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1 Introduction

When the tracheal mucosa is bypassed via endotracheal tube (ETT) intubation or from a surgically placed tracheostomy, humidification is essential to preserve tracheobronchial mucosal integrity [1]. Without humidification, the tracheal mucosa will lose ciliary function, develop inspissated secretions, and the underlying connective tissue will undergo structural changes [2–5]. Williams et al. performed a meta-analysis evaluating the relationship between the humidity and temperature of inspired gas and airway mucosal function [6]. They developed a model suggesting above or below optimal temperature, and humidity conditions can lead to impaired airway mucosal dysfunction, or, vice versa, that adequate mucociliary function is an indicator of ideal humidification. Oostdam et al. showed animals that inspired dried air demonstrated a significant reduction of extravascular water of the loose connective tissue of the airways and an increase in airways resistance to histamine [2]. ETT occlusion secondary to thickened or dried secretions is also strongly linked to suboptimal humidification [5, 7]. The most common way to avoid these and other potential complications (Box 1) is accomplished by applying humidification from non-­heated-wire humidifiers, heated-wire humidifiers, or a heat and moisture exchanger (HME) [4, 8]. The goal of each of these humidification devices is to provide tracheal humidification consisting of heat and moisture to the inspired gas with a minimum of 30 mgH2O/l or 100% relative humidity with a delivered gas at 30°C [4, 5].

Non-heated wire humidifiers are becoming increasingly less popular because of the concerns over respiratory condensation [8]. In patients requiring long-term mechanical ventilation >96 h, the heated-wire humidifiers are the device of choice [4, 8]. Other indications for heated humidifiers include contraindications for HMEs as listed in Box 2 [4]. Heater humidification devices are capable of delivering gases with 100% relative humidity near 37°C body temperature. However, heated humidifiers are more expensive than HMEs, and have been associated with a potential for electrical shock, hyperthermia, thermal injury, and nosocomial infections [4, 9, 10]. Inappropriate settings of temperature or humidification can also lead increased resistive work of breathing due to mucous plugging and/or life-threatening occlusions of endotracheal or tracheostomy tubes [8, 11].

HMEs are disposable devices that function by passively storing heat and moisture from the patient’s exhaled gas and releasing it to the inhaled gas (an artificial nose) [4, 9]. Their regulation of humidity is slightly lower than that of heat humidifiers, but their clinical efficacy is similar [12]. HMEs are hydrophobic, hygroscopic, or a combination of both. Hygrophic HMEs have the best antimicrobial properties and lower humidity retention [9]. Hygroscopic HMEs have less antimicrobial filtration but better humidity qualities, and all HMEs are able to absorb moisture in the expired air. HME humidity and temperature settings are predetermined by the manufacturer based on in vivo studies. The optimal HME performance specification is not well defined; however, Lellouche suggests an absolute humidity <30 mgH2O/l is associated with a risk of ETT occlusion (Fig. 14.1) [13]. The airway resistance has been shown to be less in hygroscopic models compared to hydrophobic or combination HMEs [14].

Fig. 14.1
figure 1_14

Recommendation of Lellouche et al.

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HMEs are more attractive than heated humidifiers for patients requiring mechani­cal ventilation of short duration because they are less costly and easier to use [9–11]. They are recommended for shorter durations <96 h or during patient transport, and have been used safely for up to 7 days [4, 10]. Most manufacturers recommend changing HMEs every 24 h despite a plethora of literature supporting use for 96 h except in COPD patients [10]. HMEs appeared to lose the ability to maintain ­humidity for more than 48 h in patients with COPD. They do not require electricity, and there is no risk of thermal injury or concerns about excessive condensation. Like heated humidifiers, they possess a unique set of complications, i.e., hypothermia, hypoventilation due to increased dead space, impaction of secretions, and increased work of breathing. HMEs also have contraindications (Box 2) [4].

HMEs that possess a large dead space present another problem that clinicians need to be aware of, particularly when using low tidal volume for lung protective strategies [3]. Resultant hypercarbia and/or increased minute ventilation may ensue. Some experts recommend removing HMEs in patients with acute respiratory distress syndrome and changing to heated humidifiers.

Ventilator-associated pneumonia (VAP) has received much attention over the past several years and has been related to multiple variables, including humidifiers [15]. Other etiologies with stronger relationships to VAP include microaspirations from around the endotracheal cuff, colonization of the GI tract, poor provider hand hygiene, inadequate oral care, prolonged sedation, and patients in recumbent positions of less than 30° [15]. Nevertheless, the humidifier can be a real threat if not managed correctly. Routine inspection of the humidification device for secretions and or condensate in the patient circuit should be scheduled [4]. Increasing evidence is showing that the rate of VAP is lower when using a passive (HME) rather than active humidification device at a relative risk of 0.7 [16]. The risk of occlusion from secretions, hypothermia, and prolonged intubation often prohibits the continued use of HMEs. The exact mechanism by which HMEs reduce VAP is not entirely clear, but most experts recommend HMEs in all patients when there are no contraindications.

Once the selected humidification device has been chosen, determining the best level of heat and humidification, as previously mentioned, is difficult. Sottiaux and Branson suggest a minimum of 32°–34°C and 100% relative humidity [5]. The AARC recommends that the absolute humidity cutoff should probably be  <  30 mgH2O/l; however, there is no evidence that any specific humidity level improves outcome [3]. Nevertheless, the deviation of tracheal mucosa function relative to suboptimal humidity cannot be ignored [1, 16]. The AARC recommends the ideal inspired gas temperature should be near the patient’s airway opening and the inspiratory gas should not exceed 37°C [4]. In general, HMEs probably improve baseline variables of humidification and temperature, but whether they perform according to specification is arduous to actually measure [3].

Lemmens et al. reviewed three different HMEs during routine anesthesia practice to determine whether the devices actually performed according to the manufacturers’ specifications [17]. Their main findings showed that following the manufacturers’ specifications did not reliably predict performance during routine anesthesia procedures. Only one HME performed according to specifications. The other two performed at less than the values specified by the manufacturers, but did provide humidification. Solomita evaluated non-heated-wire humidification, heated-wire humidification, and HMEs to determine the effects of humidification on the volume of airway secretions in mechanically ventilated patients [8]. Their results revealed that measurement of temperature alone was inadequate to predict ­humidification and non-heated-wire humidification was associated with a greater secretion volume. In practice, humidity is measured by a combination of ­temperature and arbitrary clinical observation of secretion accumulation because formal measurements of humidity are limited by technology and cost [3, 5].

4 Conclusion

The tracheal mucosa and ciliary function can be severely impaired without adequate humidification in patients who are mechanically intubated or undergo tracheostomy [1]. Resultant complications can occur from increased secretions and infection, or endotracheal tube occlusions can ensue [2, 5]. Although the exact amount of humidification and temperature has not been defined, there is a large body of literature supporting that the minimal physiological conditions of 32°–34°C and 100% relative humidity should be maintained [5]. Objective assessment of humidity is difficult to make, so routine monitoring of the quantity of secretions is recommended [4]. HMEs are generally preferred if there are no contraindications in patients who require mechanical intubations of short duration [4, 10]. If mechanical ventilation is prolonged, optimal humidification is required to ensure tracheal mucosal function. Ultimately, the humidification devices and target settings will change as more studies emerge, but the indications will remain the same.