Egan's Fundamentals of Respiratory Care - Chapter 38 - Humidity & Aerosol Therapy PDF

Humidity and Bland Aerosol Therapy CHAPTER 38 821 V apors and mists have been used for millenia to treat respiratory disease. Modern respiratory care still uses these treatments at the bedside, in the form of water...

Humidity and Bland Aerosol Therapy CHAPTER 38 821 V apors and mists have been used for millenia to treat respiratory disease. Modern respiratory care still uses these treatments at the bedside, in the form of water T = 30° vapor (humidity) and bland water aerosols. Concepts of abso- T = 22° C RH = 95% RH = 50% AH = 30 mg/L lute and relative humidity are essential for understanding AH = 10 mg/L humidity therapy and are covered in Chapter 6. This chapter Isothermic saturation reviews the principles, methods, equipment, and procedures for boundary using these concepts appropriately. T = 37° RH = 100% AH = 43.9 mg/L HUMIDITY THERAPY Humidity deficit: Humidity therapy involves adding water vapor and (some- 43.9 -mg/L times) heat to the inspired gas. To understand the need for 10.0 mg/L humidity therapy, clinicians must understand the normal 33.9 mg/L control of heat and moisture exchange. FIGURE 38-1 As a person breathes typical ambient air, the Physiologic Control of Heat and upper airway adds 20 mg/L of water vapor and the lower airway Moisture Exchange adds 13.9 mg/L. If all of that humidity were exhaled, this would Heat and moisture exchange is a primary function of the upper represent a 33.9 mg/L humidity deficit. AH, Absolute humidity; RH, relative humidity; T, temperature. (From Fink J: Humidity and respiratory tract, mainly the nose.1 The nose heats and humidi- aerosol therapy. In Cairo J, Pilbeam S, editors: Mosby’s respiratory fies gas on inspiration and cools and reclaims water from gas equipment, ed 8, St. Louis, 2010, Mosby.) that is exhaled. The nasal mucosal lining is kept moist by secre- tions from mucous glands, goblet cells, transudation of fluid through cell walls, and condensation of exhaled humidity. The Above the ISB, temperature and humidity decrease during nasal mucosa is very vascular, actively regulating temperature inspiration and increase during exhalation. Below the ISB, tem- changes in the nose and serving as an active element in promot- perature and relative humidity remain constant (BTPS). ing effective heat transfer. Similarly, the mucosa lining the Numerous factors can shift the ISB deeper into the lungs. sinuses, trachea, and bronchi aid in heating and humidifying The ISB shifts distally when a person breathes through the inspired gases. mouth rather than the nose; when the person breathes cold, dry During inspiration through the nose, the tortuous path of air; when the upper airway is bypassed (breathing through an gas through the turbinates increases contact between the artificial tracheal airway); or when the minute ventilation is inspired air and the mucosa. As the inspired air enters the nose, higher than normal. When this shift of ISB occurs, additional it warms (convection) and picks up water vapor from the moist surfaces of the airway are recruited to meet the heat and humid- mucosal lining (evaporation), cooling the mucosal surface. ity requirements of the lung. This recruitment of airways that During exhalation, the expired gas transfers heat back to the do not typically provide this level of heat and humidity can have cooler tracheal and nasal mucosa by convection. As the satu- a negative impact on epithelial integrity. These shifts of the ISB rated gas cools, it holds less water vapor. Condensation occurs can compromise the body’s normal heat and moisture exchange on the mucosal surfaces during exhalation, and water is reab- mechanisms, and humidity therapy is indicated. sorbed by the mucus (rehydration). In cold environments, the formation of condensate may exceed the ability of the mucus Indications for Humidification and to reabsorb water (resulting in a “runny nose”). Warming of Inspired Gases The mouth is less effective at heat and moisture exchange The primary goal of humidification is to maintain normal than the nose because of the low ratio of gas volume to moist physiologic conditions in the lower airways. Proper levels of and warm surface area and the less vascular squamous epithe- heat and humidity help ensure normal function of the muco- lium lining the oropharynx and hypopharynx. When a person ciliary transport system. Humidity therapy is also used to treat inhales through the mouth at normal room temperature, pha- abnormal conditions. Box 38-1 summarizes the indications and ryngeal temperatures are approximately 3° C less than when the contraindications for humidity therapy. person breathes through the nose, with 20% less relative humid- Administration of dry medical gases at flows greater than ity. During exhalation, the relative humidity of expired gas 4 L/min to the upper airway causes immediate heat and water varies little between mouth breathing and nose breathing, but loss and, if prolonged, causes structural damage to the epithe- the mouth is much less efficient in reclaiming heat and water.2 lium. As the airway is exposed to relatively cold, dry air, ciliary As inspired gas moves into the lungs, it achieves BTPS condi- motility is reduced, airways become more irritable, mucus pro- tions (i.e., body temperature, 37° C; barometric pressure; satu- duction increases, and pulmonary secretions become inspis- rated with water vapor [100% relative humidity at 37° C]) sated (thickened owing to dehydration). (Figure 38-1). This point, normally approximately 5 cm below As listed in Box 38-2, the hazard of breathing dry gas is even the carina, is called the isothermic saturation boundary (ISB).3 greater when the normal heat and water exchange capabilities 822 SECTION V Basic Therapeutics Box 38-1 Indications for Humidification of the upper airway are lost or bypassed, as occurs with endo- Therapy tracheal intubation. Breathing dry gas through an endotracheal tube (ETT) can cause damage to tracheal epithelium within PRIMARY minutes. However, as long as the inspired humidity is at least Humidifying dry medical gases 60% of BTPS conditions, no injury occurs in normal lungs.4,5 Overcoming humidity deficit created when upper airway is bypassed Prolonged breathing of improperly conditioned gases through a tracheal airway can result in hypothermia (reduced body SECONDARY temperature), inspissation of airway secretions, mucociliary Treating bronchospasm caused by cold air Contraindications for humidification therapy dysfunction, destruction of airway epithelium, and atelectasis.6 There are no contraindications to providing physiologic Box 38-3 summarizes the signs and symptoms associated with conditioning of inspired gas during mechanical ventilation. breathing cold, dry gases. A reduction of 20 mg/L below BTPS However, a heat and moisture exchanger (HME) is (44 mg/L) is less than 60% relative humidity at BTPS. contraindicated for patients: The amount of heat and humidity that a patient needs With thick, copious, or bloody secretions With an expired tidal volume (VT) less than 70% of the depends on the site of gas delivery (e.g., nose or mouth, hypo- delivered VT (e.g., patients with large bronchopleural fistulas pharynx, trachea). Table 38-1 summarizes the recommended or incompetent or absent endotracheal tube cuffs) levels based on standards.7 With body temperature less than 32° C Warmed, humidified gases are used to prevent or treat With high spontaneous minute volumes (>10 L/min) various abnormal conditions. For treatment of hypothermia, Receiving noninvasive ventilation with large mask leaks, because the patient does not exhale enough VT to replenish heating and humidifying the inspired gas is one of several tech- heat and moisture to adequately condition the inspired gas. niques used to raise core temperatures back to normal.8,9 Heated Also, the resistance and dead space of the HME may humidification is used to prevent intraoperative hypothermia.10 negate the effects of the noninvasive positive pressure and Of possibly greater clinical significance, warming and humidi- add additional work of breathing. fying the inspired gas can help alleviate bronchospasm in Receiving lung protective ventilation strategies such as in acute respiratory distress syndrome (additional dead space of patients who develop airway narrowing after exercise or when HME may increase the ventilation requirement and PaCO2). they breathe cold air. Although the cause of this condition is Receiving in-line aerosol drug treatments (a standard HME not known for certain, the primary stimulus is probably a com- must be removed from the patient circuit during treatments. bination of airway cooling and drying, which leads to hyperto- An HME designed for aerosol delivery must be switched to nicity of airway lining fluid and the release of chemical the aerosol bypass mode) mediators.11 The incidence of cold air–induced bronchospasm can be reduced by wearing a scarf over the nose and mouth in cold weather; the scarf becomes a crude passive heat and mois- Box 38-2 Hazards and Complications for ture exchanger. Humidification Therapy Hazards and complications associated with the use of heated humidifier (HH) and HME devices during mechanical ventilation TABLE 38-1 include the following: Potential electrical shock (HH) Recommended Heat and Humidity Levels Potential for burns to caregivers from hot metal (HH) Delivery Site Temperature Relative Absolute Hypothermia (HME or inadequately set HH) Range (° C) Humidity (%) Humidity (mg/L) Hyperthermia (HH) Thermal injury (HH) Nose/mouth 20-22 50 10 Underhydration and mucous impaction (HME or HH) Hypopharynx 29-32 95 28-34 Hypoventilation and/or alveolar gas trapping resulting Trachea 32-35 100 36-40 from mucus plugging of airways (HME or HH) Hypoventilation secondary to hypercapnia caused by the From Chatburn R, Primiano F: A rational basis for humidity therapy. Respir increase in dead space (HME) Care 32:249, 1987. Increased work of breathing (HME) Possible hypoventilation resulting from hypercapnia caused by the increase in dead space (HME) Inadvertent overfilling or pooled condensate resulting in Box 38-3 Clinical Signs and Symptoms of unintentional tracheal lavage (HH) Inadequate Airway Humidification High flow rates during disconnect may aerosolize contaminated condensate (HH) Atelectasis Elevated airway pressures caused by condensation (HH) Dry, nonproductive cough Ineffective low-pressure alarm during disconnection (HME) Increased airway resistance Patient-ventilator dyssynchrony and improper ventilator Increased incidence of infection function caused by condensation in the circuit (HH) Increased work of breathing Airway burns or tubing meltdown if heated wire circuits Patient complaint of substernal pain and airway dryness are covered or incompatible with humidifier (HH) Thick, dehydrated secretions  Humidity and Bland Aerosol Therapy CHAPTER 38 823 The delivery of cool humidified gas is used to treat upper mately 9.4 mg/L water vapor, equivalent to approximately 21% airway inflammation resulting from croup, epiglottitis, and of body humidity. Simply heating the humidifier to 40° C postextubation edema. This technique is used most often in (Figure 38-2, right) increases its output to 51 mg/L, which is conjunction with bland aerosol delivery (see the section on sufficient to meet BTPS conditions. Bland Aerosol Delivery). Surface Area. The greater the area of contact between water and gas, the more opportunity there is for evaporation to occur. Equipment Passover humidifiers pass gas over a large surface area of water. A humidifier is a device that adds molecular water to gas. This More space-efficient ways to increase the ratio of water to gas process occurs by evaporation of water from a surface (see surface area include bubble diffusion, aerosol, and wick Chapter 6), whether the water is in a reservoir, a wick, or a technologies. sphere of water in suspension (aerosol). Bubble-diffusion directs a stream of gas underwater, where it is broken up into small bubbles. As the gas bubbles rise to the Physical Principles Governing surface, evaporation increases the water vapor content within Humidifier Function the bubble. The smaller the bubble, the greater is the ratio of The following four variables or principles affect the quality of water to air surface area. 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 Box 38-4 Physical Principles Governing devices (Box 38-4). Humidifier Function Temperature. Temperature is an important factor affecting Temperature: The higher the temperature of a gas, the more humidifier performance. The greater the temperature of a gas, water vapor it can hold (increased capacity) or vice versa. the more water vapor it can hold (increased capacity). As gas Surface area: The greater the surface area of contact between expansion and evaporation cool water in unheated humidifiers water and gas, the more opportunity there is for evaporation to 10° C below ambient temperature, the humidifiers become to occur. less efficient. Contact time: The longer a gas remains in contact with water, the greater is the opportunity for evaporation to occur. Figure 38-2 shows this concept, where, owing to evaporative Thermal mass: The greater the mass of water or the core cooling, the unheated humidifier on the left is operating at 10° element of a humidifier, the greater is its capacity to hold C. Although the humidifier fully saturates the gas, the low oper- and transfer heat. ating temperature limits total water vapor capacity to approxi- Pressure Dry gas Dry gas release valve Gas temperature 10° C Gas temperature 40° C Relative humidity 100% Relative humidity 100% Absolute humidity 9.4 mg/L Absolute humidity 51 mg/L Room temperature 23° C Room temperature 23° C Relative humidity 50% Relative humidity 50% Absolute humidity 10 mg/L Absolute humidity 30 mg/L Reservoir temp 10° C Reservoir temp 40° C Heater outlet Electric outlet FIGURE 38-2 Effects of reservoir temperature on humidity output with unheated (left) and heated (right) bubble-type humidifiers. (Modified from Fink J, Cohen N: Humidity and aerosols. In Eubank D, Bone R, editors: Principles and applications of cardiorespiratory care equipment, St. Louis, 1994, Mosby.) 824 SECTION V Basic Therapeutics An alternative to dispersing gas bubbles in water is spraying Types of Humidifiers water particles (aerosol) into the gas. The higher the aerosol Humidifiers are either active (actively adding heat or water density (number of particles per volume of gas), the greater is or both to the device-patient interface) or passive (recycling the gas to water surface area available for evaporation. exhaled heat and humidity from the patient). Active humidifiers Wicks use porous water-absorbent materials to draw water typically include (1) bubble humidifiers, (2) passover humidi- (similar to a sponge) into its fine honeycombed structure by fiers, (3) nebulizers of bland aerosols, and (4) vaporizers. Passive means of capillary action. The surfaces of the wick increase humidifiers refer to typical heat and moisture exchangers the area of contact between the water and gas, which aids (HMEs). Specifications covering the design and performance evaporation. requirements for medical humidifiers are established by the Contact Time. The longer a gas remains in contact with American Society for Testing and Materials (ASTM).12 water, the greater the opportunity is for evaporation to occur. Active Humidifiers For bubble humidifiers, contact time depends on the depth of Bubble. A bubble humidifier breaks (diffuses) an underwa- the water column; the deeper the column, the greater is the time ter gas stream into small bubbles (Figure 38-3). Use of a foam of contact as the bubbles rise to the surface. In passover and or mesh diffuser produces smaller bubbles than an open lumen, wick-type humidifiers, the flow rate of gas through the humidi- allowing greater surface area for gas/water interaction. Unheated fier is inversely related to contact time, with high flow rates bubble humidifiers are commonly used with oxygen (O2) deliv- reducing the time available for evaporation to occur. Aerosols ery systems (see Chapter 41) to raise the water vapor content suspended in a gas stream have extended contact time (and of the gas to ambient levels. opportunity for evaporation) as the aerosol and gas travel to As indicated in Table 38-2, unheated bubble humidifiers the patient. can provide absolute humidity levels between approximately Thermal Mass. The greater the amount of water in a 15 mg/L and 20 mg/L.13,14 At room temperature, 10 mg/L abso- humidifier, the greater is the thermal mass. Increased thermal lute humidity corresponds to approximately 80% relative mass equates to increased capacity to hold and transfer heat to humidity but only approximately 25% body humidity (see therapeutic gases. Larger reservoir humidifiers can provide Chapter 6). As gas flow increases, the reservoir cools and contact more consistent heat and humidification with a broader range time is reduced, limiting effectiveness at flow rates greater than of gas flow. 10 L/min. Heating the reservoirs can increase humidity content, Bubble Aerosol Passover Wick Membrane FIGURE 38-3 Primary types of active humidifiers. Gas passes through the water (bubble), around drops of water (aerosol) or over the surface of water (Passover), a saturated material (wick) or a semipermeable membrane (membrane). (From Fink J: Humidity and aerosol therapy. In Cairo J, Pilbeam S, editors: Mosby’s respiratory equipment, ed 8, St. Louis, 2010, Mosby.)  Humidity and Bland Aerosol Therapy CHAPTER 38 825 TABLE 38-2 Hydrophobic Condenser Absolute Humidity (mg/L) Provided by Unheated Expiration Bubble Humidifiers L/min Aquapak 301 (Hudson Traveral 500 (Baxter-Travenol, RCI, Dunham, NC) Deerfield, IL) T 10°, RH 100% T 35°, RH 100% 2 17.6 20.4 AH 8 mg/L AH 40 mg/L 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 Inspiration water-vapor output from four brands of unheated, prefilled bubble humidifiers. Respir Care 27:41, 1982. T 20°, RH 50% T 30°, RH 100% but this is not recommended because cooling produces conden- AH 9 mg/L AH 30 mg/L sate that obstructs small-bore delivery tubing. To warn of flow-path obstruction and prevent bursting of the humidifier bottle, bubble humidifiers incorporate a simple pressure-relief valve, or pop-off. The pop-off is commonly a FIGURE 38-4 Process of humidification with a hydrophobic condenser humidifier. AH, Absolute humidity; RH, relative humidity; gravity or spring-loaded valve that releases pressures greater T, temperature. than 2 psi. Humidifier pop-offs should provide both an audible and a visible alarm and resume normal position when pressures return to normal.12 The pop-off can be used to test an O2 deliv- ery system for leaks by obstructing delivery tubing at or near membrane, but liquid water (and pathogens) cannot. As with a the patient interface. If the pop-off sounds, the system is leak- wick humidifier, bubbling does not occur. If a membrane-type free; failure of the pop-off to sound may indicate a leak (or a humidifier were to be inspected while it was in use, no liquid faulty pop-off valve). water would be seen in the humidifier chamber. As gas flow increases, bubble humidifiers can produce aero- Compared with bubble humidifiers, passover humidifiers sols. Although invisible to the naked eye, these water droplet offer several advantages.15,16 First, in contrast to bubble devices, suspensions can transmit pathogenic bacteria from the humidi- passover humidifiers can maintain saturation at high flow rates. fier reservoir to the patient.15 Because any device that generates Second, they add little or no flow resistance to spontaneous an aerosol poses a high risk for spreading infection, strict infec- breathing circuits. Third, they do not generate any aerosols, and tion control procedures must be followed when using these they pose a minimal risk for spreading infection. systems (see Chapter 4). Vaporizer Humidifiers. Simple vaporizers heat water to the Passover. Passover humidifiers direct gas over a surface con- point of expansion as a gas. Simple room vaporizers have been taining water. There are three common types of passover used in ambulatory settings for years as room humidifiers. A humidifiers: (1) simple reservoir type, (2) wick type, and (3) capillary force vaporizer is a thin-film, high-surface-area boiler membrane type (see Figure 38-3). that combines capillary force and phase transition to impart The simple reservoir device directs gas over the surface of pressure onto an expanding gas (water vapor) and ejects it into a volume of water (or fluid). The surface for gas-fluid interface the gas stream. is limited. Typically used with heated fluids with invasive Heat and Moisture Exchangers. An HME is a passive mechanical ventilation, room temperature fluids may be used humidifier, also described as an “artificial nose.” Similar to the with noninvasive ventilatory support (nasal continuous positive nose, an HME captures exhaled heat and moisture and returns airway pressure or bilevel ventilation). up to 70% of the heat and humidify to the patient during the A wick humidifier uses an absorbent material to increase the next inspiration. In contrast to the nose, with its rich vascula- surface area for dry air to interface with heated water. Typically, ture and endothelium, most HMEs do not actively add heat or a wick is placed upright with the gravity-dependent end in a water to the system.6 heated water reservoir. Heating elements might be below or Traditionally, use of HMEs has been limited to providing surrounding the wick. Capillary action draws water up from the humidification to patients receiving ventilatory support via reservoir and keeps the wick saturated. As dry gas enters the endotracheal or tracheostomy tubes. More recently, HMEs have chamber, it flows around the wick, quickly picking up heat and been used successfully in meeting the short-term humidifica- moisture and leaving the chamber saturated with water vapor. tion needs of spontaneously breathing patients with tracheos- No bubbling occurs, so no aerosol is produced. tomy tubes.17-22 Reports of increased incidence of blocked A membrane-type humidifier separates the water from the tracheal tubes associated with long duration of HME use in the gas stream by means of a hydrophobic membrane (Figure intensive care unit,23 are in contrast with evidence supporting 38-4). Water vapor molecules can easily pass through this long-term use of HMEs for spontaneously breathing patients.24 826 SECTION V Basic Therapeutics The three basic types of HMEs are (1) simple condenser of these devices is comparable to that of hygroscopic condenser humidifiers, (2) hygroscopic condenser humidifiers, and HMEs (approximately 70%). However, some hydrophobic (3) hydrophobic condenser humidifiers. Simple condenser HMEs that provide bacterial filtration may reduce the risk for HMEs contain a condenser element with high thermal conduc- pneumonia but be unsuitable for patients with limited respira- tivity, usually consisting of metallic gauze, corrugated metal, or tory reserve or who are prone to airway blockage because they parallel metal tubes. Inspired air cools the condenser element, may increase artificial airway occlusion.25,26 HMEs that deliver and expired water vapor condenses directly on its surface and at least 30 mg H2O/L should be used because they are associated rewarms it. On the next inspiration, cool, dry air is warmed and with a lower incidence of ETT occlusion.18 humidified as its passes over the condenser element. Simple Design and performance standards for HMEs are set by the condenser humidifiers are able to recapture only approximately International Organization for Standardization (ISO).27 The 50% of a patient’s exhaled moisture. ideal HME should operate at 70% efficiency or better (provid- Hygroscopic condenser HMEs provide higher efficiency by ing at least 30 mg/L water vapor); use standard connections; (1) using a condensing element of low thermal conductivity have a low compliance; and add minimal weight, dead space, (e.g., paper, wool, or foam) and (2) impregnating this material and flow resistance to a breathing circuit.28 HME performance with a hygroscopic salt (calcium or lithium chloride). By using varies from brand to brand and may differ from manufacturers’ an element with low thermal conductivity, hygroscopic con- specifications.29 Insufficient heat and humidification can occur denser HMEs can retain more heat than simple condenser with some HMEs causing complications.30,31 Table 38-3 com- systems while hygroscopic salt helps capture extra moisture pares performance of several commercially available HMEs from the exhaled gas. The lower water vapor pressure in the inspired gas liberates water molecules directly from the hygro- scopic salt, without cooling. Figure 38-5 depicts the overall TABLE 38-3 process of humidification with a hygroscopic condenser humid- Comparison of 25 Heat and Moisture Exchangers ifier, showing the changes in temperature and the relative and Device Manufacturer Measured AH/ml Measured absolute humidity occurring during the cycle of breathing. As AH (mg of Dead Resistance shown, these devices typically achieve approximately 70% effi- H2O/L) Space at 60 L/min ciency (40 mg/L exhaled, 27 mg/L returned). cm H2O Hydrophobic condenser HMEs use a water-repellent element Hygrovent Peters 31.9 ± 0.6 0.34 1.8 with a large surface area and low thermal conductivity (see Hygrobac Mallinckrodt 31.7 ± 0.7 0.33 2.1 Hygrovent S Peters 31.7 ± 0.5 0.58 2.8 Figure 38-4). During exhalation, the condenser temperature Hygrobac S Mallinckrodt 31.2 ± 0.2 0.69 2.3 increases to approximately 25° C because of conduction and the 9000/100 Allégiance 31.2 ± 1.4 0.35 2.7 latent heat of condensation. On inspiration, cool gas and evapo- Servo Siemens 30.9 ± 0.3 0.56 NA ration reduce the condenser temperature down to 10° C. This Humidifier large temperature change results in the conservation of more 172 Humid Vent Hudson 30.8 ± 0.3 0.88 2.3 water to be used in humidifying the next breath. The efficiency Filter Hygroster Mallinckrodt 30.7 ± 0.6 0.32 2.3 Humid Vent 2 Hudson 29.7 ± 0.4 1.03 NA Hygroscopic Condenser Servo Siemens 29.7 ± 0.8 0.78 NA Expiration Humidifier 162 Humid Vent Hudson 29.2 ± 0.4 1.01 NA 2S T 22°, RH 100% T 35°, RH 100% 9040/01 Allégiance 28.6 ± 1.1 0.61 2.4 AH 22 mg/L AH 40 mg/L 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 Hudson 23.6 ± 0.3 0.47 2.4 Hepa Light Inspiration 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 T 20°, RH 50% T 28°, RH 100% 4444/66 Allégiance 16.4 ± 0.6 0.35 3.4 AH 9 mg/L AH 27 mg/L 4000/01 Allégiance 15.1 ± 0.9 0.40 2.2 Barrierbac S Mallinckrodt 13.2 ± 0.2 0.38 2.1 Modified from Lellouche F, Taille S, Lefrancois F, et al: Humidification FIGURE 38-5 Process of humidification with a hygroscopic performance of 48 passive airway humidifiers: comparison with manufacturer condenser humidifier. AH, Absolute humidity; RH, relative humidity; data. Chest 135:276, 2009. T, temperature. AH, Absolute humidity; NA, not available.  Humidity and Bland Aerosol Therapy CHAPTER 38 827 according to their moisture output, flow resistance, and dead Compared with active humidification systems, HMEs reduce space.29 bacterial colonization of ventilator circuits.35 However, circuit As shown in Table 38-3, the moisture output of HMEs tends colonization plays a minor role in the development of nosoco- to decrease at high volumes and rates of breathing. In addition, mial infections, provided that usual maintenance precautions high inspiratory flows and high FiO2 levels can decrease HME are applied.36 Although there is no evidence of an overall dif- efficiency.28 Flow resistance through the HME also is important. ference between HMEs and heated humidifiers in preventing When an HME is dry, resistance across most devices is minimal. mortality and other complications in patients who are mechan- However, because of water absorption, HME flow resistance ically ventilated,26 and previous research indicates no difference increases after several hours of use.32 For some patients, the in incidence of ventilator-associated infections, with HMEs and increased resistance imposed by the HME may not be well toler- heated humidifiers.26,29,30,35,37-39 The position of the HME relative ated, particularly if the underlying lung disease already causes to the patient’s airway can affect its ability both to heat and to increased work of breathing. An increase in work of breathing humidify inhaled gas. Secretions can foul HMEs attached through the HME may lead to elevated airway pressures and directly to the airway. The use of devices such as closed suction possible disconnect.33 catheters and airway monitor ports requires placement of the Because HMEs eliminate the problem of breathing circuit HME closer to the ventilator. Previous research tested perfor- condensation, many clinicians consider these devices (especially mance of HMEs placed directly at the airway, 10 cm away from hydrophobic filter HMEs) to be helpful in preventing nosoco- ETT and proximal to the ventilator circuit.40 It was reported mial infections and ventilator-associated pneumonia (VAP).34 that HME performance was best at the airway (Figure 38-6).40 hygrometer 10 cm corrugated tube to ventilator to patient Site 1 capnometer Site 2 (°C) (mg/L) 40 40 35 35 30 30 25 25 20 20 1 2 (sites) 1 2 FIGURE 38-6 Placement of heat and moisture exchangers (HMEs) (Hygrobac S [Mallinckrodt-Dar, Mirandola, Italy, blue circle] or Thermovent HEPA [Smiths Medical International, Kent, U.K., red circle]) at the airway (site 1) or proximal to the ventilator circuit (site 2). Temperature mean ± SD (TEMP; left) and absolute humidity were significantly higher with both HMEs. P <.05 placed at site 1 compared with site 2. (Modified from Inui D, Oto J, Nishimura M: Effect of heat and moisture exchanger [HME] positioning on inspiratory gas humidification. BMC Pulm Med 6:19, 2006.) 828 SECTION V Basic Therapeutics Clinicians should select HMEs that perform adequately when element, which matches a preset or adjustable temperature. placed at the intended position. Although use of HMEs has They also may use a thermistor placed at the outlet of the been associated with thickened and increased volume of humidifier, with a heater set to control output temperature. secretions in some patients, the incidence of ETT occlusion Servo-controlled heating systems monitor the temperature at when HMEs are used is equivalent to that with heated the humidifier’s outlet and at the patient’s airway using a therm- humidifiers.38,41 istor probe. The controller adjusts the heater power to reach the HMEs are not recommended for use with infants and small desired airway temperature and incorporates alarms and an children for several reasons. First, HMEs add 30 to 90 ml of alarm-activated heater shutdown function. mechanical dead space, exceeding the tidal volume of the infant. An electrical heating element provides the needed energy. In addition, infants are commonly ventilated through uncuffed Five types of heating elements are common: (1) a hotplate ETTs, which allow exhaled gas to leak around the tube and element at the base of the humidifier; (2) a wraparound type bypass the HME reducing recovered heat and humidity. that surrounds the humidifier chamber; (3) a yolk, or collar, Active Heat and Moisture Exchangers. Active HMEs add element that sits between the water reservoir and the gas outlet; humidity or heat or both to inspired gas by chemical or electri- (4) an immersion-type heater, with the element placed in the cal means.42 The Humid-Heat (Louis Gibeck AB, Upplands water reservoir; (5) a heated wire in the inspiratory limb Väsby, Sweden) consists of a supply unit with a microprocessor, warming a saturated wick or hollow fiber; and (6) a thin-film, water pump, and humidification device, which is placed between high surface area broiler. the Y-piece and the ETT. The humidification device is based on Humidifier heating systems have a controller that regulates a hygroscopic HME, which absorbs the expired heat and mois- the element’s electrical power. In the simplest systems, the con- ture and releases it into the inspired gas. External heat and water troller monitors the heating element, varying the delivered are added to the patient side of the HME, so the inspired gas current to match either a preset or an adjustable temperature. should reach 100% humidity at 37° C (44 mg H2O/L air). The In these systems, the temperature of the patient’s airway has no external water is delivered to the humidification device via a effect on the controller. Conversely, a servo-controlled heating pump onto a wick and evaporated into the inspired air by an system monitors temperature at or near the patient’s airway electrical heater. The microprocessor controls the water pump using a thermistor probe. The controller adjusts heater power and the heater by an algorithm using the minute ventilation to achieve the desired airway temperature. Systems usually have (which is fed into the microprocessor) and the airway tempera- alarms and alarm-activated heater shutdown. Box 38-5 outlines ture measured by a sensor mounted in the flex-tube on the key features of modern heated humidification systems. patient side of the humidification device. The HME Booster (King Systems, Noblesville, IN) has a T-piece containing an electrically heated element that was designed for use as an RULE OF THUMB adjunct to a passive HME. The heating element heats water so Place heated humidifier thermistor probes in the that water vapor passes into the airway between the artificial inspiratory limb of a ventilator circuit far enough from airway and ETT, via a Gore-Tex membrane and aluminum. the patient Y adaptor to ensure that warm exhaled gas Using a gravity feedbag via a flow regulator that limits flow to does not fool the controller system. Never place a thermistor probe in an isolette or a radiant warmer, 10 mL/hr, water is fed to the heater, which operates at 110° C where the probe is warmed externally and the and adds 3 to 5.5 mg/L of humidity and 3° C to 4° C to inspired humidifier is fooled into shutting down, reducing the gas compared with the HME alone. The Humid-Booster was humidity available to the patient. designed for patients with minute volumes of 4 to 20 L, and it is not appropriate for use with pediatric patients or infants. Active HMEs add weight and complexity at the patient airway. Reservoir and Feed Systems Heated humidifiers operating continuously in breathing cir- RULE OF THUMB cuits can evaporate more than 1 L of water per day. An ideal HMEs should be replaced if secretions have reservoir or feed system should be safe, dependable, easy to set contaminated the filter and/or if flow resistance has up, and use allowing continuity of therapy, even when the res- increased causing an increase in the work of breathing. ervoir is being replenished. Manual Systems. Simple large-reservoir systems are manu- ally refilled (with sterile or distilled water). If a manual system Heated Humidifiers is used, momentary interruption of humidifier operation and Heat improves the water output of humidifiers. Heated humidi- mechanical ventilation is required for refilling. Because the fiers are used to increase the heat and water content of inspired system must be “opened” for refilling, cross contamination can gas for patients with bypassed upper airways and patients occur. Water levels in manually filled systems are constantly receiving noninvasive mechanical ventilatory support.6 Humid- changing, and changes in the humidifier fill volume alter ifier heating systems generally have a controller that regulates the gas compression factor and the delivered volume during the power to the heating element by monitoring the heating mechanical ventilation.  Humidity and Bland Aerosol Therapy CHAPTER 38 829 Gas inlet Box 38-5 Key Features for Heated Humidification Systems Gas temperature delivered to the patient should not be Gas outlet greater than 40° C. When temperatures greater than 40° C are reached, audible and visual alarms should indicate an overly high temperature condition and interrupt power to the heater. Audible and visual alarms should indicate when remote temperature sensors are disconnected, absent, or defective, and power to the heater should be interrupted to prevent overheating. Temperature overshoot should be minimized. Overshoot can Wick occur when servo-controlled units warm up without flow through the circuit, when the temperature probe is not Water reservoir inserted in the circuit (or becomes dislodged), or when flow Water changes during normal operation. Non–servo-controlled units Heater can overshoot when temperature controls are set too high or when gas flow is abruptly reduced. Indicators for delivered gas temperature should be accurate to ± 3° C of the indicated value. FIGURE 38-7 Schematic of the Concha-Column wick-type Humidifier temperature output should not vary more than 2° humidifier with level-compensated reservoir feed system (Hudson C from the set value (proximal to the patient). RCI, Temecula, CA). (Modified from Fink J, Cohen N: Humidity and Warmup time should not exceed 15 minutes. aerosols. In Eubank D, Bone R, editors: Principles and applications The water level should be readily visible in either the of cardiorespiratory care equipment, St. Louis, 1994, Mosby.) humidifier or the remote reservoir. Humidifiers should be able to withstand ventilation pressures greater than 100 cm H2O. Internal compliance should be low and stable so that A small inlet that can be attached to a gravity-fed intrave- changes in the water level do not significantly alter the delivered tidal volume. nous bag and line allows refilling without interruption of ven- The exposed surface of a humidifier should not be too hot to tilation. Such systems still require constant checking and manual touch during operation. Readily accessible surfaces should replenishment by opening the line valve or clamp. If not checked not be greater than 37.5° C. A warning label is needed for regularly, the reservoir in these systems can go dry, placing the hotter surfaces. patient at considerable risk. Operator, or feed, systems must not be able to overfill the humidifier to the point that water can block gas flow through Automatic Systems. Automatic feed systems avoid the need the humidifier or ventilator circuit. Humidifiers should not be for constant checking and manual refilling of humidifiers. The damaged by spilled fluids. simplest type of automatic feed system is the level-compensated Electromagnetic interference from other devices should not reservoir (Figure 38-7). In these systems, an external reservoir affect humidifier performance. The unit should not be is aligned horizontally with the humidifier, maintaining rela- damaged by 95 to 135 Volts. Fuses or circuit breakers should be clearly labeled and easily tively consistent water levels between the reservoir and the reset or replaced. The unit should have adequate humidifier chamber. overcurrent protection to prevent ventilator shutdown or loss With flotation-type systems, a float rises and falls with the of power to other equipment on the same branch circuit water level. As the water level falls below a preset value, the float because of internal equipment failures. opens the feed valve; as the water rises back to the set fill level, It should be impossible to assemble the unit in a way that would be hazardous to the patient. The direction of gas flow the float closes the feed valve. Alternatively, optical sensors can should be indicated on interchangeable components, for be used to sense water level, driving a solenoid valve to allow which proper direction is essential. refilling of the humidifier reservoir. The humidifier should be assembled and filled in a manner that minimizes the introduction of infectious materials or foreign objects. RULES OF THUMB Service and operation manuals should be provided with the Humidification of inspired gas is mandatory in humidifier and should cover all aspects of its use and mechanically ventilated patients with ETT or service. tracheostomy tube. During noninvasive ventilation, Modified from Emergency Care Research Institute: Heated humidifiers, active humidification is suggested to improve comfort. Health Devices. 1987. http://www.fda.gov/oc/po/firmrecalls/ Vapotherm2000i_01_06.html. Accessed March 2, 2011. Setting Humidification Levels The American National Standards Institute (ANSI) recom- mends minimum levels of humidity for intubated patients (>30 mg/L). However, optimum humidity targets the tempera- ture and humidity for normal conditions at the point that the 830 SECTION V Basic Therapeutics M I N I CLINI Selecting the Appropriate Therapy to Condition a Patient’s Inspired Gas PROBLEM: A survivor of near drowning has just been intu- cannot overfill and they require only an open gravity feed system. bated and placed on mechanical ventilatory support. Her body Two examples are the Vapotherm (Vapotherm, Stevensville, MD) temperature is 31° C, and her minute ventilation is high. What membrane cartridge system and the Hummax II (Metran Medical would be the appropriate humidification system to recommend Instruments, Saitama, Japan), which uses a heated wire to warm for this patient? the polyethylene microporous hollow fiber placed in the inspira- SOLUTION: Normally, patients without pulmonary disease tory circuit. supported by mechanical ventilation can be started with an HME, A capillary force vaporizer is driven by software that controls unless its use is contraindicated. Using an HME with this patient a heater element and water flow. The 19-mm diameter disc can is contraindicated because (1) she is hypothermic and (2) she has deliver 2.2 mg of water vapor/min at 37° C. Data from prototypes a high minute ventilation. Based on this assessment, the best suggest temperature control from 33° C to 41° C for flows 2 to choice is a heated humidifier, preferably with servo-controlled 40 L/min (Figure 38-8).43 airway temperature. A gas temperature above 41° C may lead to a potential thermal Membrane-type humidifiers require no flow control system injury to the patient; over-temperature alarms protect the patient because the liquid water chamber underlying the membrane from thermal injury.6 A C B FIGURE 38-8 The capillary force vaporizer (CFV) is a thin-film, high-surface-area boiler that combines capillary force and phase transition. A, Inducing phase transition in a capillary environment, the CFV imparts pressure onto the expanding gas and ejects it. B, The CFV is incorporated to provide controlled heated humidity in the Hydrate (Pari, Midlothian, VA). C, Temperature probe. (Courtesy Pari.) gas is entering the airway. For example, the humidity of air emphasized the need to set humidifiers to maintain airway tem- entering the carina is typically 37 to 40 mg/L. When humidifiers peratures between 35° C and 37° C.44 run too cold (96 hours) or for patients for whom For patients with an expired tidal volume less than 70% of HME use is contraindicated. the delivered tidal volume (e.g., patients with large ASSESSMENT OF OUTCOME bronchoneural fistulas or incompetent or absent Humidification is assumed to be appropriate if, on regular, endotracheal tube cuffs) careful inspection, the patient exhibits none of the listed For patients whose body temperature is less than 32° C hazards or complications. For patients with high spontaneous minute volumes (>10 L/min) MONITORING For patients receiving in-line aerosol drug treatments (an The humidifier should be inspected during the patient- HME must be removed from the patient circuit during ventilator system check, and condensate should be removed treatments) from the circuit as needed. HMEs should be inspected and replaced if secretions have contaminated the insert or filter. HAZARDS AND COMPLICATIONS The following should be recorded during equipment Hazards and complications associated with the use of heated inspection: humidifier (HH) and HME devices during mechanical During routine use on an intubated patient, an HH should ventilation include the following: be set to deliver inspired gas at 33° C ± 2° C and should High flow rates during disconnect may aerosolize provide a minimum of 30 mg/L of water vapor. contaminated condensate (HH) Inspired gas temperature should be monitored at or near Underhydration and mucous impaction (HME or HH) the patient’s airway opening (HH). Increased work of breathing (HME or HH) Specific temperatures may vary with the patient’s condition; Hypoventilation caused by increased dead space (HME) airway temperature should never exceed 37° C. Elevated airway pressures caused by condensation (HH) For heated wire circuits used with infants, the probe must Ineffective low-pressure alarm during disconnection (HME) be placed outside the incubator or away from the radiant Patient-ventilator dyssynchrony and improper ventilator warmer. function caused by condensation in the circuit (HH) The high-temperature alarm should be set no higher than Hypoventilation or gas trapping caused by mucous 37° C, and the low setting should not be less than 30° C. plugging (HME or HH) The water level and function of automatic feed system (if Hypothermia (HME or HH) applicable) should be monitored. Potential for burns to caregivers from hot metal (HH) Quantity, consistency, and other characteristics of Potential electrical shock (HH) secretions should be noted and recorded. When using an Airway burns or tubing meltdown if heated wire circuits are HME, if secretions become copious or appear increasingly covered or incompatible with humidifier (HH) tenacious, an HH should replace the HME. *For the complete guideline, see Restrepo RD, Walsh BK: Humidification during invasive and noninvasive mechanical ventilation: 2012. Respir Care 57:782–788, 2012. 832 SECTION V Basic Therapeutics Health care personnel should treat all breathing circuit conden- Problem Solving and Troubleshooting sate as infectious waste. See Chapters 4 and 46 for more detail Common problems with humidification systems include con- on control procedures used with breathing circuits, including densation, avoiding cross contamination, and ensuring proper the American Association for Respiratory Care (AARC) Clinical conditioning of the inspired gas. Practice Guideline on changing ventilator circuits (see Clinical Practice Guideline 4-1). Condensation In standard heated humidifier systems, saturated gas cools as it leaves the point of humidification and passes through the deliv- RULES OF THUMB ery tubing en route to the patient. As gas cools, water vapor Always treat breathing circuit condensate as infectious capacity decreases, resulting in condensation or “rain out.” waste. Use standard precautions, including wearing Factors influencing the amount of condensation include (1) the gloves and goggles. Always drain the tubing away from temperature difference across the system (humidifier to airway); the patient’s airway into an infectious waste container, and dispose of the waste according to the policies and (2) the ambient temperature; (3) the gas flow; (4) the set airway procedures of the institution. temperature; and (5) the length, diameter, and thermal mass of the breathing circuit. Figure 38-9 illustrates the condensation process. Because cooling occurs as gas transits the circuit, the humidifier is set to A common method to minimize problems with condensate a higher temperature (50° C) than desired at the airway. At 50° is to place water traps at low points in the circuit (both the C, saturated gas has an absolute humidity level of 84 mg/L of inspiratory and the expiratory limbs of ventilator circuits) to water. As cooling occurs along the tubing, the capacity of the collect condensate and reduce the likelihood of gas flow obstruc- gas to hold water vapor decreases as temperature decreases to tion. Water traps should have little effect on circuit compliance, 37° C, and holds only 44 mg/L of water vapor. Although BTPS allow emptying without disrupting ventilation, and not be conditions have been achieved, 40 mg/L, half the total output prone to leakage. of the humidifier (84 mg/L − 44 mg/L = 40 mg/L), condenses Nebulizers, with medication reservoirs positioned below the in the inspiratory limb of the circuit. ventilator circuit, can act as a “water trap,” collecting contami- This condensation poses risks to patients and caregivers and nated condensate. This creates a risk that contaminated aerosols can waste a lot of water. Condensation can disrupt or occlude can be generated and pathogens delivered to deep into the lung. gas flow through the circuit, potentially altering ventilator func- To minimize this risk, nebulizers should be placed in a superior tion. Because condensate can enter the patient airway and be position so that any condensate travels downstream from the aspirated, circuits must be positioned to drain condensate away nebulizer. In addition, these nebulizers should be rinsed and from the patient and checked often, with condensate drained air dried, washed, and sterilized or disposed of and replaced from breathing circuits frequently. between treatments. Patients contaminate ventilator circuits within hours, with One way to avoid condensation problems is to prevent con- condensate colonized with bacteria posing an infection risk.46 densation from forming. Because the decrease in temperature Outlet: 50o C RH: 100% AH: 84 mg/L Gas Delivery site: 37o C source RH: 100% AH: 44 mg/L H2O condensate in tubing Patient connection Room temperature: 22o C Heated humidifier FIGURE 38-9 Gases leaving a standard heated humidifier are cooled en route to the patient. Although the gas remains saturated (100% relative humidity [RH]), cooling reduces its water vapor capacity, and condensation forms. Almost half of the original water (500 ml/day) is lost to condensation. The temperature at the patient connection (37° C) shown here is for illustrative purposes only. Heated humidifiers should be set to deliver inspired gas at 35° C ± 2° C. AH, Absolute humidity.  Humidity and Bland Aerosol Therapy CHAPTER 38 833 in gas traveling from the humidifier to the airway causes con- ture difference between humidifier output and patient airway. densation, maintaining heat in the circuit can prevent forma- When heated wire circuits are used, the humidifier heats gas to tion of condensate. Several methods, such as insulation or a lower temperature (32° C to 40° C) than with conventional increasing the thermal mass of the circuit, can reduce circuit circuits (45° C to 50° C). Reduction in condensate in the tubing cooling by keeping the circuit at a constant temperature. The results in less water use, reduced need for drainage, and less most common approach uses wire heating elements inserted infection risk for patients and health care workers. into the ventilator circuit. Most heated wire circuits use dual controllers with two tem- perature sensors: one monitoring the temperature of gas leaving RULES OF THUMB the humidifier and the other placed at or near the patient’s Heated humidifiers should be set to deliver an inspired airway47-50 (Figure 38-10). The controller regulates the tempera- gas temperature of 34° C or greater but less than 41° C at the inspiratory limb near the Y adaptor during invasive mechanical ventilation. Gas temperatures in patients receiving noninvasive ventilation should be Expiratory selected based on patient comfort, tolerance, limb of circuit adherence, and underlying pulmonary condition. Airway temp sensor Heated wire controller Unwanted levels of condensate can still occur with heated wires. Absorptive material in the inspiratory limb of the ventila- 35 CO tor circuit acts as a wick warmed by the heated wire system Ventilator Patient (Fisher & Paykel Healthcare, Irvine, CA). wye Use of heated wire circuits in neonates is complicated by the use of incubators and radiant warmers. Incubators provide a Humidifier Heated wires warm environment surrounding the infant and radiant warmers outlet sensor use radiant energy to warm objects that intercept radiant light. 35 CO In both cases, a temperature probe placed in the heated envi- Humidifier ronment would affect humidifier performance, resulting in controller reduced humidity received by the patient. Figure 38-11 shows FIGURE 38-10 Heated wire humidifier system. The dual sensor the impact of temperature probe placement, in or out of the system keeps the temperature constant throughout the inspiratory limb of the ventilator circuit, minimizing condensation. Cooling of incubator, on absolute humidity delivered to the neonate. Con- exhaled gas in the expiratory limb can cause condensation unless it sequently, temperature probes always should be placed outside also is heated. of the radiant field or incubator (Figure 38-12). 46 Temperature probe outside incubator with extension 44 Temperature probe at Y-piece inside incubator Absolute Humidity (mg/L) 42 40 38 36 34 32 Ambient temperature 26 –+ 1°C 30 29 31 33 35 37 39 Incubator Set Temperature (°C) FIGURE 38-11 Humidity achieved at the Y-piece of a neonatal humidification system when used inside an incubator (dotted line) and outside or under an incubator (solid line). 834 SECTION V Basic Therapeutics Temperature probe and heater wire outside warming environment FIGURE 38-12 Neonatal breathing circuit configuration used with an incubator, with the temperature probe placed outside of the warming environment and an unheated portion of the inspiratory circuit delivering the gases to the Y-piece. Cross Contamination Many heated wire humidification systems have a humidity Aerosol and condensate from ventilator circuits are known control. This control does not reflect either absolute or relative sources of bacterial colonization.46 However, advances in both humidity but only the temperature differential between the circuit and humidifier technology have reduced the risk for humidifier and the airway sensor. If the heated wires are set nosocomial infection when these systems are used. Wick-type warmer than the humidifier, less relative humidity is delivered or membrane-type passover humidifiers prevent formation of to the patient. To ensure that the inspired gas is being properly bacteria-carrying aerosols. Heated wire circuits reduce produc- conditioned, clinicians always should adjust the temperature tion and pooling of condensate within the circuit. In addition, differential to the point at which a few drops of condensation the high reservoir temperatures in humidifiers are bacteri- form near the patient connection, or “wye.” Lacking direct mea- cidal.51 In ventilator circuits using wick-type humidifiers with surement of humidity, observation of this minimal condensate heated wire systems, circuit contamination usually occurs from is the most reliable indicator that the gas is fully saturated at the patient to the circuit, rather than vice versa. the specified temperature. If condensate cannot be seen, there For decades, the traditional way to minimize the risk for is no way of knowing the level of relative humidity without circuit-related nosocomial infection in critically ill patients direct measurement—it could be anywhere between 99% and receiving ventilatory support was to change the ventilator 0%. HME performance can be evaluated in a similar manner.55 tubing and its attached components daily.52 It is now known that frequent ventilator circuit changes increase the risk for nosocomial pneumonia.45 There is minimal risk for VAP with RULES OF THUMB weekly circuit changes and there may be no need to change You can estimate whether an HME is performing well at circuits at all unless visibly soiled.35,36,53,54 In addition, substan- the bedside by visually confirming condensation in the tial cost savings can accrue with decreased frequency of circuit flex tube between the airway and HME. Lack of condensate may be a clue that humidification is changes. inadequate and that alternative systems may be appropriate for use with the patient. 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 BLAND AEROSOL THERAPY take the steps needed to ensure proper conditioning of the inspired gas received by patients. Humidity is simply water in the gas phase, whereas a bland The most accurate and reliable way to ensure that patients aerosol consists of liquid particles suspended in a gas (see are receiving gas at the expected temperature and humidity level Chapter 39 for details on aerosol physics). Bland aerosol therapy is to measure these parameters. Portable battery-operated involves the delivery of sterile water or hypotonic, isotonic, or digital hygrometer-thermometer systems are available for less hypertonic saline aerosols. Bland aerosol administration may be than $300 and are invaluable in ensuring proper conditioning accompanied by O2 therapy. To guide practitioners in applying of the inspired gas. When measuring high-humidity environ- this therapy, the AARC has published Clinical Practice Guide- ments, hygrometers become saturated and nonresponsive over line: Bland Aerosol Administration; excerpts appear in Clinical time and so should be used for spot checks only. Practice Guideline 38-2.56  Humidity and Bland Aerosol Therapy CHAPTER 38 835 38-2 Bland Aerosol Administration AARC Clinical Practice Guideline (Excerpts) INDICATIONS Diagnosis of laryngotracheobronchitis or croup Presence of upper airway edema—cool, bland aerosol History of upper airway irritation and increased work of Laryngotracheobronchitis breathing (e.g., smoke inhalation) Subglottic edema Patient discomfort associated with airway instrumentation Postextubation edema or insult Postoperative management of the upper airway Bypassed upper airway Presence of a bypassed upper airway Need for sputum induction (e.g., Pneumocystis pneumonia Need for sputum specimens or mobilization of secretions or tuberculosis) is an indication for administration of hypertonic saline aerosol. CONTRAINDICATIONS Bronchoconstriction ASSESSMENT OF OUTCOME History of airway hyperresponsiveness With administration of water or hypotonic or isotonic saline, HAZARDS AND COMPLICATIONS the desired outcome is one or more of the following: Wheezing or bronchospasm Decreased work of breathing Bronchoconstriction when artificial airway is used Improved vital signs Infection Decreased stridor Overhydration Decreased dyspnea Patient discomfort Improved arterial blood gas values Caregiver exposure to airborne contagions produced during Improved O2 saturation, as indicated by pulse oximetry coughing or sputum induction With administration of hypertonic saline, the desired Edema of the airway wall outcome is a sputum sample that is adequate for analysis. Edema associated with decreased compliance and gas MONITORING exchange and with increased airway resistance The extent of patient monitoring should be determined based Sputum induction by hypertonic saline inhalation can cause on the stability and severity of the patient’s condition: bronchoconstriction in patients with chronic obstructive Patient subjective response—pain, discomfort, dyspnea, pulmonary disease, asthma, cystic fibrosis, or other restlessness pulmonary diseases. Heart rate and rhythm, blood pressure ASSESSMENT OF NEED Respiratory rate, pattern, mechanics; accessory muscle The presence of one or more of the following may be an use indication for administration of a water or isotonic or Sputum production—quantity, color, consistency, odor hypotonic saline aerosol: Skin color Stridor Breath sounds Brassy, crouplike cough Pulse oximetry (if hypoxemia is suspected) Hoarseness after extubation Spirometry equipment (if adverse reaction is a concern) From Kallstrom T, American Association for Respiratory Care: Clinical practice guideline: bland aerosol administration, 2003 revision and update. Respir Care 5:529–533, 2003. of a siphon tube, where it is sheared off and shattered into liquid Equipment particles. The large, unstable particles fall out of suspension or The equipment needed for bland aerosol therapy includes an impact on the internal surfaces of the device, including the fluid aerosol generator and a delivery system. Devices used to gener- surface (baffling). The remaining small particles leave the neb- ate bland aerosols include large-volume jet nebulizers and ulizer through the outlet port, carried in the gas stream. A ultrasonic nebulizers (USNs). Delivery systems include various variable air-entrainment port allows air mixing to increase flow direct airway appliances and enclosures (mist tents). rates and to alter FiO2 levels (see Chapter 41). Similar to humidifiers, if heat is required, a hot plate, wrap- Aerosol Generators around, yolk collar, or immersion element can be added. These Large-Volume Jet Nebulizers. A large-volume jet nebulizer devices rarely have sophisticated servo-controlled systems to is the most common device used to generate bland aerosols. As control delivery temperature. They may not shut down when depicted in Figure 38-13, these devices are pneumatically the reservoir empties, resulting in the delivery of hot, dry gas to powered, attaching directly to a flowmeter and compressed gas the patient. Failure of the heating element also can cause a loss source. Liquid particle aerosols are generated by passing gas at of heating capacity, without warning to the clinician. a high velocity through a small “jet” orifice. The resulting low Depending on the design, input flow, and air-entrainment pressure at the jet draws fluid from the reservoir up to the top setting, the total water output of unheated large-volume jet 836 SECTION V Basic Therapeutics DISS flow meter inlet 7 6 Variable air entrainment port Outlet port Jet orifice 5 Siphon tube 4 2 Water reservoir 3 Filter 1 FIGURE 38-14 Functional schematic of a typical large-volume ultrasonic nebulizer. 1, Radiofrequency generator; 2, shielded cable; 3, piezoelectric crystal transducer; 4, water-filled couplant reservoir; FIGURE 38-13 All-purpose large-volume jet nebulizer. 5, solution chamber; 6, chamber inlet; and 7, chamber outlet. (Modified from Barnes TA: Core textbook for respiratory care practice, ed 2, St. Louis, 1994, Mosby.) 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.56,57 produces an aerosol with MMAD between 4 and 6 µm. Signal Larger versions of these devices (with 2-L to 3-L reservoirs) are amplitude directly affects the amount of aerosol produced; the used to deliver bland aerosols into mist tents. These enclosure greater the amplitude, the greater is the volume of aerosol systems can generate flow rates greater than 20 L/min, with output. In contrast to frequency, signal amplitude may be water outputs of 5 ml/min (300 ml/hr). Because heat buildup adjusted by the clinician. in enclosures is a problem, these systems are always run Particle size and aerosol density delivered to the patient also unheated. are affected by the source and flow of gas through the aerosol- Ultrasonic Nebulizers. A USN is an electrically powered generating chamber. Some large-volume USNs have built-in device that uses a piezoelectric crystal to generate aerosol. This fans that direct room air through the solution chamber con- crystal transducer converts radiowaves into high-frequency ducting the aerosol to the patient. The airflow may be adjusted mechanical vibrations (sound). These vibrations are transmit- by changing the fan speed or use of a simple damper valve. ted to a liquid surface, where the intense mechanical energy Alternatively, compressed anhydrous gases can be delivered creates a cavitation in the liquid, forming a standing wave, or to the chamber inlet through a flowmeter. For precise control “geyser,” that sheds aerosol droplets. Figure 38-14 provides a over delivered O2 concentrations, clinicians can attach a flow- schematic of a large volume USN. Output from a radiofre- meter with an O2 blender or air-entrainment system to the quency generator is transmitted over a shielded cable to the chamber inlet. piezoelectric crystal. Vibrational energy is transmitted either The flow and amplitude settings interact to determine indirectly through a water-filled couplant reservoir or directly aerosol density (mg/L) and total water output (ml/min). Ampli- to a solution chamber. Gas entering the chamber inlet picks up tude affects water output. At a given amplitude setting, the the aerosol particles and exits through the chamber outlet. greater the flow through the chamber, the less the density of the The properties of the ultrasonic signal determine the char- aerosol. Conversely, low flows result in aerosols of higher acteristics of the aerosol generated by these nebulizers. The density. Total aerosol output (ml/min) is greatest when both frequency at which the crystal vibrates, preset by the manufac- flow and amplitude are set at the maximum. Using these set- turer, determines aerosol particle size. Particle size is inversely tings, some units can achieve total water outputs of 7 ml/min. proportional to signal frequency. A USN operating at a fre- Particle size, aerosol density, and output are also affected by quency of 2.25 MHz may produce an aerosol with a mass the relative humidity of the carrier gas (see Chapter 39). In median aerodynamic diameter (MMAD) of approximately contrast to jet nebulizers, the temperature of the solution placed 2.5 µm, whereas another nebulizer operating at 1.25 MHz in a USN increases up to 10° C during use. Although this  Humidity and Bland Aerosol Therapy CHAPTER 38 837 increase in temperature affects water vapor capacity, its impact on aerosol output is minimal. RULES 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. A B 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 D 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.58 Although a major manufacturer of USNs (DeVilbiss) discontinued their product line, other companies in C both the United States and Europe still manufacture units for FIGURE 38-15 Airway appliances used to deliver bland aerosol clinical use. therapy. A, Aerosol mask. B, Face tent. C, Tracheostomy mask. Commercially available USNs (usually marketed as “cool” D, T-tube. 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 T-tubes, tracheostomy masks exert no traction on the airway these units are cleaned according to the manufacturer’s recom- and they allow secretions and condensate to escape from the mendations and that water is discarded from the reservoir peri- airway, reducing airway resistance. odically between cleanings. In the absence of a manufacturer’s recommendation, these units should undergo appropriate dis- Enclosures (Mist Tents and Hoods) infection at least every 6 days.59 Generally, passover and wick- Infants and small children may not readily tolerate direct airway type humidifiers present less risk than the USN as a room appliances such as masks, so enclosures such as mist tents and humidifier. aerosol hoods are used to deliver bland aerosol therapy to these patients. More recent studies have shown that aerosol hoods can Airway Appliances provide aerosol delivery with similar efficiency to a properly Airway appliances used to deliver bland aerosol therapy include fitted aerosol mask in infants, with less discomfort for the the aerosol mask, face tent, T-tube, and tracheostomy mask patient.60 (Figure 38-15). The aerosol mask and face tent are used for Mist tents were used for more than 40 years mainly to treat patients with intact upper airways. The T-tube is used for croup and thus called croup tents. The cool aerosol provided patients who are orally or nasally intubated or who have a tra- through these enclosures promotes vasoconstriction, decreases cheostomy. The tracheostomy mask is used only for patients edema, and reduces airway obstruction. who have a tracheostomy. In all cases, large-bore tubing is Any body enclosure poses two problems