Obstructive Sleep Apnea Diagnosis and Treatment - part 5 pptx

174 Robert and Argaud an individually made interface is now seldom needed even if it remains probably the best interface (10–15). There are currently four different types of interfaces: nasal mask, facial mask covering the nose and the mouth, nasal pillows, and mouthpieces. Nasal masks are predominantly used (10,16). Mouthpieces are now essentially indicated in the case of daytime ventilation (17,18). This may afford an excellent inter- face to adjunct daytime ventilation in neuromuscular patients who are unable to main- tain acceptable diurnal arterial blood gases without frequent intermittent periods of assistance. The mouthpiece is positioned close to the patient’s mouth where it is inter- mittently captured to take a few assisted breaths from the ventilator and subsequently released. An advantage is to clear the face from a face-attached interface. Thus, the patient needing assistance night and day may use a combination of interfaces. Ventilator and Mode for Noninvasive Positive Pressure Ventilation Ventilators use one of two basic methods: volume-preset and pressure-preset (10). With volume-preset, the ventilator always delivers the tidal volume which is set by the clinician, regardless of the patient’s pulmonary system mechanics (compliance, resistance, and active inspiration). However, leaks at the skin–mask interface or through the mouth when using a nasal mask, reduce the volume received by the patient. Conversely, with pressure-preset, changes in pulmonary mechanics directly influence the flow and the delivered tidal volume (lower or higher) since the venti- lator delivers the set pressure all along inspiration. Then leaks augment the flow and tend to maintain the tidal volume (19,20). It is important to understand that NIPPV is dominated both by rapid variations of nonintentional leaks and of the geometry and the resistance of the upper airway (21). Obviously, leaks and airway resistance partly interact. Facing these continuous changes the respective advan- tages and drawbacks of volume- and pressure-preset modes, which are opposite, make a predictable effect difficult. The way to begin and end inspiration is either initiated by the ventilator or in response to a patient effort to do so, allowing one to define the main modes: (i) control (ii) assist-control (iii) assist or spontaneous (pos- sible only with pressure-preset). Most of the home ventilators function uniquely in volume or in pressure preset but modern ones may deliver inspiration according to the two modes. Besides the classical circuitry including two valves (on the inspira- tory and expiratory limbs) alternatively closing and opening, bilevel positive airway pressure (BPAP) ventilators are simpler and therefore lend themselves to home mechanical ventilation (22). Inspiratory and expiratory pressures are alternatively established in a single circuit incorporating an intentional, calibrated leak located close to the patient or even on the mask. The theoretical disadvantage with such a circuit is the risk of a variable CO 2 rebreathing. Concern about the risk of CO 2 rebreathing is not definitively documented (23–25) even if the trend is to consider it as negligible (26–28) provided positive expiratory pressure is applied in order to eliminate CO 2 through the intentional leak (at least 2–4 cm H 2 O). Depending on the ventilator, all the different modes and refined settings and even closed-loop modes usually applied in the intensive care unit, are more or less available. Some ventilators may analyze ventilation in an on-going manner, keep it in an internal memory and provide the data for further assessment. The general objective is to provide many possible capabilities in order to have enough tools to adapt and optimize patient– machine synchronization. While conceptually attractive, sufficient studies have not been performed to document or refute the advantages of such complexity in the context of noninvasive home ventilation. Noninvasive Positive Ventilation 175 Choice of the Ventilator and Mode Many clinicians currently prefer a pressure-preset ventilator in assist mode as the first choice with the view to offer the better synchronization (9). In fact, in the stud- ies comparing volume- and pressure-preset ventilators no clear differences in the correction of hypoventilation in short-term studies (29–37) and in long-term out- comes (38–40) are shown. This is understandable since during NIPPV, leaks and resistance changes alternate very quickly and when the pressure target does well, the volume target does not do well, and conversely. However, it is important to remain flexible by trying alternative approaches if problems occur with one or the other type of ventilator. Besides, it should be noted that batteries are unavailable or they offer a short autonomy for BPAP ventilators and this would limit security and mobility of neuromuscular patients with hypoventilation and then drive the prefer- ence to the volume ventilator. CRITERIA TO DISCUSS NONINVASIVE POSITIVE PRESSURE VENTILATION Signs and Symptoms of Hypoventilation The presence of clinical symptoms and/or physiologic markers of hypoventilation are useful in identifying clinical severity as it relates to therapeutic decision-making with regard to initiation of nocturnal NIPPV. In the course of a typical progressive disease, two successive steps occur more or less rapidly: (i) nocturnal hypoventilation reversible during wake associated with none or a few clinical symptoms; and (ii) noc- turnal and daylight hypoventilation associated with clinical symptoms, which show a low respiratory reserve and should be considered an unstable state with increased susceptibility to life-threatening acute ventilatory failure that may be triggered by what may otherwise be trivial additional factors (41,42). A sleep study continuously recording CO 2 (end-tidal EtCO 2 or transcutaneous TcCO 2 ) and/or oxygen saturation (SpO 2 ) is required to document nocturnal hypoventilation, which may occur through- out all sleep stages but in some cases exclusively during rapid eye movement (REM) sleep. Daytime hypoventilation is defined by an abnormally elevated partial pressure of arterial carbon dioxide (PaCO 2 ), a high-serum bicarbonate level and a relatively normal pH with associated reduction of partial pressure of arterial oxygen (PaO 2 ). Chronic daytime hypoventilation is an important indicator invariably associated with sleep-related hypoventilation. Thus, in the presence of diurnal hypoventilation, the reason for overnight recording is only to rule out obstructive or central apnea. Clinical symptoms indicating consequences of hypoventilation (Table 1) must be carefully evaluated since even when modest, they are important for the appreciation of disease severity and prognosis and to indicate NIPPV. Pulmonary function tests help define and quantify the ventilatory–respiratory disease but have low predictive values for chronic sleep-related hypoventilation in individual patients except in those with neu- romuscular disease. Indeed, in Duchenne muscular dystrophy, hypoventilation appears only during REM sleep, all night, or during the daytime when supine inspira- tory vital capacity is < 40%, < 25%, and < 12%, respectively (41). Similarly a peak cough flow < 160 L/min, related to expiratory muscle deficit, means an increased risk of accumulation of secretions that may worsen hypoventilation and trigger acute fail- ure (18,43–46). It is crucial to notice that isolated reduced PaO 2 does not mean hypoventilation but only a mismatching of ventilation and perfusion adequately compensated or even overcompensated (low PaCO 2 ), which will not require support by mechanical ventilation but only by supplemental oxygen. 176 Robert and Argaud Diseases That May Potentially Be Treated with Noninvasive Positive Pressure Ventilation The principal diseases which may be addressed using NIPPV therapy are shown in Table 2. Except for those due to respiratory control or upper airway abnormalities, all may become severe enough to cause alveolar hypoventilation during sleep and daytime and eventually may impair quality of life and threaten life. In neuromuscular disorders it is important to consider the progressiveness according to each type of disease and the individual concerned. Survival with Noninvasive Positive Pressure Ventilation in Different Diseases NIPPV efficacy in terms of survival compared to control treatment is major information that one needs to adequately discuss NIPPV. Besides a few randomized control trials (47–50), information comes from retrospective series compared to the usual prognosis (14,40,51–58). In order to extend the analysis it is also possible to take into TABLE 1 Clinical Features Frequently Associated with Alveolar Hypoventilation Shortness of breath during activities of daily living in the absence of paralysis Orthopnea in patients with disordered diaphragmatic dysfunction Poor sleep quality: insomnia, nightmares, and frequent arousals Nocturnal or early morning headaches Daytime fatigue, drowsiness and sleepiness, loss of energy Decrease in intellectual performance Loss of appetite and weight loss Appearance of recurrent complications: respiratory infections Clinical signs of cor pulmonale TABLE 2 Main Diseases That Can Benefit from Noninvasive Positive Pressure Ventilation, Classified According to the Cause and Progressiveness of the Respiratory Impairment Parietal disorders: (PFT abnormal:  VC,  FEV1, → FEV1/VC,  RV,  TLC) a Chest wall: Kyphoscoliosis No worsening Sequels of tuberculosis Slow worsening Obesity hypoventilation syndrome Depends on obesity Neuromuscular disorders: Spinal muscular atrophy No worsening Acid maltase deficit Slow worsening (>15 years) Duchenne muscular dystrophy Intermediate worsening (5–15 yrs) Myotonic myopathy Intermediate worsening (5–15 yrs) Amyotrophic lateral sclerosis Rapid Worsening (0–3 yrs) Lung diseases: (PFT abnormal: → or  VC,  FEV1,  FEV1/VC,  RV,  TLC) a COPD Continuous worsening Bronchiectasis, Cystic fibrosis Continuous worsening Predominant ventilatory control abnormalities: (PFT normal) Ondine’s curse Improvement (?) Cheyne-Stokes breathing Depends on heart failure Upper airway abnormalities: (PFT normal) Pierre Robin No worsening Obstructive sleep apnea No worsening a Symbols indicate actual compared to theoretical values:  or , decrease or increase; →, normal. Abbreviations: COPD, chronic obstructive pulmonary disease; FEV1, forced expiratory volume in one second; PFT, pulmonary function test; RV, residual volume; TLC, total lung capacity; VC, vital capacity. Noninvasive Positive Ventilation 177 account the results obtained either with negative pressure ventilation (59) or with tracheostomy (4,60). Their conclusions are informative enough and generally accepted by the medical community even if these conclusions are refutable in terms of evidence-based medicine. In neuromuscular disease, NIPPV always increases survival. Approximate median prolongations of life depend on the age when starting NIPPV and the comorbidities (including extended paralysis): very long (> 20 years) in the sequelae of poliomyelitis; long (10 years) in spinal muscular atrophy (SMA) type 2 and 3, Duchenne muscular dystrophy, acid maltase deficiency; short in myotonic dystrophy (4 years); and very short in amyotrophic lateral sclerosis (ALS) (one year). In chest-wall abnormalities NIPPV also prolongs life: in kyphosis (15 years) and in the sequelae of tuberculosis (seven years). In lung diseases no data support a positive effect on survival: in chronic obstructive pulmonary disease (COPD) patients for whom randomized trials are negative (48,49,59) or in cystic fibrosis or bronchiectasis patients for whom data are too scarce. Circumstances and Indications for Noninvasive Positive Pressure Ventilation In clinical practice, NIPPV is initiated either electively or in the context of acute ven- tilatory failure initially treated invasively with translaryngeal intubation or nonin- vasively with facial interfaces (61). In the latter circumstances, the long-term necessity for NIPPV should be re-evaluated after weeks or months during follow-up since the indications for NIPPV may change as the clinical conditions improve or not. In cases of chronic and stable awake hypoventilation, the cornerstone to foresee use of NIPPV is an advanced severity with clinical symptoms of hypoventilation plus a balance of several other issues: (i) the main primary process explaining the hypoventilation: mechanical or lung deficit; (ii) the natural rate of progression appreciated as a few years or dozens of years; (iii) the clinical severity at the time of decision-making: actual symptoms and history of acute–subacute failure in the pre- vious months; and (iv) the patient’s willingness, including the family and social environment, to undertake this therapy. Indications are outlined in Table 3. NIPPV is strongly indicated in patients with chest-wall and neuromuscular disorders in the presence of clinical symptoms attributable to diurnal hypoventilation (18,62–67). There are no validated values above which NIPPV is definitely indicated; however, TABLE 3 Typical Indications for Nocturnal Noninvasive Positive Pressure Ventilation According to Disease Process and Severity Diseases Symptoms Day CO 2  a Symptoms Night CO 2  a No symptoms Day CO 2  a Usual daily duration of NIPPV (hrs) Scoliosis Yes Yes Perhaps <12 Tuberculosis Yes Yes Perhaps <12 Neuromuscular stable or slow Yes Perhaps Perhaps 18–24 Neuromuscular intermediate Yes Perhaps Perhaps 18–24 Neuromuscular rapid Yes Yes Yes 24 COPD Perhaps No No 12 Bronchiectasis, Cystic fibrosis Perhaps No No 18–24 Obesity-hypoventilation Perhaps Perhaps No <12 a , increase. Abbreviations: COPD, chronic obstructive pulmonary disease; NIPPV, noninvasive positive pressure ventilation. 178 Robert and Argaud many clinicians consider treatment in scoliosis and sequelae of tuberculosis with awake PaCO 2 > 50–55 mmHg and a PaO 2 < 60 mmHg, and in neuromuscular dis- ease, a PaCO 2 around 45–50 mmHg and a PaO 2 < 70 mmHg. In case of clear clinical symptoms less severe values may be considered as an indication to start NIPPV (65). Conversely, in COPD and probably in other lung diseases diurnal hypoventila- tion do not support the unequivocal utility of NIPPV (68,69). Nevertheless, this question remains open since the clinical trials are underpowered and secondary parameters like some components of the quality of life or hospitalization days, may have improved. Some observational series suggest better results (70–73). Presently, we may admit NIPPV as an option in COPD patients with symptoms of hypoventi- lation contributing to recurrence of acute–subacute failure, provided that long-term oxygen and drug therapy have already been optimally adjusted. During early stages with only isolated nocturnal hypoventilation, NIPPV is not mandatory but could be optional in kyphoscoliosis (74,75) and in neuromuscular diseases (75). In the latter, when worsening is both inevitable and rapid (e.g., ALS), NIPPV is valuable at an early stage provided that this is an acceptable therapeutic option for the patient. Other particular diseases may also deserve considerations related to NIPPV use even if clinical experience remains nonconclusive. Obesity hypoventilation syn- drome is dominated by morbid obesity impeding ventilation, frequent obstructive apnea and more or less reversible decreased reactivity of the respiratory centers (76). In acute–subacute as in chronic situations, NIPPV has been shown to reverse hypoventilation (77–80). But, considering the high prevalence of obstructive apnea, CPAP is also a simpler and efficient treatment. Cheyne-Stokes breathing with central and obstructive apnea in the context of severe cardiac insufficiency has been shown to negatively worsen the clinical situation and survival (81–83). Conventional NIPPV or a new modality such as adaptive servo-ventilation has been shown to alleviate apnea and improve cardiac function (84–89). Nevertheless, no conclusion about the utility of nocturnal NIPPV in terms of survival and main outcomes is available. Even more, a large study comparing O 2 and CPAP, which also alleviates apnea and improves cardiac function does not prove the clinical superiority of CPAP in term of survival even if apneas are significantly diminished (90). Pure obstructive apneas in the con- text of OSA could be suppressed with NIPPV. Some authors have proposed NIPPV as a second-line treatment in the case of CPAP failure. Such a possibility is not supported with enough conclusive study to be recommended (91–93). Ondine’s curse, in chil- dren, is characterized by the lack of metabolic response of the respiratory centers during sleep and is responsible for severe nocturnal hypoventilation. The usual treat- ment is tracheostomy and nocturnal ventilation. Some clinical experience suggests that, after years, tracheostomy might be converted in some cases to nocturnal NIPPV. Obviously such options must remain in the hand of specialized teams (94). MANAGEMENT OF NONINVASIVE POSITIVE PRESSURE VENTILATION Initiation and Settings in the Case of Nocturnal Ventilation The main goal of NIPPV, used at best uniquely during the night, includes the provision of improvement in arterial blood gases nearly up to normal values with- out discomfort and sleep disruption. The objective in case of a residual muscle ability to breathe is to provide enough improvement to allow comfortable time off the ventilator. Even if there is no absolute recommendation it is good general prac- tice to proceed in three steps. The first step consists of selecting and adjusting the ventilator settings while the patient is awake, insuring physiological adequacy, Noninvasive Positive Ventilation 179 and patient comfort for at least one or two hours. One study, done on awake cystic fibrosis patients, found that clinical observation is as efficient as the use of physio- logical measurement including esophageal pressure in setting the ventilator param- eters (95). Another study in patients with COPD and neuromuscular disease has shown that using physiological measurement does not improve ventilation during the day but improves ventilation and sleep quality during the night (96,97). In the second step, the clinician should judge adequacy when sleeping during a nap and/ or nocturnal use. To complete this step, different options according to the resources available in each center could be used. Arterial blood gas measurements would seem ideal; however, one or few samples during the night do not represent the rapid changes observed during several continuous hours of sleep, and the inva- siveness of sampling have led most clinicians to noninvasively monitor different parameters. Ideally, a complete polysomnogram recording SpO 2 and PtcCO 2 or PEtCO 2 , airflow, tidal volume, airway pressure, rib cage and abdomen excursion, and sleep staging permits a complete assessment (98). When resources are not available to perform these detailed recordings, fewer measurements during over- night recordings remain informative. However, the minimal requirement is to overnight record SpO 2 in room air assessing that the normalization of SpO 2 goes with the normalization, or at least the improvement of PaCO 2 . In addition, data related to patient tolerance, comfort, sleep quality, and well-being should be obtained. The third step consists in looking for reduction of PaCO 2 and augmenta- tion of PaO 2 , without dyspnoea, during the day in free ventilation after several NIPPV nights to confirm that the settings are adequate. This also gives information about the necessity or not to add daylight hours of NIPPV (at first during the nap and more when necessary). If the results are not satisfactory, alterations must be made to the settings and possibly the mask and the ventilator, and their effects checked again. In most cases, a few days are necessary to succeed. If one uses assist pressure-preset ventilation, 10 cm H 2 O of inspiratory pressure support is a suggested starting point. If necessary, the pressure level is progressively increased to achieve evidence of improvement. Pressure support higher than 20 cm H 2 O is rarely necessary. Nevertheless, one observational series reports good results in COPD patients ventilated with higher (28 cm H 2 O) pressure (73). In COPD, the addition of an expiratory positive pressure [positive end-expiratory pressure (PEEP) or expiratory positive airway pressure], also necessary to decrease the rebreathing with BPAP venti- lators, should conceptually improve patient triggering when intrinsic PEEP exists. But, there is no long-term study proving its clinical usefulness (99,100). Depending on the ventilator capabilities and observations made of how patient and ventilator do together, more subtle settings concerning triggers, initial flow, and inspiratory time limit could be tried. A backup frequency set close to the spontaneous frequency of the patient during sleep is a reasonable substitute to avoid central apnea induced by transitory but repeated hyperventilation overpassing the apnea threshold (101). When employing a volume-preset ventilator, the initial suggested settings may be established by adjusting the frequency of ventilator-delivered breaths so that it approxi- mates the patient’s spontaneous breathing frequency during sleep, an inspiratory time/ total breathing cycle time between 0.33 and 0.5 and a relatively high tidal volume of around 10 to 15 mL/kg to insure sufficient tidal volume in case of leaks (19). Supplemental O 2 should be added into the ventilator circuit in those patients requiring oxygen while awake due to lung parenchyma diseases (e.g., COPD, cystic fibrosis, bronchiectasis). In the absence of parenchymal disease it is only after trying to optimize all technical parameters that residual desaturation may justify addi- tional O 2 bled into the ventilator circuit during sleep (102,103). 180 Robert and Argaud Continuous Noninvasive Positive Pressure Ventilation In neuromuscular (to a lesser degree in end-stage lung diseases) the ventilator dependency may be total when starting NIPPV or may progressively increase following the gradual worsening of the disease. In the case of continuous need for ventilation, NIPPV could be used provided that the following techniques are adapted: alternate interfaces night and day, and assisted coughing available (18,104–106). Only a very well-trained team may take in charge of such an approach in patients who are completely informed and conscious of the constraints and dangers. Such application has been reported by different teams in stable neuro- muscular patients, such as those with a sequelae of poliomyelitis, high-level spinal cord injury or Duchenne muscular dystrophy (1,17,107). Alternatively, a tracheos- tomy may be performed to facilitate ventilatory assistance and secretion removal. There is no clear answer as to whether and beyond what duration a quite totally ventilator-dependent patient is better or more safely ventilated by tracheostomy or NIPPV (65,108–111). This debate will probably continue and, in the end, the decision to indicate or to convert to tracheostomy is highly dependent on the phi- losophy and capabilities of the clinical team as well as that of the patient and his/ her family environmental preferences. It is essential that discussion of such issues be started as early as possible in the patient’s course, well before the imperative arises. Besides, swallowing dysfunction, responsible for frequent and massive aspirations and pneumonia, observed during the course of ALS (frequent and due to bulbar origin) or of Duchenne muscular dystrophy (seldom and due to muscle weakness), is an imperative indication for tracheostomy to prolong survival, but it also raises major difficulties to communicate and to have enough personal interactions (locked-in state) (110,112). From this point of view, NIPPV, which may be easily stopped, could be a reasonable maximal option in case of rapidly devas- tating diseases like ALS, and can be considered both by the patient and medical team as a limitation of care or a palliative approach (113,114). This was confirmed since NIPPV in ALS patients with bulbar symptoms do not survive longer than controls (50). Follow-up Clinical follow-up and daytime arterial blood gas (ABG) measurements (or their surrogates) should be conducted regularly (two times per year for example). When possible, recordings during sleep on NIPPV, identical to those used for initiating NIPPV are useful. At any time, when there are unsatisfactory results like recurrence of clinical symptoms or hypoventilation on ABG, inadequate NIPPV must be sus- pected and objective evaluation during sleep must be undertaken. At the very least, overnight oximetry must be done. When NIPPV is determined to be suboptimal, a change in ventilator modality or setting and a review of the mask fitting may be indicated. Increasing the total duration of NIPPV use per day should also be considered, particularly when the underlying disease has progressed. Masks have to be regularly checked and changed or adapted as needed. Management of Complications Air Leaks During Noninvasive Positive Pressure Ventilation To some degree, leaks are present when using nasal NIPPV during sleep in all patients. The major potential adverse effects of such leaks are reduced efficiency of ventilation and sleep fragmentation (115–118). A variety of measures, more or less Noninvasive Positive Ventilation 181 efficacious, have been suggested to address problematic leaks. These include: pre- venting neck flexion, reclining in a semi-recumbent position, discouraging the mouth from opening by use of a chin strap (117) or a cervical collar, switching to pressure-preset mode (19), decreasing the peak inspiratory pressure, increasing the delivered volume (20), optimizing the interface (12,16), and possibly switching to nasal pillows or a full-face mask (119). The effectiveness of these measures must be confirmed during sleep recordings. Nasal Dryness, Congestion, and Rhinitis With reference to the CPAP literature, the side effects of nasal dryness, congestion, and rhinitis are related to a defect of humidification promoted by air leaks (120). For patients with nasal and mouth dryness, a cold passover or a heated humidifier (the latter is more effective) (121) can be used. Heat/moisture exchangers are not well adapted to the case of leaks since the “dry” flow from the ventilator is higher than the “dampened” flow returning from the patient. In a large series, a minority of patients needed humidifiers (10). Aerophagia Aerophagia, or swallowing air, is frequently reported by patients, but rarely intolerable (122). Minor clinical signs are: eructation, flatulence, and abdominal dis- comfort. Aerophagia is usually dependent on the level of inspiratory pressure and is more commonly seen when using volume and/or mouthpiece ventilation and in the care of patients with neuromuscular disease. The incidence decreases if the peak inspiratory pressure is kept below 25 cm H 2 O pressure. NONINVASIVE POSITIVE PRESSURE VENTILATION EFFECTS (OTHER THAN SURVIVAL) AND RELATED MECHANISMS During Ventilatory Assistance As expected, when under NIPPV, ventilation and gas exchange are improved in all types of disease (38,70,74,123–127), even if significant episodes of transient hypoventilation, related to mouth leaks, may appear (115–118). Duration of sleep is augmented without clear changes in its quality (115,116,128). Respiratory muscles are normally put at rest but there are many exceptions due to air leaks and patient-ventilator asynchrony (129–131). After Ventilation When spontaneous ventilation exists and in the absence of major lung disease, gas exchange remains improved. It may persist for hours and even days before reap- pearance of hypoventilation (132,133). The improvement reported in many studies is important in chest-wall and neuromuscular diseases but inconsistent in COPD (123,125,134,135). Certainly, it explains improvements in clinical symptoms such as general well-being, appetite, exercise capability, headaches, ankle edema, and resur- gence of acute failure, as well as decreasing hospitalization, increasing quality of life (136–138) and finally improving survival. Three main explanations have been proposed: (i) improved respiratory muscle strength; (ii) resetting of the chemoreceptors; and (iii) decrease of the ventilatory load. The first hypothesis suggests that ventilatory assistance rests the respira- tory muscles reversing fatigue. Indeed, inspiratory force [PI max (maximum insp- iratory pressure) or P es (esophageal pressure) during sniff nasal pressure testing] have been found significantly augmented in four studies (56,139–141), very close in 182 Robert and Argaud one (142) and stable in one (143). One study in which a nonvolitional objective mea- sure using bilateral anterolateral phrenic nerve magnetic stimulation was assessed and found to be negative for improvement (142). One study in which respiratory muscle endurance was measured showed significant improvement (139). The second hypothesis suggests that, in response to chronic hypercapnia and hypoxia, the che- moreceptors commanding the respiratory centers change their set point, which per- petuates hypoventilation rather than attempting to generate nonsustainable ventilatory muscle efforts (144–147). The resumption of better ventilation during NIPPV would reset the centers to more normal values. The three studies that have looked at the hypercapnic ventilatory response have actually found significant improvement (140,142,143). It is interesting to consider that even a few hours of NIPPV during daytime can have the same effect (72,148) indicating that the deter- mining factor is to resume hypoventilation for a relatively short daily duration. The third hypothesis suggests that an improvement of respiratory chest-wall and/or lung compliance, under the effects of positive pressure ventilation, would reduce the ventilatory load and increase the efficiency of the muscles. In the studies done on scoliosis, vital capacity significantly increased (56,139–141); while in the other two hypothesis, including also neuromuscular patients, the vital capacity remained unchanged. In one study, chest-wall and lung compliance did not change even though there was a nonsignificant trend toward an increase (142). In the three stud- ies, periodic hyper-insufflation using higher inspiratory pressure during a few min- utes in scoliosis (149,150) and ALS (151) patients, revealed an increase in compliance. It seems probable that, even if the mechanisms which explain the efficacy of NIPPV are imperfectly understood, it is likely that several factors, even if not individually significant, change and interact together to improve alveolar ventilation. The mini- mum mandatory duration of assistance is not clearly known. However, a relation- ship between a decrease of PaCO 2 and the pressure to ventilate has been found (141). Finally, one study reports a significant improvement of pulmonary arterial hyperten- sion, which obviously favors clinical improvement (152). In COPD patients, the absence of clinical results compared to scoliosis and neuromuscular disease, even if resetting of the respiratory centers has been shown (125,153), could be explained by the relatively low impairment of respiratory muscles and the importance of the lesions of the lung itself and its progressiveness. 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