Generally, clinicians agree that humidification should be used for all patients placed on mechanical ventilation. Whether to provide active or passive humidification, however, is always a debate.

Image of monitoring equipment.

In 2012, two researchers from Texas reviewed scores of clinical trials and articles related to humidification during invasive and noninvasive mechanical ventilation that were published between 1990 and 2011.1 Following the Grading of Recommendations Assessment, Development, and Evaluation (GRADE) scoring system, the researchers updated the list of the American Association for Respiratory Care’s (AARC) clinical practice guidelines (CPGs), which addresses humidification in general, appropriate use of active and passive humidification, heat and moisture exchangers (HMEs) and contraindications for use.

The Agency for Healthcare Research and Quality (AHRQ) asserts that providing humidification helps to “prevent hypothermia, disruption of the airway epithelium, bronchospasm, atelectasis and airway obstruction.”  For the most part, clinicians agree that humidification should be used for all patients placed on mechanical ventilation. Whether to provide active or passive humidification, however, depends on individual circumstances, patient condition and facility protocol.

Joseph Killam, senior product manger for AirLife Respiratory Care at BD [formerly CareFusion Corp], explained that passive humidification involves “capturing exhaled breath and redelivering it to the patient on mechanical ventilation via an HME.” According to the updated guidelines, HME should be given at a minimum of 30 mg H2O/L. “BD offers a range of devices that meet this standard,” he said. HMEs help to mimic normal conditions in the patient’s airway, and they have also been found to be cost effective, easy to use and maintain.

Passive humidification may also be used effectively with patients who require aerosolized medication. A bypass HME features a closed system that allows the clinician to administer the drug without breaking the circuit. Killam said, “This device has a rotating arm. You move the media out of the way, deliver the medication and then move the arm back in place.”

For the most part, passive humidification presents no significant risks; however, if a patient has “frank, bloody, thick secretions,” this method would be contraindicated, said Killam, noting that use of an HME adds dead space to the circuit, which may increase work of breathing.

Hygroscopic Condensers

In spite of the updated guidelines, controversy continues to swirl around the amount of humidification a patient should receive. Hal Norris, president, ARC Medical Inc, supports the use of humidification in patients that are intubated and on mechanical ventilation. However, he questions the need for excessive amounts of water. “Intubation without humidification dries the patient out and causes endotracheal tube occlusion. The effect of water is to provide enough humidification to maintain the mucus membranes and epithelium so there is no drying of the epithelium or lungs,” he said.

Norris explained that HME devices have a range of performance. “Like gas mileage, there are some low performing devices and some high performing devices,” he said, adding that ARC Medical’s ThermoFlow, which acts as an “artificial nose,” is the only product on the market that complies with the AARC guidelines. ThermoFlow’s hygroscopic condensing humidifier (HCH) makes the difference, he asserted.

A poster presentation2 at the AARC meeting in 1996 demonstrated that a hygroscopic condensing humidifier provides adequate heat and humidity for mechanically ventilated patients “by focusing on the change in the consistency of the patient’s secretions.” The study involved 58 patients, and 286-patient days of sputum categorization—161 in the HCH group and 125 in the heated wire circuit (HWC) group—which actively added heat and humidification to the inspiratory limb of a heated circuit after passing through a heated water chamber. Sputum was recorded as watery, moderate or tenacious. The results were similar for each group (HCH group: 12.4% watery, 79.5% moderate, 11.2% tenacious. HWC group: 11.2% watery, 88% moderate, 13.6% tenacious).

Another study3 found similar results regarding secretion consistency, but also evaluated the incidence of bacterial growth. The ThermoFlo HCH with filter, which is placed between the Y connector and the endotracheal tube, was changed every 24 hours; the HWC, every 48 hours. After seven days of use, the HCH showed no bacterial growth, while the HWC cultures did have bacterial growth after 48 hours of use. More important, no tube occlusions occurred throughout the course of the study. “We have used this in hospitals with as many as 1300 beds and from the time of intubation to extubation, we’ve never had an occlusion,” Norris said, adding that regardless of minute ventilation and medical condition, humidification with a hygroscopic condenser at 30 mg H2O/L is better for the patient.

Higher Levels of Humidification

While 30 mg H2O/L serves as the minimum requirement for humidification, higher levels have been found to be beneficial as well. Researchers conducted an animal study4 in 2015 to define the relationship between airway humidification and mechanical ventilation-induced lung inflammatory responses. The study authors measured the inflammatory cytokines in the bronchi alveolar lavage fluid (BALF) and examined the integrity of the airway cilia and the tracheal epithelium of 40 rabbits, which were divided into control, dry gas and experimental groups. The dry gas group received mechanical ventilation for eight hours without humidification and the experimental group received mechanical ventilation with humidification at 30, 35, 40 and 45° C.

At the conclusion of the study the dry gas group demonstrated increased tumor necrosis factor alpha levels in BALF compared with the control group. The animals treated with humidification at 40° C showed reduced inflammation and pathology similar to that found in the control group. The authors concluded that appropriate humidification reduced inflammatory response, consequential to mechanical ventilation and also reduced damage to the cilia and water loss in the airway.

Passive vs Active

Mechanically ventilated patients who do not receive some type of humidification may experience thickening of secretions, changes in the lung lining and obstruction of the airway, according to Madhu Sasidhar, MD, section head for respiratory care at the Cleveland Clinic. Whether an active or passive humidification device is used, the goal is to provide humidified warm air that mimics the patient’s body temperature. He noted, though, that since passive humidification devices do not rely on an external water or energy source or have a heating element, they are more convenient to use than an active system. Also, patients expected to be on a mechanical ventilator for 24 hours or less and who do not have significant structural lung disease or need for frequent medication treatment present as ideal candidates for passive humidification. He added that passive humidification can also be clinically effective for patients who are mobile.

In spite of their convenience, passive humidification devices do have some disadvantages, according to Sasidhar. “Their use is contraindicated for long periods of time in mechanically ventilated patients. They need to be changed frequently. Secretions can increase and the system might not provide enough humidification, which can dry out the patient,” he said. “Also, the system needs to be removed if the patient is receiving medication. There is also a slight risk of air leaks.”

From personal experience, Sasidhar indicated that it’s not uncommon to see a mechanically ventilated patient with a tracheotomy. In such cases, humidification can increase the risk of complications related to mechanical ventilation associated infections, especially pseudomonas.

On the other hand, active devices, which have a heated circuit and require a water source, are almost always clinically superior, Sasidhar noted. Active humidification is used for patients with underlying lung diseases, such as chronic obstructive pulmonary disease (COPD), bronchial asthma or pneumonia. He pointed out in some cases patients with underlying lung disease need long-term mechanical ventilation. “The exception would be patients with respiratory failure due to neuromuscular causes, such as multiple sclerosis or Lou Gehrig’s disease. In that case, an HME could be sufficient,” he said.

According to Jay Graham, MBA, RRT, senior quality care consultant for AirLife Respiratory Care for BD, active humidification is preferred for patients with non-invasive ventilation. “The flow rate on a passive system is so high it dries out the patient’s airways and could cause an increase in secretions and tenacity,” he said.

Humidification Challenges

Sasidhar pointed out that there is a dearth of good, solid literature on the outcomes of long-term mechanical ventilation with either passive or active humidification with regard to VAP and other lung-related infections. He reported that some evidence shows an association between breaking the circuit to change the HME and an increased risk of developing VAP. Since active humidification doesn’t require the clinician to break the circuit, there are empirical benefits for this type of humidification over passive systems. For this reason, Sasidhar recommends an active humidification system for the patient who needs long-term mechanical ventilation and is at risk of developing VAP.

One of the greater challenges Sasidhar has witnessed is operationalizing practice and protocols related to the use of humidification systems. As the director of a respiratory service, he understands firsthand the importance of following diligent protocols when using HMEs. “When an RT is working with a patient, the HME has to be changed routinely. Also, if the patient is transferred or his condition changes, the HME may no longer be appropriate so you have to transition from the HME to active humidification. You have to be careful of the true technical specifications and bench performance of HMEs. Many don’t meet the minimum requirements,” he said.

Also, when using HMEs during transport, the therapist must keep in mind ambient temperature changes, which could affect the effectiveness of the HME. Sasidhar pointed out that in the hospital, temperatures remain at relatively the same levels. But during transport a patient is exposed to an open environment, moving from the hospital to an ambulance or aircraft and back again to the hospital. “The performance of the HME can vary. In some patients this could have a significant impact,” he said.

Adverse effects are hard to detect in a patient with underlying lung disease, according to Sasidhar. “You need to be aware of the technical issues regarding the device interaction with the environment. Under these circumstances, you would use active humidification or heated circuits.” RT

Phyllis Hanlon is a contributing writer to RT. For further information, contact [email protected]


  1. Restrepo RD, Walsh BK. “Humidification during invasive and noninvasive mechanical ventilation: 2012.” Resp Care. 2012 May;57(5): 782-788. doi:10.4187/respcare.01766.

  2. Day S, McConnell R, Fredericksen H, MacIntyre NR. “Comparison of long term use of a hygroscopic condensing humidifier versus heated wire circuit.” Presented at the AARC, November 1996.

  3. Stout KK, Blatt MW. “The effects of the ThermoFlo filter hygroscopic  condensing humidifier on sputum characteristics.” Poster presentation at the American College of Chest Physicians. 1994.

  4. Jiang M, Song JJ, Guo XL, Tang YL, Li HB. “Airway humidification reduces the inflammatory response during mechanical ventilation.” Respir Care. 2015 Dec;60(12): 1720 – 8. doi: 10.4187/respcare.93640. Epub 2015 Sept 1.

  5. Hernandez G, et al. Effect of postextubation high-flow nasal cannula vs conventional oxygen therapy on reintubation in low-risk patients.JAMA. 2016;315(13):1354-1361. doi:10.1001/jama.2016.2711.