The human airway has an important role in heating and humidifying inspired gas, and recovering heat and moisture from expired gas. The amount of water vapor in a gas mixture can be measured as absolute humidity (AH) or relative humidity (RH) in relation to the temperature. AH is the total water present in the gas (mg H2
O/L) and RH is the amount of water present expressed as the percentage of maximum carrying capacity at a given temperature [1
]. The human airway must provide gas at core temperature and 100% RH at the alveolar surface in order to optimize gas exchange and protect lung tissue [2
Non-invasive ventilation (NIV) is a mechanical ventilation modality that does not utilize an invasive artificial airway (endotracheal tube or tracheostomy tube) [3
]. NIV is usually delivered through a nasal or oro-nasal mask so the inspired gas passes through the upper airway where it is conditioned. Like during spontaneous breathing, patients under NIV require adequate humidification and heating of the inspired air (that is, gas conditioning) [3
]. NIV delivers inspired air at high flow rates, which may overwhelm the usual airway humidification mechanisms. Inadequate gas conditioning has been associated with anatomical and functional deterioration of nasal mucosa (ciliary activity, mucus secretion, local blood flow, nasal resistance). In addition, there are also negative effects on tolerance to NIV when a patient breathes inadequately humidified air [1
] (Table ).
Effects of inefficient gas conditioning during non-invasive ventilation
Metaplastic changes and keratinization of the nasal epithelium and submucosa have been reported in patients on home-NIV when the level of humidification was inadequate for long periods [5
]. These histopathological findings were confirmed by our recent survey, which found similar structural changes of the nasal mucosa in four patients with acute respiratory failure treated for 7 days with NIV without a humidification system added (unpublished data; Figure ). This suggests that changes in the nasal mucosa occur relatively early after starting NIV in an acute setting and that humidification should be considered even when only short-term use of NIV is expected.
Biopsy of nasal mucosa. Metaplasia (1) and keratinization (2) in nasal respiratory mucosa in one patient without humidification during non-invasive ventilation.
Inadequate airway gas conditioning may have serious consequences in critically ill patients using NIV [6
]. Difficulties were recently reported with intubation in patients failing a trial of NIV delivered at high inspiratory oxygen fraction with a low level of humidification [6
]. One case study showed ispissated secretions, causing life-threatening airway obstruction in a patient using NIV for hypoxemic respiratory failure [7
]. A high fraction of inspired oxygen was also entrained into the NIV circuit and this extra anhydrous gas may have contributed to the airway obstruction.
Inadequate humidification can also cause significant discomfort for chronic NIV users. In an experimental setting, Wiest and colleagues [8
] showed that dryness-related symptoms started to appear when AH was lower than 15 mgH2
O/L. Normal subjects undergoing NIV without humidification scored discomfort significantly higher on a visual analogue scale than when heated humidification (HH) was used, and this was associated with increased nasal airway resistance (NAWR). HH improved comfort and compliance to NIV [6
Similarly, Lellouche and colleagues [10
], in healthy subjects under continuous positive airway pressure (CPAP) delivered by a bucco-nasal mask, found the level of comfort was significantly lower when no humidification was used. When a humidification device was applied and AH was higher than 10 to 12 mgH2
O/L comfort was significantly better [10
In a very comprehensive compliance analysis in patients in home-NIV, Nava and colleagues [11
] compared two humidification systems, HH and a heat and moisture exchange filter (HME); compliance was much better (75% of patients) with the former. However, other symptoms, such as dry throat, the number of hospital admissions and the rate of complications caused by infection (mainly pneumonia), were similar with the two systems [11
]. Similar results were published by Massie and colleagues [12
], who stressed the importance of 'early' humidification from the very start of ventilatory treatment, so as to ensure the best possible compliance in home-NIV patients. Some other authors, however, maintain that humidification has no really significant effect on adherence to chronic NIV [13