Oxygen Delivery Systems, Inhalation Therapy, and Respiratory Therapy

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Chapter 14 Oxygen Delivery Systems, Inhalation Therapy, and Respiratory Therapy

II Oxygen Therapy

A Indications

O2 is one of the most common therapeutic substances used in the practice of critical care medicine. O2 therapy may improve outcomes of patients undergoing surgery. Use of O2 concentrations greater than 50% FIO2 has reduced the incidence of wound infections in patients undergoing colorectal or spinal surgery.14 This section reviews some of the indications, goals, and modes of O2 therapy in the adult patient.

Treatment or prevention of hypoxemia is the most common indication for O2 therapy, and the final goal of effective treatment is avoidance or resolution of tissue hypoxia. Tissue hypoxia exists when delivery of O2 is inadequate to meet the metabolic demands of the tissues. O2 content (Box 14-1) depends on the arterial partial pressure of O2 (PaO2), the hemoglobin concentration of arterial blood, and the saturation of hemoglobin with O2. O2 delivery (DO2) is calculated by multiplying cardiac output (liters per minute) by the arterial O2 content. DO2 is measured in milliliters of O2 per minute, and for a 70-kg, healthy patient, it is approximately 1000 mL/min (Box 14-2).

Hypoxia may result from a decrement of any of the determinants of DO2, including anemia, low cardiac output, hypoxemia, or abnormal hemoglobin affinity (e.g., carbon monoxide toxicity) out of proportion to demand. Hypoxia may also arise from a failure of O2 use at the tissue level (e.g., microvascular perfusion defect of shock) or at the cellular level (e.g., cyanide poisoning).

Aerobic metabolism requires a balance between DO2 and O2 consumption. Inspiration of enriched concentrations of O2 may increase the PaO2, the percentage of saturation of hemoglobin, and the O2 content, thereby augmenting DO2 until the underlying cause of the hypoxia can be corrected (e.g., transfusing the anemic patient, reversing cardiac dysfunction). The clinical situation in which O2 therapy is most effective, however, is in the treatment of hypoxemia.

Hypoxemia may be defined as a deficiency of O2 tension in the arterial blood, typically defined as a PaO2 value less than 80 mm Hg. The most common perioperative causes of hypoxemia include decreased alveolar O2 tension (e.g., ventilation-perfusion mismatch, hypoventilation) and capillary shunt (e.g., atelectasis). Less common causes of perioperative hypoxemia include decreased mixed venous O2 content (image) and diffusion defect.

Mismatch of ventilation and perfusion (image) is essentially an uncoupling of alveolar blood supply and ventilation. In an area of low image, hypoxemia results when mixed venous blood flowing past the alveolar-capillary membrane (ACM) takes away O2 molecules faster than ventilation to the alveolus can replace them. The resultant partial pressure of O2 in the alveolus (PAO2) is too low to oxygenate the blood flowing past it. In a true intrapulmonary shunt, the ventilation decreases to zero, with image = 0. Anatomic shunt occurs when blood flows from the right side of the heart to the left side without traversing the pulmonary capillaries. A small percentage of physiologic shunt results from bronchial and thebesian circulation. True intrapulmonary shunts cause hypoxemia that is poorly responsive to O2 therapy. Therapy for “oxygen-refractory” hypoxemia is aimed at reducing the shunt. Different levels of shunting, such as low-ventilation areas, often cause blood to flow through capillaries adjacent to alveoli that do not participate in ventilation. Atelectasis is a common cause of this type of shunt. Respiratory therapy, such as tracheobronchial toilet, to remove mucous plugging of a lobar bronchus or adjusting an endotracheal tube (ETT) that has advanced into a main stem bronchus, may be effective at reversing causes of relative shunt. Positive airway pressure therapy can reduce intrapulmonary shunting in certain disease states associated with a diffuse reduction in functional residual capacity.

In a situation that is the opposite of a high image ratio, a portion of ventilation does not participate in gas exchange. Dead space ventilation occurs when the perfusion becomes zero, and the image ratio approaches infinity. Anatomic dead space is an area of the lungs that does not participate in gas exchange, such as the larger airways. Physiologic dead space is the total dead space that contributes to elevated image ratio. Dead space ventilation does not contribute significantly to hypoxemia unless perfusion is significantly disrupted, as occurs in a pulmonary embolus.

Hypoventilation causes hypoxemia when an increase in alveolar carbon dioxide (CO2) displaces the O2 molecules and decreases PAO2, as demonstrated in the alveolar gas equation:

Clinical entities associated with low PAO2 values include chronic obstructive pulmonary disease (COPD), asthma, retained secretions, sedative or narcotic administration, acute lung injury syndrome, and early or mild pulmonary edema. Inspiration of enriched concentrations of O2 under these circumstances increases PAO2, which increases the O2 gradient across the ACM, resulting in faster equilibration of mixed venous blood exposed to the ACM and a higher pulmonary venous, left atrial, left ventricular, and arterial PO2.

Even small increases in inspired O2 tension can affect hypoxemia when caused by low PaO2. Drug-induced alveolar hypoventilation resulting in hypoxemia on room air is exquisitely sensitive to increases in inspired O2 concentration. Appropriate initial management of patients with alterations in mental status includes the use of O2 therapy as long as ventilatory needs are also monitored.

Cases of hypoxemia caused by true shunt or image mismatch share a common phenomenon, which is exaggerated by a decreased mixed venous hemoglobin saturation (low image). Because hemoglobin saturation is the major determinant of O2 content in blood, a low image leads to a low venous O2 content (image). Low image causes hypoxemia by worsening the hypoxemic effect of any existing shunt or areas of low image by presenting more desaturated blood to the left atrium. Decreased image arises from low O2 delivery (e.g., low cardiac output, anemia, hypoxemia) or increased O2 consumption (e.g., high fever, increased minute ventilation and work of breathing).

The consequences of untreated hypoxemia include tachycardia, acidosis, and increased myocardial O2 demand, as well as increased minute volume and work of breathing. By treating hypoxemia, supplemental O2 restores homeostasis and greatly decreases the stress response and its attendant cardiopulmonary sequelae.

B Oxygen Delivery Systems

With the exception of anesthetic breathing circuits, virtually all O2 delivery systems are nonrebreathing. In nonrebreathing circuits, the inspiratory gas is not made up in any part by the exhaled tidal volume (VT), and the only CO2 inhaled is that in any entrained room air. To avoid rebreathing, exhaled gases must be sequestered by one-way valves, and inspired gases must be presented in sufficient volume and flow to satisfy the high peak flow rates and minute ventilation demonstrated in critically ill patients. Inspiratory entrainment of room air or the use of inspiratory reservoirs (including the anatomic dead space of the nasopharynx, oropharynx, and non–gas-exchanging portion of the bronchial tree) and one-way valves typifies nonrebreathing systems and defines them as two groups.57 Low-flow systems depend on inspiration of room air to meet inspiratory flow and volume demands. High-flow systems attempt to provide the entire inspiratory demand. High-flow systems use reservoirs or very high flow rates to meet the large peak inspiratory flow demands and the exaggerated minute volumes found in many critically ill patients.

1 Low-Flow Oxygen Systems

A low-flow, variable-performance system depends on room air entrainment to meet the patient’s peak inspiratory and minute ventilatory demands that are not met by the inspiratory gas flow or O2 reservoir alone. Low-flow devices include the nasal cannula, simple face mask, partial rebreathing mask, nonrebreathing mask, and tracheostomy collar. Low-flow systems are characterized by the ability to deliver high and low values of FIO2. The FIO2 becomes unpredictable and inconsistent when these devices are used for patients with abnormal or changing ventilatory patterns.8 Low-flow systems produce FIO2 values of 21% to 80%. The FIO2 may vary with the size of the O2 reservoir, O2 flow, and the patient’s ventilatory pattern (e.g., VT, peak inspiratory flow, respiratory rate, minute ventilation). With a normal ventilation pattern, these devices can deliver a relatively predictable and consistent FIO2 level.

Low-flow systems do not mean low FIO2 values. With changes in VT, respiratory rate, O2 reservoir size, and so on, the FIO2 can vary dramatically at comparable O2 flow rates. The following examples are theoretical mathematical estimates of an FIO2 produced by a low-flow system (e.g., nasal cannula) in two clinical conditions.

The example for estimation of FIO2 from a low-flow system is based on the standard normal patient and ventilatory pattern. Several assumptions are used for the FIO2 calculation. The anatomic reservoir for a nasal cannula consists of nose, nasopharynx, and oropharynx, and it is about one third of the entire normal anatomic dead space (including trachea). For example, 150 mL ÷ 3 = 50 mL; assume a nasal cannula O2 flow rate of 6 L/min (100 mL/sec), VT of 500 mL, respiratory rate of 20 breaths/min, inspiratory (I) time of 1 second, and expiratory (E) time of 2 seconds. If the terminal 0.5 second of the 2-second expiratory time has negligible gas flow, the anatomic reservoir (50 mL) completely fills with 100% O2, assuming an O2 flow rate of 100 mL/sec. Using the preceding normal variables, the FIO2 is calculated for a patient with a 500 mL and a 250 mL VT (Tables 14-1 and 14-2).

TABLE 14-1 Example 1: VT Is Decreased to 500 mL

Cannula 6 L/min VT, 500 mL
Mechanical reservoir None I/E ratio = 1 : 2
Anatomic reservoir 50 mL Rate = 20 breaths/min
100% O2 provided/sec 100 mL Inspiratory time = 1 sec
Volume inspired O2    
 Anatomic reservoir 50 mL  
 Flow/sec 100 mL  
 Inspired room air
(0.20 × 350 mL)
70 mL  
O2 inspired 220 mL  
  image  

FIO2, Fraction of inspired oxygen; I/E ratio, inspiration/expiration ratio; VT, tidal volume.

TABLE 14-2 Example 2: If VT Is Decreased to 250 mL

Volume inspired O2  
 Anatomic reservoir 50 mL
 Flow/sec 100 mL
 Inspired room air (0.20 × 100 mL) 20 mL
O2 inspired 170 mL
  image

FIO2, Fraction of inspired oxygen; VT, tidal volume.

The preceding 50% variability in FIO2 at 6 L/min of O2 flow clearly demonstrates the effects of a variable ventilatory pattern. In general, the larger the VT or faster the respiratory rate, the lower the FIO2. The smaller the VT or lower the respiratory rate, the higher the FIO2.

Low-flow O2 devices are the most commonly employed O2 delivery systems because of simplicity, ease of use, familiarity, economics, availability, and acceptance by patients. In most clinical situations (see “High-Flow Oxygen Systems” and “High-Flow Devices”), these systems should be initially employed.

C Oxygen Delivery Devices

1 Low-Flow Devices

a Nasal Cannulas

Because of their simplicity and the ease with which patients tolerate them, nasal cannulas are the most frequently used O2 delivery devices. The nasal cannula consists of two prongs, with one inserted into each naris, that deliver 100% O2. To be effective, the nasal passages must be patent, but the patient need not breathe through the nose. The flow rate settings range from 0.25 to 6 L/min. The nasopharynx serves as the O2 reservoir (Fig. 14-1). Gases should be humidified to prevent mucosal drying if the O2 flow exceeds 4 L/min. For each 1 L/min increase in flow, the FIO2 is assumed to increase by 4% (Table 14-3).

image

Figure 14-1 The three reservoirs of low-flow oxygen therapy.

(From Vender JS, Clemency MV: O2 delivery systems, inhalation therapy, and respiratory care. In Benumof JL, editor: Clinical procedures in anesthesia and intensive care, Philadelphia, 1992, JB Lippincott, pp 63–87.)

TABLE 14-3 Approximate FIO2 Delivered by Nasal Cannula

Flow Rate (L/min) Approximate FIO2*
1 0.24
2 0.28
3 0.32
4 0.36
5 0.40
6 0.44

FIO2, Fraction of inspired oxygen.

* Based on normal ventilatory patterns.

An FIO2 of 0.24 to 0.44 can be delivered predictably if the patient breathes at a normal minute ventilation rate with a normal respiratory pattern. Increasing flows to more than 6 L/min does not significantly increase the FIO2 above 0.44 and is often poorly tolerated by the patient.

The components of a nasal cannula are nasal cannula prongs, delivery tubing, and an adjustable, restraining headband. Additional equipment includes an O2 flowmeter to provide controlled gas delivery from a wall outlet; a humidification system increases patients’ comfort at higher flows (≥4 L/min).

Procedurally, the initiation of O2 therapy should be preceded by a review of the chart and documentation of the O2 concentration and device ordered. If a humidifier (typically prefilled, single-use, disposable) is used, it should be filled to the appropriate level with sterile water and connected to the flowmeter. The nasal prong should be secured in the patient’s naris and the cannula secured around the patient’s head by a restraining strap.

Avoidance of undue cutaneous pressure is essential. Gauze may be needed to pad pressure points around the cheeks and ears during prolonged use. The flowmeter should be adjusted to the prescribed liter flow to attain the desired FIO2 (see Table 14-3).

Although nasal cannulas are simple and safe, several potential hazards and complications exist. O2 supports combustion, and any type of O2 therapy is a fire hazard. Nasal trauma from prolonged use of or pressure from the nasal prongs can cause tissue damage. With poorly humidified, high gas flows, the airway mucosal surface can become dehydrated. This mucosal dehydration can result in mucosal irritation, epistaxis, laryngitis, ear tenderness, substernal chest pain, and bronchospasm.5,7,9 Because this is a low-flow system, the FIO2 can be inaccurate and inconsistent, leading to the potential for underoxygenation or overoxygenation. Overoxygenation may induce respiratory distress in patients with severe COPD by reversing protective hypoxic pulmonary vasoconstriction, depressing ventilation, and minimizing the Haldane effect (see “Complications”). Underoxygenation potentiates any problems associated with hypoxemia.

b Simple Face Mask

To provide a higher FIO2 value than that provided by nasal cannula with low-flow systems, the size of the O2 reservoir must increase (see Fig. 14-1). A simple face mask consists of a mask with two side ports. The mask serves as an additional O2 reservoir of 100 to 200 mL. The side ports allow room air entrainment and exit for exhaled gases. The mask has no valves. An FIO2 of 0.40 to 0.60 can be achieved predictably when patients exhibit normal respiratory patterns. Gas flows greater than 8 L/min do not significantly increase the FIO2 above 0.60 because the O2 reservoir is filled. A minimum flow of 5 L/min is necessary to prevent CO2 accumulation and rebreathing. The delivered O2 value depends on the ventilatory pattern of the patient, similar to the situation with nasal cannulas.

The equipment needed is identical to that used for nasal cannula O2 administration. The only difference is the use of a face mask. The predicted FIO2 can be estimated from the O2 flow rate (Table 14-4). Appropriate mask application is needed with all masks to maximize the FIO2 and the patient’s comfort. The mask should be positioned over the nasal bridge and the face, restricting O2 escape into the patient’s eye, which can cause ocular drying and irritation. If FIO2 values above 0.60 are required, a partial rebreathing mask, nonrebreathing mask, or high-flow system should be employed. All O2 devices that deliver higher values of FIO2 increase the potential of O2 toxicity (see “Complications”).

TABLE 14-4 Approximate FIO2 Delivered by Simple Face Mask

Flow Rate (L/min) FIO2*
5–6 0.4
6–7 0.5
7–8 0.6

FIO2, Fraction of inspired oxygen.

* Based on normal ventilatory patterns.

c Partial Rebreathing Mask

To deliver an FIO2 level of more than 60% with a low-flow system, the O2 reservoir system must be increased (see Fig. 14-1).7 A partial rebreathing mask adds a reservoir bag with a capacity of 600 to 1000 mL. Side ports allow entrainment of room air and the exit of exhaled gases. The distinctive feature of this mask is that the first 33% of the patient’s exhaled volume fills the reservoir bag. This volume is derived from the anatomic dead space and contains little CO2. During inspiration, the bag should not completely collapse. A deflated reservoir bag results in a decreased FIO2 because of entrained room air. With the next breath, the first exhaled gas (which is in the reservoir bag) and fresh gas are inhaled—accounting for the name partial rebreather. Fresh gas flows should be 8 L/min or greater, and the reservoir bag must remain inflated during the entire ventilatory cycle to ensure the highest FIO2 and adequate CO2 evacuation. An FIO2 of 0.60 to 0.80 or more can be delivered with this device if the mask is applied appropriately and the ventilatory pattern is normal (Table 14-5). This mask’s rebreathing capacity allows O2 conservation and may be useful during transportation, when O2 supply may be limited. Complications with partial rebreathing O2 delivery systems are similar to those with other mask devices with low-flow systems.

TABLE 14-5 Approximate FIO2 Delivered by Mask with Reservoir Bag

Flow Rate (L/min) FIO2*
6 0.6
7 0.7
8 0.8
9 0.8+
10 0.8+

FIO2, Fraction of inspired oxygen.

* Based on normal ventilatory patterns.

d Nonrebreathing Mask

A nonrebreathing mask (Fig. 14-2) is similar to a partial rebreathing mask but adds three unidirectional valves. One valve is located on each side of the mask to permit the venting of exhaled gases and to prevent room air entrainment. The third unidirectional valve is situated between the mask and the reservoir bag and prevents exhaled gases from entering the bag.

image

Figure 14-2 A nonrebreathing oxygen mask. In addition to the exhalation valve, the mask has a one-way inhalation valve.

(From Vender JS, Clemency MV: Oxygen delivery systems, inhalation therapy, and respiratory care. In Benumof JL, editor: Clinical procedures in anesthesia and intensive care, Philadelphia, 1992, JB Lippincott.)

The bag must be inflated throughout the ventilatory cycle to ensure the highest FIO2 and adequate CO2 evacuation. Typically, the FIO2 level is 0.80 to 0.90. Fresh gas flow is usually 15 L/min (range, 10 to 15 L/min). If room air is not entrained, an FIO2 value approaching 1.0 can be achieved. If fresh gas flows or reservoir volume do not meet ventilatory needs, many masks have a spring-loaded tension valve that permits room air entrainment if the reservoir is evacuated. This spring valve is often called a safety valve. The spring valve tension should be checked periodically. If such a valve is not present, one of the unidirectional valves on the mask should be removed to allow room air entrainment if needed to meet ventilatory demands. This may be required to meet the increased inspiratory drive of critically ill patients. If the total ventilatory needs are met without room air entrainment, the rebreathing mask performs like a high-flow system. The operational application of a nonrebreathing mask is similar to that of other mask devices. To optimize the system, the mask should fit snugly (without excessive pressure) to avoid air entrainment around the mask, which would dilute the delivered gas and lower the FIO2. If the mask fit is appropriate, the reservoir bag responds to the patient’s inspiratory efforts. The high flows often employed increase the potential for several problems. Gastric distention, cutaneous irritation, and distention of the venting valves in the open position allowing room air entrainment can occur with excessive gas flows.

2 High-Flow Devices

a Venturi Masks

High-flow systems have flow rates and reservoirs large enough to provide the total inspired gases reliably. Most high-flow systems use gas entrainment at some point in the circuit to provide the flow and FIO2 needs. Venturi masks entrain air by the Bernoulli principle and constant pressure-jet mixing.10 This physical phenomenon is based on a rapid velocity of gas (e.g., O2) moving through a restricted orifice. This action produces viscous shearing forces that create a decreased pressure gradient (subatmospheric) downstream relative to the surrounding gases. The pressure gradient causes room air to be entrained until the pressures are equalized. Figure 14-3 illustrates the Venturi principle.

image

Figure 14-3 Application of the Venturi principle.

(From Vender JS, Clemency MV: Oxygen delivery systems, inhalation therapy, and respiratory care. In Benumof JL, editor: Clinical procedures in anesthesia and intensive care, Philadelphia, 1992, JB Lippincott.)

Altering the gas orifice or entrainment port size causes the FIO2 value to vary. The O2 flow rate determines the total volume of gas provided by the device. It provides predictable and reliable FIO2 values of 0.24 to 0.50 that are independent of the patient’s respiratory pattern. These masks come in two varieties:

image

Figure 14-4 Graded air entrainment by the Ventimask provides specific FIO2 levels through the jet orifices.

(From Vender JS, Clemency MV: Oxygen delivery systems, inhalation therapy, and respiratory care. In Benumof JL, editor: Clinical procedures in anesthesia and intensive care, Philadelphia, 1992, JB Lippincott.)

To use any air entrainment device properly to control the FIO2, the standard air-O2 entrainment ratios and minimum recommended flows for a given FIO2 level must be used (Table 14-6). The minimum total flow requirement should result from entrained room air added to the fresh O2 flow and equal three to four times the minute ventilation. This minimal flow is required to meet the patient’s peak inspiratory flow demands. As the desired FIO2 increases, the air-O2 entrainment ratio decreases with a net reduction in total gas flow. The higher the desired FIO2, the greater the probability of the patient’s needs exceeding the total flow capabilities of the device.

Venturi masks are often useful when treating patients with COPD who may develop worsening respiratory distress and dead space ventilation with supplemental increases in O2 fraction.11,12 The Venturi mask’s ability to deliver a high flow with no particulate H2O makes it beneficial in treating asthmatics, in whom bronchospasm may be precipitated or exacerbated by aerosolized H2O administration.

Several specific concerns regarding the application of a Venturi mask must be recognized to provide appropriate function. Obstructions distal to the jet orifice can produce back pressure and an effect referred to as Venturi stall. When this occurs, room air entrainment is compromised, causing a decreased total gas flow and an increased FIO2. Occlusion or alteration of the exhalation ports can also produce this situation. Aerosol devices should not be used with these devices. Water droplets can occlude the O2 injector. If humidity is needed, a vapor-type humidity adapter collar should be used.

c Aerosol Masks and T-Pieces with Nebulizers or Air-Oxygen Blenders

Large-volume nebulizers and wide-bore tubing are optimal for delivering FIO2 levels greater than 0.40 with a high-flow system. Aerosol masks, in conjunction with air entrainment nebulizers or air-O2 blenders, deliver consistent and predictable FIO2 levels, regardless of the patient’s ventilatory pattern. A T-piece is used in place of an aerosol mask for patients with an artificial airway.

Air entrainment nebulizers can deliver FIO2 of 0.35 to 1.00 and produce an aerosol. The maximum gas flow through the nebulizer is 14 to 16 L/min. As with the Venturi masks, less room air is entrained with higher FIO2 values. As a result, total flow at high FIO2 values is decreased. To meet ventilatory demands, two nebulizers may feed a single mask to increase the total flow, and a short length of corrugated tubing may be added to the aerosol mask side ports to increase the reservoir capacity (Fig. 14-5). If the aerosol mist exiting the mask side ports disappears during inspiration, room air is probably being entrained, and flow should be increased.

image

Figure 14-5 Single-unit and double (tandem)-unit mechanical aerosol systems.

(From Vender JS, Clemency MV: Oxygen delivery systems, inhalation therapy, and respiratory care. In Benumof JL, editor: Clinical procedures in anesthesia and intensive care, Philadelphia, 1992, JB Lippincott.)

Circuit resistance can increase as a result of water accumulation or kinking of the aerosol tubing. The increased pressure at the Venturi device decreases room air entrainment, increases the FIO2 level, and decreases total gas flow. If a predictable FIO2 level of more than 0.40 is desired, an air-O2 blender should be used. Air-O2 blenders can deliver consistent and accurate FIO2 values from 0.21 to 1.0 and flows of up to 100 L/min with humidification. The higher flows tend to produce excessive noise through the large-bore tubing. Air-O2 blenders are recommended for patients with increased minute ventilation who require a high FIO2 level and in whom bronchospasm may be precipitated or worsened by a nebulized H2O aerosol. With an artificial airway, a 15- to 20-inch reservoir tube should be added to the Briggs T-piece (Hudson, RCI, Temecula, CA) to prevent the potential of entraining air into the system.

D Humidifiers

Humidity is the water vapor in a gas. When air is 100% saturated at 37° C, it contains 43.8 mg of H2O/L. The amount of water vapor a volume of gas contains depends on the temperature and water availability. The vapor pressure exerted by the water vapor is equal to 47 mm Hg. Alveolar gases are 100% saturated at 37° C. When the inspired atmosphere contains less than 43.8 mg of H2O/L or has a vapor pressure of less than 47 mm Hg, a gradient exists between the respiratory mucosa and the inhaled gas. This gradient causes water to leave the mucosa and to humidify the inhaled gas.

Room air that has a relative humidity of 50% at 21° C has a relative humidity of 21% at 37° C. Under normal conditions, the lungs contribute about 250 mL of H2O per day to maximally saturate inspired air.7

The administration of dry O2 lowers the water content of the inspired air. The upper respiratory tract filters, humidifies, and warms inspired gases. Nasal breathing is more efficient than oral breathing for conditioning inspired gases. The use of an artificial airway bypasses the nasopharynx and oropharynx, where a significant amount of warming and humidification of inspired gases are accomplished. As a result, O2 administration and the use of artificial airways increase the demand on the lungs to humidify the inspired gases.

The increased demand ultimately leads to mucosal drying, inspissated secretions, and decreased mucociliary clearance, which can eventually result in bacterial infections, mucous plugging, atelectasis, and pneumonia. To prevent these complications, a humidifier or nebulizer should be used to increase the water content of the inspired gases.

Indications for humidity therapy include high-flow therapeutic gas delivery to nonintubated patients, delivery of gases through artificial airways, and reduction of airway resistance in asthma. Low flows (1 to 4 L/min) usually do not need humidification except in specific individuals, but all O2 delivered to infants should be humidified.

A humidifier increases the heated or unheated water vapor in a gas. This can be accomplished by passing gas over heated water (heated passover humidifier); by fractionating gas into tiny bubbles as gas passes through water (bubble diffusion or jet humidifiers); by allowing gas to pass through a chamber that contains a heated, water-saturated wick (heated wick humidifier); and by vaporizing water and selectively allowing the vapor to mix with the inspired gases (vapor-phase humidifier). Other variations of humidification systems exist but are beyond the scope of this chapter.16

Bubble humidifiers can be used with nasal cannulas, simple face masks, partial and nonrebreathing masks, and air-O2 blenders. They increase the relative humidity of gas from 0% to 70% at 25° C, which is approximately equal to 34% at 37° C.17,18 Large-volume bubble-through humidifiers are available for use with ventilator circuits or high-gas-flow delivery systems.

A heated humidifier may be used when delivering dry gases to patients with ETTs because it allows delivery of gases with an increased water content and relative humidity exceeding 65% at 37° C. When heated humidifiers are used, proximal airway temperature should be monitored to ensure a gas temperature that allows maximum moisture-carrying capacity but prevents mucosal burns.

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