Physical Principles Governing Humidifier Function

The following four variables or principles affect the quality of performance of a humidifier: (1) temperature, (2) surface area, (3) time of contact, and (4) thermal mass. These factors are exploited to various degrees in the design of humidification devices (Box 35-3).

Temperature

Temperature is an important factor affecting humidifier performance. The greater the temperature of a gas, the more water vapor it can hold (increased capacity). As gas expansion and evaporation cool water in unheated humidifiers to 10° C below ambient temperature, the humidifiers become less efficient.

Figure 35-3 shows this concept, where, owing to evaporative cooling, the unheated humidifier on the left is operating at 10° C. Although the humidifier fully saturates the gas, the low operating temperature limits total water vapor capacity to approximately 9.4 mg/L water vapor, equivalent to approximately 21% of body humidity. Simply heating the humidifier to 40° C (see Figure 35-3, right) increases its output to 51 mg/L, which is sufficient to meet BTPS conditions.

Types of Humidifiers

Humidifiers are either active (actively adding heat or water or both to the device-patient interface) or passive (recycling exhaled heat and humidity from the patient). Active humidifiers typically include (1) bubble humidifiers, (2) passover humidifiers, (3) nebulizers of bland aerosols, and (4) vaporizers. Passive humidifiers refer to typical heat and moisture exchangers (HMEs). Specifications covering the design and performance requirements for medical humidifiers are established by the American Society for Testing and Materials (ASTM).¹³

Active Humidifiers

Bubble

A bubble humidifier breaks (diffuses) an underwater gas stream into small bubbles (Figure 35-4). Use of a foam or mesh diffuser produces smaller bubbles than an open lumen, allowing greater surface area for gas/water interaction. Unheated bubble humidifiers are commonly used with oxygen (O₂) delivery systems (see Chapter 38) to raise the water vapor content of the gas to ambient levels.

As indicated in Table 35-2, unheated bubble humidifiers can provide absolute humidity levels between approximately 15 mg/L and 20 mg/L.^14–16 At room temperature, 10 mg/L absolute humidity corresponds to approximately 80% relative humidity but only approximately 25% body humidity (see Chapter 6). As gas flow increases, these devices become less efficient as the reservoir cools and contact time is reduced, limiting their effectiveness at flow rates greater than 10 L/min. Heating the reservoirs of these units can increase humidity content, but this is not recommended because the resulting condensate tends to obstruct the small bore delivery tubing to which these units connect.

TABLE 35-2

Absolute Humidity (mg/L) Provided by Unheated Bubble Humidifiers

L/min	Aquapak 301 (Hudson RCI, Corp, Dunham, NC)	Traveral 500 (Baxter-Travenol, Corp, Deerfield, IL)
2	17.6	20.4
4	17.7	19.5
6	16.9	16.2
8	14.9	15.7

Modified from Darin J, Broadwell J, MacDonell R: An evaluation of water-vapor output from four brands of unheated, prefilled bubble humidifiers. Respir Care 27:41, 1982.

To warn of flow-path obstruction and to prevent bursting of the humidifier bottle, bubble humidifiers incorporate a simple pressure-relief valve, or pop-off. Typically, the pop-off is either a gravity or spring-loaded valve that releases pressures greater than 2 psi. Humidifier pop-offs should provide both an audible and a visible alarm and should automatically resume normal position when pressures return to normal.¹³ The pop-off also can be used to test an O₂ delivery system for leaks. If the system is obstructed at or near the patient interface and the pop-off sounds, the system is leak-free; failure of the pop-off to sound may indicate a leak (or a faulty pop-off valve).

At high flow rates, bubble humidifiers can produce aerosols. Although invisible to the naked eye, these water droplet suspensions can transmit pathogenic bacteria from the humidifier reservoir to the patient.¹⁷ Because any device that generates an aerosol poses a high risk of spreading infection, strict infection control procedures must be followed when using these systems (see Chapter 4).

Passover

Passover humidifiers direct gas over a surface containing water. There are three common types of passover humidifiers: (1) simple reservoir type, (2) wick type, and (3) membrane type (see Figure 35-4).

The simple reservoir device directs gas over the surface of a volume of water (or fluid). The surface for gas-fluid interface is limited. These systems are typically used with heated fluids for use with mechanical ventilation, but they may also be used with room temperature fluids with noninvasive ventilatory support (nasal continuous positive airway pressure or bilevel ventilation).

A wick humidifier uses an absorbent material to increase the surface area for dry air to interface with heated water. Typically, a wick is placed upright with the gravity-dependent end in a heated water reservoir. Heating elements might be below or surrounding the wick. Capillary action continually draws water up from the reservoir and keeps the wick saturated. As dry gas enters the chamber, it flows around the wick, quickly picking up heat and moisture and leaving the chamber fully saturated with water vapor. No bubbling occurs, so no aerosol is produced.

A membrane-type humidifier separates the water from the gas stream by means of a hydrophobic membrane (see Figure 35-4). Water vapor molecules can easily pass through this membrane, but liquid water (and pathogens) cannot. As with a wick humidifier, bubbling does not occur. If a membrane-type humidifier were to be inspected while it was in use, no liquid water would be seen in the humidifier chamber.

Compared with bubble humidifiers, passover humidifiers offer several advantages.17,18 First, in contrast to bubble devices, passover humidifiers can maintain saturation at high flow rates. Second, they add little or no flow resistance to spontaneous breathing circuits. Third, they do not generate any aerosols, and they pose a minimal risk for spreading infection.

Vaporizer Humidifiers

Simple vaporizers heat water to the point of expansion as a gas. Simple room vaporizers have been used in ambulatory settings for years as room humidifiers. A capillary force vaporizer is a thin-film, high surface area boiler that combines capillary force and phase transition to impart pressure onto an expanding gas (water vapor) and ejects it into the gas stream.

Heat and Moisture Exchangers

An HME is most often a passive humidifier, also described as an “artificial nose.” Similar to the nose, an HME captures exhaled heat and moisture and uses it to heat and humidify the next inspiration. In contrast to the nose, with its rich vasculature and endothelium, most HMEs do not actively add heat or water to the system. A typical HME is a passive humidifier, capturing both heat and moisture from expired gas and returning up to 70% of both to the patient during the next inspiration.

Traditionally, use of HMEs has been limited to providing humidification to patients receiving invasive ventilatory support via endotracheal or tracheostomy tubes. More recently, HMEs have been used successfully in meeting the short-term humidification needs of spontaneously breathing patients with tracheostomy tubes.¹⁹ Kapadia²⁰ reviewed airway accidents in the intensive care unit for a 4-year period and noted an increasing trend in the incidence of blocked tracheal tubes, which was associated with an increased duration of HME filter use. More recent evidence supports long-term use of HMEs for spontaneously breathing patients.²¹

The three basic types of HMEs are (1) simple condenser humidifiers, (2) hygroscopic condenser humidifiers, and (3) hydrophobic condenser humidifiers. Simple condenser humidifiers contain a condenser element with high thermal conductivity, usually consisting of metallic gauze, corrugated metal, or parallel metal tubes. On inspiration, inspired air cools the condenser element. On exhalation, expired water vapor condenses directly on its surface and rewarms it. On the next inspiration, cool, dry air is warmed and humidified as its passes over the condenser element. Simple condenser humidifiers are able to recapture only approximately 50% of a patient’s exhaled moisture (50% efficiency).

Hygroscopic condenser humidifiers provide higher efficiency by (1) using a condensing element of low thermal conductivity (e.g., paper, wool, foam) and (2) impregnating this material with a hygroscopic salt (calcium or lithium chloride). By using an element with low thermal conductivity, hygroscopic condenser humidifiers can retain more heat than simple condenser systems. In addition, the hygroscopic salt helps capture extra moisture from the exhaled gas. During exhalation, some water vapor condenses on the cool condenser element, whereas other water molecules bind directly to the hygroscopic salt. During inspiration, the lower water vapor pressure in the inspired gas liberates water molecules directly from the hygroscopic salt, without cooling. Figure 35-5 depicts the overall process of humidification with a hygroscopic condenser humidifier, showing the changes in temperature and the relative and absolute humidity occurring during the cycle of breathing. As shown, these devices typically achieve approximately 70% efficiency (40 mg/L exhaled, 27 mg/L returned).

Hydrophobic condenser humidifiers use a water-repellent element with a large surface area and low thermal conductivity (Figure 35-6). During exhalation, the condenser temperature increases to approximately 25° C because of conduction and latent heat of condensation. On inspiration, cool gas and evaporation cools the condenser down to 10° C. This large temperature change results in the conservation of more water to be used in humidifying the next breath. The efficiency of these devices is comparable to hygroscopic condenser humidifiers (approximately 70%). However, some hydrophobic humidifiers that provide bacterial filtration may reduce the risk of pneumonia but be unsuitable for patients with limited respiratory reserve or who are prone to airway blockage because they may increase artificial airway occlusion.^22,23

Design and performance standards for HMEs are set by the International Organization for Standardization (ISO).²⁴ The ideal HME should operate at 70% efficiency or better (providing at least 30 mg/L water vapor); use standard connections; have a low compliance; and add minimal weight, dead space, and flow resistance to a breathing circuit.²⁵ According to Lellouche and colleagues,²⁶ HME performance varies from brand to brand, and only 37.5% of 32 HMEs tested in the study performed well. Table 35-3 compares performance of several commercially available HMEs according to their moisture output, flow resistance, and dead space.²⁶

TABLE 35-3

Comparison of 25 Heat and Moisture Exchangers

Device	Manufacturer	Measured AH (mg H2O/L)	AH/ml of Dead Space	Measured Resistance at 60 L/min cm H2O
Hygrovent	Peters	31.9 ± 0.6	0.34	1.8
Hygrobac	Mallinckrodt	31.7 ± 0.7	0.33	2.1
Hygrovent S	Peters	31.7 ± 0.5	0.58	2.8
Hygrobac S	Mallinckrodt	31.2 ± 0.2	0.69	2.3
9000/100	Allégiance	31.2 ± 1.4	0.35	2.7
Servo Humidifier 172	Siemens	30.9 ± 0.3	0.56	NA
Humid Vent Filter	Hudson	30.8 ± 0.3	0.88	2.3
Hygroster	Mallinckrodt	30.7 ± 0.6	0.32	2.3
Humid Vent 2	Hudson	29.7 ± 0.4	1.03	NA
Servo Humidifier 162	Siemens	29.7 ± 0.8	0.78	NA
Humid Vent 2S	Hudson	29.2 ± 0.4	1.01	NA
9040/01	Allégiance	28.6 ± 1.1	0.61	2.4
9000/01	Allégiance	28.5 ± 0.8	0.32	3.9
BB100E	Pall	27.2 ± 0.7	0.32	1.4
BB100	Pall	26.8 ± 0.5	0.30	2.0
Stérivent	Mallinckrodt	23.8 ± 0.9	0.26	1.9
Iso Gard Hepa Light	Hudson	23.6 ± 0.3	0.47	2.4
Stérivent S	Mallinckrodt	22.2 ± 0.2	0.36	1.7
BB25	Pall	19.6 ± 1.4	0.56	2.6
BB2000AP	Pall	18.9 ± 0.4	0.54	3.1
Stérivent Mini	Mallinckrodt	16.6 ± 1.0	0.47	2.2
4444/66	Allégiance	16.4 ± 0.6	0.35	3.4
4000/01	Allégiance	15.1 ± 0.9	0.40	2.2
Barrierbac S	Mallinckrodt	13.2 ± 0.2	0.38	2.1

AH, Absolute humidity; NA, not available.

Modified from Lellouche F, Taille S, Lefrancois F, et al: Humidification performance of 48 passive airway humidifiers: comparison with manufacturer data. Chest 135:276, 2009.

As shown in Table 35-3, the moisture output of HMEs tends to decrease at high volumes and rates of breathing. In addition, high inspiratory flows and high FiO₂ levels can decrease HME efficiency.²⁵ Flow resistance through the HME also is important. When an HME is dry, resistance across most devices is minimal. However, because of water absorption, HME flow resistance increases after several hours of use.^27,28 For some patients, the increased resistance imposed by the HME may not be well tolerated, in particular, if the underlying lung disease already causes increased work of breathing.

Because HMEs eliminate the problem of breathing circuit condensation, many clinicians consider these devices (especially hydrophobic filter HMEs) to be helpful in preventing nosocomial infections and ventilator-associated pneumonia.²⁹ Compared with active humidification systems, HMEs do reduce bacterial colonization of ventilator circuits.³⁰ However, circuit colonization plays a minor role in the development of nosocomial infections, provided that usual maintenance precautions are applied.³¹ Evidence indicates no difference in incidence of ventilator-associated pneumonia, mortality, morbidity, and respiratory complications among patients managed with HMEs and heated humidifiers.^{23,26,30,32,33}

The position of the HME relative to the patient’s airway can affect its ability both to heat and to humidify inhaled gas. Secretions can foul HMEs attached directly to the airway. The use of devices such as closed suction catheters and airway monitor ports requires placement of the HME closer to the ventilator. Inui and colleagues³⁴ tested performance of HMEs placed directly at the airway, 10 cm away from endotracheal tube and proximal to the ventilator circuit. HME performance was best at the airway for both HMEs tested, but one HME model exceeded recommended performance standards (≥30 mg/L absolute humidity and ≥30° C) at both sites, whereas the other model did not perform adequately at either position (Figure 35-7). Clinicians should select HMEs that perform adequately when placed at the intended HME site. Although use of HMEs has been associated with thickened and increased volume of secretions in some patients, the incidence of endotracheal tube occlusion when HMEs are used is lower than when heated humidifiers are used.^35,36

HMEs are not recommended for use with infants for several reasons. First, HMEs add 30 to 90 ml of mechanical dead space, exceeding the tidal volume of the infant. In addition, infants are commonly ventilated through uncuffed endotracheal tubes, which allow exhaled gas to leak around the tube and bypass the HME reducing recovered heat and humidity.

Heating Systems

Heat improves the water output of humidifiers. Heated humidifiers are used mainly for patients with bypassed upper airways and patients receiving mechanical ventilatory support. Although heating a humidifier provides benefits, it also presents additional risks. Humidifier heating systems generally have a controller that regulates the power to the heating element by monitoring the heating element, which matches a preset or an adjustable temperature. They may also use a thermistor placed at the outlet of the humidifier, with a heater set to control output temperature. Servo-controlled heating systems monitor the temperature at the humidifier’s outlet and at the patient’s airway using a thermistor probe. The controller adjusts the heater power to reach the desired airway temperature and incorporates alarms and an alarm-activated heater shutdown function.

An electrical heating element provides the needed energy. Five types of heating elements are common: (1) a hot plate element at the base of the humidifier; (2) a wraparound type that surrounds the humidifier chamber; (3) a yolk, or collar, element that sits between the water reservoir and the gas outlet; (4) an immersion-type heater, with the element placed in the water reservoir; (5) a heated wire in the inspiratory limb warming a saturated wick or hollow fiber; and (6) a thin-film, high surface area broiler.

Humidifier heating systems have a controller that regulates the element’s electrical power. In the simplest systems, the controller monitors the heating element, varying the delivered current to match either a preset or an adjustable temperature. In these systems, the temperature of the patient’s airway has no effect on the controller. Conversely, a servo-controlled heating system monitors temperature at or near the patient’s airway using a thermistor probe. The controller adjusts heater power to achieve the desired airway temperature. Both types of controller units usually incorporate alarms and alarm-activated heater shutdown. Box 35-4 outlines key features of modern heated humidification systems.³⁸

Reservoir and Feed Systems

Heated humidifiers operating continuously in breathing circuits can evaporate more than 1 L of water per day. To avoid constant refilling, these devices either incorporate a large water reservoir or use a gravity feed system. An ideal reservoir or feed system should be safe, dependable, and easy to set up and use and should allow for continuity of therapy, even when the reservoir is being replenished.

Automatic Systems

Automatic feed systems avoid the need for constant checking and manual refilling of humidifiers. The simplest type of automatic feed system is the level-compensated reservoir (Figure 35-8). In these systems, an external reservoir is aligned horizontally with the humidifier, maintaining relatively consistent water levels between the reservoir and the humidifier chamber.

Flotation valve controls can be used to maintain humidifier reservoir fluid volume. In flotation-type systems, a float rises and falls with the water level. As the water level falls below a preset value, the float opens the feed valve; as the water rises back to the set fill level, the float closes the feed valve. An optical sensor can also be used to sense water level, driving a solenoid valve to allow refilling of the humidifier reservoir.

Mini Clini

Selecting the Appropriate Therapy to Condition a Patient’s Inspired Gas

 Problem

A survivor of near drowning has just been intubated and placed on mechanical ventilatory support. Her body temperature is 31° C, and her minute ventilation is high. What would be the appropriate humidification system to recommend for this patient?

Solution

Normally, patients supported by mechanical ventilation can be started with an HME, unless its use is contraindicated. According to AARC Clinical Practice Guideline 35-1, using an HME with this patient is contraindicated because (1) she is hypothermic, and (2) she has a high minute ventilation. Based on this assessment, the best choice is a heated humidifier, preferably with servo-controlled airway temperature.

Membrane-type humidifiers do not require a flow control system. Because the liquid water chamber underlying the membrane cannot overfill, these devices require only an open gravity feed system to ensure proper function. Two examples are the Vapotherm (Vapotherm Inc, Stevensville, MD) membrane cartridge system and the Hummax II (Metran Medical Instruments Mfg Co, Ltd, Saitama, Japan), which uses a heated wire to warm the polyethylene microporous hollow fiber placed in the inspiratory circuit.

The Hydrate (Pari Respiratory Equipment, Midlothian, VA) capillary force vaporizer is driven by software that controls a heater element and water flow. The 19-mm diameter disc can deliver 2.2 mg of water vapor/min at 37° C. Data from prototypes suggest temperature control from 33° C to 41° C for flows 2 to 40 L/min (Figure 35-9).³⁹

To guide practitioners in applying humidity therapy during ventilatory support, the American Association for Respiratory Care (AARC) has published Clinical Practice Guideline: Humidification During Mechanical Ventilation; excerpts from the AARC guideline appear in Clinical Practice Guideline 35-1.⁷

35-1   Humidification During Mechanical Ventilation

AARC Clinical Practice Guideline (Excerpts)*

Indications

Humidification of inspired gas during mechanical ventilation is mandatory when an endotracheal or a tracheostomy tube is present.

Contraindications

There are no contraindications to providing physiologic conditioning of inspired gas during mechanical ventilation. However, an HME is contraindicated in the following circumstances:

• For patients with thick, copious, or bloody secretions

• For patients with an expired tidal volume less than 70% of the delivered tidal volume (e.g., patients with large bronchopleural fistulas or incompetent or absent endotracheal tube cuffs)

• For patients whose body temperature is less than 32° C

• For patients with high spontaneous minute volumes (>10 L/min)

• For patients receiving in-line aerosol drug treatments (an HME must be removed from the patient circuit during treatments)

Hazards and Complications

Hazards and complications associated with the use of heated humidifier (HH) and HME devices during mechanical ventilation include the following:

• High flow rates during disconnect may aerosolize contaminated condensate (HH)

• Underhydration and mucous impaction (HME or HH)

• Increased work of breathing (HME or HH)

• Hypoventilation caused by increased dead space (HME)

• Elevated airway pressures caused by condensation (HH)

• Ineffective low-pressure alarm during disconnection (HME)

• Patient-ventilator dyssynchrony and improper ventilator function caused by condensation in the circuit (HH)

• Hypoventilation or gas trapping caused by mucous plugging (HME or HH)

• Hypothermia (HME or HH)

• Potential for burns to caregivers from hot metal (HH)

• Potential electrical shock (HH)

• Airway burns or tubing meltdown if heated wire circuits are covered or incompatible with humidifier (HH)

• Possible increased resistive work of breathing caused by mucous plugging (HME or HH)

• Inadvertent overfilling resulting in unintended tracheal lavage (HH)

• Inadvertent tracheal lavage from pooled condensate in circuit (HH)

Assessment Of Need

Either an HME or an HH can be used to condition inspired gases:

• HMEs are better suited for short-term use (≤96 hours) and during transport.

• HHs should be used for patients requiring long-term mechanical ventilation (>96 hours) or for patients for whom HME use is contraindicated.

Assessment Of Outcome

Humidification is assumed to be appropriate if, on regular, careful inspection, the patient exhibits none of the listed hazards or complications.

Monitoring

The humidifier should be inspected during the patient-ventilator system check, and condensate should be removed from the circuit as needed. HMEs should be inspected and replaced if secretions have contaminated the insert or filter. The following should be recorded during equipment inspection:

• During routine use on an intubated patient, an HH should be set to deliver inspired gas at 33° C ± 2° C and should provide a minimum of 30 mg/L of water vapor.

• Inspired gas temperature should be monitored at or near the patient’s airway opening (HH).

• Specific temperatures may vary with the patient’s condition; airway temperature should never exceed 37° C.

• For heated wire circuits used with infants, the probe must be placed outside the incubator or away from the radiant warmer.

• The high-temperature alarm should be set no higher than 37° C, and the low setting should not be less than 30° C.

• Water level and function of automatic feed system (if applicable) should be monitored.

• Quantity, consistency, and other characteristics of secretions should be noted and recorded. When using an HME, if secretions become copious or appear increasingly tenacious, an HH should replace the HME.

---

^*For the complete guideline, see American Association for Respiratory Care: Clinical practice guideline: humidification during mechanical ventilation, Respir Care 37:887, 1992.

Setting Humidification Levels

Although the American National Standards Institute (ANSI) recommends minimum levels of humidity for intubated patients (>30 mg/L), there is less guidance regarding appropriate settings for optimal humidification. One suggestion is to target the temperature and level of humidity for normal conditions at the point that the gas is entering the airway. For example, the humidity of air entering the carina is typically 35 to 40 mg/L. When humidifiers run too cold (<32° C), humidity can be reduced to the point of increased airway plugging. Not all active heated humidifiers perform the same under all conditions. Nishida⁴⁰ compared the performance of four active humidifiers (MR290 with MR730 [Fisher & Paykel Healthcare Inc, Laguna Hills, CA], MR310 with MR730 [Fisher & Paykel Healthcare Inc], ConchaTherm IV [Hudson RCI, Temecula, CA], and Hummax II), which were set to maintain the temperature of the airway opening at 32° C and 37° C under various ventilator parameters. The greater the minute ventilation, the lower the humidity delivered with all devices except the Hummax II. When the airway temperature control of the devices was set at 32° C, the ConchaTherm IV, the MR 310, and the MR 730 all failed to deliver 30 mg/L of vapor, which is the value recommended by ANSI. This study emphasizes the need to set humidifiers to maintain airway temperatures between 35° C and 37° C.

Controversy exists regarding the appropriate temperature and humidity of inspired gas delivered to mechanically ventilated patients with artificial airways. The current AARC Clinical Practice Guideline recommends 33° C, within 2° C, with a minimum of 30 mg/L of water vapor. In a comprehensive review, Williams⁴¹ suggested that inspired humidity be maintained at an optimal level, 37° C with 100% relative humidity and 44 mg/L, to minimize mucosal dysfunction. Theoretically, optimal humidity offers improved mucociliary clearance. The benefits of this strategy are theory based but have yet to be shown conclusively in the clinical setting. Further controlled studies are needed to support better the need for optimal humidity.

Condensation

In all standard heated humidifier systems, saturated gas cools as it leaves the point of humidification and passes through the delivery tubing en route to the patient. As the gas cools, its water vapor capacity decreases, resulting in condensation or “rain out.” Factors influencing the amount of condensation include (1) the temperature difference across the system (humidifier to airway); (2) the ambient temperature; (3) the gas flow; (4) the set airway temperature; and (5) the length, diameter, and thermal mass of the breathing circuit.

Figure 35-10 provides an example of the condensation process. In this case, because of cooling along the circuit, the humidifier temperature has to be set to a higher level (50° C) than desired at the airway. At 50° C, the humidifier fully saturates the gas to an absolute humidity level of 84 mg/L of water. As cooling occurs along the tubing, the capacity of the gas to hold water vapor decreases. By the time the gas reaches the patient, the temperature of the gas has decreased to 37° C, and it is holding only 44 mg/L of water vapor. Although BTPS conditions have been achieved, 40 mg/L, half the total output of the humidifier (84 mg/L − 44 mg/L = 40 mg/L), has condensed in the inspiratory limb of the circuit.

The condensation process poses risks to patients and caregivers and can waste a lot of water. First, condensation can disrupt or occlude gas flow through the circuit, potentially altering fractional inspired oxygen (FiO₂) or ventilator function or both. Condensate can work its way toward the patient and be aspirated. For these reasons, circuits must be positioned to drain condensate away from the patient and must be checked often, and excess condensate must be drained from heated humidifier breathing circuits on a regular basis.

Typically, patients contaminate ventilator circuits within hours, and condensate is colonized with bacteria and poses an infection risk.⁴² To avoid problems in this area, health care personnel should treat all breathing circuit condensate as infectious waste. See Chapters 4 and 43 for more detail on control procedures used with breathing circuits, including the AARC Clinical Practice Guideline on changing ventilator circuits (see Clinical Practice Guideline 4-1).

Several techniques are used to minimize problems with breathing circuit condensate. One common method is to place water traps at low points in the circuit (both the inspiratory and the expiratory limbs of ventilator circuits). This method aids drainage of condensate and reduces the likelihood of gas flow obstruction. When used in ventilator circuits, water traps should have little effect on circuit compliance, allow emptying without disrupting ventilation, and not be prone to leakage.

Nebulizers, with medication reservoirs below the aerosol generator and placed in the ventilator circuit, can act as a “water trap,” collecting contaminated condensate. There is a tremendous risk that contaminated aerosols can be generated and pathogens delivered to the deep lung. To minimize this risk, nebulizers should be placed in a superior position so that any condensate travels downstream from the nebulizer. In addition, these nebulizers should be removed from the ventilator circuit between treatments, rinsed and air dried, washed, and sterilized or disposed of and replaced.

One way to avoid condensation problems is to prevent condensation from forming. Because the decrease in temperature in gas traveling from the humidifier to the airway causes condensation, maintaining an appropriate temperature in the circuit can prevent formation of condensate. Several methods, such as insulation or increasing the thermal mass of the circuit, can reduce circuit cooling by keeping the circuit at a constant temperature. The most common approach uses wire heating elements inserted into the ventilator circuit.

Most heated wire circuits use dual controllers with two temperature sensors: one monitoring the temperature of gas leaving the humidifier and the other placed at or near the patient’s airway (Figure 35-11). The controller regulates the temperature difference between humidifier output and patient airway. When heated wire circuits are used, the humidifier heats gas to a lower temperature (32° C to 40° C) than with conventional circuits (45° C to 50° C). The reduction in condensate in the tubing results in less water use, reduced need for drainage, and less infection risk for patients and health care workers.

Even heated wire circuits can produce unwanted levels of condensate. One strategy is to provide absorptive material in the inspiratory limb of the ventilator circuit, which acts as a wick warmed by the heated wire system (Fisher & Paykel Healthcare Inc).

Use of heated wire circuits in neonates is complicated by the use of incubators and radiant warmers. Incubators provide a warm environment surrounding the infant and radiant warmers use radiant energy to warm objects that intercept radiant light. In both cases, a temperature probe placed in the heated environment would affect humidifier performance, resulting in reduced humidity received by the patient. Figure 35-12 shows the impact of temperature probe placement, in or out of the incubator, on absolute humidity delivered to the neonate. Consequently, temperature probes should always be placed outside of the radiant field or incubator (Figure 35-13).

Cross Contamination

Aerosol and condensate from ventilator circuits are known sources of bacterial colonization.⁴² However, advances in both circuit and humidifier technology have reduced the risk of nosocomial infection when these systems are used. Wick-type or membrane-type passover humidifiers prevent formation of bacteria-carrying aerosols. Heated wire circuits reduce production and pooling of condensate within the circuit. In addition, the high reservoir temperatures in humidifiers are bactericidal.⁴³ In ventilator circuits using wick-type humidifiers with heated wire systems, circuit contamination usually occurs from the patient to the circuit, rather than vice versa.

For decades, the traditional way to minimize the risk of circuit-related nosocomial infection in critically ill patients receiving ventilatory support was to change the ventilator tubing and its attached components every 24 hours.⁴⁴ It is now known that frequent ventilator circuit changes increase the risk of nosocomial pneumonia.⁴⁵ Current research indicates that there is minimal risk of ventilator-associated pneumonia with weekly circuit changes and that there may be no need to change circuits at all.^30,31,46,47 In addition, substantial cost savings can accrue with decreased frequency of circuit changes.

Proper Conditioning of Inspired Gas

All respiratory therapists (RTs) are trained to measure patient inspired FiO₂ levels regularly and, in ventilatory care, to monitor selected pressures, volumes, and flows. However, few clinicians take the steps needed to ensure proper conditioning of the inspired gas received by patients.

The most accurate and reliable way to ensure that patients are receiving gas at the expected temperature and humidity level is to measure these parameters. Portable battery-operated digital hygrometer-thermometer systems are available for less than $300 and are invaluable in ensuring proper conditioning of the inspired gas. When measuring high-humidity environments, hygrometers become saturated and nonresponsive over time and so should be used for spot checks only. These devices should be as common at the bedside as O₂ analyzers.

Many heated wire humidification systems have a humidity control. This control does not reflect either absolute or relative humidity but only the temperature differential between the humidifier and the airway sensor. If the heated wires are set warmer than the humidifier, less relative humidity is delivered to the patient. To ensure that the inspired gas is being properly conditioned, clinicians should always adjust the temperature differential to the point that a few drops of condensation form near the patient connection, or “wye.” Lacking direct measurement of humidity, observation of this minimal condensate is the most reliable indicator that the gas is fully saturated at the specified temperature. If condensate cannot be seen, there is no way of knowing the level of relative humidity without direct measurement—it could be anywhere between 99% and 0%. HME performance can be evaluated in a similar manner.⁴⁸

Aerosol Generators

Large Volume Jet Nebulizers

A large volume jet nebulizer is the most common device used to generate bland aerosols. As depicted in Figure 35-14, these devices are pneumatically powered, attaching directly to a flowmeter and compressed gas source. Liquid particle aerosols are generated by passing gas at a high velocity through a small “jet” orifice. The resulting low pressure at the jet draws fluid from the reservoir up to the top of a siphon tube, where it is sheared off and shattered into liquid particles. The large, unstable particles fall out of suspension or impact on the internal surfaces of the device, including the fluid surface (baffling). The remaining small particles leave the nebulizer through the outlet port, carried in the gas stream. A variable air-entrainment port allows air mixing to increase flow rates and to alter FiO₂ levels (see Chapter 38).

Similar to humidifiers, if heat is required, a hot plate, wraparound, yolk collar, or immersion element can be added. However, in contrast to heated humidifiers, these devices rarely have sophisticated servo-controlled systems to control delivery temperature. Many systems do not shut down when the reservoir empties, resulting in the delivery of hot, dry gas to the patient. Failure of the element can also cause a loss of heating capacity, without warning to the clinician.

Depending on the design, input flow, and air-entrainment setting, the total water output of unheated large volume jet nebulizers varies between 26 mg H₂O/L and 35 mg H₂O/L. When heated, output increases to between 33 mg H₂O/L and 55 mg H₂O/L, mainly because of increased vapor capacity.49,50 Larger versions of these devices (with 2-L to 3-L reservoirs) are used to deliver bland aerosols into mist tents. These enclosure systems can generate flow rates faster than 20 L/min, with water outputs of 5 ml/min (300 ml/hr). Because heat buildup in enclosures is a problem, these systems are always run unheated.

Ultrasonic Nebulizers

A USN is an electrically powered device that uses a piezoelectric crystal to generate aerosol. This crystal transducer converts radio waves into high-frequency mechanical vibrations (sound). These vibrations are transmitted to a liquid surface, where the intense mechanical energy creates a cavitation in the liquid, forming a standing wave, or “geyser,” which sheds aerosol droplets. Figure 35-15 provides a schematic of a large volume USN. Output from a radiofrequency generator is transmitted over a shielded cable to the piezoelectric crystal. Vibrational energy is transmitted either indirectly through a water-filled couplant reservoir or directly to a solution chamber. Gas entering the chamber inlet picks up the aerosol particles and exits through the chamber outlet.

The properties of the ultrasonic signal determine the characteristics of the aerosol generated by these nebulizers. The frequency at which the crystal vibrates, preset by the manufacturer, determines aerosol particle size. Particle size is inversely proportional to signal frequency. A USN operating at a frequency of 2.25 MHz may produce an aerosol with a mass median aerodynamic diameter (MMAD) of approximately 2.5 µm, whereas another nebulizer operating at 1.25 MHz produces an aerosol with MMAD between 4 µm and 6 µm. Signal amplitude directly affects the amount of aerosol produced; the greater the amplitude, the greater the volume of aerosol output. In contrast to frequency, signal amplitude may be adjusted by the clinician.

Particle size and aerosol density delivered to the patient are also affected by the source and flow of gas through the aerosol-generating chamber. Some large volume USNs have built-in fans that direct room air through the solution chamber conducting the aerosol to the patient. The airflow may be adjusted by changing the fan speed or use of a simple damper valve. Alternatively, compressed anhydrous gases can be delivered to the chamber inlet through a flowmeter. For precise control over delivered O₂ concentrations, clinicians can attach a flowmeter with an O₂ blender or air-entrainment system to the chamber inlet.

The flow and amplitude settings interact to determine aerosol density (mg/L) and total water output (ml/min). Amplitude affects water output. At a given amplitude setting, the greater the flow through the chamber, the less the density of the aerosol. Conversely, low flows result in aerosols of higher density. Total aerosol output (ml/min) is greatest when both flow and amplitude are set at the maximum. Using these settings, some units can achieve total water outputs of 7 ml/min.

Particle size, aerosol density, and output are also affected by the relative humidity of the carrier gas (see Chapter 36). In contrast to jet nebulizers, the temperature of the solution placed in a USN increases during use. Although this increase in temperature affects water vapor capacity, its impact on aerosol output is minimal.

 Rule Of Thumb

To produce a high-density aerosol using a USN (useful for sputum induction), set the amplitude high and the flow rate low. To maximize aerosol delivery per minute (when trying to help mobilize secretions), set the flow rate to match and slightly exceed patient inspiratory flow rate, and set the amplitude at the maximum.

Although USNs have some unique capabilities, in most cases of bland aerosol administration, their relative advantages over jet nebulizers are outweighed by their high cost and erratic reliability. Exceptions include the use of a USN for sputum induction, where the high output (1 to 5 ml/min) and aerosol density seem to yield higher quantity and quality of sputum specimens for analysis, although at some cost in increased airway reactivity.⁵¹ Although a major manufacturer of USNs (DeVilbiss) discontinued their product line, other manufacturers in both the United States and Europe still manufacture units for clinical use.

Commercially available USNs (usually marketed as “cool” mist devices) have found a place in the home, being used as room humidifiers. As with any nebulizer, the reservoirs of these devices can easily become contaminated, resulting in airborne transmission of pathogens. Care should be taken to ensure that these units are cleaned according to the manufacturer’s recommendations and that water is discarded from the reservoir periodically between cleanings. In the absence of a manufacturer’s recommendation, these units should undergo appropriate disinfection at least every 6 days.⁵² Generally, passover and wick-type humidifiers present less risk than the USN as a room humidifier.

Airway Appliances

Airway appliances used to deliver bland aerosol therapy include aerosol mask, face tent, T-tube, and tracheostomy mask (Figure 35-16). The aerosol mask and face tent are used for patients with intact upper airways. The T-tube is used for patients who are orally or nasally intubated or who have a tracheostomy. The tracheostomy mask is used only for patients who have a tracheostomy. In all cases, large bore tubing is required to minimize flow resistance and prevent occlusion by condensate.

For short-term therapy to patients with intact upper airways, the aerosol mask is the device of choice. However, some patients cannot tolerate masks and may do better with a face tent. No data support preferential use of an open aerosol mask versus a face tent.

Although the T-tube is the most common application for tracheostomy patients, unless moderate to high FiO₂ levels are needed, a tracheostomy mask is a better choice. In contrast to T-tubes, tracheostomy masks exert no traction on the airway, and they allow secretions and condensate to escape from the airway, reducing airway resistance.

Enclosures (Mist Tents and Hoods)

Infants and small children may not readily tolerate direct airway appliances such as masks, so enclosures such as mist tents and aerosol hoods are used to deliver bland aerosol therapy to these patients. More recent studies have shown that aerosol hoods can provide aerosol delivery with similar efficiency to a properly fitted aerosol mask in infants, with less discomfort for the patient.⁵³

Because mist tents were used for more than 40 years mainly to treat croup, clinicians may still refer to these devices as croup tents. The cool aerosol provided through these enclosures promotes vasoconstriction, decreases edema, and reduces airway obstruction.

Any body enclosure poses two problems: carbon dioxide (CO₂) buildup and heat retention. CO₂ buildup can be reduced by providing sufficiently high gas flow rates. These high flows of fresh gas circulate continually through the enclosure and “wash out” CO₂, while helping maintain desired O₂ concentrations. Heat retention is handled differently by each manufacturer. Some devices use high fresh gas flows to prevent heat buildup. Others incorporate a separate cooling device. Some tent devices use a simple ice compartment to cool the aerosol. The Ohmeda Ohio Pediatric Aerosol Tent (Ohmeda Ohio Corp., Gurnee, IL) and other similar units use electrically powered refrigeration units to cool the circulating air.

The cooling from these refrigeration units produces a great deal of condensation, which must be drained into a collection bottle outside of the tent. Units such as the Mistogen CAM-3M have overcome some of these problems with a thermoelectric cooling system, in which an electrical current passing through a semiconductor augments heat absorption and release. As warm air is taken from the tent, heat is transferred and released in the room, and cool air is returned to the tent.

Cross Contamination

Rigorous adherence to the infection control guidelines detailed in Chapter 4, especially guidelines covering solutions and equipment processing, should help minimize the cross contamination and infection risks involved in using these systems. In addition, the water should be changed regularly, and the couplant compartments and nebulizer chambers of USNs should be disinfected or replaced regularly.

Environmental Exposure

Environmental safety issues from secondhand and exhaled aerosol arise mainly when aerosol therapy is prescribed for immunosuppressed patients or for patients with tuberculosis. A survey suggested that RTs may be at increased risk for developing asthma-like symptoms, attributed partly to secondhand exposure to aerosols such as ribavirin or albuterol.⁵⁸ To minimize problems in this area, all clinicians should strictly follow U.S. Centers for Disease Control and Prevention standards and airborne precautions, including precautions specified for control of exposure to tuberculosis (see Chapter 4). Additional methods for dealing with environmental control of drug aerosols are described in Chapter 36.

Inadequate Aerosol Output

Inadequate mist production is a common problem with all nebulizer systems. With pneumatically powered jet nebulizers, poor mist production can be caused by inadequate input flow of driving gas, siphon tube obstruction, or jet orifice misalignment. With the exception of inadequate driving gas flow, these problems require unit repair or replacement. If a USN is not functioning properly, the electrical power supply (cord, plug, and fuse or circuit breakers) should be checked first. The clinician next should check to confirm that (1) carrier gas is flowing through the device and (2) the amplitude, or output, control is set above minimum. If there is still no visible mist output, the clinician should inspect the couplant chamber to confirm proper fill level and the absence of any visible dirt or debris. Finally, the clinician must ensure that the couplant chamber solution meets the manufacturer’s specifications (most units do not function properly with distilled water).

Overhydration

Overhydration is a problem with continuous use of heated jet nebulizers and USNs. With USNs capable of such extraordinarily high water outputs, they should never be used for continuous therapy. The risk of overhydration is highest for infants, small children, and patients with preexisting fluid or electrolyte imbalances. Even if used only to meet BTPS conditions, bland aerosol therapy effectively eliminates insensible water loss through the lungs and should be equated to a daily water gain (approximately 200 ml/day for an average adult). In addition to overhydration of the patient, inspissated pulmonary secretions can swell after high-density aerosol therapy, worsening airway obstruction. Careful patient selection and monitoring can prevent most potential problems with overhydration.

Bronchospasm

Even bland water aerosols can cause bronchospasm in some patients. Ultrasonic nebulization of distilled water is used in some pulmonary function laboratories to provoke bronchospasm and to assess bronchial hyperactivity.⁵⁷ To avoid this problem at the bedside, the clinician should always carefully review the patient’s history and diagnosis before administering any bland aerosol, especially a hypotonic water solution. As indicated in the AARC practice guideline (see Clinical Practice Guideline 35-2), patients receiving continuous bland aerosol therapy should be initially monitored carefully (including breath sounds and subjective response) and reevaluated every 8 hours or with any change in clinical condition.⁴⁹ If bronchospasm occurs during therapy, treatment must be stopped immediately, O₂ must be provided, and appropriate bronchodilator therapy should be initiated as soon as possible. If the physician still requests bland aerosol therapy for such a patient, pretreatment with a bronchodilator may be needed. In addition, isotonic solutions (0.9% saline) may be better tolerated by these patients than water.

A problem unique to large volume, air entrainment jet nebulizers is the noise they generate, especially at high flows. The American Academy of Pediatrics recommends that sound levels remain less than 58 dB to avoid hearing loss for infants being cared for in incubators and O₂ hoods. Because many commercial nebulizers exceed this noise level when in operation, careful selection of equipment is necessary. However, the best way to avoid this problem and minimize infection risks further is to use heated passover humidification instead of nebulization.