chapter 11.humidification equipment
TRANSCRIPT
Chapter 11 Humidification Equipment General Considerations Terminology Humidi ty is a general term used to describe the amount of water vapor in a gas. I t
may be expressed several ways.
Absolute Humidity Absolute humidi ty is the mass of water vapor present in a volume of gas. It is
commonly expressed in mill igrams of water per l i te r of gas.
Humidity at Saturation The max imum amount of water vapor that a volume of gas can ho ld is the humid ity
at satura tion. This wi l l vary
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wi th the tempera ture. The warmer the temperature, the more water vapor can be
held in a gas. Table 11.1 shows the abso lute humidi ty of saturated gas at various
temperatures. At a body tempera ture of 37°C, i t is 44 mg H2O/L.
TABLE 11.1 Water Vapor Pressure and Absolute Humidity in Moisture-saturated Gas
Temperature °Cmg H2O/L mm Hg
0 4.84 4.58
1 5.19 4.93
2 5.56 5.29
3 5.95 5.69
4 6.36 6.10
5 6.80 6.54
6 7.26 7.01
7 7.75 7.51
8 8.27 8.05
9 8.81 8.61
10 9.40 9.21
11 10.01 9.84
12 10.66 10.52
13 11.33 11.23
14 12.07 11.99
15 12.82 12.79
16 13.62 13.63
17 14.47 14.53
18 15.35 15.48
19 16.30 16.48
20 17.28 17.54
21 18.33 18.65
22 19.41 19.83
23 20.57 21.07
24 21.76 22.38
25 23.04 23.76
26 24.35 25.21
27 25.75 26.74
28 27.19 28.35
29 28.74 30.04
30 30.32 31.82
31 32.01 33.70
32 33.79 35.66
33 35.59 37.73
34 37.54 39.90
35 39.57 42.18
36 41.53 44.56
37 43.85 47.07
38 46.16 49.69
39 48.58 52.44
40 51.03 55.32
41 53.66 58.34
42 56.40 61.50
Relative Humidity Relat ive humidity, or percent saturation , is the amount of water vapor at a
part icula r tempera ture expressed as a percentage of the amount that wou ld be he ld
i f the gas were satura ted.
Water Vapor Pressure Humidi ty may also be expressed as the pressure exerted by water vapor in a gas
mixture. Table 11.1 shows the vapor pressure of water in saturated gas a t various
temperatures.
Inter-relationships I f a gas saturated with water vapor is heated, its capaci ty to hold moisture
increases and i t becomes unsaturated (has <100% relat ive humid ity). Its absolu te
humidi ty remains unchanged. Gas tha t is 100% satura ted at room temperature and
warmed to body temperature wi thout addit ional humidi ty wi l l absorb water by
evaporat ion f rom the surface of the respiratory tract mucosa unti l i t becomes
sa turated.
I f gas sa turated wi th water vapor is coo led , it wil l condense (rain out) water. The
absolute humid ity wil l fall, but the relat ive humidi ty wi l l remain at 100%.
I f inspired gas is to have a rela tive humidi ty o f 100% at body temperature , it must
be maintained at body temperature af te r leaving the humidif ier or heated above
body tempera ture at the humidif ier and allowed to coo l as i t f lows to the patient.
Coo ling wi l l result in condensat ion (ra in out) in the breathing system.
The specif ic heat of gas is low. As a consequence, i t quick ly assumes the
temperature of the surround ing environment. Inhaled gases quick ly approach body
temperature , and gases in corrugated tubes rap idly approach room temperature.
The heat of vaporizat ion of water is relat ively high. Evaporat ion of water, therefore ,
requires cons iderably more heat than warming of gases . Likewise , condensation of
water y ields more heat than cool ing of gases.
Considerations for Anesthesia Water is intent ional ly removed from medical gases so tha t gases delivered f rom the
anesthesia mach ine are dry and at room tempera ture. As gases f low to the a lveol i,
inspired gas is brought to body temperature (ei ther by heating or cool ing) and 100%
rela tive humidi ty (ei ther by evaporat ion or condensation). In the unintubated
patient, the upper respiratory tract (especial ly the nose) funct ions as the princ ipa l
heat and moisture exchanger (HME). During normal nasal breathing, the
temperature in the upper trachea is between 30°C and 33°C, wi th a
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rela tive humidi ty of approximately 98%, providing a water content of 33 mg/L (1).
Tracheal tubes and supraglott ic ai rway devices bypass the upper a irway, modifying
the patte rn of heat and mois ture exchange so tha t the tracheobronchial mucosa
must assume a greater role in heating and humid ifying gases.
Effects of Inhaling Dry Gases The importance of humid if icat ion in anesthesia remains uncertain . It is of g reates t
benef it in ped iatric pat ients, pa tients at increased risk for developing pulmonary
complica tions , and long procedures.
Damage to the Respiratory Tract
As the respiratory mucosa dries and i ts temperature drops, secretions thicken,
c il iary funct ion is reduced, surfactant act iv ity is impaired, and the mucosa becomes
more suscept ib le to injury (2,3). If secret ions are not cleared, ate lec tasis o r ai rway
obstruct ion can resul t . Thickened plugs may prov ide loc i for infec tion. Dry gases
can cause bronchoconstric t ion, further compromis ing respiratory funct ion.
Humidifying gases may decrease the incidence of respira tory complications
(coughing and breath holding) associated wi th an inhalat ion induct ion (4,5).
There is no agreement about the minimum humidity necessary to prevent
patho log ical changes. Recommendations have ranged f rom 12 to 44 mg H2O/L
absolute humid ity (2,3,6,7,8 ,9). The dura tion of exposure is important. I t is unlike ly
that a brief exposure to dry gases wil l damage the tracheobronchial tree. As time
increases, the like lihood tha t significant tracheobronchial damage wi l l become
greater.
Body Heat Loss Body temperature is lowered as the ai rways bring the inspired gas into thermal
equil ibrium and sa turate it wi th water. The use of a humidif ica tion dev ice can
decrease the heat loss tha t occurs during anesthesia and may provide some heat
input (10 ,11,12,13,14,15,16,17). Contro ll ing inspired gas temperature and humidi ty
is not an ef f ic ien t method of maintaining body temperature. Means to increase body
temperature are discussed in Chapter 31.
Absorbent Desiccation The effects of dry absorbent on the composi tion of inspired gases are discussed in
Chapter 9. Whi le HMEs preserve patien t heat and humidi ty , they also may
contribute to absorbent desicca tion (18).
Tracheal Tube Obstruction Thickened secretions in a tracheal tube increase i ts res is tance and can result in
comple te obstruction (19 ,20,21,22,23,24).
Consequences of Excessive Humidity An increased water load can cause c i l iary degenerat ion and paralysis, pulmonary
edema, al tered alveolar-arterial oxygen gradien t, decreased v ital capacity and
compliance, and a decrease in hematocri t and serum sodium (25).
Sources of Humidity
Carbon Dioxide Absorbent
The reac tion of absorbent with carbon diox ide l iberates water (Chapter 9). Water is
also conta ined in the absorbent granu les . Since the react ion is exothermic, heat is
produced. I f the absorbent granu les desiccate , they may react wi th certain
anesthet ics and produce extreme heat. This is discussed in detail in Chapter 9.
Exhaled Gases There is some rebreath ing in the tracheal tube, the supraglott ic a irway dev ice, and
the connections to the breathing sys tem. A lmos t half of the humidi ty in expired gas
is preserved in this manner (26).
In systems that allow rebreathing of exhaled gases (Chapter 8 ), the humidi ty and
temperature of the insp ired gases depend on the relat ive proport ions of f resh and
expired gases. This wil l depend on the sys tem and the fresh gas f low. As the f resh
gas f low is increased, the inspired temperature and humidi ty are reduced. Use of
the sys tem by a previous patient wil l increase the humidi ty .
Moistening the Breathing Tubes and Reservoir Bag Rinsing the ins ide of the breathing tubes and reservoir bag wi th water before use
increases the inspired humidi ty (27).
Low Fresh Gas Flows Using low fresh gas f lows wi th a ci rcle breathing sys tem wi ll conserve mois ture.
This is d iscussed in more detail in Chapter 9.
Coaxial Breathing Circuits Coaxial ci rc le systems, when combined wi th low f lows , wi l l inc rease the humid ity
more quickly than a system with two separate l imbs (2), bu t th is is not very eff ic ien t
in terms of heat or humidi ty improvement (28).
The Bain system (see Chapter 8) is a coax ial version of the Mapleson D. It does not
meet opt imal
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humidif ica tion requirements because of the high fresh gas f low required (26).
Heat and Moisture Exchangers An HME conserves some exhaled water and heat and returns them to the patien t in
the inspired gas. Many HMEs also perform bacterial /v iral f i l t ra tion and prevent
inhalat ion of small part ic les. The HME is also known as a condenser humid if ie r,
Swedish nose, art i fic ial nose, nose humidif ier, passive humidif ier, regenerat ive
humidif ier, moisture exchanger, and vapor condenser. When combined wi th a f i l te r
for bac teria and v i ruses, i t is cal led a heat and moisture exchanging f i l ter (HMEF).
Two international s tandards on HMEs have been published (29,30).
Description HMEs are disposable devices wi th the exchanging medium enclosed in a plast ic
housing. They vary in s ize and shape. Typical ones are shown in Figure 11.1. Each
has a 15-mm female connec tion port at the patient end and a 15-mm male port at
the other end. The patient port may also have a concentric 22-mm male f i tt ing
(Fig .11 .1C,D). There may be a port to attach the gas sampling l ine for a respiratory
gas monitor (Fig. 11 .1B,D) or an oxygen line. One type uti l izes a ceramic heat ing
element with a water input port , a membrane, and an aluminum grid that vaporizes
the water (31).
The dead space of HMEs varies. Pediatric and neonatal HMEs wi th low dead space
are avai lable (32 ,33). Mos t modern HMEs are one of two types, as shown in Table
11.2.
Hydrophobic Hydrophobic HMEs have a hydrophobic membrane with smal l pores. The membrane
is pleated to increase the surface area. A pleated hydrophobic membrane provides
moderately good inspired humidi ty . The performance of this type of HME may be
impaired by h igh ambient temperatures (1,21).
Hydrophobic HMEs are eff ic ient bacterial and v iral f i l ters (34,35,36,37,38,39). A
pleated hydrophobic f i l te r wi l l consistent ly prevent the hepat it is C v i rus from
passing whi le a hygroscop ic f i l ter may be inef fective (40). They a llow the passage
of water vapor but not l iquid water a t usual venti lato ry pressures (34 ,36). They are
associated wi th small inc reases in res istance even when wet (12,41,42).
Hygroscopic Hygroscopic HMEs conta in a wool , foam, or paperl ike material coated with
moisture-retain ing chemicals. The medium may be impregnated with a bacteric ide
(43). Composite hygroscopic HMEs consist of a hygroscopic layer plus a layer of
thin , nonwoven f iber membrane that has been subjected to an e lectrical f ie ld to
increase i ts polari ty. This improves fi l trat ion e ff ic iency and hydrophobic ity.
Most studies have shown that compos ite hygroscopic HMEs are more eff icient a t
moisture and tempera ture conservat ion than hydrophobic ones
(1,22,44,45,46,47,48,49,50,51,52,53). They wil l lose their ai rborne f i l t ra tion
ef fic iency if they become wet, and microorganisms held by the f i l te r medium can be
washed through the device. Their res istance can inc rease great ly when they
become wet (42).
Indications An HME can be used to increase inspired heat and humidi ty during both short- and
long-term ventila tion. HMEs may be especially usefu l when transport ing intubated
patients, because transport vent i la tors frequently have no means for humid ifying
inspired gases.
An HME can be used to supply supp lemental oxygen to an intubated patien t or
patient wi th a supraglott ic airway by connecting oxygen tubing to the gas sampling
port (54,55,56).
Contraindications Contraindicat ions inc lude pat ients with thick , cop ious, or bloody secre tions and
patients with a leak tha t prevents exhaled gas f rom travers ing the passive
humidif ier (e.g., bronchopleuralcutaneous f is tula or leaking or absent tracheal tube
cuff ). HMEs should be used with caut ion when weaning a patient from respiratory
support (57,58).
Factors Affecting Moisture Output
Heat and Moisture Exchanger Type Composi te hygroscop ic HMEs have bette r heat and moisture exchanging properties
than do hydrophobic ones.
Initial Humidity Increasing the humidi ty in the gas entering the HME f rom the breathing sys tem wi ll
increase the inspired humidi ty (59).
Inspiratory and Expiratory Flows The faster that gas passes through the HME, the less time there is to evaporate or
deposi t moisture, so a la rge t idal volume may cause the humidi ty of the inspired
gas to fa ll (45,48,59,60,61).
System Continuity A leak around the tracheal tube wi l l resul t in decreased inspired humidi ty (32,62).
Use The HME selec ted should be of an appropriate size fo r the patien t's tidal volume. If
a small HME is used in a large pat ient, the HME wi ll be ineff ic ient (63). Connecting
more than one HME in series wil l improve perfo rmance (64 ,65). Care must be taken
that the uni ts a re pushed f irmly together and that the increase in dead space is not
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excess ive for that particular patien t. Added dead space is especially important in
smal l patien ts.
View Figure
Figure 11.1 Heat and moisture exchangers. A,E: Straight variety. B: Right angle HME with port for aspiration of respiratory gases on the breathing system side. C: The flexible tube attached to the HME extends the distance between the patient and the breathing system and allows the angle between the breathing system and the patient to be altered. Because this HME has significant dead space, it should be used only with high tidal volumes and controlled ventilation with monitoring of inspired and exhaled carbon dioxide. D: Hydrophobic HME with respiratory gas aspiration port. (Pictures C, D, and E courtesy of Gibeck Respiration, Pall Biomedical Products Corp. and ARC Medical Inc.)
An HME shou ld be v isible and accessib le at all times in o rder to de tec t
contamination or d isconnection. The greates t inspired relative humidity occurs wi th
the HME posi tioned next to the tracheal tube, mask, o r suprag lott ic ai rway device .
Some gas moni tors (Chapter 22) a re part icula rly sensit ive to water. I f the sampl ing
l ine is on the machine s ide of the HME, the amount of moisture to which the
moni tor is exposed wi l l be reduced.
An HME can be used with any breathing system. With the Mapleson sys tems, dead
space can be reduced by
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ut i l izing the gas sampl ing port as the fresh gas inlet (66). Oxygen can be
administered through the gas sample port on the HME (55,56). An HME may be
used for pat ien ts who have a tracheostomy (67).
TABLE 11.2 Comparison of Hygroscopic and Hydrophobic Heat and Moisture Exchangers
Type Hygroscopic Hydrophobic
Heat and moisture exchanging efficiency
Excellent Good
Effect of increased tidal volume on heat and moisture exchange
Slight decrease Significant decrease
Filtration efficiency when dry Good Excellent
Filtration efficiency when wet Poor Excellent
Resistance when dry Low Low
Resistance when wet Significantly increased
Slightly increased
Effect of nebulized medications Greatly increased resistance
Little effect
An HME may be used as the so le source of humidi ty o r may be combined wi th
another source such as an unheated humidif ier (68) but should not be used wi th a
heated humidif ier.
I f a nebu lizer o r metered-dose inhaler (Chapter 7) is used to deliver medication, i t
should be inserted between the HME and the pat ient or the HME removed f rom the
c i rcui t during aerosol treatment.
An HME shou ld be replaced i f contaminated with secretions.
Advantages HMEs are inexpensive, easy to use, small, l ightweight, re liable, s imple in design,
and s ilent in operation. They have low resistance when dry. They do not require
water, an external source of energy, a temperature monitor, or alarms. There is no
danger of overhydra tion, hyperthermia, sk in or respira tory tract burns, or electrical
shock. Their use may inc rease the correlat ion between esophageal and core
temperatures (69). They act as a barrie r to large partic les, and some are eff ic ient
bacterial and v iral f i l ters, al though their role in reducing nosocomial infec tions
remains controvers ial . They may reduce problems caused by humidi ty in the
breathing sys tem such as obs truct ion of l ines and venti lato r mal-funct ion (70).
Disadvantages The main disadvantage of HMEs is the limi ted humidity tha t these dev ices can
deliver. The ir contribution to temperature preservation is not sign if icant.
Tempera ture management is discussed in Chapter 31. Ac tive heating and
humidif ica tion are more effective than an HME in retaining body heat, allev ia ting
thick sec retions, and preventing tracheal tube blockage
(19,21,22,23,71,72,73,74,75). The d if fe rence is more apparent af te r intubation
last ing for several days.
Placing an HME between the breathing system and the pat ient increases dead
space. This may necessi ta te an increase in tida l volume and can lead to dangerous
rebreathing (76). I t also increases the work of breathing during both inspirat ion and
exhalation (77).
Hazards
Excessive Resistance The use of an HME increases resistance, although usual ly it is not a major
component of the to ta l work of breathing (78). Resis tance increases with use
(9,33,35,38,58,78,79,80,81,82,83,84,85,86). Heavy v iscous sec retions can greatly
increase resistance. An HME should no t be used with a heated humidif ier, as this
can cause a dangerous increase in resistance. Nebulized medication increases the
resistance of hygroscopic HMEs (34,38).
With a Mapleson system, increased resis tance may cause divers ion of f resh gas
down the expiratory l imb (87,88,89).
High resis tance may resul t in suffic ient back pressure to prevent the low ai rway
pressure alarm f rom be ing activa ted if there is a disconnec tion between the pat ient
and the HME (90,91).
I f increased resistance is suspec ted during control led vent ilation, the peak pressure
should be measured wi th and without the HME in p lace. Spontaneously breathing
patients should be observed for s igns o f increased work of b reathing .
Airway Obstruction
An HME can become obs tructed by f luid, b lood, secretions, a manufac turing defect,
or nebulized drugs (18,92,93,94,95,96,97,98,99,100,101,102). Parts may become
detached and block the breathing system (103). The HME's weight may cause the
tracheal tube to kink.
I f an HME is used for long-term ventilat ion , tracheal tube occlusion may occur
(20,21,22,23,24,73).
Inefficient Filtration Liquid can break through a hygroscop ic HME, result ing in poor f i l trat ion (34,36).
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Foreign Particle Aspiration Parts of the HME may become detached (64,104,105,106,107). The parts may then
be inhaled by the pat ient.
Rebreathing The HME dead space may cause excessive rebreathing, especial ly wi th small tidal
volumes. Special low-volume dev ices are availab le for pedia trics. Even these smal l
devices may be too la rge for infants under 15 kg (108). HMEs should not be used
for mask vent ilation in smal l infants (109).
Leaks and Disconnections Adding an HME to a breathing system increases the potent ia l fo r disconnect ions
and leaks (18 ,92,110,111,112).
Hypothermia Patient warming is discussed in Chapter 31. HMEs are a means to conserve
temperature , but the amount of heat p reserved by th is method is smal l
(15,113,114,115).
Dry Carbon Dioxide Absorbent HMEs wi l l decrease the amount of humidity available to the absorbent. The ex tent
to wh ich HMEs wil l lead to absorbent desiccat ion is unclear. The effec t of dry
carbon d ioxide absorbents on the produc tion of Compound A and carbon monoxide
is discussed in Chapter 9. In add it ion, dry absorbent wi l l absorb some volati le
agents. This can impede anesthetic induct ion with these agents (18).
Humidifiers
A humidif ier (vaporizer o r vaporizing humidif ier) passes a stream of gas over water
(pass -over), ac ross wicks dipped in water (b low-by), o r through water (bubble or
cascade). Humidif iers may be heated or unheated.
Unheated Most unheated humidif iers are disposable, bubble-th rough devices that are used to
increase the humidi ty in oxygen suppl ied to patients v ia a face mask or nasa l
cannula. They cannot deliver more than about 9 mg H2O/L.
Heated Heated humidif ie rs incorporate a device to warm the water in the humidif ier. Some
also heat the inspiratory tube.
Description Humidification Chamber The humid if icat ion chamber contains the l iquid water. It may be d isposable or
reusable . A clear chamber makes i t easy to check the water level . Some humidifiers
have an in tegral or remote reservoir that supplies l iqu id water to the humid if icat ion
chamber (Fig. 11.2).
Heat Source Heat may be supplied by heated rods immersed in the water o r a pla te at the
bottom of the humidif icat ion chamber (Fig. 11.3).
Inspiratory Tube The insp iratory tube conveys humidif ied gas f rom the humidif ier ou tlet to the
patient. If i t is not heated, the gas wil l cool and lose some of i ts moisture as i t
t ravels to the patien t. This water wil l co llect in the inspiratory tub ing. A water trap
wi l l be necessary to collec t the condensed water.
Heating or insulating the inspiratory tube allows more prec ise control of the
temperature and humidity del ivered to the pat ient and avoids moisture rainout
(116,117). A heated wire ins ide the inspira tory tubing is mos t of ten used (Fig.
11.3). I t should ex tend as c lose to the patient connect ion as possible. Disposab le
wi res in preassembled disposable breathing systems are available. A reusable wire
must be inserted manual ly into the insp iratory tube by using a draw wire (118). If
the gas temperature de livered to the patient is set above the temperature of the
humidif ica tion chamber outlet , less than 100% rela tive humidi ty wi l l be del ivered
(119,120,121,122).
Temperature Monitor(s) Most heated humidif iers have a means to measure the gas temperature at the
patient end of the breath ing system. Usually , there are temperature sensors in the
water reservoir o r in contact wi th the heater pla te to activate alarms and shut off
heater power when necessary.
Thermostat
Servo-controlled Units A servo-controlled unit automatically regulates power to the heating element in the
humidif ier in response to the temperature sensed by a probe near the pat ient
connec tion or the humidifier outlet (123,124,125). These devices are equipped wi th
alarms to warn of high temperature. Often, there are two thermos ta ts so that if one
fails , the other wil l cu t off the power before a dangerous temperature is reached.
Nonservo-controlled Units A nonservo-controlled un it p rov ides power to the heat ing element according to the
se tt ing of a contro l, irrespec tive of the delivered tempera ture. I t may inc lude a
servo-control led ci rcuit , but the servo-control led uni t controls the heater rather than
the del ivered tempera ture (123,125).
Controls Most humidif iers a llow temperature selec tion at the end of the delivery tube or at
the humidi ficat ion chamber out let. Some al low less than 100% relative humidi ty to
be de livered. Some models genera te saturated vapor only at a preset tempera ture
(8).
Alarms Alarms may warn when the temperature at the patient end of the c i rcuit deviates
f rom the set temperature by a f ixed amount, when the temperature probe is no t in
place, when the heater wire is not connected, when the water level in the
humidif ica tion chamber is low, or when the ai rway tempera ture probe does not
sense an increase in temperature wi thin a certa in t ime af te r the humid if ie r is turned
ON. A low temperature a la rm wil l help to detect problems wi th the heater element.
I t
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also may be the means of detect ing lack of gas f low in the c i rcu it (126).
View Figure
Figure 11.2 Heated humidifier with separate water reservoir. (Picture courtesy of Hudson RCI.)
Action Some humidif iers heat the gas to a temperature exceed ing the desired patient
ai rway tempera ture (superheat ing) so that the cooling that occurs as i t f lows to the
patient wi l l resul t in the desired temperature at the patient connection. Coo ling in
the tube wil l resul t in water ra ining out in the tube. In other humidif iers,
temperature increases as i t passes th rough the inspiratory tube so tha t gas wi th
less than 100% relative humidi ty is del ivered.
View Figure
Figure 11.3 Heated humidifier. Heat is supplied from a heated plate below the humidification chamber. The heating wire at the left fits inside the delivery tube.
I f the delivery tube is not heated, the tempera ture wil l d rop as i t f lows to the
patient. The magnitude of the drop depends on many factors , including ambient
temperature ; gas f low; and the length, diameter, and thermal mass of the breathing
system. Cool ing can be reduced by shortening or insula ting the del ivery tube or by
us ing higher inspiratory f lows. I f the gas is saturated at the humidif ier outlet , the
temperature drop wi l l cause water vapor condensation (rainout) to occur. A water
trap wi l l be necessary to col lec t the condensed water.
Standard Requirements An international and a U.S. standard on humidif iers have been pub lished (127,128).
They contain the fol lowing prov is ions.
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• Humidif iers must be capable of p roducing an output of at least 10 mg H2O/L.
Those in tended for use wi th patien ts whose supraglottic ai rways have been
bypassed must be capab le of producing an output of at least 33 mg H2O/L.
• The average temperature at the delivery tube out le t shal l not f luc tuate by
more than 2°C from the se t temperature af te r a s ta te of thermal equil ibrium
has been established following a change in gas f low or set temperature. If
the measured gas tempera ture differs f rom the set temperature by more than
the range specif ied by the manufacturer, an ala rm must be activated.
• The volume of l iquid ex iting the humidif ie r shal l no t exceed 1 mL/minute or
20 mL/hour for humidif iers intended for use wi th neonates or 5 mL/minute or
20 mL/hour for all other humidif iers.
• I f the humid if ie r is heated, the gas temperature a t the delivery tube outlet
shall not exceed 41°C or the gas temperature a t the humidif ier out let shall be
indicated continuously and the tempera ture-measuring dev ice shal l ac tivate
an alarm when the temperature exceeds 41°C. The humidif ie r shal l interrupt
heating when the measured gas temperature exceeds 41°C.
• The accessible surface temperature of the del ivery tube mus t not exceed
44°C within 25 cm of the patien t connection port.
• When the humidifier is ti l ted 20° from its normal operating posit ion, there
shall be no water sp il led into the breathing system.
• All calibrated contro ls and ind ica tors sha ll be accurate to wi thin 5% of their
full -scale values, except for temperature displays and contro ls. The
measured gas temperature shall be accura te to ±2°C.
• The direct ion of f low must be marked on humidif iers with f low-direc tion–
sensi tive components .
Use In the c i rc le system, a heated humidif ie r is placed in the inspira tory limb
downstream of the unidirect ional valve by using an accessory breath ing tube. A
heated humidif ier must not be placed in the exp iratory l imb (129,130). I f a fi l ter is
used in the breathing system, i t mus t be placed upstream of the humidif ie r to
prevent i t f rom becoming c logged.
In Mapleson systems (Chapter 8), the humid if ie r is usually placed in the f resh gas
supply tube (131,132). Using a large-diameter tubing and plac ing the humidif ier
near the end of the tube wil l decrease condensation. The delivery tube temperature
probe may be placed ei ther between the fresh gas supp ly tube and the T-piece or
between the T-piece and the pat ient.
The humid if ie r must be lower than the patient to avoid the risk of water running
down the tubing in to the pat ient. The condensate mus t be drained periodical ly o r a
water trap inserted in the most dependent part of the tubing to prevent blockage or
aspirat ion.
The heater wire in the delivery tube should no t be bunched, but s trung evenly along
the length of the tube. The de livery tube should no t rest on other surfaces or be
covered with sheets , blankets , or other materials . A boom arm or tube tree may be
used for support (133).
Advantages Most heated humidif iers are capable of delivering sa turated gas at body
temperature or above, even with high f low rates. A heated humidif ier can produce
more effect ive humidif icat ion than an HME (74,134). Some (but not all ) can be used
for spontaneously breath ing and tracheotomized pat ients (67).
Disadvantages Humidif iers are bu lky and somewhat complex. These devices involve high
maintenance cos ts, electrical hazards , and increased work (temperature control ,
ref i l l ing the reservoir, draining condensate , c lean ing , and steri l ization). Their use is
associated wi th higher cos ts than HMEs.
Compared wi th c irculat ing water and forced-air warming (Chapter 31), the heated
humidif ier offers relat ively l i tt le protect ion against heat loss during anesthes ia
(135).
Hazards Infection Bac terial growth can occur in water stored in a reservoir or the condensate in the
delivery tube. The use of a heated c ircui t reduces the amount of condensate, which
may reduce the infect ion risk.
Breathing System Problems Reported breathing system problems include st icking valves, leaks , disconnec tions ,
incorrect connec tions , obs tructed f resh gas l ine or inspiratory l imb, no ise, and
c logged f i l ters and HMEs (136,137,138,139,140,141).
The delivery tubing may mel t, resu lt ing in an obstruction or leak
(118,142,143,144,145,146,147,148,149,150). Fires have been reported
(123,151,152). A charred breathing sys tem may resul t in fumes entering the
patient 's lungs. Overheat ing of breath ing c ircui ts with melt ing may be caused by
defec ts in or damage to the heated wire; bunching of the heated-wire coi ls within
the breathing system; elec trical incompatibil i ty between the heated-wire breathing
c i rcui t and the humidi fier (153); opera ting the device outside the specified range of
f lows or minute volumes; or covering the del ivery tube wi th sheets, blankets, or
other materials (133,144,147). Problems may occur if a c ircui t that is not from the
humidif ier manufacturer is used. Electrical connectors for heated-wi re c ircui ts tha t
are physical ly compatible may not be electrically interchangeable (133). When
venti lation is in terrupted, as when cardiac bypass is begun, the humid if ier should
be turned OFF. Without a gas a ir f low, the temperature at the pat ient end wi l l drop
and the heating element may increase i ts ou tput, causing the attached breathing
c i rcui t to mel t .
P.305
Adding a humidif ie r may change the breathing system volume and compliance
s ignif icant ly (154). This can result in less accurate small tidal volume del ivery.
Venti la to rs that ca lculate the sys tem compliance and gas compression mus t
perform their checkout procedure wi th the humidif ier in place s ince the humidif ier
can affect vent ilator accuracy.
Water Aspiration There is danger of l iquid water entering the trachea and drowning the pat ient or
causing a burn in the respiratory tract. These risks can be decreased by instal l ing a
water trap in both the inspiratory and exha la tion sides in the mos t dependent
port ion of the breathing tube, draining condensate frequently, and placing the
humidif ier and breathing tubes below the patien t.
Overhydration A heated humidi f ier can produce a posi tive water balance and even overhydration .
Al though mos t anesthetics are of suff icient ly short duration that this is no t a
s ignif icant problem, i t can be a prob lem wi th infants.
Thermal Injury Delivering overheated gases into the trachea can cause hyperthermia or damage to
the tissues l in ing the tracheobronchial tree. Skin burns have been reported f rom
administering heated oxygen nasally and when cont inuous posi t ive airway pressure
was de livered (125,155,156). Burns can also occur when t issue is in contact with
heated breathing ci rcui ts (157,158).
Overheating inspired gas may be caused by omitting , misplacing, d islodging, or not
fully inserting the ai rway temperature probe or by turn ing the humidif ier ON wi th a
low gas f low (123,155,156,159). A temporary increase in inspired gas tempera ture
may occur following a period of interrupted f low or an inc reased f low rate (160).
Increased Work of Breathing A heated humidi f ier inc reases resistance (26,161,162). Most cannot be used wi th
spontaneously breathing pat ien ts.
Monitoring Interference A humidif ier may add enough resis tance to prevent a low a irway pressure alarm
f rom being ac tivated if the sensor is upstream of the humidif ie r (91). Some f low
sensors are a ffected by condensat ion, produc ing a false posit ive alarm (163).
Pressure and f low monitoring are discussed in Chapter 23.
High humidi ty can cause prob lems wi th s idestream (asp irat ing) respiratory gas
moni tors (Chapter 22).
Altered Anesthetic Agents Halothane may be altered if i t passes through a humidif ie r whose heating e lement
is in di rect contact wi th the gas at a temperature of 68°C or higher (164).
Equipment Damage or Malfunction
Venti la to rs are sensit ive to rainout caused by water condensation. Signs inc lude
increased res is tance to exhalation, inaccura te pressure and volume measurements ,
autocycl ing, and vent ilator shutdown (70,165). In o rder to prevent these problems ,
a water trap should be used i f water is l ikely to condense in the breathing system.
These need to be inspected regu larly and emptied as needed. The humidif ier needs
to be lower than the patient and venti lato r.
Nebulizers Description A nebu lizer (aerosol generator, a tomizer, nebuliz ing humid if ier) emits water in the
form of an aerosol mist (water vapor plus part iculate water) (166). The most
commonly used are the pneumatical ly d riven (gas-driven, jet , high pressure,
compressed gas) and u ltrasonic nebulizers. Both can be heated. In addi tion to
providing humidif ica tion, nebul izers may be used to deliver drugs to the breathing
system.
A pneumatic nebulizer works by pushing a jet of high-pressure gas into a l iquid,
inducing shearing forces and breaking the water up in to f ine part ic les. An ul trasonic
nebulizer produces a f ine mis t by subject ing the l iqu id to a h igh-f requency,
electrically driven ultrason ic resonator. The frequency of osci l la tion determines the
s ize of the droplets. There is no need for a driv ing gas. Ul trasonic nebulizers create
a denser mist than pneumatic ones (124).
Use Because a high f low of gas must be used wi th a pneumatic nebul izer, it shou ld be
placed in the fresh gas l ine. An ul trasonic nebulizer can be used in the fresh gas
l ine or the inspiratory l imb.
Hazards Nebulized drugs may obstruct an HME or f i l ter in the breathing sys tem (95,96,167).
Overhydat ion can occur. If the droplets a re not warmed, hypothermia may resul t .
Infection can be transmitted because mic roorganisms can be suspended in the
water droplets.
There are reported cases where a nebulizer was connected di rectly to a trachea l
tube wi thout provis ion for exhalation (168). In one case, th is resu lted in a
pneumothorax.
Advantages
Nebulizers can del iver gases saturated with water without heat and, if desi red, can
produce gases carrying more water.
Disadvantages Nebulizers are somewhat costly. Pneumatic nebu lizers require high gas f lows.
Ul trasonic nebul izers require a source of electric ity and may present electrical
hazards. There may be considerable water deposi tion in the tubings, requiring
f requent d raining, water traps in both the insp iratory and exha lat ion tubes, and
posing the dangers of water d raining into the pat ient or blocking the tub ing.
P.306
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P.308
Questions For the fol lowing quest ions, select the correct answer.
1. Which is not an effect of inhaling dry gases?
Drying of the mucosa
Decreased compliance
Development of loci for infection
Impairment of surfactant act iv ity
Decreased alveolar-arteria l oxygen difference
View Answer2. Where should a heated humidifier be located in the circle system? Between the exhalation tubing and the carbon dioxide absorber
Between the Y-piece and tracheal tube
Between the inspiratory tubing and the Y-p iece
Between the absorber and the inspiratory tubing
Between the Y-piece and the mask
View Answer3. If a bacteria l fi lter is used in a circ le system that has a heated humidifier, where should the fi lter be placed?
Between the Y-piece and the tracheal tube
Between the Y-piece and the inhalat ional tubing
Between the inhalat ional tubing and the humidif ier
Between the inhalat ional unid irectional valve and the humidif ier
Between the Y-piece and the exhala tion tubing
View Answer4. Which is the definition of absolute humidity? The pressure exerted by water vapor in a gas mixture
The amount o f water vapor at a part icular temperature as a percentage of the
amount that would be held if the gas were saturated
The max imum amount of water vapor that a volume of gas can ho ld
The mass of water vapor present in a volume of gas.
The humid ity of gases at body temperature
View Answer5. What w ill cause water to rain out in the breathing hose?
Using coaxial tubes in the breath ing system
Heated humidif ied gas that cools in the breathing tube
Warming cooled humidif ied gases in the breathing tube
Maintaining the temperature of heated gas in the breathing tubes
Using an ex tra-long breath ing tube
View AnswerFor the following quest ions, answer
• i f A, B, and C are correct
• i f A and C are correct
• i f B and D are correct
• is D is correct
• i f A, B, C, and D are correct.
6. Which of the following statements about humidity and ambient conditions are true?
I f a gas saturated with water is heated, i t can hold more water
Gas that is 100% saturated at room tempera ture and warmed to body temperature
wi l l be about 60% saturated
I f a gas saturated with water is heated, the absolute humid ity remains the same
If a gas saturated with water is heated, the rela tive humidi ty inc reases
View Answer7. A result of the low specific heat of gases is Inhaled gas wi l l quick ly assume body tempera ture
Water wi l l condense in the exhalat ion s ide of a ci rcle sys tem
Gas in breath ing tubes wil l quick ly assume room tempera ture
Gas has a tendency to change temperature s lowly
View Answer8. Heat and moisture are normally lost during anesthesia because Dry gases are suppl ied f rom the anesthesia machine
Gases are heated by the body
Tracheal intubat ion bypasses normal humidif ica tion mechanisms
The trachea l tube does not act to conserve heat or moisture
View Answer9. Sources of humidity in the breathing system include Carbon dioxide absorbent
Exhaled water from a prev ious pat ient
Rebreathing of p rev iously exhaled gases
Fresh gas
View Answer10. Advantages of a hydrophobic HME over a composite hygroscopic HME include Better fi l trat ion
Less res is tance when wet
Nebulized drugs have l it t le ef fect on resistance
Better humidif ica tion
View Answer11. Using a heated breathing tube with a heated humidifier will result in Rainout in the delivery tube
A higher temperature at the Y-piece
Drying of secre tions
A del ivered relat ive humidity of 100%
View Answer12. Hazards of heated humidifiers include
Sticking valves
Overhydra tion
Al te rat ion of anesthet ic agents
Obs truct ion of sides tream gas moni tors
View Answer13. Advantages of HMEs include Fluctuat ing tempera ture and humidi ty
Decrease in dead space
Low compliance
Decreased resistance to breathing
View Answer14. Contraindications to use of an HME include
Bloody secret ions
Pat ient temperature of less than 35°C
Bronchopleurocutaneous f is tula
Uncuffed tracheal tube
View Answer15. When using an HME, the inspired humidity can be in-creased by Use of an uncuffed tube
Increasing the minute volume
Lowering the humidi ty of gas entering the HME
Use of a hygroscopic rather than a hydrophobic HME
View AnswerP.309
16. Which patients or conditions derive the greatest benefit from humidification of inspired gases? Long anes thetics
Pat ients who are l ikely to develop pulmonary complicat ions
Pediatric pa tients
Elderly patients
View Answer17. Which of the following are respiratory complications of dry respiratory mucosa? Bronchoconstric t ion
Airway obstruction
Infection
Atelectasis
View Answer18. Consequences of excessive humidity include which of the following? Pulmonary edema
An increased serum sodium
Ci liary degeneration
Increased pulmonary compl iance
View Answer19. Hazards associated with using a nebulizer inc lude
Overhydra tion
Fi lter obstruction
Bac teria infection
Hypothermia
View Answer