By Phyllis Hanlon

Image of monitoring equipment.

In 2012, the American Association for Respiratory Care (AARC) released updated clinical practice guidelines that highly recommend humidification during invasive and noninvasive mechanical ventilation.1 Based on 184 clinical trials and systematic reviews, and 10 related articles, humidification has been shown to improve efficacy of treatment, and increase patient comfort and compliance. 

Invasively ventilated patients who receive humidification through a passive system, ie, heat and moisture exchangers (HMEs), should receive a minimum of 30 mg H2O/L, according to the guidelines. Note that HMEs are not recommended for patients with low tidal volumes, because these devices contribute additional dead space, which can increase the ventilation requirement and arterial carbon dioxide tension (PaCO2). Additionally, the guidelines do not advise using HMEs as a prevention strategy for ventilator-associated pneumonia (VAP).

Furthermore, the guidelines suggest that active devices, ie, heated humidification (HH), for patients invasively ventilated should be between 33 mg H2O/L and 44 mg H2O/L, and gas temperature should range between 34°C and 41°C at the circuit Y-piece, with a relative humidity of 100%. The guidelines also suggest active humidification for noninvasively ventilated (NIV) patients, since it may improve adherence and patient comfort; HMEs are not recommended for these patients. 

Additionally, studies indicate that essential humidity should be 31°C, 32 mg/L, when used with face mask oxygen to achieve ideal results. Optimal humidity should be 37°C, 44 mg/L, with low flow oxygen through nasal cannula.

Matching Humidity and Patient Physiology

Research indicates that actively matching humidity to the patient’s physiology attains the best results, according to Rob Cornell, humidification products manager at Fisher & Paykel Healthcare Inc. He explained that the end goal for patients with bypassed upper airways is to deliver heat and moisture at a level that mimics the normal physiologic conditions in the lungs, body temperature, and 100% saturation with moisture. “This facilitates gas exchange in the lungs and helps maintain the airway defense mechanism called the mucociliary transport system. When a patient is artificially ventilated, it’s important to maintain mucus quality so it flows normally,” he said. “If gas is not delivered at body temperature and saturated with moisture, the mucus in the airways can lose moisture, thicken, and become difficult to transport. This can result in blocked small airways and potential for blocked endotracheal or tracheostomy tubes. This can affect gas exchange and oxygen delivery as well.” 

A study by Ryan et al supports Cornell’s statements.2 The researchers determined that gases delivered to the airway at body temperature and saturated with moisture did not result in moisture loss to the body or airway, having no effect on mucociliary transport. Cornell said, “If gases were delivered at lower temperatures, and therefore, lower humidity—the two go hand-in-hand—the body has to make up the heat and humidity deficit, with consequences for mucociliary transport.” Ryan’s study also reported that gases delivered at lower temperatures resulted in this gas having a low relative humidity at body temperature, which should be at 100%. 

Cornell explained that HMEs currently serve roughly half the market. “These devices do not add heat and moisture to inspired air, but reflect a portion of the patient’s own exhaled heat and moisture,” he said. “What is returned to the patient with the next breath is at a temperature and humidity level that Ryan showed could not mimic normal physiologic conditions in the lungs.”

There are many challenges with humidifiers for creating and then delivering gas conditioned to body temperature and saturated with moisture. Ambient environmental conditions can cause condensation in the humidification system; condensation is not humidity delivered to patients. Cornell noted that drafts from air conditioner vents or fans, as well as cold rooms, might drain heat energy from a humidification system. Radiant heat sources, such as infant warmers or the sun shining through a window, also can influence humidifier performance at controlling condensation. 

To ensure effective humidification delivery, Fisher & Paykel developed a system that includes a heater base with a temperature protocol set at body temperature, an autofeed humidification chamber, and a high performance heated breathing circuit that creates and delivers body temperature and saturated gases. “The high performance circuit is key to minimize heat loss and effectively control condensate. The latest high performance circuits have an insulated inspiratory limit to regulate heat loss and therefore condensation,” Cornell said. “These new circuits also include the Evaqua expiratory limb that is vapor permeable to allow water vapor to leave the circuit before condensation can form.”

Enhancing Respiratory Efficiency with High Flow Therapy 

Another effective active humidification delivery system developed for high flow therapy by nasal cannula enhances respiratory efficiency by purging nasopharyngeal anatomical dead space and supporting the work of breathing.3 This system allows a patient to breathe high fractions of oxygen, or the intended gas mixture without dilution by ambient gas because room air is not entrained during inspiration, said Tom Miller, PhD, director of clinical research and education at Vapotherm. “The humidity should be as close to body conditions as you can get: 35° to 37°C and saturated with H2O vapor,” he said, adding that flows up to 40 liters per minute through a nasal cannula allow for greater comfort and compliance than alternative noninvasive ventilator support modalities. 

Vapotherm’s system features a patented vapor transfer cartridge system with semipermeable membranes through which the H2O passes in vapor form. The pore size is large enough to allow one molecule at a time to push water into the gas stream, generating a high flow of warm vapor. When the vapor is administered via nasal cannula, the maximum amount of humidity is delivered deep into the patient’s airway with minimal condensation or dilution with room air.

This system differs from conventional ones, according to Miller, and incorporates advanced alarm and monitoring features for optimal safety and efficacy. “There is a big advantage. Because we’re not boiling water, we’re not overheating it. The gas is 37° when it emerges from the vapor transfer cartridge,” he said. “The delivery tube system acts like an insulator; thermistors and heating elements are not needed. Our gas is at body temperature and is saturated with energetically stable vapor.”

Vapotherm’s specially designed water jacket does not depend on water coils, so is not affected by changes in the environment. “Cold in the room won’t cause rain-out, like in a conventional system,” said Miller. 

Additionally, the Vapotherm system is designed for the specific challenge of being a high flow nasal cannula system. “When we blow high flow through a cannula, it provides resistance that increases the pressure inside the device,” Miller said. He pointed out that pressure in a ventilator circuit is typically no more than 60 to 65 cm/H2O internal pressure, but when pushing high flow through a small nasal cannula circuit, pressure can be greater than the 200 cm/H2O range. “Other systems can use wide board cannulae to decrease the pressure that builds. The large prongs fit more snugly in the nose, but offer less resistance than the optimal smaller bore cannulae,” he said. 

Another unique high flow delivery system delivers blended helium and oxygen gas mixtures (heliox) through a nasal cannula. “Heliox reduces a patient’s resistive work of breathing by using a gas that is easier to draw in and out of the airways,” Miller said. “However, helium has greater thermal conductance than air, and so it is even more important to make sure it is adequately warmed and humidified.”

Choosing a Humidification System 

Michael Nibert, RRT, BSRT, of Nibert Consulting, a respiratory consulting firm in College Station, Tex, noted that each facility makes its own decisions regarding which method of humidification it will use. While he sees no safety issue with either method, he pointed out that many health care facilities are currently trending toward heated humidification instead of HMEs. “There are too many considerations when using HMEs to get optimal humidity. For instance, some patients can lose lung volume and may be left on a dry circuit. Infection control is also more of an issue,” he said.

William Howard, RRT, equipment coordinator at Brigham and Women’s Hospital in Boston and clinical specialist at Outcomes Solutions Inc, pointed out that for patients who do not require frequent change-out, HMEs could be an inexpensive option. “But in some patients who require frequent changing to avoid contamination, you could find you’re paying more money. In fast turnover patients, like those who’ve had a heart procedure, it’s common to use HMEs. They won’t be on a ventilator for a long time,” Howard said, adding that some hospitals use this treatment for patients with pneumonia and lung disease also.

Howard added, though, that not all patients are suitable candidates for HMEs. “If the patient has frank, bloody, copious secretions or if the patient is hypothermic, fresh out of the operating room, with a body temperature of less than 32 degrees, such as a near-drowning victim, HME would not be appropriate,” he said. 

Howard also prefers heated humidification, which he said has no downsides. “It’s a natural representative of humidity and secretion in a person. The patient doesn’t have to make up for humidity deficit,” he said, adding that this therapy can be used indefinitely and in all age groups. 

As an example of this, Jeri Eiserman, RRT, director of clinical support and education for Teleflex, cites the company’s ConchaTherm Neptune brand system. “The heated-wire circuits that are used with the Neptune do not have a limited period of use (ie, a labeled change interval). Rather, they can be used for the duration of the patient’s therapy, regardless of the length of time, unless they become soiled or fail to function.” As a result, she said, “the need for circuit manipulation and change” is reduced. RT


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


References

1. Restrepo RD, Walsh BK. Humidification during invasive and noninvasive mechanical ventilation: 2012. Respir Care. 2012;57(5):782-8.

2. Ryan SN, Rankin N, Meyer E, Williams R. Energy balance in the intubated human airway is an indicator of optimal gas conditioning. Crit Care Med. 2002;30(2):355-61.

3. Dysart K, Miller TL, Wolfson MR, Shaffer TH. Research in high flow therapy: mechanisms of action. Respir Med. 2009;103(10):1400-5.