Inhibitory and excitatory effects on respiration by phrenic nerve afferent stimulation in cats

Respiratory effects of electrical stimulation of phrenic nerve afferents were studied in anesthetized cats, either spontaneously breathing or paralyzed and ventilated. The type of phrenic afferent fibers activated was controlled by recording the evoked action potentials from dorsal root fibers. In both preparations, stimulation at a strength sufficient to activate small diameter myelinated phrenic nerve afferents induced a biphasic response. The first phase lasted a few respiratory cycles and was inhibitory and consisted of a decrease in tidal volume (VT) or phrenic activity (NA), inspiratory time (TI), respiratory duty cycle (TI/Ttot) and instantaneous ventilation (VE) or minute phrenic activity (NMA). Expiratory time (TE) increased and breathing frequency (f) and mean inspiratory flow (VT/TI) or mean inspiratory neural activity (NA/TI) did not change. This short-term response was suppressed in animals pretreated with bicuculline. The second phase was a long-term excitation in which VT or NA, f, VE or NMA and VT/TI increased whereas both TI and TI/Ttot decreased and TE did not change. Unlike published data, our results suggest that small-diameter myelinated phrenic nerve afferents are involved in these responses. These phrenic fibers, like afferents from other muscles, affect respiratory output and may play a role in the control of breathing.     

Modulation of respiratory reflexes by an excitatory amino acid mechanism in the ventrolateral medulla.

Results from several studies suggest that the ventrolateral medulla (VLM) is involved in modulating the respiratory response to central and/or peripheral chemoreceptor stimulation. Furthermore, the excitatory amino acid (EAA) glutamate has been shown to have marked effects on respiration when administered to VLM sites. The purpose of this study was to determine if an excitatory amino acid mechanism in the VLM modulates the respiratory responses to hypoxia or hypercapnia in anesthetized rats. Exposure to hypoxic or hypercapnic gas under control conditions elicited increases in respiratory activity (diaphragmatic EMG activity and breathing frequency). Bilateral injection of kynurenic acid (KYN), an EAA antagonist, into rostral VLM sites evoked significant increases in breathing frequency; injections more caudal in the VLM typically caused apnea. Significantly larger increases in respiratory output were elicited by both hypoxia and hypercapnia after rostral VLM microinjections of KYN. The accentuated responses returned to control levels after a recovery of approximately 100 min. Microinjection of xanthurenic acid (XAN), an inactive analog of kynurenic acid, into the VLM prior to KYN had only slight effects on resting respiratory activity and no effects on the responses to hypoxia or hypercapnia. These results suggest two separate VLM sites which modulate respiration by EAA mechanisms. A more rostral site tonically inhibits respiratory activity and the respiratory responses to chemoreceptor stimulation and more caudal VLM sites may be required for the maintenance of respiratory activity.     

Premotor input to hypoglossal motoneurons from Kölliker-Fuse neurons in decerebrate cats

Experiments were performed in 18 paralysed, ventilated, decerebrate adult cats to characterise projections from the K?lliker-Fuse nucleus(KFN) to hypoglossal(HG) motoneurons. Efferent neural activity was recorded from the medial branch of both HG nerves, right recurrent laryngeal(RL) nerve, and C3 branch of the left phrenic nerve. Electrical stimulation(1 Hz, 1 msec pulse duration) of discrete areas within the KFN coordinates elicited a preferential, predominantly ipsilateral burst of HG action potentials with an average stimulus-response latency of 8 msec. Tonic stimulation(5-20 Hz) frequently produced considerable HG temporal summation and the appearance of phasic inspiratory HG activity. Injection of 40-100 nl kainic acid(6.37 mM) into the rostral pontine site with the lowest electrical threshold for HG activation elicited a prolonged tonic HG activation. Following kainate injection, the electrical stimulation threshold for HG excitation increased. Pressure injections of 5-100 nl of 2 mM N-methyl-D-aspartic acid (NMDA)and 2 mM(+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid hydrobromide(AMPA) into the KFN were associated with activation and/or suppression of HG motor output. The results indicate that HG motoneurons innervating protrusor tongue muscles receive a selective projection from the KFN that can be activated by glutamate receptors on KF neurons.     

Functions of the retrofacial nucleus in chemosensitivity and ventilatory neurogenesis

The hypothesis was evaluated that neurons within the retrofacial nucleus of medulla integrate afferent stimuli from the central chemoreceptors. In decerebrate, vagotomized, paralyzed and ventilated cats, activity of the phrenic nerve was monitored. Peak integrated phrenic activity increased in hypercapnia; the frequency of phrenic bursts typically declined slightly. The retrofacial nucleus was ablated by radio-frequency lesions or neurons within this nucleus were destroyed by microinjections of kainic acid. Results were similar following lesions or injections. Following unilateral ablations, peak phrenic activity was greatly reduced at normocapnia and hypercapnia; the frequency of phrenic bursts typically rose. Both frequency and peak phrenic activity fell further after the contralateral destruction with a cessation of all phasic phrenic discharge being observed in most animals. Injections of kainic acid in regions rostral, caudal or medial to the retrofacial nucleus produced no consistent changes in phrenic activity. We conclude that neuronal activities in the region of the retrofacial nucleus are important both in the integration of stimuli from the central chemoreceptors and in defining the discharge patterns of respiratory neurons.     

Roles of the pontine pneumotaxic and micturition centers in respiratory inhibition during bladder contractions

In decerebrate or anesthetized cats with moderately distended urinary bladders, spontaneous bladder contractions (SBCs) have been shown to decrease phrenic and hypoglossal nerve activities. To determine the involvement of both the pontine micturition center (PMC) and the pneumotaxic center in the respiratory response to SBCs, we recorded phrenic and hypoglossal nerve activities in decerebrate, paralyzed, vagotomized, artificially ventilated cats. Electrical stimulation of the PMC in cats with subthreshold bladder volumes below the threshold for SBCs elicited both increases in intravesical pressure (IVP) and attenuation of respiratory motor nerve activities. Respiration was not altered after PMC lesions, which abolished SBCs, contractions in response to PMC stimulation, and respiratory inhibition due to passive bladder distension. Electrical stimulation of the pneumotaxic center altered respiratory motor nerve activities and increased IVP in cats with subthreshold bladder volumes. Pneumotaxic center lesions caused apneusis, but did not abolish the SBCs, which continued to attenuate the apneustic respiratory motor nerve activity. These results indicate that the PMC is an important component of the reflex pathway from urinary bladder distension to respiratory inhibition, whereas the pneumotaxic center does not appear to be an essential part of this pathway.     

Phrenic afferent input to the lateral medullary reticular formation of the cat

The afferent inputs from phrenic nerve stimulation to the lateral reticular formation of the lower brain stem were studied in anesthetized spontaneously breathing cats. The activity of reticular neurons was recorded by means of extracellular tungsten microelectrodes. Electrical stimulation of the central end of the right phrenic nerve evoked excitatory or inhibitory responses in the lateral reticular nucleus (LRN), in the nucleus ambiguus (AMB) and in a region dorsal to the AMB of ipsi- and contralateral sides. Phrenic afferents belonging to the flexor reflex afferent group were involved in these responses. The discharge pattern of the respiratory related units (RRU) of the AMB were exceptionally affected by phrenic nerve stimulations. It is concluded that high threshold phrenic afferents relay in the LRN before projecting to the cerebellar cortex. The overlapping of respiratory and non-respiratory afferents in the reticular formation may participate to the adaptations of respiratory and somatomotor functions during specific behaviors.     

Medullary relay neurons in the carotid-body chemoreceptor pathway of cats.

Central terminations of the carotid body chemoreceptor afferents were localized by recording field potentials and unit activity evoked by electrical stimulation of the carotid sinus nerve, and unit activity during chemical stimulation of the chemoreceptors. Areas where evoked responses with short latency could be recorded and which contained neurons that responded to carotid body excitation were located in two regions of the medulla, about the level of the obex: a dorsal region which included the nucleus tractus solitarius and the reticular formation just below this nucleus; and a ventrolateral region which included the nucleus ambiguus and the ventrolateral reticular formation around that nucleus. The evoked field responses in the two regions were similar. In these two regions, the only neurons which increased their firing both to NaCN and a decreased PIO2 had a respiratory, bursting activity which was phase locked with phrenic nerve firing. The authors' findings suggest that there is a direct synaptic input from carotid body chemoreceptor afferents onto medullary respiratory neurons.     

Effects of carotid denervation on interactions between lung inflation and PaCO2 in modulating phrenic activity

Hypercapnia attenuates the effects of static airway pressure (Paw) on phrenic burst frequency (f) and the expiratory duration. We examined the role of carotid chemoreceptors in this response using an experimental preparation that allowed independent control of lung inflation and CO2 reflexes. Experiments were conducted in intact (n = 6) and carotid denervated (CBX; n = 12) chloralose/urethane anesthetized dogs. Integrated phrenic amplitude (Phr), f, and the inspiratory (TI) and expiratory durations (TE) were measured as a function of Paw (2-12 cm H2O) at levels of PaCO2 between 30 and 80 mm Hg. In intact dogs: (1) f decreased as Paw increased, and elevated PaCO2 decreased the slope of this relationship; (2) neither PaCO2 nor Paw affected TI; and (3) TE increased hyperbolically with Paw, and elevated PaCO2 attenuated this relationship. In CBX dogs: (1) f decreased as Paw increased, but this relationship was not affected by PaCO2; (2) TI increased as PaCO2 increased but was unaffected by Paw; and (3) TE increased as Paw increased but was unaffected by PaCO2. The results indicate that carotid chemoreceptors are necessary in the mechanism whereby hypercapnia attenuates the effects of Paw on f and TE. Furthermore, carotid denervation reveals an effect of hypercapnia on TI, an effect that is not evident in dogs with functional carotid chemoreceptors.     

Influence of lung volume on phrenic, hypoglossal and mylohyoid nerve activities

In decerebrate, paralyzed cats, ventilated by a servo-respirator in accordance with phrenic nerve activity, we examined the influence of lung volume on the activities of the phrenic, hypoglossal and mylohyoid nerves. When lung inflation was briefly withheld, the durations of inspiration (TI) and expiration (TE) and the activities of all three nerves increased. The relative increase in hypoglossal activity greatly exceeded that of phrenic activity and was apparent earlier in the course of inspiration. This hypoglossal response was enhanced by hypercapnia and isocapnic hypoxia. The responses of mylohyoid activity were quite variable: withholding lung inflation augmented inspiratory activity in some cats, but expiratory discharge in others. Sustained increases in end-expiratory lung volume were induced by application of 3-4 cm H2O of positive end-expiratory pressure (PEEP). Steady-state PEEP did not influence nerve activities or the breathing pattern. Bilateral vagotomy increased TI, TE, and the activities of all three nerves. No response to withoholding lung inflation could be discerned after vagal section. The results provide further definition of the influence of vagally mediated, lung volume dependent reflexes on the control of upper airway muscles. These reflexes are well suited to relieve or prevent upper airway obstruction.     

Progesterone stimulates respiration through a central nervous system steroid receptor-mediated mechanism in cat

We have examined the effect on respiration of the steroid hormone progesterone, administered either intravenously or directly into the medulla oblongata in anesthetized and paralyzed male and female cats. The carotid sinus and vagus nerves were cut, and end-tidal PCO2 and temperature were kept constant with servo-controllers. Phrenic nerve activity was used to quantitate central respiratory activity. Repeated doses of progesterone (from 0.1 to 2.0 micrograms/kg, cumulative) caused a sustained (greater than 45 min) facilitation of phrenic nerve activity in female and male cats; however, the response was much more variable in females. Progesterone injected into the region of nucleus tractus solitarii, a respiratory-related area in the medulla oblongata, also caused a prolonged stimulation of respiration. Progesterone administration at high concentration by both routes also caused a substantial hypotension. Identical i.v. doses of other classes of steroid hormones (17 beta-estradiol, testosterone, and cortisol) did not elicit the same respiratory effect. Pretreatment with RU 486, a progesterone-receptor antagonist, blocked the facilitatory effect of progesterone. We conclude that progesterone acts centrally through a steroid receptor-mediated mechanism to facilitate respiration.     

Respiratory response to baroreceptor stimulation and spontaneous contractions of the urinary bladder

Spontaneous contractions of the urinary bladder (SBCs) and experimental elevations of carotid sinus pressure (CSP) have been shown to result in respiratory inhibition with preferential reduction in hypoglossal (HG) nerve activity as compared with that of phrenic nerve discharge. We assessed the interaction between these respiratory inhibitory stimuli in decerebrate, vagotomized, paralyzed and artificially ventilated cats. We denervated the right carotid sinus and pressurized the isolated left carotid sinus region within the linear range of the baroreflex, while maintaining systemic arterial pressure at approximately 100 mmHg. We monitored the HG and phrenic nerve responses to SBCs, to elevations in CSP between SBCs, and to elevations in CSP during SBCs. Our results show that superimposing these stimuli results in respiratory inhibition, especially of HG activity, that exceeds that resultant from either stimulus alone. We speculate that the combined presence of SBCs and episodic hypertension may contribute to the development of periodic breathing or obstructive apnea, particularly during sleep.     

Effect of blocking medial area of nucleus retrofacialis on respiratory rhythm

Experiments were performed on anaesthetized, vagotomized rabbits. Respiratory movement and phrenic rhythmical discharge were reversibly abolished by the symmetrical injection of 1% procaine into the medial area of the nucleus retrofacialis (mNRF). Blocking other areas of the medulla had no obvious effect on respiratory rhythm, with the exception of the rostral portion of the ventral respiratory group (VRG), which overlaps with the mNRF. When the mNRF was blocked, most inspiratory and expiratory neurons recorded in the VRG and DRG (dorsal respiratory group) gradually started to fire continuously, and no longer exhibited respiratory rhythm. A minority of respiratory neurons was inactivated during apnea. Stimulation of the caudal portion of the DRG and VRG evoked only a short cluster of phrenic discharges instead of rhythmical firing, indicating that the respiratory neurons situated in these areas cannot generate rhythmic activity by themselves. This suggests that the mNRF plays an important role in the genesis and maintenance of basic respiratory rhythm.     

The effects of hypercapnia and cooling the ventral medullary surface on capsaicin induced respiratory reflexes

The effect of right atrial (RA) injection of 3 micrograms/kg capsaicin on phrenic, hypoglossal and recurrent laryngeal activities was studied in chloralose anesthetized, paralyzed and artificially ventilated cats. Within 2 sec following capsaicin injection, the phrenic and hypoglossal activities completely disappeared (apnea), while the recurrent laryngeal activity markedly increased. Similar responses were also obtained with RA injection of phenyldiguinide (PDG), suggesting that the respiratory responses of both drugs are essentially similar. Sino-aortic denervation did not affect the capsaicin induced respiratory responses. Bilateral vagotomy abolished the responses, suggesting that vagal sensory receptors are responsible for the reflex effects. Hyperoxic hypercapnia (3 and 7% CO2 in O2) reduced the apneic duration of phrenic and hypoglossal nerves. The magnitude of the recurrent laryngeal excitation was decreased during CO2 breathing. Graded focal cooling of the intermediate area (Is area) of the ventral medullary surface (to inhibit central chemoreceptor activity) significantly prolonged capsaicin induced apneic duration of hypoglossal nerves more than the phrenic. The recurrent laryngeal responses, however, were unaffected by cooling of the ventral medullary surface. The results show that capsaicin and PDG, presumably by stimulating C fibers, affect cranial nerves as well as the phrenic. The reflex responses to C fiber stimulation seem to be altered by intervention which stimulate (hypercapnia) or depress (Is cooling) 'central chemoreceptors.'     

Respiratory resetting induced by activation of inspiratory bulbo-spinal neurons

The respiratory effects elicited by spinal (C2-C3) stimulation at the level of descending inspiratory axons were studied in paralysed, non-vagotomized and artificially ventilated cats anaesthetized with urethane-chloralose. The activation of inspiratory bulbospinal axons in the ventrolateral quadrant was confirmed by recording the ipsilateral phrenic excitation following a single pulse. Brief stimulus trains delivered at the same locus during expiration elicited short- and long-term phrenic activations. The short-term activation consisted of a tetanic orthodromic response. The long-term activation, of central origin, exhibited the same pattern as a spontaneous inspiration and consisted of an inspiratory resetting which necessitated weak anaesthesia and light hypocapnia. Control experiments (restricted lesions of the medulla and the cervical cord, recording of afferent activity in thalamic sensory nuclei, medullary stimulation) revealed that this inspiratory resetting could not be related to appreciable activation of either non-respiratory efferents or spinal afferent pathways studied but was likely to depend on the activation of the descending inspiratory axons. We conclude that the respiratory resetting obtained by spinal stimulation resulted from mass antidromic activation of the inspiratory bulbospinal neurons which thus appear to be involved in the generation of the respiratory rhythm.     

Properties of inspiratory termination by superior laryngeal and vagal stimulation.

Electrical stimulation of two respiratory afferent nerves, the vagus and the internal branch of the superior laryngeal, was used to terminate inspiration. The short latency responses of phrenic motoneurones to these stimuli were studied to determine if inspiratory termination was preceded by a characteristic phrenic motoneurone discharge pattern, reflecting changes in brainstem inspiratory neurone discharge and inspiratory terminating mechanisms. Stimulus trains of sufficient intensity delivered to the superior laryngeal nerve terminated inspiration within 50 ms and were preceded by a stereotyped pattern of phrenic motoneurone discharge. This consisted of a short latency (disynaptic), predominantly contralateral excitation in response to the first shock of the train, followed by a marked and long lasting inhibition. In contrast, vagal stimulation typically terminated inspiration hundreds of milliseconds after the onset of the stimulus train and was not preceded by a stereotyped pattern of phrenic motoneurone responses to single shocks. Transient short latency responses were obtained but were extremely small, requiring considerable excitation followed by a moderate bilateral depression of activity. Inspiration could be terminated with or without the presence of these short latency responses. These results indicate that superior laryngeal and vagal (presumably pulmonary stretch receptor) afferents have different projections to brainstem inspiratory neurones and may exert their effects on inspiratory duration through different, but as yet undefined, neural mechanisms.     

Die Rolle der Schlafmedizin bei Herz-Kreislauf-Erkrankungen

Sleep-disordered breathing (SDB) is receiving more attention within the cardiology community. While obstructive sleep apnea (OSA) is thought to be an independent risk factor for the development for and prognosis of various cardiac and cardiovascular diseases, central sleep apnea (CSA) and Cheyne-Stokes respiration (CSR) in particular are thought to be comorbidities with an independent prognostic impact in patients with established cardiovascular diseases. Simplified screening tools enable the cardiologist to easily screen for or at least rule out significant SDB in their patients. Even the diagnosis of OSA or CSA seems to be feasible using multichannel cardiorespiratory polygraphy or polysomnography. In addition, apnea screening is becoming more and more integrated into various cardiac devices, e. g. Holter electrocardiogram (ECG) and implantable cardiac rhythm devices. Modern therapeutic devices using automatic adjustment of positive airway pressure facilitate treatment of patients with SDB and fully implantable respiratory devices using transvenous approaches, e. g. transvenous phrenic nerve stimulation to treat Cheyne-Stokes respiration, might open up a new field in cardiology. Once therapeutic interventions to treat SDB are proven to improve quality of life, cardiovascular function and prognostic outcome in patients with cardiovascular diseases, new pathways to diagnose and treat these patients need to be determined.     

A comparative analysis of contractile characteristics of the diaphragm and of respiratory system mechanics

The mean inspiratory flow rate (VT/TI) is used as an index of central respiratory 'drive', and, at rest, it varies interspecifically in proportion to body weight (BW) raised to the 0.74 power (Boggs and Tenney, 1984). VT/TI is determined by the level of central neural respiratory output, the velocity of contraction of respiratory muscles, and the mechanical characteristics of the respiratory system. We have examined the last two factors in 13 species ranging in weight from 0.025 to 515 kg. We determined the 'effective' inspiratory mechanical characteristics of the respiratory system (time constant, resistance, and compliance) and the time course of diaphragmatic contraction during bilateral supramaximal phrenic nerve stimulation in anesthetized animals. We also measured passive expiratory mechanical variables and made morphometric measurements of the diaphragm. We found that VT/TI during phrenic nerve stimulation was proportional to BW0.82. The 'effective' respiratory time constant (tau'rs) and passive expiratory time constant (tau rs) scaled in proportion to body weight with nearly similar exponents: tau'rs alpha BW0.26 and tau rs alpha BW0.21. In addition, the time constant of diaphragmatic contraction (tau mc) was proportional to BW0.20. Inspiratory time is proportional to tau'rs and tau mc, and tidal volume during stimulation was almost directly proportional to body weight. Thus, interspecific changes in VT/TI during stimulation were related to interspecific changes in the mechanical characteristics of the respiratory system and the velocity of muscular contraction. We conclude that interspecific changes in VT/TI need not reflect interspecific variation in central respiratory drive under resting conditions. We found that diaphragm weight and volume and diaphragm muscle thickness were geometrically similar in all species studied. Inspiratory pressure is an interspecific constant; therefore, by the Law of Laplace, smaller animals must develop greater tension per unit of muscle mass.     

Comparison of phrenic motoneuron activity in eupnea and apneusis

Our purpose was to characterize activities of phrenic motoneurons during apneusis. In decerebrate, cerebellectomized, vagotomized, paralyzed and ventilated cats, we recorded activities of phrenic nerve and single phrenic fibers during eupnea and apneusis. Reversible apneusis was obtained by cooling the rostral pons with a fork thermode. Phrenic motoneurons were defined as 'early' or 'late' during eupnea. Early units commenced activity before or during the first 20% of neural inspiration. The onset of discharge of late units extended throughout the rest of inspiration. In apneusis, some late units ceased activity entirely; others commenced activity at the end of the rising phase of phrenic activity or during the apneustic plateau. Early units commenced activities at the same time as in eupnea and generally maintained the same discharge frequency. Hence, the ramp phase of phrenic discharge in apneusis is generated largely by activities of early motoneurons. Our results imply that the level of bulbospinal activity impinging upon the phrenic nucleus is reduced in apneusis. The integration of efferent activity within the phrenic nucleus is discussed.     

Responses of hypoglossal and phrenic nerves to decreased respiratory drive in cats

Agents which depress respiration, such as alcohol, seem to increase the occurrence of obstructive apneas during sleep. It has been proposed that upper airway obstruction can result from an imbalance in the activity (or forces) produced by the upper airway muscles versus the chest wall muscles so that upper airway passages might be blocked when a disproportionate decrease in upper airway muscle activity occurs. This study examines the hypothesis that depression of respiration affects the activity of the hypoglossal nerve (the motor nerve to the tongue) more than the activity of the phrenic nerve (the motor nerve to the diaphragm). In addition, we examined the role of the putative central chemoreceptor area on the ventrolateral medullary surface (VMS) in maintaining phrenic and hypoglossal discharge. In chloralose-anesthetized, artificially ventilated, paralyzed cats, three methods of reducing respiratory drive were studied: hyperoxic hypocapnia (produced by mechanical hyperventilation), the application to the intermediate area of the ventral medullary surface of the respiratory depressant GABA and its agonist muscimol, and cooling the same area of the VMS (using a water-cooled thermode). All these interventions decreased hypoglossal nerve activity more than phrenic nerve activity (range of p values: p less than 0.001 to p less than 0.01). Moreover, the reduction in hypoglossal activity was greater with GABA and muscimol than with the other two maneuvers; this was statistically significant for both GABA versus VMS cooling (p less than 0.02) and muscimol versus VMS cooling (p less than 0.01). These results show that respiratory depression can differentially affect hypoglossal and phrenic nerve activity.(ABSTRACT TRUNCATED AT 250 WORDS)     

Transmission fatigue of the rabbit diaphragm

This study evaluates the role of transmission fatigue of the diaphragm in rabbits subjected to inspiratory resistive loading (IRL) sufficiently severe to increase peak tidal airway pressure to about 50% of that elicited by 100 Hz phrenic nerve stimulation. After 58 +/- 14 min of IRL, the transdiaphragmatic pressure (Pdi) responses to phrenic nerve stimulation at 20, 60, and 100 Hz were reduced by approximately one third. In contrast, IRL induced no significant change in the response to direct diaphragm stimulation (in the presence of transient neuromuscular blockade). Although respiratory acidosis occurred during IRL (pH 7.04 +/- 0.04, PCO2 90 +/- 10, PO2 131 +/- 38), it was not sufficient to explain the reduced contractility. In a separate series of experiments, the diaphragm compound action potential elicited by unilateral phrenic nerve stimuli was recorded by implanted diaphragm electrodes and the Pdi elicited by contralateral phrenic nerve stimulation at 100 Hz was measured. Both action potential amplitude and Pdi declined during IRL and both improved after 10 min of recovery. These findings demonstrate that transmission fatigue plays a major role in rabbit diaphragm fatigue induced by spontaneous breathing against inspiratory resistance.