One of the most important developments in the field of mechanical ventilation has been the increased utilization of noninvasive ventilation (NIV) as an important weapon in the critical care arsenal. The increased use of NIV today has been fueled by its relative ease of application compared to historical forms of noninvasive ventilation, such as the iron lung and cuirass, as well as its demonstrated ability to improve patient outcomes in certain forms of acute respiratory failure compared to past standard therapy, including endotracheal intubation.1

When Is NIV Right?

Many applications of noninvasive ventilation have been attempted in the critical care arena, but so far, only four are supported by multiple randomized controlled trials and meta-analysis. Successful application has been noted in the exacerbations of COPD,2 treatment of acute cardiogenic pulmonary edema,3 facilitation of early ventilatory liberation of the COPD patient population,4 and utilization in patients who are at high risk for infectious process secondary to tracheal intubation and who are immunocompromised.5

In the COPD patient population, NIV application resulted in a more rapid improvement in vital signs and gas exchange, as well as in a reduction in the need for tracheal intubation.6 Based on recent clinical trials and observation, NIV should be considered the ventilatory modality of first choice to treat acute respiratory failure caused by exacerbations in COPD patients. Recent meta-analyses on the application of NIV to treat acute pulmonary edema have revealed that both continuous positive airway pressure (CPAP) and NIV lowered intubation and mortality rates compared to conventional diuresis therapy along with oxygen administration.7 Patients with a history of COPD, who were extubated despite repeated failed spontaneous breathing trials with conventional methods and placed on NIV, had shorter ICU stays and decreased incidence of reintubation.8 The utilization of NIV is well documented for immunocompromised patients who are high risk for infectious complications from endotracheal intubation, such as those with hematologic malignancies or AIDS, or following solid organ or bone marrow transplants.9

Successful application of noninvasive ventilation is dependent upon a well-fitting and comfortable patient interface. Although nasal masks have certain advantages over full face masks—including greater comfort, less likelihood of causing claustrophobia, and easier speech and expectoration—they also permit more air leakage through the mouth and have been associated with a higher rate of initial intolerance during acute application of NIV.10

Another important aspect of successful NIV administration is the ability to deliver humidified gas to patients receiving high flow during prolonged use. Noninvasive ventilation lowers the humidity of the delivered gas, and humidification is important to bring the delivered gas to near ambient levels.

Humidification Is Key

During noninvasive positive pressure ventilation, the upper airways are not bypassed; so theoretically they should adequately heat and humidify medical gases. However, out of a group of patients with severe obstructive sleep apnea syndrome who were treated with nasal CPAP and administered nonhumidified ambient air, up to 60% suffered from nasal congestion, stuffiness, and dryness.11 These airway symptoms were mainly caused by the considerable one-way flow of ambient air through the nose and out of the mouth during mouth leaks, causing the release of several vasoactive amines and leukotrienes. The use of a heated humidifier reduced nasal congestion, upper airway dryness, and dry mouth or nose. In addition, the use of a heated humidifier increased patient satisfaction and the number of hours with CPAP application per night, and the patients indicated that they felt more refreshed on awakening.

Humidification and warming of the inspired gas may be needed to help prevent the adverse effects of cool, dry gases on the airway epithelium. The nasal mucosa can lose its capacity to heat and humidify inspired air, and the mucosa progressively dries and releases inflammation mediators such as leukotrienes, with an increased vascularity. If the gases delivered from the noninvasive ventilation unit are not humidified, the humidity level will be lower than that of ambient air; this is especially true if the pressure support level has to be increased. The consequences of inadequate humidification can be detrimental and can lead to increased mucus viscosity and retained secretions, resulting in increased airway resistance, diminished pulmonary compliance, and possible atelectasis.10 During sleep apnea, warm, humid air may have a bronchodilation effect on patients with abnormal airway caliber and facilitate improved gas distribution.12

In the acute setting, NIV is usually delivered via a face mask, but this may cause discomfort, skin lesions, and gas leaks. To enhance patient comfort and to permit longer periods of NIV, a new device—the helmet— has been introduced. Similar to the carbon dioxide rebreathing that occurs with use of the helmet, the high internal gas volume could also serve as a mixing chamber between the heated and humidified expired gases and the dry medical gases entering the helmet. This could raise the levels of heat and humidity of the medical gases, thus avoiding the need for a heated humidifier. The final humidity inside the helmet will depend mainly on two factors: the amount of humidity in the patient’s expired gases and the flow of fresh medical gases into the helmet. In addition, the humidifying capability of the respiratory tract may also be influenced by the presence of airway or pulmonary disease.

Choosing the Best Device

Two types of humidification devices—heated humidifiers and heat and moisture exchangers (HMEs)—have been utilized for both acute and chronic application of NIV.13 Use of the heat and moisture exchangers is widely accepted in intubated patients receiving mechanical ventilation. HMEs are easy to use and may be less expensive than heated humidifiers. Heat and moisture exchangers combined with microbiological filters (HMEFs), also called artificial noses, work by passively retaining warmth and humidity in air leaving the trachea during expiration and by recycling it during the next inspiration. Artificial humidification of inspired air serves to maintain or restore physiological heat and moisture conditions in the bronchial system in intubated or tracheotomized patients. The need to condition the respiratory gases in these patients is undisputed, but what are the advantages and disadvantages of the various methods and techniques of humidifying inspired gases? Present-day medical knowledge indicates that passive artificial humidifying of respiratory gases (heat and moisture exchanger) is adequate to meet most requirements for warming and moistening the inspiratory air in patients whose upper airways are unable to naturally condition respiratory gases as a consequence of intubation and tracheotomy. This applies to artificial ventilation in prehospital situations, artificial ventilation in anesthesia, and long-term artificial ventilation on the intensive care unit. With appropriate restrictions, the respiratory air of patients who breathe spontaneously via an artificial air vent (eg, tracheal cannula) can also be conditioned by HME. However, HMEs, which are generally placed between the Y-piece and the noninvasive ventilation interface, can add an important amount of dead space compared to a heated humidifier, which is placed in the inspiratory limb. Heated humidifiers comprise an external source producing heat and water vapor from sterile water. Active humidification with heated humidifiers has long been considered the gold standard for adequate inspired gas conditioning during mechanical ventilation, since these devices can—theoretically—deliver gas at 37°C with 44 mg H2O/L absolute humidity to the patient. Such values, however, have not been found in the clinical setting, even with the most efficient heated humidifiers. Measurements showed considerably lower values, between 35 and 40 mg H2O/L. Heated passover humidifiers have minimal effects on delivered pressures, whereas heat and moisture exchangers are to be avoided because they can interfere with the NIV to reduce work of breathing.14

Two studies conducted in 2002 that compared the physiologic effects of HME and heated humidifier found similar results. Jaber et al15 found in 24 patients that, compared to a heated humidifier, HME was associated with an elevated higher Paco2. HME was also associated with a larger minute volume and higher mouth occlusion pressure at 0.1 second. In a second study, Lellouche and coworkers14 found that hypercapnic patients’ inspiratory efforts were greatly increased with an HME in-line. Alveolar ventilation was maintained only at the expense of an increased work of breathing with a HME compared to a heated humidification system.

Both studies noted that humidification devices can strongly affect some physiological variables. It should also be noted that in both studies, the NIV interface was provided via a mask. Based on the two outcomes, it is recommended that during acute application of NIV, a heated humidification system should be used to help reduce the work of breathing and maximize carbon dioxide clearance. In the chronic application of NIV, however, both a heated humidification system and HME had similar tolerance and adverse effects, so at this time there is no recommendation favoring one device or another.


Successful application of noninvasive ventilation may depend more on patient comfort than on setting the optimal parameters. Selecting the proper patient-ventilator interface and providing adequate humidification can increase the likelihood of successful application. Based on the current literature review and clinical experience, there is strong evidence for the need to humidify the inspired high-flow gas that noninvasive ventilators generate. The jury verdict appears to be somewhat mixed on which type of humidification device is superior. Good clinical assessment and the utilization of objective criteria will guide the respiratory care practitioner to make the best decision on which humidification device will optimize the patient’s outcome during the application on noninvasive ventilation.

Kenneth Miller, MEd, RRT-NPS, is clinical educator for respiratory care, Lehigh Valley Hospital, Allentown, Pa. For further information, contact [email protected]


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