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RESPIRATORY SYSTEM: Edited by Arthur P. Wheeler

Noninvasive ventilation

practical advice

Bello, Giuseppe; De Pascale, Gennaro; Antonelli, Massimo

Author Information
Current Opinion in Critical Care: February 2013 - Volume 19 - Issue 1 - p 1-8
doi: 10.1097/MCC.0b013e32835c34a5
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Noninvasive ventilation (NIV), the provision of ventilatory assistance by means of techniques that do not bypass the upper airway, has assumed a central role in the treatment of patients with both hypoxemic and hypercapnic acute respiratory failure (ARF) [1,2▪]. The most convincing evidence supporting NIV as a first-line treatment for ARF has been shown in selected patients with either a severe chronic obstructive pulmonary disease (COPD) exacerbation or acute cardiogenic pulmonary edema (CPE) [1,2▪]. The main theoretical advantage of NIV is avoiding the side effects and complications related to endotracheal intubation (ETI) [3], improving patient comfort and preserving airway defense mechanisms.

Once the decision to start NIV has been taken, it is essential to choose a proper NIV interface and ventilation mode, and plan a close monitoring in an adequate hospital location. This overview reports on the use of NIV in patients with ARF or who are at risk of ARF, focusing on the practical aspects of using NIV in the acute care setting. In particular, the areas covered here include criteria for patient selection, choice of interface, ventilation modes, and monitoring.


Interfaces connect ventilator tubing to the patient, allowing the delivery of pressurized gas into the airway during NIV. Currently available interfaces include nasal and face masks, helmet, nasal pillows, and mouthpieces [4,5▪] (Fig. 1). The choice of an appropriate interface that fits properly is a key issue for the success of NIV.

Interfaces used to deliver noninvasive ventilation. (a) nasal mask; (b) oronasal mask; (c) total face mask; (d) helmet; (e) nasal pillows; (f) mouthpiece. Photographs printed with the permission of the patients.

Data obtained from physiologic evaluations of NIV delivered by different types of interfaces have been useful to improve devices and patient–ventilator synchrony. A recent study on 60 patients undergoing NIV to treat ARF found total face mask to be more comfortable compared with traditional oronasal mask, allowing patients to tolerate NIV for a longer period [6▪]. In this study, the rate of ETI was similar with either interface, as was the evolution of clinical parameters, blood gases, and levels of ventilatory support. In clinical practice, alternating different interfaces may be the best strategy to improve patients’ tolerance.

Box 1:
no caption available


Choosing the right ventilation mode is crucial for achieving physiological and clinical benefit during NIV. Each ventilation mode has theoretical advantages and limitations.

Spontaneously breathing patients with respiratory failure of various causes may benefit from continuous positive airway pressure (CPAP) to correct hypoxemia. In acute CPE, CPAP can result in early physiological improvement and reduce the need for intubation and mortality [7,8]. A growing body of evidence supports the benefit of NIV techniques in the management of CPE, and their use as a first-line intervention in CPE patients is becoming more widely used. Although both CPAP and noninvasive intermittent positive pressure ventilation with or without positive end-expiratory pressure (PEEP) showed similar efficacy in decreasing the need for ETI and mortality without increasing the risk of acute myocardial infarction, CPAP could be considered the preferred mode in CPE patients, as it is cheaper and easier to use in various clinical settings.

Selection of ventilation mode may have significant impact in reducing the work of breathing in patients with ARF. In a physiologic study [9] performed in hypoxemic patients with ARF, noninvasive pressure support ventilation (PSV) combined with PEEP improved dyspnea and gas exchange, and lowered neuromuscular drive and inspiratory muscle effort. In these patients, CPAP used alone improved oxygenation but failed to unload the respiratory muscles.

As in the intubated mechanically ventilated patients, application of external PEEP is effective in counterbalancing the effects of dynamic hyperinflation in patients with acute hypercapnic exacerbations of COPD. In these patients, NIV delivered through different ventilator modes can provide respiratory muscle rest and improve respiratory physiologic parameters. No difference in clinical outcome or arterial blood gases between patients ventilated in assist control ventilation (ACV) and PSV modes has been found, even though PSV is in general better accepted by the patients and associated with fewer side effects in comparison with ACV mode [10].

Volume control ventilation can be useful in patients with changing respiratory impedance. It can be preferred for those patients with severe chest wall deformity or obesity who may need higher inflation pressures.

Triggering systems are critical to the success of NIV in both assist and control modes. During assisted ventilation, flow triggering reduces breathing effort more effectively as compared with pressure triggering, obtaining a better patient–ventilator interaction [11].

There are no clear recommendations or specific requirements from bench studies on the performance of NIV ventilators and interfaces [12▪]. In the absence of specific recommendations, the choice of ventilation mode should be dictated by factors such as personal experience, clinical setting, cause and severity of the pathologic process responsible for ARF. However assisted modes, particularly PSV, are more often used. As regard to pressure-targeted ventilation, it is suggested starting at low pressures to facilitate patient tolerance (appropriate initial pressures are a CPAP of 3–5 cm H2O and an inspiratory pressure of 8–12 cm H2O above CPAP) and, if necessary, gradually increase pressures as tolerated to obtain alleviation of dyspnea, decreased respiratory rate, adequate exhaled tidal volume, and good patient–ventilator interaction. Pressures commonly used to administer CPAP in patients with ARF range from 5 to 12 cm H2O. In patients with hypoxemic ARF and bilateral pulmonary infiltrates, undergoing 10 cm H2O CPAP delivered via a helmet, adding a sigh with 25 cm H2O for 8 s, once a minute, improved oxygenation [13▪]. Oxygen supplementation should be targeted to achieve an oxygen saturation above 92% or between 85 and 90% in patients at risk of worsening hypercapnia. A modality that provides a backup rate is necessary for patients with inadequate ventilatory drive.


NIV interfaces behave differently in respect to carbon dioxide (CO2) exchange. The face mask constitutes an additional mechanical dead space, and its effect on CO2 rebreathing is proportional to its internal volume [14]. As this volume is small compared with the patient's tidal volume, the amount of CO2 that is rebreathed is also small. By contrast, CO2 exchange during helmet ventilation follows the model of a semi-closed environment with an air exchange system [15].

During helmet CPAP, the inspired partial pressure of CO2 is independent from the level of CPAP and inversely correlated to the fresh gas flow delivered [16]. Gas flows above 45–60 l/min render the CO2 rebreathing clinically irrelevant [16]. As compared to helmet CPAP, helmet-delivered NIV in PSV mode can provide a more efficient CO2 washout, probably due to the phasic administration of inspiratory flow [17]. In addition, the analysis of CO2 rebreathing during helmet-delivered PSV does not show significant reductions in inspired partial pressure of CO2 by increasing the level of inspiratory assistance [17]. In a sophisticated computational fluid dynamic model to evaluate the effective dead space between different NIV interfaces, Fodil et al.[18▪▪] showed that the dead space between the face mask and the helmet differed only modestly (110–370 ml), whereas their internal volumes were markedly different (110–10 000 ml), thus confirming that effective dead space is not related to the internal gas volume included inside the interface. To minimize CO2 rebreathing during disconnection of the fresh gas supply while performing helmet CPAP, it has been advised to utilize large helmets with a large antisuffocation valve [19▪].


Optimal synchrony between the patient's spontaneous breathing activity and the ventilator parameters is one of the key factors determining tolerance to NIV. The lack of an optimal patient–ventilator interaction can lead to an increase in the work of breathing and patient discomfort [20]. Patient–ventilator asynchrony may be determined by a number of events including ineffective triggering, double-triggering, auto-triggering, premature cycling, and delayed cycling.

When PSV is used as a noninvasive ventilatory assistance mode, some forms of patient–ventilator asynchrony may occur, causing breathing discomfort. In a prospective multicenter observation study on ARF patients receiving NIV via a face mask in PSV mode, the level of pressure support and the magnitude of leaks were significantly associated with asynchrony [21]. Indeed, eventual air leaks during noninvasive PSV may impede the inspiratory flow decay required to open the expiratory valve, thereby prolonging inspiratory flow. In these circumstances, air leaks should be minimized by optimizing the fitting or size of the interface, or even switching to another type of interface. To reduce leaks, it may also be helpful to decrease ventilator pressure settings as much as allowed by clinical parameters. In older machines, when an air leak occurs, an option to obtain a better patient–ventilator interaction is to select pressure-limited, time-cycled ventilation modes, or even PSV mode with a maximal inspiratory time. With ventilators that allow changing the cycling off criteria (expiratory trigger), raising the cycling off airflow threshold (i.e., the percentage of peak inspiratory flow at which transition from inspiration to expiration occurs) can activate an earlier switchover to expiration, thus avoiding prolonged insufflations and patient–ventilator asynchrony.

In the presence of significant air leaks, pressure-targeted modes are preferred to deliver NIV as they can maintain delivered tidal volume better than volume-targeted modes [22]. In new ventilators, a NIV algorithm, usually referred to as ‘NIV mode’, measures and compensates leaks in order to minimize their detrimental impact on patient–machine synchrony [23,24▪]. Neurally adjusted ventilatory assist (NAVA) seems to be a very promising mode to help improve adaptation during NIV [25▪▪–27▪▪].

An optimal ventilator setting that also takes into account the type of interface used to deliver NIV is crucial in determining patient–ventilator interaction. It has been advised that the highest PEEP and pressure support levels clinically indicated and tolerated by the patient should be applied when NIV is administered with the helmet, in order to increase the elastance of the system, enhancing the trigger sensitivity [28]. Vargas et al.[29] suggested that increasing both PEEP and pressure support level and using the highest pressurization rate may be suitable when providing NIV through this interface. In their study, the helmet with the same settings as the face mask was associated with less inspiratory-muscle unloading and with worse patient–ventilator asynchrony. In contrast, specific settings with a fast ramp and higher pressures provided results similar to the mask, ameliorating the inspiratory trigger delay, without discomfort.

There is a wide variability in the estimation of leaks and tidal volume among current bilevel positive-pressure systems used for home mechanical ventilation [30▪].


In the acute care setting, NIV should be considered early when patients develop signs of incipient respiratory failure. Clinical indicators of need for ventilatory assistance include dyspnea, tachypnea, accessory muscle use, paradoxical abdominal breathing, and gas exchange deterioration.

To date, the best-established indication for NIV is ARF related to COPD exacerbation or CPE. Various categories of hypoxemic non-COPD patients, particularly those with immunosuppression or in the postoperative setting may also benefit from NIV, providing they are managed in centers with extensive experience. NIV is also frequently proposed to assist the ventilator-weaning process in selected patients under invasive mechanical ventilation.

NIV should be avoided in the following conditions:

  1. coma, seizures or severe central neurological disturbances;
  2. inability to protect the airway or clear respiratory secretions;
  3. unstable hemodynamic conditions (blood pressure or rhythm instability);
  4. upper airway obstruction;
  5. severe upper gastrointestinal bleeding;
  6. recent facial surgery, trauma, burns, deformity, or inability to fit the interface (unless a helmet is used);
  7. recent gastro-esophageal surgery;
  8. undrained pneumothorax;
  9. vomiting.

Patients with altered levels of consciousness due to hypercapnic ARF associated with COPD exacerbation are exposed to high risk of NIV failure, but a cautious attempt with NIV may be performed in these patients, provided that a careful monitoring is available and prompt ETI is accessible [31]. In treatment-responsive patients, recovery of consciousness occurs within 45–60 min after NIV application.


Identification of predictors of success or failure may help in recognizing patients who are likely to benefit from NIV and exclude those for whom NIV would be unsafe or ineffective, avoiding dangerous delays before ETI. Moreover, knowing the factors affecting the likelihood of success of NIV may be useful to decide the duration of NIV trial.

Predictors of NIV failure observed in COPD patients with ARF are the following:

  1. lower arterial pH at baseline [32,33];
  2. greater severity of illness, as indicated by Acute Physiology and Chronic Health Evaluation (APACHE) II score [34];
  3. inability to coordinate with the ventilator [34];
  4. inability to minimize the amount of mouth leak with nasal mask ventilation [34];
  5. less efficient or less rapid correction of hypercapnia, pH, or tachypnea in the early hours [34];
  6. functional limitations caused by COPD before ICU admission, evaluated using a score correlated to home activities of daily living (ADL) [33];
  7. higher number of medical complications (particularly hyperglycemia) on ICU admission [33].

Predictors of NIV failure observed in hypoxemic patients with ARF are the following:

  1. higher severity score [Simplified Acute Physiology Score (SAPS) II ≥35 [35] / SAPS II >34 [36]/higher SAPS II [37▪]]
  2. older age (> 40 years) [35];
  3. presence of acute respiratory distress syndrome or community-acquired pneumonia [35,37▪,38];
  4. failure to improve oxygenation after 1 h of treatment (PaO2:FiO2 ≤146 [35]/PaO2:FiO2 ≤175 [36]);
  5. higher respiratory rate under NIV [38];
  6. need for vasopressors [38];
  7. need for renal replacement therapy [38].


Once it has been decided to start NIV, the initial approach should consist in illustrating the various pieces of equipment to the patient and fitting the NIV interface. Patients should be motivated and reassured by the clinician, instructed to coordinate their breathing with the ventilator, and encouraged to communicate any discomfort or fears. Although sedation is infrequently required during NIV, caution is advised if benzodiazepines or opiates are administered to prevent hypoventilation or loss of airway protection. After an initial period of continuous administration, NIV can be intermittently applied, with variable periods of discontinuation, depending on the patient's respiratory conditions. Collaboration among caregivers including physicians, respiratory therapists, and nurses, is critical to the success of NIV (Fig. 2).

Sequential steps for delivering noninvasive ventilation in patients with acute respiratory failure.


Monitoring of patients undergoing NIV has the aim to determine whether NIV is being performed safely and effectively. NIV should be initiated in a protected environment such as the ICU or the step-down unit. The early use of NIV for less acutely ill patients with COPD on a medical ward seems to be feasible, but if pH is lower than 7.30, admission to the ICU is highly recommended [39]. The ICU should not be the place for starting palliative care or for limiting life-sustaining treatments in patients who have no available therapeutic options. Alike, in selected situations in which the prognosis is unclear, a full code management including all supportive care, such as mechanical ventilation, vasoactive agents, and renal replacement therapy, should be guaranteed. In these cases an ICU trial, that is, an unlimited treatment for a limited period of at least 3 days before making end-of-life decisions, may be considered.

Monitoring of patients receiving NIV in the acute care setting includes:

  1. level of consciousness;
  2. comfort;
  3. chest wall motion;
  4. accessory muscle recruitment;
  5. patient–ventilator synchrony;
  6. respiratory rate;
  7. exhaled tidal volume;
  8. flow and pressure waveforms;
  9. heart rate;
  10. blood pressure;
  11. continuous electrocardiography;
  12. continuous oximetry;
  13. arterial blood gas at baseline, after 1–2 h, and as clinically indicated.

The potential clinical benefit of close observation of ventilator graphics in patients under NIV has been recently evaluated in patients with acute exacerbation of COPD. The analysis of flow and pressure waveforms on the ventilator screen led to a more rapid normalization of pH at 2 h, a significant improvement of the patient's tolerance and a higher decrease of arterial partial pressure of CO2 (PaCO2) at 2 and 6 h [40▪].

ETI must be rapidly assured, when indicated. Criteria used to perform ETI in ARF patients undergoing NIV are as follows:

  1. patient intolerance;
  2. inability to improve gas exchange;
  3. inability to improve dyspnea or respiratory muscle fatigue;
  4. appearance of severe hemodynamic or electrocardiographic instability;
  5. severe neurological deterioration.


Humidification and warming of the inspired gases during NIV may be required to prevent the detrimental effects of cool, dry gases on the tracheobronchial epithelium. Two humidifying devices are commonly used with ICU ventilators: heated humidifiers, and heat and moisture exchangers (HMEs). The latter are most commonly used due to their simplicity and cost-effectiveness. However, because the HMEs are placed between the Y-piece and the patient, they add a substantial amount of dead space, compared with a heated humidifier. In addition, HMEs may increase flow resistance [41].

Heated humidification during NIV in patients with ARF can minimize work of breathing and improve CO2 clearance. Lellouche et al.[42] showed that use of an HME greatly increased work of breathing in comparison with heated humidifier during NIV delivered through a face mask in nine patients with hypercapnic ARF. In these patients, alveolar ventilation was maintained only at the expense of a greater work of breathing with the HME compared with the heated humidifier. In another study on 24 patients with ARF under face mask NIV, Jaber et al.[43] found that HME was associated with significantly higher PaCO2 compared with heated humidifier.

Unlike the acute care setting, no firm conclusion can be drawn on the type of humidification system to be used in COPD patients under long-term NIV. A randomized crossover 12-month study on 16 COPD patients receiving NIV with either heated humidifier or HME, showed that compliance with treatment and occurrence of infections were similar with heated humidifier and HME, albeit patients with heated humidifier showed less dryness of the throat [44]. Of note, at the end of this study, a higher number of patients decided to continue NIV with heated humidifier.


A considerable amount of evidence supports the use of NIV to improve gas exchange and avoid ETI and its attendant complications in selected patients with ARF. However, several issues are critical to the success or failure of NIV. Identifying patients who are proper candidates for NIV and those in whom NIV is not likely to be effective can help to avoid inappropriate NIV application or unnecessary delays before starting invasive ventilation. Also, choosing the right interface and ventilation mode, and performing a close monitoring in an appropriate location are of primary importance for NIV success. Finally, effective patient–healthcare provider communication should be an integral part of clinical practice before and during NIV, as it may enhance favorable NIV outcome.



Conflicts of interest

There are no conflicts of interest.

None of the authors received any funds for conducting this study.


Papers of particular interest, published within the annual period of review, have been highlighted as:

  • ▪ of special interest
  • ▪▪ of outstanding interest

Additional references related to this topic can also be found in the Current World Literature section in this issue (pp. 65–66).


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2▪. Keenan SP, Sinuff T, Burns KE, et al. Canadian Critical Care Trials Group/Canadian Critical Care Society Noninvasive Ventilation Guidelines GroupClinical practice guidelines for the use of noninvasive positive-pressure ventilation and noninvasive continuous positive airway pressure in the acute care setting. CMAJ 2011; 183:E195–E214.

This is a comprehensive search and appraisal of the current literature, using the Grading of Recommendations Assessment, Development and Evaluation methodology to assess the quality of the research and to generate clinical recommendations on the use of NIV in the critical care setting.

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5▪. Bello G, Mercurio G, Antonelli M. Helmet noninvasive ventilation: clinical applications. ICU Manage 2011; 11:21–23.

This is an article that focuses on the physiologic aspects of NIV delivered by the helmet.

6▪. Chacur FH, Vilella Felipe LM, Fernandes CG, et al. The total face mask is more comfortable than the oronasal mask in noninvasive ventilation but is not associated with improved outcome. Respiration 2011; 82:426–430.

In this study conducted on 60 patients undergoing NIV because of ARF, total face mask seemed to be more comfortable compared with traditional oronasal mask, even though the rate of ETI was similar with either interface, as was the evolution of clinical parameters, blood gases, and levels of ventilatory support.

7. Masip J, Roque M, Sanchez B, et al. Noninvasive ventilation in acute cardiogenic pulmonary edema: systematic review and meta-analysis. JAMA 2005; 294:3124–3130.
8. Winck JC, Azevedo LF, Costa-Pereira A, et al. Efficacy and safety of noninvasive ventilation in the treatment of acute cardiogenic pulmonary edema: a systematic review and meta-analysis. Crit Care 2006; 10:R69.
9. L’Her E, Deye N, Lellouche F, et al. Physiologic effects of noninvasive ventilation during acute lung injury. Am J Respir Crit Care Med 2005; 172:1112–1118.
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These authors evaluated bench studies on the performance of ventilators and interfaces for NIV, suggesting that no recommendation or specific requirement does result from these studies, whose findings are sometimes inconsistent and even conflicting.

13▪. Cammarota G, Vaschetto R, Turucz E, et al. Influence of lung collapse distribution on the physiologic response to recruitment maneuvers during noninvasive continuous positive airway pressure. Intensive Care Med 2011; 37:1095–1102.

In patients with hypoxemic ARF undergoing CPAP at 10 cm H2O delivered via a helmet, adding a sigh with 25 cm H2O for 8 s once a minute improved oxygenation in bilateral but not in unilateral distribution of lung involvement.

14. Criner GJ, Travaline JM, Brennan KJ, et al. Efficacy of a new full face mask for noninvasive positive pressure ventilation. Chest 1994; 106:1109–1115.
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18▪▪. Fodil R, Lellouche F, Mancebo J, et al. Comparison of patient-ventilator interfaces based on their computerized effective dead space. Intensive Care Med 2011; 37:257–262.

This is a sophisticated computational fluid dynamic model to evaluate the effective dead space between different NIV interfaces. These authors showed that the dead space differed only modestly (110–370 ml) between the face mask and the helmet, whereas their internal volumes were markedly different (110–10 000 ml), thus confirming that effective dead space is not related to the internal gas volume included inside the interface.

19▪. Milan M, Zanella A, Isgrò S, et al. Performance of different continuous positive airway pressure helmets equipped with safety valves during failure of fresh gas supply. Intensive Care Med 2011; 37:1031–1035.

These authors suggested utilizing large helmets with a large antisuffocation valve to minimize carbon dioxide rebreathing during disconnection of the fresh gas supply while performing CPAP helmet.

20. Kondili E, Prinianakis G, Georgopoulos D. Patient-ventilator interaction. Br J Anaesth 2003; 91:106–119.
21. Vignaux L, Vargas F, Roeseler J, et al. Patient-ventilator asynchrony during noninvasive ventilation for acute respiratory failure: a multicenter study. Intensive Care Med 2009; 35:840–846.
22. Mehta S, McCool FD, Hill NS. Leak compensation in positive pressure ventilators: a lung model study. Eur Respir J 2001; 17:259–267.
23. Vignaux L, Tassaux D, Carteaux G, et al. Performance of noninvasive ventilation algorithms on ICU ventilators during pressure support: a clinical study. Intensive Care Med 2010; 36:2053–2059.
24▪. Carteaux G, Lyazidi A, Cordoba-Izquierdo A, et al. Patient-ventilator asynchrony during noninvasive ventilation: a bench and clinical study. Chest 2012; 142:367–376.

In this bench and clinical study, dedicated NIV ventilators allowed better patient–ventilator synchrony than ICU and transport ventilators, even when the NIV algorithm was engaged, especially regarding the risk of auto-triggering. The NIV algorithm improved, however, at least slightly and with a wide variation among ventilators, triggering and/or cycling off synchronization.

25▪▪. Schmidt M, Dres M, Raux M, et al. Neurally adjusted ventilatory assist improves patient-ventilator interaction during postextubation prophylactic noninvasive ventilation. Crit Care Med 2012; 40:1738–1744.

In patients receiving prophylactic postextubation NIV, the combination of NIV algorithm and NAVA was found to be the best compromise between a good patient–ventilator synchrony and a low level of leaks.

26▪▪. Bertrand PM, Futier E, Coisel Y, et al. Neurally adjusted ventilator assist versus pressure support ventilation for noninvasive ventilation during acute respiratory failure: a cross-over physiological study. Chest 2012 [Epub ahead of print] doi:10.1378/chest.12–0424.

This is a prospective physiological crossover study of 13 patients with ARF given two 30-min trials of face mask NIV with PSV and NAVA in random order, showing that NAVA significantly improved patient–ventilator interaction by reducing patient–ventilator asynchrony, the trigger delay, and the inspiratory time in excess, while the two modes resulted in similar improvements in gas exchange.

27▪▪. Cammarota G, Olivieri C, Costa R, et al. Noninvasive ventilation through a helmet in postextubation hypoxemic patients: physiologic comparison between neurally adjusted ventilatory assist and pressure support ventilation. Intensive Care Med 2011; 37:1943–1950.

This short-term physiologic study in 10 patients with postextubation hypoxemic ARF showed that compared with PSV, helmet NIV delivered with NAVA improved patient–ventilator interaction and synchrony, with no differences in gas exchange and neural effort.

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In this study, the authors highlight the wide variability in the estimation of leaks and tidal volume among current bilevel positive-pressure systems used for home mechanical ventilation.

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From the data collected over 5 years in patients with hematologic malignancies admitted with ARF to 158 Italian ICUs, the success of NIV (roughly half the NIV trials) was associated with shorter periods of mechanical ventilation and ICU stays, as well as less severe postadmission infectious complications, and lower mortality rate, compared with invasive mechanical ventilation.

38. Adda M, Coquet I, Darmon M, et al. Predictors of noninvasive ventilation failure in patients with hematologic malignancy and acute respiratory failure. Crit Care Med 2008; 36:2766–2772.
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In patients with acute exacerbation of COPD, the analysis of flow and pressure waveforms on the ventilator screen led to a more rapid normalization of pH at 2 h, to a significant improvement of the patient's tolerance to ventilation at 2 h, and to a higher decrease of arterial partial pressure of carbon dioxide at 2 and 6 h.

41. Iotti GA, Olivei MC, Palo A, et al. Unfavorable mechanical effects of heat and moisture exchangers in ventilated patients. Intensive Care Med 1997; 23:399–405.
42. Lellouche F, Maggiore SM, Deye N, et al. Effect of the humidification device on the work of breathing during noninvasive ventilation. Intensive Care Med 2002; 28:1582–1589.
43. Jaber S, Chanques G, Matecki S, et al. Comparison of the effects of heat and moisture exchangers and heated humidifiers on ventilation and gas exchange during noninvasive ventilation. Intensive Care Med 2002; 28:1590–1594.
44. Nava S, Cirio S, Fanfulla F, et al. Comparison of two humidification systems for long-term noninvasive mechanical ventilation. Eur Respir J 2008; 32:460–464.

acute respiratory failure; equipment and supplies; noninvasive ventilation

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