basal activity quantitated as a percentage of this maximal value. Therefore, direct comparisons of basal activity between individuals was possible. We observed apnea patients to have significantly greater basal genioglossal activity compared to controls (40.6 +/- 5.6% vs. 12.7 +/- 1.7% of maximum). This difference persisted when size-matched subsets were compared. This augmented GG activity in apnea patients could be reduced with positive airway pressure. We speculate that this neuromuscular compensation present during wakefulness in apnea patients may be lost during sleep leading to airway collapse.

Effects of hypoxia and hypercapnia on cricothyroid muscle response to airway pressure

We studied the effects of hypercapnia (FICO2 = 0.07) and hypoxia (FIO2 = 0.13) on the cricothyroid muscle response (peak integrated CT EMG) to graded inspiratory resistance (RI). Five anesthetized dogs spontaneously breathed through the upper airway (UAB) or a side arm of a tracheotomy cannula bypassing the larynx (TB). With room air UAB, graded increases in RI increased CT EMG, upper airway pressure change (Paw) and esophageal pressure change (Pes, indicating increased inspiratory drive), but decreased VT. For room air TB, the effects of RI were much less but still significant. Hypercapnia increased VT, fR, Pes, Paw and CT EMG for any given RI during UAB more than during TB. Hypoxia increased fR but did not increase VT, Pes, Paw, or CT EMG. The results are consistent with a model predicting CT activity as additive functions of inspiratory drive and laryngeal receptor stimulation by Paw and no direct effects of chemoreceptor stimulation on the CT, independent of those resulting from changes in inspiratory drive and upper airway pressure.

Respiratory muscle activation by limb muscle afferent stimulation in anesthetized dogs

In 10 chloralose anaesthetized and spontaneously breathing dogs, we assessed the effect of limb muscle afferents on the peak integrated EMG activities of the genioglossus, alae nasi, costal diaphragm, parasternal intercostal, triangularis sterni, and transverse abdominis muscles. The influence of vagal and baroreceptor afferents were eliminated by vagotomy and perfusion of carotid sinuses at a constant pressure. Muscle afferents were activated by stimulating the central end of the gastrocnemius nerve for 1 min at 40 Hz and at different voltages. Stimulation at voltages equal to 5, 10 and 20 times twitch-threshold increased minute ventilation to 165, 216 and 250% of pre-stimulation values, respectively, which was achieved by increasing breathing frequency (shortening of the inspiratory and expiratory times) and tidal volume. The activity of the parasternal intercostal and alae nasi muscles increased by a similar degree to that of the diaphragm while the activities of the genioglossus and transverse abdominis were augmented to a greater degree than that of the diaphragm. On the other hand, the motor drive to triangularis sterni increased significantly only at 20 times twitch-threshold and to a lesser degree than that to the diaphragm. These results suggest that upper airway, inspiratory and expiratory rib cage and abdominal muscles may be independently regulated. Differences in the sensitivity of these muscles to the activation of limb muscle afferents can be explained by a complex pattern of central projections of these afferents on the central respiratory controllers or by intrinsic properties of the motor output of these controllers.

Evidence for reflex upper airway dilator muscle activation by sudden negative airway pressure in man

1. To determine if negative upper airway pressure causes reflex pharyngeal dilator muscle activation, we used intra-oral bipolar surface electrodes to record genioglossus electromyogram (EMG) activity in response to 500 ms duration pressure stimuli of 0, -2.5, -5, -15, -25 and -35 cm H2O (0-90% rise time less than 30 ms) in ten normal, conscious, supine subjects. 2. With the subjects relaxed at end-expiration, stimuli were applied in each of three conditions: (i) glottis open (GO), (ii) glottis closed (GC) and (iii) controls with the mouth and nose closed. 3. Six rectified and integrated EMG responses were bin averaged for each pressure in each experimental condition. Response latency was defined as the time when the EMG activity significantly increased above pre-stimulus levels. Response magnitude was quantified as the ratio of the EMG activity for 80 ms post-stimulus to 80 ms prestimulus; data from after the subject's voluntary reaction time (for tongue protrusion) were not analysed. 4. Negative airway pressure activated the genioglossus. The median latency of activation (34 ms) was much faster than the time for voluntary activation (184 ms) indicating a reflex response. 5. Significant activation, compared to 0 cmH2O controls and controls with mouth and nose closed, occurred with pressures of at least -5 cm H2O (GC) and -15 cm H2O (GO). At -25 and -35 cm H2O, responses with GO were significantly greater than with GC. 6. The magnitude ('strength') of the responses differed between subjects; these differences were repeatable. 7. We conclude that negative airway pressure causes reflex pharyngeal dilator muscle activation in man. Responses with GC suggest that upper airway receptors can mediate the response but larger responses with GO indicate a contribution from subglottal receptors.

Prolonged augmentation of respiratory discharge in hypoglossal motoneurons following superior laryngeal nerve stimulation

Experiments were conducted to investigate long-lasting effects of brief superior laryngeal nerve (SLN) stimulation on respiratory discharge in the hypoglossal nerve. In paralyzed, decerebrate and artificially ventilated cats, SLN stimulation (Hz, 3-5, s, 3-5 times threshold for inhibition of phrenic nerve discharge) immediately increase hypoglossal activity. Following stimulation, the amplitude of respiratory activity in the hypoglossal nerve was augmented (478 +/- 205%), and slowly decayed to prestimulus levels with a time constant of 106 +/- 16 s. In contrast, phrenic nerve activity was completely inhibited during the SLN stimulation and for several seconds thereafter. After activity resumed, phrenic burst frequency remained depressed (33 +/- 6%). Stimulation of the carotid sinus nerve elicited similar effects on hypoglossal nerve activity. Intracellular recordings from hypoglossal motoneurons indicated that SLN stimulation increased central respiratory drive potentials (CRDPs) following a stimulus train, but had inconsistent effects on resting membrane potential. Intracellular depolarizing current pulses (5-15 nA; 2 s) had no prolonged effects on membrane potential or CRDPs. The possible role of serotonin in prolonged augmentation of hypoglossal activity following SLN stimulation was investigated. Intracellular injection of horseradish peroxidase (HRP) into hypoglossal motoneurons and immunohistochemistry for serotonin revealed some close appositions between serotonin immunoreactive boutons and HRP-labeled neurons, but such appositions were sparse. Pretreatment with methysergide had little effect on prolonged augmentation of hypoglossal discharge following SLN stimulation. These results indicate that: (1) SLN stimulation causes prolonged augmentation of hypoglossal activity probably via increased synaptic inputs to hypoglossal motoneurons; and (2) serotonin is not necessary in the underlying mechanism.

Some factors affecting the maintenance of upper airway patency in man

This paper describes some of the anatomical and physiological factors affecting the maintenance of upper airway patency in man. Such factors have particular relevance to the mechanisms responsible for maintaining upper airway patency during sleep, and the failure of these mechanisms in patients with the clinical syndrome of obstructive sleep apnoea: a condition in which repeated episodes of sleep-related inspiratory oropharyngeal collapse lead to recurrent hypoxaemia, disturbed sleep patterns and other clinical sequelae. The relationships between upper airway geometry, negative intrapharyngeal pressure, activation of upper airway dilator muscles, and sleep state are important factors affecting the maintenance of upper airway patency. The aim of this paper is not to consider such factors in isolation but to consider their interaction in affecting the adequacy of the upper airspace as a conduit for airflow.

Carbon dioxide-responsive laryngeal receptors in the dog

The purpose of this study was to relate the carbon dioxide (CO2) response of laryngeal receptors to their behavior during the breathing cycle (i.e. their response to transmural pressure changes, laryngeal movement or decreases in temperature) or during exposure to irritant stimuli (water or cigarette smoke). In 9 anesthetized mongrel dogs breathing spontaneously through a tracheostomy, unit activity from the superior laryngeal nerve was recorded while warmed and humidified gas mixtures (air or 10% CO2 in O2) were passed, for 1 min, through the functionally isolated upper airway in the expiratory direction. None of the 10 cold receptors studied were affected by CO2. Eleven of 20 laryngeal non-modulated mechano-receptors were stimulated (from 0.3 to 1.6 imp/sec) by exposure to CO2. These CO2-responsive receptors were also stimulated by known irritant stimuli (cigarette smoke, water), although not all receptors which responded to these irritants were stimulated by CO2. Twelve of 33 respiratory-modulated receptors were affected by CO2; 4 were stimulated and 8 inhibited. Receptors inhibited by CO2 were also inhibited by negative pressure while receptors stimulated by CO2 were also stimulated by negative pressure. These results show that CO2-responsive laryngeal receptors are not specialized endings. Although it is not clear to what extent each separate group of laryngeal receptors is involved, each may contribute to the reflex bradypnea which has been observed during exposure of the upper airway to elevated levels of CO2. However, the importance of CO2-responsive laryngeal receptors in physiological conditions remains unclear.

Time course of laryngeal aperture response to expiratory resistance loading in humans

Expiratory laryngeal restriction in normal humans is augmented immediately following expiratory flow resistance loading applied to the mouth. The time course of this increased narrowing reflects the dynamics of the physiological stimuli that govern laryngeal control during expiration. Lung volume, expiratory airflow and upper airway pressure are possible factors influencing the larynx, and a comparison of their dynamics with those observed for laryngeal narrowing during control and loaded expirations ought to indicate the possible contribution of each source. We examined the dynamics of lung volume, airflow and laryngeal aperture subsequent to single breath applications of small flow resistance loads (2 and 5 cm H2O.L-1.sec) in 3 subjects using video image analysis techniques to define accurately the laryngeal aperture variation with time. The earliest deviation of the loaded expired volume time course from the control trajectory lagged the earliest occurrence of a significant increase in laryngeal narrowing with the larger load. Augmented laryngeal narrowing occurred simultaneously with deviation of the expired flow from the control trajectory. The responses to the smaller load were noisier, but were consistent with the hypothesis that information other than lung volume modulates the early laryngeal response to these loads, and receptors responding to the rate of change of lung volume (airflow) and/or upper airway pressure may be involved.

Nasal influences on snoring and obstructive sleep apnea

Although the relationship between nasal obstruction and sleep disturbance is variable, either partial or total obstruction of the nasal passages can cause snoring, obstructive sleep apnea, and the sequelae of alveolar hypoventilation. In addition, nasal obstruction can cause sleep fragmentation, sleep deprivation, and the known sequelae of disturbed sleep architecture, including associated daytime tiredness and alterations in normal behavior patterns. Nasal obstruction may produce greater physiologic effects during sleep than during the awake state. A complete examination of the upper respiratory tract should be done in all patients with obstructive sleep apnea and snoring. The degree of nasal obstruction is not directly correlated with the severity of symptoms and findings.

Water-responsive laryngeal receptors in the dog are not specialized endings

The primary purpose of this study was to ascertain whether laryngeal receptors activated by water are specialized endings or whether they also respond to other stimuli, such as pressure, temperature and laryngeal motion as they occur during the breathing cycle. In 35 anesthetized mongrel dogs, breathing spontaneously through a lower cervical tracheostomy, water and other test solutions at approximately 37 degrees C were injected into the functionally isolated larynx with a small catheter. Of the 130 receptors studied, none of the cold receptors (N = 13) responded to water, whereas approximately 60% of all laryngeal mechanoreceptors (72 of 117) responded with either a short delay, short duration or a long delay, long duration response. In general the former pattern of response was exhibited by nonrespiratory-modulated receptors, whereas the latter was typical of respiratory-modulated receptors. The specific nature of the stimulus (hypotonicity or lack of chloride ion) of the water response was further studied in 53 receptors with isoosmotic solutions of dextrose and sodium gluconate. The long delay, long duration response was dependent on a decreased osmolality, while the short delay, short duration response was dependent on the lack of chloride ion of the test solutions. All water-responsive receptors tested (N = 17) were blocked within 50 sec by topically applied 2% lidocaine and thus presumed to be superficial. However, 10 receptors which did not respond to water were also blocked within 50 sec, suggesting that not all superficial receptors are stimulated by water. Based on these observations, we propose that changes in osmolality or ionic composition of the laryngeal surface liquid could play an important role in modifying reflexes involved in the maintenance of upper airway patency.

Motor unit regulation of mammalian pharyngeal dilator muscle activity

The present study examined the cellular regulation of one of the pharyngeal dilator muscles, the geniohyoid, by assessing its motor unit (MU) behavior in anesthetized cats. During spontaneous breathing, MU that (a) were active during inspiration only (I-MU) and (b) were active during both inspiration and expiration (I/E-MU) were identified. I-MU had a later inspiratory onset time and a shorter duration of inspiratory firing than did I/E-MU (P less than 0.002 and P less than 0.0001, respectively). I-MU were usually quiescent whereas I/E-MU were usually active during the last 20% of inspiration. I/E-MU fired more rapidly (P less than 0.00001) and for relatively longer periods of time (P less than 0.00001) during inspiration than during expiration. End-expiratory airway occlusion (preventing lung expansion during inspiration) augmented the inspiratory activity of both I-MU and I/E-MU. Conversely, end-expiratory airway occlusion reduced the absolute and relative firing durations (P less than 0.002 and P less than 0.00002, respectively) and the firing frequency (P less than 0.001) of I/E-MU activity during expiration. These results indicate that (a) the complex pattern of pharyngeal dilator muscle activity is due to the integrated activity of a heterogeneous group of MU, (b) changes in the degree to which pharyngeal dilator muscles are active result from combinations of MU recruitment/decruitment and modulations of the frequency and duration of MU firing, and (c) gating of lung-volume afferent information occurs during the respiratory cycle.

Influence of lung volume in sleep apnoea

The influence of a constant increase in functional residual capacity on apnoea characteristics was studied in patients with the sleep apnoea syndrome. Pulmonary inflation was achieved by applying a continuous negative extrathoracic pressure into a Poncho type respirator. Nine patients slept in the Poncho for two consecutive nights, negative extrathoracic pressure being applied during the second night. There was no difference in the total sleep time, its composition within the different sleep stages, the apnoea and apnoea-hypopnoea indices, or the sleep time spent in apnoea between the two nights. The mean (SD) apnoea duration increased with negative extrathoracic pressure from 25.3 (2) to 30.5 (3) seconds (p = 0.003) and time spent in obstructive apnoea (percentage of apnoea time) from 56 (13) to 75 (8) (p = 0.02). The mixed apnoea time (%) decreased from 37 (7) to 21 (7) (p = 0.02). Despite the increase in apnoea duration, less time was spent below each oxygen saturation value during negative extrathoracic pressure. The results were similar for apnoeic episodes during non-REM (non-rapid eye movement) sleep, whereas no significant modifications were seen during REM sleep. It is concluded that the composition of apnoea time and resulting oxygen desaturation are influenced by lung volume.

Respiratory activity of the cricothyroid muscle

Single motor units of the cricothyroid muscle (CT), the contralateral CT electromyogram, and the posterior cricoarytenoid muscle (PCA) electromyogram were recorded in anesthetized, spontaneously breathing dogs. In quiet breathing the CT was active predominantly in inspiration, and distinct phasic expiratory activity was observed at lighter levels of anesthesia. Both the CT and PCA increased their inspiratory and expiratory activity with hypercapnia, whereas only their inspiratory activity increased in response to negative pressure and/or absence of volume feedback. Cold blockade of either the recurrent laryngeal nerves or the external branch of the superior laryngeal nerves did not modify CT or PCA activity. In general, activity of CT motor units reflected the behavior of the whole muscle, but different units were recruited at different levels of CT activity. Even though the majority exhibited similar thresholds for inspiration and expiration, some units showed a lower threshold for either one, suggesting some degree of specialization. However, for a few units with high threshold for inspiration the expiratory threshold could not be determined, since a comparable level of CT activity was not achieved in expiration.

Laryngeal paralysis on receptor and reflex responses to negative pressure in the upper airway

Mechanoreceptors affected by changes in transmural pressure and/or contraction of intrinsic muscles are present in the larynx. This study is designed to test the hypothesis that laryngeal paralysis alters laryngeal mechanoreceptor and reflex responses to collapsing pressure. Experiments were carried out on anesthetized, spontaneously breathing dogs. The activity of 65 mechanoreceptors was recorded from the superior laryngeal nerve during upper airway occlusion before and during laryngeal paralysis induced by cold block of both recurrent laryngeal nerves (RLN). Esophageal and upper airway pressures were also recorded. Thirty-three laryngeal mechanoreceptors stimulated by negative pressure decreased their inspiratory activity during upper airway occlusion from 48.1 to 30.4 imp/sec when the RLNs were blocked. In contrast, 21 inspiratory modulated mechanoreceptors inhibited by negative pressure and 11 responding only to negative pressure did not change their activity during RLN block. The effect of laryngeal paralysis on the cricothyroid muscle response to negative pressure was assessed in 7 dogs and found to be minimal. These results suggest that 'drive' receptors stimulated by negative pressure do not play a significant role in the reflex activation of upper airway dilating muscles.

Comparison of the response of diaphragm and upper airway dilating muscle activity in sleeping cats

The steady state and transient effects of hyperoxic hypercapnia on the electromyographic activities of the genioglossus (GG), posterior cricoarytenoid (PCA), and diaphragm (D) were studied in cats instrumented with chronically implanted electrodes during non-rapid eye movement sleep. Hypercapnia (inhalation of 3.4 and 7.4% CO2 in O2) increased the phasic electrical activity occurring during inspiration of all three muscles. Activities of the PCA and D increased in a parallel fashion with increasing CO2. The GG responded to steady state CO2 inhalation alinearly, with larger increases in activity occurring when CO2 was raised from 3.4 to 7.4% than when CO2 was increased from 0 to 3.4%. When gas mixtures containing CO2 were given, the GG reached its new steady state level more slowly than the D or PCA, and when CO2 was rapidly removed from the inspired gas mixture, the GG attained its steady state sooner than the D. These results suggest that in sleeping cats, hypercapnia does not affect either transient or steady state responses of the GG in the same way as the D or the PCA. These differences seem to be explained largely by different threshold characteristics of hypoglossal and phrenic neurons.

Respiratory activity in the superior laryngeal nerve of the rabbit

We studied the respiratory modulation of laryngeal afferents and their response to transmural pressure in 24 anesthetized, spontaneously breathing rabbits. Laryngeal afferent activity has a predominant inspiratory augmentation during tracheal breathing or tracheal occlusion that can be accounted for by the respiratory movement transmitted to the larynx through the trachea. During upper airway breathing or upper airway occlusion SLN afferent activity increases in expiration and decreases in inspiration. This respiratory modulation is due to changes in upper airway pressure (Pua). In fact, positive pressure stimulates SLN afferent activity, while negative pressure inhibits it. Mechanical restriction of epiglottal movement reduced the response to Pua changes during upper airway occlusion and application of maintained positive (0.1-0.5 kPa) and negative (-0.1 to -0.5 kPa) pressures (P less than 0.005). Furthermore, surgical removal of epiglottis decreased the baseline activity of SLN to 16.5% of control. These experiments suggest that in the rabbit the epiglottis is the main source of SLN afferent activity and that its displacement, due to changes in Pua, is the most important factor for modulating SLN activity. Most of the laryngeal receptors showed an inspiratory augmentation with tracheal breathing and occlusion, were stimulated by positive pressure and inhibited by negative pressure, reflecting the behavior observed in the whole nerve.

Respiratory and upper airway responses to nasal obstruction in awake lambs and ewes

The aims of this study were to compare the ability of awake newborn lambs and adult sheep to breathe orally when the nasal route was blocked and to determine the means by which it was accomplished. Chronic EMG electrodes (diaphragm, genioglossus, geniohyoid, posterior crico-arytenoid, digastric, thyroarytenoid) and fibre-optic catheters were implanted in 10 lambs and 4 ewes. Before each study soft tubes were fixed into the nostrils allowing rapid blockade of the nasal ventilatory pathway. During nasal blockade inspiratory upper airway dilator EMG activity increased. SaO2 fell until mouth opening occurred, then returned to near control values; oral breathing then ceased leading to desaturation again. In lambs and ewes there was significant hypoxia, hypercapnia and acidaemia. In lambs 2-14 days old, PO2 and pH fell to lower levels than in older lambs (15-30 days) or ewes and PCO2 rose more. In lambs PCO2 gradually increased during the period of obstruction. We conclude that maintenance of blood gas homeostasis in newborn lambs is more severely impaired by nasal obstruction than in older lambs or ewes, possibly due to an immaturity of neural mechanisms controlling the creation of an oral airway.

[Mechanisms of supralaryngeal airway obstruction in normal persons and habitual mouth breathers]

We examined oronasal flow partitioning in 27 volunteers with normal or slightly increased nasal resistance (mean +/- SD, 0.24 +/- 0.19 kPa/l/s). Mean percentage of inspiratory nasal flow contribution was measured during spontaneous oronasal breathing. The averaged nasal admixture of airflow differed considerably within and between all subjects (mean +/- SD, 20.9% +/- 16.5%; range 1%-70%), showing no correlation to nasal resistance. Five of 27 subjects with a history of habitual mouth breathing had a significantly lower nasal admixture as compared with controls (2.5% +/- 1.7% vs 25.1% +/- 15.4%; P less than 0.005), but with no statistical difference in nasal resistance. To evaluate the hypothesis that velopharyngeal narrowing is due to an increased tone of the soft palate, measurements were also performed under positive nasal pressure, inspiratory resistive loading at the mouth, and during breath-holding. There was no significant difference of airflow distribution between these modifications and unloaded breathing in either group. These data suggest, therefore, that oronasal flow distribution is due to active positioning of the soft palate, and that habitual mouth breathing without any nasal obstruction may be associated with closure of the velopharyngeal isthmus as a consequence of disturbed neural control mechanisms.

Effect of upper airway pressure pulses on breathing pattern

Importance of the time of application of upper airway pressure pulses on breathing pattern was investigated in 19 anesthetized, spontaneously breathing rabbits. The upper airway was functionally isolated into a closed system. A servo-respirator, triggered by the inspiratory activity of the diaphragm, was used to apply pressure pulses to the isolated upper airway. Negative pressure pulses of -5, -10, and -15 cm H2O when applied in early inspiration (within the first half) produced a reversible inhibition of inspiration in most trails (86.2%). This resulted in a prolongation of inspiratory duration (TI) and a decrease in mean inspiratory drive (P.Dia/TI) whereas peak diaphragm (P.Dia) activity and expiratory duration (TE) remained largely unaffected. In the remaining 13.8% of trials, an irreversible inhibition with short TI and reduced P.Dia activity was observed. In contrast, with late application of negative pressure pulses the only significant change was a shortening of TI. When positive pressure pulses were applied during expiration, no significant change in TE occurred with either early or late application. A significant prolongation of subsequent TI was seen irrespective of the time of positive pressure application. These results indicate that time of application during the respiratory cycle is an important variable in determining the response to upper airway pressure pulses.

Effects of cooling on laryngeal reflexes in the dog

We investigated the reflex effects of laryngeal cooling on posterior cricoarytenoid (PCA) muscle activity, breathing pattern, arterial blood pressure and heart rate. We performed experiments on 9 anesthetized, spontaneously breathing dogs. Laryngeal temperature was decreased by passing cold air through the functionally isolated larynx while the dog was breathing through a tracheostomy. Inspiratory and expiratory durations, esophageal pressure, peak PCA activity, heart rate and blood pressure did not change significantly during laryngeal cooling. Upon interruption of cold airflow, while the laryngeal temperature was returning to control values, we assessed PCA response to upper airway occlusion. At laryngeal temperatures of 20-25 degrees C the peak PCA activity during upper airway occlusion was approximately 2/3 of that observed at control temperature (approximately equal to 33 degrees C). This difference was abolished by topically applied anesthetics or by superior laryngeal nerve section. In addition, we recorded from 4 laryngeal mechanoreceptors stimulated by negative pressure; their response to upper airway occlusion was reduced to 1/2 by laryngeal cooling. These results indicate that laryngeal cooling has a marked depressive effect on the PCA response to collapsing pressure in the larynx, thereby compromising the mechanism subserving upper airway patency.

Influence of negative pressure applied to the upper airway on the breathing pattern in unanesthetized awake dogs

We examined the influence of changes in upper airway pressure on the breathing pattern in 5 unanesthetized awake dogs. The dogs breathed through an endotracheal tube or through a comfortably fitting fiberglass snout mask. With matched resistances and volume of the dead space, the inspiratory duration, tidal volume, and minute ventilation were higher during nasal breathing compared to tracheal breathing. Nasal and tracheal occlusion produced prolongation of inspiration in the first occluded breathing attempt, but the prolongation was more marked in nasal occlusion tests. Augmentation of genioglossus muscle activity occurred on the first occluded breath in nasal but not tracheal occlusion. In another series of experiments, negative pressure was applied to the isolated upper airway while the dog breathed through a tracheostomy tube. Negative pressure caused a prolongation of inspiratory duration which was proportional to the level of the applied pressure. However, the prolongation of inspiratory duration was significantly more marked when application of negative pressure was timed simultaneously with tracheal occlusion. Our results demonstrate that the upper airway has a powerful effect on the control of breathing, which becomes more evident during tracheal occlusion.

Load compensation during positive pressure breathing in anesthetized man

To investigate the mechanisms by which human subjects prevent or compensate for the change in respiratory muscle length imposed by applying continuous positive pressure to the airways, six men were studied under general anesthesia with methoxyflurane at the end of a minor surgical procedure (rhinoplasty). Ventilatory and occlusion pressure response to carbon dioxide was measured by a rebreathing technique with no bias pressure, or with 16 cm H2O positive pressure produced by adding weights to a spirometer bell. Static pressure-volume curves of the respiratory system were obtained while the subjects were paralyzed with succinyl choline. In contrast to awake subjects described in other studies, the anesthetized patients did not activate expiratory muscles to combat the rise in end-expiratory level caused by pressure, and showed little evidence of enhanced activation of inspiratory muscles that in the conscious state compensates for the disadvantage of their shorter length. A change in the shape of the occlusion pressure wave, however, suggested that positive pressure had some effect on the neural discharge to inspiratory muscles. The mechanisms by which the respiratory system defends itself against a pressure load that tends to change end-expiratory level are sensitive to anesthesia and may require consciousness.