By following guidelines and being informed of humidification device options, RCPs can ensure that their patients get the best fit.
In some respiratory care programs, a laboratory exercise is performed wherein an oxygen cylinder is opened and some of its contents flow into a tightly closed bag, which contains a hygrometer. After the bag is full, the hygrometer appears to be stuck at the “0.” This exercise reinforces what the instructor and the textbook have told the students about how dry these gasses are and how a humidity deficit can develop. This is particularly true for intubated patients where the anatomical humidifier has been bypassed. At the end of class, the decision to add some form of humidity for mechanically ventilated patients is a fairly easy one to make.
Unfortunately, the simple question of whether to humidify is now rapidly followed by the more complex question of “how.” Heated humidifiers, heated wire circuits, and dozens of heat moisture exchangers (HMEs) are all at the disposal of today’s clinicians. There is very little argument that if inhaled air is not adequately humidified, a host of respiratory system compromises will follow, such as the destruction of cilia and mucus membranes, and an increased incidence of pneumonia.1-3 Yet each of the humidifying devices mentioned above could be harmful to the patient if it is provided under the wrong circumstances. In the paragraphs that follow, information on these devices and some guidelines on their use will be provided to assist in this decision-making process.
Considering the number of options available to today’s respiratory care practitioner, it could be said that the selection of a humidifier is much like the concept of humidity itself: It’s all relative.
Types of Humidifiers
Passive Humidifiers. The passive humidifier or “artificial nose” mimics the same process that occurs in the human nose. These devices are placed between the patient “Y” and the connection to the endotracheal tube. The ebb and flow of air across its surface allow a recurring transference of moisture from the patient’s exhaled air to the HME and back to the patient. The term “HME” is used to describe both a particular type of device and the entire category of passive humidifiers.
HMEs have become popular for several reasons:
•They have no moving parts
•They do not require electricity
• If they are changed only once every 24 to 48 hours, they can be a less expensive method for providing an acceptable level of humidity to invasively ventilated patients.7
The most basic HMEs consist of layered aluminum inside a low-compliance case. A newer version of HME, the hygroscopic condenser humidifier (HCH), is conceptually similar to the basic HME, but its interior surface is treated with a moisture-repelling hygroscopic salt (lithium chloride, calcium chloride). This increases its ability to extract moisture.7 The basic HME may return only 10 mg to 14 mg H2O/L. The HCH has been shown to return 22 mg to 34 mg H2O/L. The reported ranges for both devices were obtained at tidal volumes of 500 mL to 1,000 mL.7
Either type is available with a viral/bacterial filter. The filter provides a degree of safety to the patient, equipment, and caregivers from whatever organisms may inhabit the ventilator circuit. A word of caution comes from one source who urges the buyer to scrutinize the research that supports the filter’s efficiency. Demers8 reports that the performance of commonly used HMEs with filters (HMEF) can range from 1/50 to > 30-fold the efficiency of a high-efficiency particulate air (HEPA)-grade device.
Standards for HMEs have been developed by groups like the AARC,3 American National Standards Institute (ANSI),9 and International Organization for Standardization (ISO).10 AARC’s minimum standard for an HME states that it should deliver no less than 30 mg H2O/L at 30°C.3 Some of these devices are designed to be used for a few hours, others for as long as 5 days. Consequently, characteristics such as size, weight, the amount of moisture they return, and their resistance to airflow can differ greatly.7 No single device is appropriate for all circumstances, and their relative merits should be compared to patients’ needs.
Considering patient safety, HMEs may have an additional benefit over the traditional heated humidifier. Due to differences in circuit and ambient temperatures, heated humidifying devices typically produce condensation in the ventilator circuit, and it presents several problems. The condensate can very easily become contaminated,9 and if a bolus of this fluid were introduced through an inadvertent lavage, it could be the mechanism for the development of ventilator associated pneumonia (VAP).10
When an HME is used, very little, if any, condensation collects in the patient’s ventilator circuit. The presumption has been that if there is no condensate to accumulate, the threat to the patient is minimized. Interestingly, several studies have failed to demonstrate statistically significant differences in VAP rates when HMEs and heated humidifiers are used.11-14 Perhaps the reason is explained by Hess15 in his review on ventilator circuit change and VAP. He reports that aspiration of oral secretions is the most common mechanism for nosocomial pneumonia, and thus, it is the patient, not the circuit, that is the real source of contamination.
Regardless of the type of HME used, it can return only so much moisture to the patient. If the patient has a low core temperature or is dehydrated, this form of humidifying device would have little chance of meeting the body’s estimated need for water intake. Additionally, the amount of water returned to the patient will change as the patient’s ventilation changes. Increases in tidal volume, respiratory rate, and even peak flow will all decrease the amount of humidification the patient receives.
If the HME becomes occluded with blood, secretions, or condensate, the associated increase in resistance to airflow can cause increases in peak pressure, auto-PEEP, and work of breathing.15,16 Other ventilatory concerns relate to the physical size of the HME. Some may have a dead space volume as high as 90 cc. Use of an HME of this size and dead space would be contraindicated in patients being ventilated with low tidal volumes because it may exacerbate hypercapnic states.17
Other contraindications relating to the use of HMEs have been described in the AARC’s Clinical Practice Guidelines.
Active Humidifiers. Use of a heated humidifier is generally considered to be the most efficient way to deliver humidity to the mechanically ventilated patient.18,19 Active humidifiers include heated humidifiers or a combination of a heater and a heated wire circuit (HWC). The only regulated parameter is the system’s temperature, not the humidity. Temperature is used as a proxy for humidity. The optimal temperature setting at the proximal airway is recommended to be 37°C to 40°C (yielding 44 mg H2O/L of inhaled gas), but the scientific basis for this is debated.20 As the gas travels through the circuit, ambient temperature changes cause the moisture to “rain out.” The condensation that develops presents a challenge to ventilator operation: It may obstruct airflow, trigger ventilator breaths, and promote patient/ventilator dyssynchrony. As it accumulates, the condensate must be disposed of in an aseptic manner. Disconnecting the circuit to drain the condensate (“breaking the circuit”) may contribute to VAP,15 and placement of an inline water trap may be an acceptable alternative.
Use of heated wire circuits offers a partial solution to the condensation problem, as a temperature gradient is created by increasing the temperature in the distal aspect of the inspiratory limb. Heating the interior of the circuit in this way greatly minimizes the rainout.
The cost of a heated wire system is reported as a drawback to its use.14 If the circuit does not require changing, costs will decrease for each day it is used. There are, however, operational issues that should be addressed.
Operational Issues
Temperature gradients To maintain optimal humidity delivery, gradients need to be adjusted as ambient temperature, ventilator settings, and water reservoir levels change. These settings will need to be changed if the patient 1) is getting small volume nebulizer treatments; and/or 2) is in a room where temperature fluctuates (bedside fans or heating/air-conditioning problems). It can be both intellectually challenging and time-consuming to have to adjust the equipment based on ambient conditions.
Unfortunately, the concept of setting and adjusting negative or positive gradients is difficult for some to comprehend. Setting these levels incorrectly with one system creates a new set of problems. The alarms package in earlier versions of some devices was very sensitive, alerting the staff to problems very quickly. The audible alarms sound so frequently that there is a great temptation to either adjust the heater to a level that could be subtherapeutic or just turn it off. Newer systems use compensatory algorithms to make these adjustments automatically, but in one study the devices produced humidity levels lower than advertised.6
Occluded endotracheal tube from patient ventilated with HWC.
Occlusions. One problem observed with HMEs is that they become occluded and obstruct airflow in and out of the circuit. The occlusion problem has also been reported in the endotracheal tubes of patients using HME. Kapadia21 reported 13 episodes of endotracheal tube occlusion in patients using HME, with eight of these patients requiring cardiopulmonary resuscitation. Occlusion has been attributed to a low level of inhaled humidity from an inefficient HME, but, ironically, this problem has also been reported in patients using heated humidification and heated wire circuits.22 These may have occurred because the settings were subtherapeutic. Lellouche et al6 report that under some conditions, they found absolute humidity levels <20 mg/L delivered by heated wire systems. These levels are actually lower than the amount of humidity reportedly delivered by the HMEs attributed to endotracheal tube occlusion.
Other factors contributing to humidity delivery:
• ambient temperature
• the ventilator’s outlet temperature
• humidification system efficiency
• humidifier settings.
Is the Humidifier Working?
This is a question that all RCPs should ask themselves. It has certainly been asked by researchers.22-24 Regardless of what type of system is being used, the clinician should question its effectiveness. Since no system reports the actual amount of humidity being delivered, other signs must be relied on. These could include observation of the circuit itself for signs of small droplets of moisture or “beading.” When heated humidifiers have been used, the presence of these small droplets in the chamber has been used as an indicator that the gas is fully saturated.6 This is probably not an accurate method, since the temperature of the gas that leaves the ventilator can be quite high (35.0°-51.0°C in some models) and will artificially raise the point at which condensation appears.6 High or low ambient room temperature would influence the presence of moisture in the circuit.
Beydon et al24 found a good correlation between absolute humidity and the presence of condensation in the flex tube leading to the patient’s endotracheal tube. This data was obtained in patients with minute volumes as high as 15 L and patients using HWC and HME.
There is clearly a diversity of opinion on how humidity therapy is best applied. The diversity is not limited to HME vs HWC. It includes which HME and which HWC should be used, and how they should be set. The clinician can clarify the conflict by 1) evaluating the patient’s secretions and the circuit with the same diligence as any ventilator parameter and also by using an algorithm such as that developed by Branson et al25 as guides when selecting a system or device; and 2) staying informed.
The role of humidity therapy in the prevention of VAP, the optimal level of humidity, and which is the best humidification system are still being debated. As these questions are answered, better techniques and new technology are sure to follow, but it will still be up to the clinician to make the correct selection.
Mark Grzeskowiak, RRT, RCP, is clinical supervisor, adult critical care, respiratory care department, Long Beach Memorial Medical Center, Long Beach, Calif, and clinical advisor/adjunct faculty instructor, respiratory care program, Orange Coast College, Costa Mesa, Calif.
References
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