Reversibility of Airflow Obstruction by Hypoglossus Nerve Stimulation in Anesthetized Rabbits

Exploring hypoglossal nerve stimulation therapy for obstructive sleep apnea: A comprehensive review of clinical and physiological upper airway outcomes

Obstructive sleep apnea (OSA) is a chronic disorder characterized by recurrent episodes of upper airway collapse during sleep, which can lead to serious health issues like cardiovascular disease and neurocognitive impairments. While positive airway pressure serves as the standard treatment, intolerance in some individuals necessitates exploration of alternative therapies. Hypoglossal nerve stimulation (HGNS) promises to mitigate OSA morbidity by stimulating the tongue muscles to maintain airway patency. However, its effectiveness varies, prompting research for optimization. This review summarizes the effects of HGNS on upper airway obstruction from human and animal studies. It examines physiological responses including critical closing pressure, maximal airflow, nasal and upper airway resistance, compliance, stiffness, and geometry. Interactions among these parameters and discrepant findings in animal and human studies are explored. Additionally, the review summarizes the impact of HGNS on established OSA metrics, such as the apnea-hypopnea index, oxygen desaturation index, and sleep arousals. Various therapeutic modalities, including selective unilateral or bilateral HGNS, targeted unilateral HGNS, and whole unilateral or bilateral HGNS, are discussed. This review consolidates our understanding of HGNS mechanisms, fostering exploration of under-investigated outcomes and approaches to drive advancements in HGNS therapy.

Pleural and transpulmonary pressures to tailor protective ventilation in children

This review aims to: (1) describe the rationale of pleural (P_PL) and transpulmonary (P_L) pressure measurements in children during mechanical ventilation (MV); (2) discuss its usefulness and limitations as a guide for protective MV; (3) propose future directions for paediatric research. We conducted a scoping review on P_L in critically ill children using PubMed and Embase search engines. We included peer-reviewed studies using oesophageal (P_ES) and P_L measurements in the paediatric intensive care unit (PICU) published until September 2021, and excluded studies in neonates and patients treated with non-invasive ventilation. P_L corresponds to the difference between airway pressure and P_PL Oesophageal manometry allows measurement of P_ES, a good surrogate of P_PL, to estimate P_L directly at the bedside. Lung stress is the P_L, while strain corresponds to the lung deformation induced by the changing volume during insufflation. Lung stress and strain are the main determinants of MV-related injuries with P_L and P_PL being key components. P_L-targeted therapies allow tailoring of MV: (1) Positive end-expiratory pressure (PEEP) titration based on end-expiratory P_L (direct measurement) may be used to avoid lung collapse in the lung surrounding the oesophagus. The clinical benefit of such strategy has not been demonstrated yet. This approach should consider the degree of recruitable lung, and may be limited to patients in which PEEP is set to achieve an end-expiratory P_L value close to zero; (2) Protective ventilation based on end-inspiratory P_L (derived from the ratio of lung and respiratory system elastances), might be used to limit overdistention and volutrauma by targeting lung stress values < 20-25 cmH₂O; (3) P_PL may be set to target a physiological respiratory effort in order to avoid both self-induced lung injury and ventilator-induced diaphragm dysfunction; (4) P_PL or P_L measurements may contribute to a better understanding of cardiopulmonary interactions. The growing cardiorespiratory system makes children theoretically more susceptible to atelectrauma, myotrauma and right ventricle failure. In children with acute respiratory distress, P_PL and P_L measurements may help to characterise how changes in PEEP affect P_PL and potentially haemodynamics. In the PICU, P_PL measurement to estimate respiratory effort is useful during weaning and ventilator liberation. Finally, the use of P_PL tracings may improve the detection of patient ventilator asynchronies, which are frequent in children. Despite these numerous theoritcal benefits in children, P_ES measurement is rarely performed in routine paediatric practice. While the lack of robust clincal data partially explains this observation, important limitations of the existing methods to estimate P_PL in children, such as their invasiveness and technical limitations, associated with the lack of reference values for lung and chest wall elastances may also play a role. P_PL and P_L monitoring have numerous potential clinical applications in the PICU to tailor protective MV, but its usefulness is counterbalanced by technical limitations. Paediatric evidence seems currently too weak to consider oesophageal manometry as a routine respiratory monitoring. The development and validation of a noninvasive estimation of P_L and multimodal respiratory monitoring may be worth to be evaluated in the future.

Systemic Administration of Tempol, a Superoxide Dismutase Mimetic, Augments Upper Airway Muscle Activity in Obese Zucker Rats

Obstructive sleep apnea (OSA) is characterized by repetitive partial/complete collapse of the pharynx during sleep, which results in apnea/hypopnea leading to arterial oxygen desaturations and arousals. Repetitive apnea/hypopnea-arousal episodes cause hypoxia/reoxygenation cycles, which increase free radical generation and oxidative stress that cause motor/sensory nerve impairments and muscle damage. We hypothesize that antioxidants may protect and/or reverse from oxidative stress-induced damage in OSA patients. To understand the acute protective effects of antioxidants on respiratory muscles, we studied the systemic effects of a membrane permeable superoxide dismutase mimetic, Tempol, on genioglossus (EMG_GG) and diaphragmatic (EMG_DIA) electro-myographic activities, hypoglossal motoneuron (HMN) nerve activity and cardiorespiratory parameters (mean arterial blood pressure, heart rate) in adult isoflurane-anesthetized obese Zucker rats (OZR) and age-matched lean Zucker rats (LZR). Tempol dose-dependently (1-100 mg/kg) increased EMG_GG without changing EMG_DIA in OZR and LZR. Tempol increased respiratory rate and tidal volume in OZR and LZR. Tempol (1-25 mg/kg) dose-dependently increased HMN nerve activity in healthy Sprague Dawley rats. Tempol (100 mg/kg) increased EMG_GG output by 189% in OZR and 163% in LZR. With respect to mechanisms of effect, Tempol (100 mg/kg) did not augment EMG_GG after bilateral HMN transection in Sprague Dawley rats. Although future studies are warranted, available data suggest that in addition to its antioxidant and antihypertensive properties, Tempol can selectively augment EMG_GG through modulating HMN and this effect may prevent collapsibility and/or improve stability of the upper airway pharyngeal dilator muscles during episodes of partial and/or complete collapse of the upper airway in OSA human subjects.

Heliox administration in anesthetized rabbits with spontaneous inspiratory flow limitation

We investigated the effects of heliox administration (80% helium in O₂) on tidal inspiratory flow limitation (tIFL) occurring in supine anesthetized spontaneously breathing rabbits, regarded as an animal model of obstructive apnea-hypopnea syndrome. 22 rabbits were instrumented to record oro-nasal mask flow, airway opening, tracheal and esophageal pressures, and diaphragm and genioglossus electromyographic activities while breathing either room air or heliox, and, in 12 rabbits, also during the application of continuous positive airway pressure (CPAP; 6 cmH₂O). For the group, heliox increased peak inspiratory flow, ventilation (18 ± 11%), peak inspiratory tracheal and dynamic transpulmonary pressures, but in no animal eliminated tIFL, as instead CPAP did in all. Muscle activities were unaffected by heliox. In the presence of IFL the increase in flow with heliox (ΔV̇ifl) varied markedly among rabbits (2 to 49%), allowing the distinction between responders and non-responders. None of the baseline variables discriminated responders and non-responders. However, fitting the Rohrer equation (R = K₁ + K₂V̇) to the tracheal pressure-flow relationship over the first 0.1 s of inspiration while breathing air allowed such discrimination on the basis of larger K₂ in responders (0.005 ± 0.002 versus 0.002 ± 0.001 cmH₂O·s²·ml^-2; P < 0.001), suggesting a corresponding difference in the relative contribution of laminar and turbulent flow. The differences in ΔV̇ifl between responders and non-responders were simulated by modeling the collapsible segment of the upper airways as a non-linear resistor and varying its pressure-volume curve, length, and diameter, thus showing the importance of mechanical and geometrical factors in determining the response to heliox in the presence of tIFL.NEW & NOTEWORTHY In an obstructive sleep apnea rabbit model, heliox never abolishes tidal inspiratory flow limitation (IFL), but increases inspiratory flow and tidal volume, substantially in some and nearly nil in other animals. Positive response to heliox cannot be predicted on the basis of breathing pattern characteristics or upper airway resistance that preceded IFL onset, but is related to the mechanical and geometrical features of upper airway collapsible segment, as indicated by model simulation.

Electrical stimulation of the whole hypoglossal nerve in patients with obstructive sleep apnea

PURPOSE

Electrical stimulation of the whole hypoglossal nerve (HGp-ES) has been demonstrated to enlarge the pharynx and improve pharyngeal stability and patency to airflow in all animals studied, but not in humans. The present study was undertaken to better understand the effect of HGp-ES on the human pharynx.

METHODS

Eight patients with obstructive sleep apnea who had implanted stimulators with electrodes positioned proximally on the main truck of the hypoglossus were studied under propofol sedation. Pharyngoscopy and air flow measurements at multiple levels of continuous positive airway pressure (CPAP) were performed before and during Hgp-ES.

RESULTS

HGp-ES that activates both tongue protrusors and retractors narrowed the pharyngeal lumen at the site of collapse (velopharynx in all subjects) from 1.38 ± 0.79 to 0.75 ± 0.44 cm2, p < 0.05 (measured at mid-range of CPAP levels) and lowered airflow (from 8.88 ± 2.08 to 6.69 ± 3.51 l/min, p < 0.05). Changes in critical pressure (Pcrit) and velopharyngeal compliance were not significant, but oropharyngeal compliance decreased (from 0.43 ± 0.18 to 0.32 ± 0.13 cm2/cmH2O, p < 0.05). No correlation was found between the pattern of change in luminal shape (determined as the ratio of a-p vs. lateral diameter when lowering CPAP) or changes in cross-sectional area and airflow during Hgp-ES.

CONCLUSIONS

Our findings indicate that human retractors dominate when stimulated together with the protrusors during HGp-ES. While co-activation of retractors may be beneficial, it should be limited. We speculate that exercises that augment protrusor force may improve the response to hypoglossal stimulation. The exclusion of patients with concentric pharyngeal obstruction should be re-evaluated.

Hypoglossal nerve stimulation in a rabbit model of obstructive sleep apnea reduces apneas and improves oxygenation

Based on a prior anesthetized model, we developed an unanesthetized model to evaluate the effects of hypoglossal nerve stimulation (HNS) during sleep. We prepared three rabbits with injections of hyaluronic acid in the base of tongue to produce upper airway obstruction followed by HNS implant. Two rabbits were saline controls, and one, a passive control. Measures were sleep, airflow, effort, oxygen saturation, and heart rate. HNS with electrodes around the right hypoglossal nerve were adjusted to a level without behaviorally disturbing the animal. During HNS stimulation in the tongue-base injected rabbits, obstructive apneas and hypopneas of intermediate (3 to 7 cycles of respiratory effort) or longer (≥8 cycles) duration were largely eliminated while less clinically relevant shorter events (<3) were unaffected, and oxygen saturation was improved. Control animals exhibited no intermediate or long events. In this model HNS can relieve induced sleep apnea, without disturbing the animal: however, despite being non-canine and of substantial size, the model has its challenges.NEW & NOTEWORTHY This report describes a rabbit model for testing the impact of hypoglossal nerve stimulation (HNS) on obstructive apneas. Obstructive sleep apnea (OSA) is induced by injecting hyaluronic acid (as a filler) into the base of the tongue. HNS reduced the length and rate of obstructions and improved oxygenation during sleep. Our efforts with this model advanced understanding of the complexities of this OSA preclinical model for neurostimulation reversal of sleep-disordered breathing.

Neural Control of the Upper Airway: Respiratory and State-Dependent Mechanisms

Upper airway muscles subserve many essential for survival orofacial behaviors, including their important role as accessory respiratory muscles. In the face of certain predisposition of craniofacial anatomy, both tonic and phasic inspiratory activation of upper airway muscles is necessary to protect the upper airway against collapse. This protective action is adequate during wakefulness, but fails during sleep which results in recurrent episodes of hypopneas and apneas, a condition known as the obstructive sleep apnea syndrome (OSA). Although OSA is almost exclusively a human disorder, animal models help unveil the basic principles governing the impact of sleep on breathing and upper airway muscle activity. This article discusses the neuroanatomy, neurochemistry, and neurophysiology of the different neuronal systems whose activity changes with sleep-wake states, such as the noradrenergic, serotonergic, cholinergic, orexinergic, histaminergic, GABAergic and glycinergic, and their impact on central respiratory neurons and upper airway motoneurons. Observations of the interactions between sleep-wake states and upper airway muscles in healthy humans and OSA patients are related to findings from animal models with normal upper airway, and various animal models of OSA, including the chronic-intermittent hypoxia model. Using a framework of upper airway motoneurons being under concurrent influence of central respiratory, reflex and state-dependent inputs, different neurotransmitters, and neuropeptides are considered as either causing a sleep-dependent withdrawal of excitation from motoneurons or mediating an active, sleep-related inhibition of motoneurons. Information about the neurochemistry of state-dependent control of upper airway muscles accumulated to date reveals fundamental principles and may help understand and treat OSA. © 2016 American Physiological Society. Compr Physiol 6:1801-1850, 2016.

Human tongue neuroanatomy: Nerve supply and motor endplates

The human tongue has a critical role in speech, swallowing, and respiration, however, its motor control is poorly understood. Fundamental gaps include detailed information on the course of the hypoglossal (XII) nerve within the tongue, the branches of the XII nerve within each tongue muscle, and the type and arrangement of motor endplates (MEP) within each muscle. In this study, five adult human tongues were processed with Sihler's stain, a whole-mount nerve staining technique, to map out the entire intra-lingual course of the XII nerve and its branches. An additional five specimens were microdissected into individual muscles and stained with acetylcholinesterase and silver staining to study their MEP morphology and banding patterns. Using these techniques the course of the entire XII nerve was mapped from the main nerve to the smallest intramuscular branches. It was found that the human tongue innervation is extremely dense and complex. Although the basic mammalian pattern of XII is conserved in humans, there are notable differences. In addition, many muscle fibers contained multiple en grappe MEP, suggesting that they are some variant of the highly specialized slow tonic muscle fiber type. The transverse muscle group that comprises the core of the tongue appears to have the most complex innervation and has the highest percentage of en grappe MEP. In summary, the innervation of the human tongue has specializations not reported in other mammalian tongues, including nonhuman primates. These specializations appear to allow for fine motor control of tongue shape.

Neural control of the upper airway: integrative physiological mechanisms and relevance for sleep disordered breathing

The various neural mechanisms affecting the control of the upper airway muscles are discussed in this review, with particular emphasis on structure-function relationships and integrative physiological motor-control processes. Particular foci of attention include the respiratory function of the upper airway muscles, and the various reflex mechanisms underlying their control, specifically the reflex responses to changes in airway pressure, reflexes from pulmonary receptors, chemoreceptor and baroreceptor reflexes, and postural effects on upper airway motor control. This article also addresses the determinants of upper airway collapsibility and the influence of neural drive to the upper airway muscles, and the influence of common drugs such as ethanol, sedative hypnotics, and opioids on upper airway motor control. In addition to an examination of these basic physiological mechanisms, consideration is given throughout this review as to how these mechanisms relate to integrative function in the intact normal upper airway in wakefulness and sleep, and how they may be involved in the pathogenesis of clinical problems such obstructive sleep apnea hypopnea.

Respiratory diseases and their effects on respiratory function and exercise capacity

Given that aerobic metabolism is the predominant energy pathway for most sports, the respiratory system can be a rate-limiting factor in the exercise capacity of fit and healthy horses. Consequently, respiratory diseases, even in mild forms, are potentially deleterious to any athletic performance. The functional impairment associated with a respiratory condition depends on the degree of severity of the disease and the equestrian discipline involved. Respiratory abnormalities generally result in an increase in respiratory impedance and work of breathing and a reduced level of ventilation that can be detected objectively by deterioration in breathing mechanics and arterial blood gas tensions and/or lactataemia. The overall prevalence of airway diseases is comparatively high in equine athletes and may affect the upper airways, lower airways or both. Diseases of the airways have been associated with a wide variety of anatomical and/or inflammatory conditions. In some instances, the diagnosis is challenging because conditions can be subclinical in horses at rest and become clinically relevant only during exercise. In such cases, an exercise test may be warranted in the evaluation of the patient. The design of the exercise test is critical to inducing the clinical signs of the problem and establishing an accurate diagnosis. Additional diagnostic techniques, such as airway sampling, can be valuable in the diagnosis of subclinical lower airway problems that have the capacity to impair performance. As all these techniques become more widely used in practice, they should inevitably enhance veterinarians' diagnostic capabilities and improve their assessment of treatment effectiveness and the long-term management of equine athletes.

Treating obstructive sleep apnea with hypoglossal nerve stimulation

PURPOSE OF REVIEW

Obstructive sleep apnea (OSA) is a common disorder characterized by recurrent pharyngeal collapse secondary to sleep-induced hypotonia of peri-pharyngeal structures. Therapy for OSA is sometimes poorly tolerated and not always effective. The current study reviews a new treatment modality, hypoglossus stimulation, recently evaluated by multiple physiological studies and currently assessed by several clinical studies.

RECENT FINDINGS

A phase-I, implantable hypoglossus nerve stimulation multicenter study was published in 2001. Significant reduction in apnea-hypopnea index (AHI) was reported in seven of the eight implanted OSA patients, but technical faults precluded prolonged follow-up. Over the past 2 years, three new hypoglossus nerve stimulation systems have been evaluated in more than 60 OSA patients. In adequately selected patients, a more than 50% reduction in AHI was observed. Usually, a decrease in OSA severity from moderate-severe to mild-minimal can be achieved.

SUMMARY

Ongoing research, including recent initiation of a large multicenter phase-III study, suggests that hypoglossus nerve stimulators are likely to be available as a new treatment modality within a few years. Additional data are needed to define which OSA patients are most likely to benefit from hypoglossus nerve stimulation. Continuous refinement of electrodes design is likely to improve stimulation efficacy in coming years.

A new animal model of obstructive sleep apnea responding to continuous positive airway pressure

STUDY OBJECTIVES

An improved animal model of obstructive sleep apnea (OSA) is needed for the development of effective pharmacotherapies. In humans, flexion of the neck and a supine position, two main pathogenic factors during human sleep, are associated with substantially greater OSA severity. We postulated that these two factors might generate OSA in animals.

DESIGN

We developed a restraining device for conditioning to investigate the effect of the combination of 2 body positions-prone (P) or supine (S)-and 2 head positions-with the neck flexed at right angles to the body (90°) or in extension in line with the body (180°)-during sleep in 6 cats. Polysomnography was performed twice on each cat in each of the 4 sleeping positions-P180, S180, P90, or S90. The effect of continuous positive airway pressure (CPAP) treatment was then investigated in 2 cats under the most pathogenic condition.

SETTING

NA.

PATIENTS OR PARTICIPANTS

NA.

INTERVENTIONS

NA.

MEASUREMENTS AND RESULTS

Positions P180 and, S90 resulted, respectively, in the lowest and highest apnea-hypopnea index (AHI) (3 ± 1 vs 25 ± 2, P < 0.001), while P90 (18 ± 3, P<0.001) and S180 (13 ± 5, P<0.01) gave intermediate values. In position S90, an increase in slow wave sleep stage 1 (28% ± 3% vs 22% ± 3%, P<0.05) and a decrease in REM sleep (10% ± 2% vs 18% ± 2%, P<0.001) were also observed. CPAP resulted in a reduction in the AHI (8 ± 1 vs 27 ± 3, P<0.01), with the added benefit of sleep consolidation.

CONCLUSION

By mimicking human pathogenic sleep conditions, we have developed a new reversible animal model of OSA.

Role of the hypoglossal nerve in equine nasopharyngeal stability

The equine upper airway is highly adapted to provide the extremely high oxygen demand associated with strenuous aerobic exercise in this species. The tongue musculature, innervated by the hypoglossal nerve, plays an important role in airway stability in humans who also have a highly adapted upper airway to allow speech. The role of the hypoglossal nerve in stabilizing the equine upper airway has not been established. Isolated tongues from eight mature horses were dissected to determine the distal anatomy and branching of the equine hypoglossal nerve. Using this information, a peripheral nerve location technique was used to perform bilateral block of the common trunk of the hypoglossal nerve in 10 horses. Each horse was subjected to two trials with bilateral hypoglossal nerve block and two control trials (unblocked). Upper airway stability at exercise was determined using videoendoscopy and measurement of tracheal and pharyngeal pressure. Three main nerve branches were identified, medial and lateral branches and a discrete branch that innervated the geniohyoid muscle alone. Bilateral hypoglossal block induced nasopharyngeal instability in 10/19 trials, and none of the control trials (0/18) resulted in instability ( P < 0.001). Mean treadmill speed (± SD) at the onset of instability was 10.8 ± 2.5 m/s. Following its onset, nasopharyngeal instability persisted until the end of the treadmill test. This instability, induced by hypoglossal nerve block, produced an expiratory obstruction similar to that seen in a naturally occurring equine disease (dorsal displacement of the soft palate, DDSP) with reduced inspiratory and expiratory pharyngeal pressure and increased expiratory tracheal pressure. These data suggest that stability of the equine upper airway at exercise may be mediated through the hypoglossal nerve. Naturally occurring DDSP in the horse shares a number of anatomic similarities with obstructive sleep apnea. Study of species with extreme respiratory adaptation, such as the horse, may provide insight into respiratory functioning in humans.

Obstructive sleep apnea - management update

Obstructive sleep apnea (OSA) is a highly significant condition based both on the high prevalence in community and significant consequences. Obstructive sleep apnea syndrome (OSAS), OSA together with hypersomnolence, is seen in 4% of middle-aged men and 2% of middle-aged women. OSA is associated with impaired quality of life and increased risks of motor vehicle accidents, cardiovascular disease (including hypertension and coronary artery disease), and metabolic syndrome. There is some evidence for the use of conservative interventions such as weight loss and position modification. CPAP remains the mainstay of treatment in this condition with high-level evidence supporting its efficacy. Continuous positive airway pressure (CPAP) is an intrusive therapy, with long-term adherence rates of less than 70%. Dental appliances have been shown to be effective therapy in some subjects but are limited by the inability to predict treatment responders. Alternative treatments are discussed but there is little role for upper airway surgery (except in a select few experienced institutions) or pharmacological treatment. The current levels of evidence for the different treatment regimens are reviewed.

Effect of positional changes of anatomic structures on upper airway dilating muscle shortening during electro- and chemostimulation

Positional changes of anatomic structures surrounding the upper airway are known to affect pharyngeal mechanics and collapsibility. We hypothesized that these alterations also affect the ability of the upper airway dilator muscles to enlarge the pharynx by altering their ability to shorten when activated. Using sonomicrometry, we evaluated in seven anesthetized dogs the effects of changes in tracheal and head position on the length of the genioglossus (GG) and the geniohyoid (GH) and the effects of these positional changes on the magnitude of shortening of the two muscles in response to electro- (ES) and chemostimulation (CS). Caudal traction of the trachea lengthened the GG and GH in all dogs, whereas cranial displacement of the trachea and flexion of the head to a vertical position shortened the muscles. Compared with the magnitude of ES-induced shortening in the neutral position, ES-induced shortening of the GG was 144.7 ± 14.6, 49.3 ± 4.3, and 33.5 ± 11.6% during caudal and cranial displacement of the trachea and during head flexion, respectively. Similar effects of the positional changes were found for the GH, as well as for both muscles during respiratory stimulation with Pco2 of 90 Torr at the end of CO2 rebreathing, although inspiratory muscle shortening during CS reached only one-quarter to one-third of the magnitude observed during ES. We conclude that positional alterations of anatomic structures in the neck have a dramatic effect on the magnitude of shortening of the activated GG and GH, which may reduce substantially their ability to protect pharyngeal patency.

Physiological responses during a T-piece weaning trial with a deflated tube

RATIONALE

T-piece trials and spontaneous breathing trials through the tracheostomy tube are often used as weaning techniques. They are usually performed with the cuff inflated, which may increase the inspiratory load and/or influence the tidal volume generated by the patient. We assessed diaphragmatic effort during T-piece trials with or without cuff inflation.

SETTINGS

Respiratory intensive care unit

METHODS

We measured breathing pattern, transdiaphragmatic pressure (Pdi), the pressure-time product of the diaphragm, per minute (PTPdi/min) and per breath (PTPdi/b), and lung mechanics (lung compliance and resistance) in 13 tracheotomized patients ready for a weaning trial. V(T) was recorded with respiratory inductive plethysmography (RIP-V(T)) or pneumotachography PT-V(T)). Patients completed two T-piece trials of 30[Symbol: see text]min each with or without the cuff inflated.

RESULTS

RIP-V(T) and PT-V(T) values were similar with the cuff inflated, but PT-V(T) significantly underestimated RIP-V(T) when the cuff was deflated, and therefore the RIP-V(T) was chosen as the reference method. The RIP-V(T) was significantly greater and the Pdi and PTPdi/min significantly lower when the cuff was deflated than when it was inflated. The efficiency of the diaphragm, calculated by the ratio of PTPdi/b over RIP-V(T), was also improved, while no changes were observed in lung mechanics.

CONCLUSIONS

Diaphragmatic effort is significantly lower during a T-piece trial with a deflated cuff than when the cuff is inflated, while RIP-V(T) is higher, so that the diaphragm's efficiency in generating tidal volume is also improved.