Oxygen Delivery Systems, Inhalation Therapy, and Respiratory Therapy

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

Arthur J. Tokarczyk, Steven B. Greenberg, Jeffery S. Vender

I. Introduction

II. Oxygen Therapy

A. Indications

B. Oxygen Delivery Systems

1. Low-Flow Oxygen Systems

2. High-Flow Oxygen Systems

C. Oxygen Delivery Devices

1. Low-Flow Devices

2. High-Flow Devices

D. Humidifiers

E. Manual Resuscitation Bags

F. Complications

III. Techniques of Respiratory Care

B. Chest Physical Therapy

1. Postural Drainage and Positional Changes

2. Percussion and Vibration Therapy

3. Incentive Spirometry

C. Intermittent Positive-Pressure Breathing

1. Indications

2. Administration

D. Noninvasive Ventilation

1. Indications

2. Limitations

IV. Inhalation Therapy

A. Basic Pharmacologic Principles

B. Aerosolized Drug Delivery Systems

C. Pharmacologic Agents

1. Mucokinetic Drugs

2. Bronchodilators and Antiasthmatic Drugs

V. Conclusion

VI. Clinical Pearls

I Introduction

Many patients suffer from acute and chronic dysfunction of the pulmonary system. Management of the surgical and critical care patient requires an understanding of pulmonary pathophysiology and the appropriate prophylactic and therapeutic techniques. Oxygen (O₂) therapy is one of the most commonly employed medical interventions. An understanding of O₂ delivery systems and devices is essential for optimal care. Bronchial hygiene has become a cornerstone in prophylactic care of the perioperative surgical patient, and these techniques may restore normal pulmonary physiology in patients with compromised function. Many medications that are specifically intended to treat pulmonary pathophysiology can be delivered by inhalation therapy with greater efficiency and less toxicity than oral or parenteral methods. This chapter reviews the various procedures and techniques used in the respiratory care of patients.

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.¹^–⁴ 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).

Box 14-1 Calculation of Arterial Oxygen Content

CaO₂= arterial O₂ content (mL/dL)

Hg= hemoglobin (g/dL)

1.34= O₂-carrying capacity of hemoglobin

PaO₂= arterial partial pressure of O₂ (mm Hg)

0.0031 = solubility coefficient of O₂ in plasma

Box 14-2 Calculation of Oxygen Delivery

CaO₂= arterial O₂ content per 100 mL of blood mL/dL. This value is approximately 20 mL/dL in the normal adult with a hemoglobin of 15 g/dL.

CO= cardiac output (L/min). This value is approximately 5 L/min in the healthy, 70-kg adult.

DO₂= O₂ delivery (mL/min). This value is approximately 1000 mL/min in the healthy, 70-kg adult.

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 () and diffusion defect.

Mismatch of ventilation and perfusion () is essentially an uncoupling of alveolar blood supply and ventilation. In an area of low , 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 = 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 ratio, a portion of ventilation does not participate in gas exchange. Dead space ventilation occurs when the perfusion becomes zero, and the 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 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 mismatch share a common phenomenon, which is exaggerated by a decreased mixed venous hemoglobin saturation (low ). Because hemoglobin saturation is the major determinant of O₂ content in blood, a low leads to a low venous O₂ content (). Low causes hypoxemia by worsening the hypoxemic effect of any existing shunt or areas of low by presenting more desaturated blood to the left atrium. Decreased 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 (V_T), 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.⁵^–⁷ 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., V_T, 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 V_T, 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), V_T 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 V_T (Tables 14-1 and 14-2).

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

Cannula	6 L/min	V_T, 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

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

TABLE 14-2 Example 2: If V_T 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

FIO₂, Fraction of inspired oxygen; V_T, 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 V_T or faster the respiratory rate, the lower the FIO₂. The smaller the V_T 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.

2 High-Flow Oxygen Systems

High-flow, fixed-performance systems are nonrebreathing systems that provide the entire inspiratory atmosphere needed to meet the peak inspiratory flow and minute ventilatory demands of the patient. The flow rate and reservoir are essential to meet the patient’s peak inspiratory flow. Flows of 30 to 40 L/min (or three to four times the measured minute volume) are often necessary. High-flow devices include aerosol masks and T-pieces that are powered by air-entrainment nebulizers or air-O₂ blenders and Venturi masks (see “Oxygen Delivery Devices”). Regardless of the patient’s respiratory pattern, high-flow systems are expected to deliver predictable, consistent, and measurable high and low FIO₂ values. High-flow systems also can control the humidity and temperature of the delivered gases. The primary limitations of these systems are cost, bulkiness, and patients’ tolerance.

There are two primary indications for high-flow O₂ devices:

1. Patients who require a consistent, predictable, minimal FIO₂ to reverse hypoxemia but prevent respiratory compromise due to excessive O₂ delivery (see “Complications”)

2. The patient with increased minute ventilation and abnormal respiratory pattern who needs predictable and consistent high FIO₂ values

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).

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.

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.

e Tracheostomy Collars

Tracheostomy collars are used primarily to deliver humidity to patients with artificial airways. O₂ may be delivered with these devices, but as with other low-flow delivery systems, the FIO₂ is unpredictable, inconsistent, and dependent on ventilatory pattern due to the limited reservoir volume. The consistency of delivered FIO₂ by tracheostomy also depends on the patient breathing entirely through the tracheostomy. Cuff deflation or tube fenestration allows entrainment of air through the upper airways, and this may alter the concentration of delivered O₂.

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.

Figure 14-3 Application of the Venturi principle.

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:

1. A fixed FIO₂ model, which requires specific inspiratory attachments that are color coded and have labeled jets that produce a known FIO₂ with a given flow

2. A variable FIO₂ model (Fig. 14-4), which has a graded adjustment of the air entrainment port that can be set to allow variation in delivered FIO₂

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

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.

TABLE 14-6 Approximate Air Entrainment Ratio and Gas Flow (FIO₂)

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.

b High-Flow Nasal Cannulas

High-flow nasal cannulas have been developed that can provide humidified gas flows up to 50 L/min while achieving 72% to 100% FIO₂.¹³ Consistent delivery of humidification is also maintained at 72% to 99.9% relative humidity through the use of specialized nasal cannulas,¹⁴ larger tubing, high-flow humidifiers, and O₂ blenders. These high-flow nasal cannulas can generate moderate levels of continuous positive airway pressure (CPAP) in nasal- and mouth-breathing patients.¹⁵ It does not require a face mask, unlike most other high-flow systems, and this affords patients the opportunity to verbally communicate and eat in a normal manner.

Two types of high-flow cannulas are available for adults: the High Flow Adult Cannula (Salter Labs, Arvin, CA) and the Nasal High Flow (Fisher & Paykel Healthcare, New Zealand). Both can achieve high relative humidity and high FIO₂ values. The High Flow Adult Cannula appears similar to a regular nasal cannula. Key differences include larger, three-channel tubing that is colored green; an enlarged reservoir at the prongs; and flow rates up to 15 L/min. The Nasal High Flow employs small-diameter corrugated tubing throughout the length of the supply tubing to achieve flow rates up to 50 L/min.