The fraction of inspired gas that is oxygen is abbreviated as FiO2 and is often expressed as a percentage.
Research has shown that routine use of supplemental oxygen in cardiac patients may have unto- ward effects, including increased coronary vascular resistance, reduced coronary blood flow, and increased risk of mortality ( Amsterdam, et al., 2014). Indications for supplemental oxygen administra-tion include clinically significant hypoxemia (ie, oxygen saturaadministra-tion less than 90%), heart failure, dyspnea, cyanosis, or when other high-risk features of hypoxemia are present ( Amsterdam, et al., 2014;
O’Gara, et al., 2013).
Fig. 2.6 Colorimetric exhaled carbon dioxide detector. (Copyright ©2016 Medtronic. All rights reserved. Used with the per-mission of Medtronic.)
32 CHAPTER 2 Airway Management
Nasal Cannula [Objective 3]
A nasal cannula , which is also called nasal prongs, is a piece of plastic tubing with two soft prongs that project from the tubing. The prongs are inserted into the patient ’s nostrils, and the tubing is then secured to the patient ’s face (Fig. 2.7). Oxygen flows from the cannula into the patient ’s nasopharynx, which acts as an anatomic reservoir. Factors that influence the FiO2 delivered by a nasal cannula include the oxygen flow rate, the patient ’s ventilatory rate and tidal volume, and the anatomy and geometry of the patient ’s nasal cavity, nasopharynx, and oropharynx ( Ward, 2013).
For many years it was thought that for every liter-per-minute (L/min) increase in oxygen flow when using a nasal cannula, the effective FiO2 increased by about 4 percentage points. For example, giving supplemental O2 at 1 L/min by cannula would raise the FiO2 to about 24%, 2 L/min would raise it to 28%, and up to 6 L/min would raise it to 44% (Markovitz, et al., 2010). Research has shown these estimates of cannula performance to be overly optimistic ( Ward, 2013). In a 2010 study, the FiO2 levels produced in the trachea at oxygen flow rates of 1, 3, and 5 L/min were measured while subjects breathed at a normal rate and pattern. Researchers found the delivered FiO2 to be about 23% at 1 L/min, about 28% at 3 L/min, and about 32% at 5 L/min (Markovitz, et al., 2010). Delivered FiO2 decreases consid-erably during conditions associated with dyspnea ( Ward, 2013). Advantages and disadvantages of using a nasal cannula are shown in Box 2.2.
Fig. 2.7 Low-flow nasal cannula. (From Potter PA & Perry AG: Fundamentals of nursing: Concepts, process, and practice, ed 8, St. Louis, 2013, Mosby.)
BOX 2.2 Low-Flow Nasal Cannula
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Advantages and DisadvantagesADVANTAGES DISADVANTAGES
• Comfortable and well tolerated by most patients
• Does not interfere with patient assessment or impede patient communication with health care personnel
• Allows for talking and eating
• No rebreathing of expired air
• Can be used with mouth breathers
• Useful for patients who are predisposed to carbon dioxide retention
• Can be used for patients who require oxygen but who cannot tolerate a nonrebreather mask
• Can only be used in a spontaneously breathing patient
• Easily displaced
• Nasal passages must be open
• Drying to mucous membranes; may cause sinus pain
• Tubing may cause skin breakdown or irritation
• Deviated septum and mouth breathing may reduce FiO2
• Oxygen flow rates of more than 6 L/min do not enhance delivered oxygen concentration
High-flow nasal cannula (HFNC) systems are being used with increasing frequency for some critically ill patients. Components needed to provide HFNC oxygen include a nasal cannula that can accommo-date high inlet flow, a high-flow oxygen flowmeter, and a humdifier ( Ward, 2013). Commercially avail-able humidified HFNC systems use flow rates of 5 to 40 L/min and deliver an FiO2 of close to 100%
(Reardon, et al., 2014a).
Simple Face Mask [Objective 3]
A simple face mask , which is also called a standard mask, is a plastic reservoir that has been designed to fit over the nose and mouth of a spontaneously breathing patient. The mask is secured around the patient ’s head by means of an elastic strap. The internal capacity of the mask produces a reservoir effect. Small holes on each side of the mask allow for the passage of inspired and expired air.
Supplemental oxygen is delivered through a small-diameter tube connected to the base of the mask (Fig. 2.8).
When using a simple face mask, the oxygen flow rate must be higher than 5 L/min to flush the buildup of the patient ’s exhaled carbon dioxide from the mask. At 5 to 10 L/min, the simple face mask can deliver an inspired oxygen concentration of approximately 35% to 60%. The patient ’s actual inspired oxygen concentration will vary, because the amount of air that mixes with supplemental oxygen is depen-dent on the patient ’s inspiratory flow rate. Advantages and disadvantages of using a simple face mask are shown in Box 2.3.
Exhalation ports
Oxygen inlet
Fig. 2.8 Simple face mask. (From Kacmarek, Stoller, Heuer: Egan's fundamentals of respiratory care , ed 10, St. Louis, 2013, Mosby.)
BOX 2.3 Simple Face Mask
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Advantages and DisadvantagesADVANTAGES DISADVANTAGES
• Higher oxygen concentration delivered than by nasal cannula
• Can only be used in a spontaneously breathing patient
• Not tolerated well by severely dyspneic patients
• Can be uncomfortable
• Difficult to hear the patient speaking when the device is in place
• Must be removed at meals
• Requires a tight face seal to prevent the leakage of oxygen
• Side holes in the mask permit inhalation of room air
• Oxygen flow rates of more than 10 L/min do not enhance delivered oxygen concentration
34 CHAPTER 2 Airway Management
Partial Rebreather Mask [Objective 3]
A partial rebreather mask is similar to a simple face mask, but it has an attached oxygen-collecting device (ie, reservoir) at the base of the mask that is filled before patient use (Fig. 2.9A). When the patient breathes in, 100% oxygen is drawn into the mask from the reservoir (bag). When the patient breathes out, oxygen enters the bag from the oxygen source and some of the patient ’s exhaled air enters the bag (ie, an amount that is approximately equal to the volume of the patient ’s anatomic dead space). The amount ofCO2that is rebreathed is negligible as long as the oxygen flow keeps the bag from collapsing more than about one-third during inhalation (Heuer, 2013).
Valves
Reservoir bag
A B
Reservoir bag
Fig. 2.9 A, Partial rebreather mask. B, Nonrebreather mask. (From Kacmarek, Stoller, Heuer: Egan's fundamentals of respi- ratory care , ed 10, St. Louis, 2013, Mosby.)
BOX 2.4 Partial Rebreather Mask
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Advantages and DisadvantagesADVANTAGES DISADVANTAGES
• Higher oxygen concentration delivered than by nasal cannula
• Can only be used in a spontaneously breathing patient
• Not tolerated well in severely dyspneic patients
• Can be uncomfortable
• Difficult to hear the patient speaking when the device is in place
• Must be removed at meals
• Requires a tight face seal to prevent the leakage of oxygen
• May cause skin irritation
• Lacks inspiratory valve; thus exhaled air mixes with inspired air
The oxygen concentration of the patient ’s exhaled air, in combination with the supply of 100% oxy-gen, allows for the use of oxygen flow rates that are lower than those that are necessary for a nonrebreather mask. Depending on the patient ’s breathing pattern, the mask fit, and the oxygen flowmeter setting, oxygen concentrations of 35% to 60% can be delivered when an oxygen flow rate is used that prevents the reservoir bag from completely collapsing on inspiration (ie, typically 6 to 10 L/min). Advantages and disadvantages of using a partial rebreather mask are shown in Box 2.4.
Nonrebreather Mask [Objective 3]
A nonrebreather mask, also called a nonrebreathing mask, is similar to a partial rebreather mask, but it does not permit the mixing of the patient ’s exhaled air with 100% oxygen. A one-way valve between the mask and the reservoir bag and a flap over one of the exhalation ports on the side of the mask prevent the inhalation of room air (Fig. 2.9B). When the patient breathes in, oxygen is drawn into the mask from the reservoir (ie, bag) through the one-way valve that separates the bag from the mask. When the patient breathes out, the exhaled air exits through the open side port on the mask. The one-way valve prevents the patient ’s exhaled air from returning to the reservoir bag (thus the name nonrebreather ). This ensures a supply of 100% oxygen to the patient, with minimal dilution from room air.
A nonrebreather mask is the delivery device of choice when high concentrations of oxygen are needed for the spontaneously breathing patient. Depending on the patient ’s breathing pattern, the fit of the mask, and the oxygen flowmeter setting, oxygen concentrations of 60% to 80% can be delivered when an oxygen flow rate (typically a minimum of 10 L/min) is used that prevents the reservoir bag from col-lapsing completely on inspiration (Heuer, 2013). Inflate the reservoir bag with oxygen before placing the nonrebreather mask on the patient. Advantages and disadvantages of using a nonrebreather mask are shown in Box 2.5. A summary of oxygen percentages by device is shown in Table 2.1.
ACLS Pearl
When using a partial rebreather or nonrebreather mask, make sure that the bag does not collapse when the patient inhales. Should the bag collapse, increase the delivered oxygen by 2 L increments until the bag remains inflated during inhalation. The reservoir bag must remain at least two-thirds full so that sufficient supplemental oxygen is available for each breath.
BOX 2.5 Nonrebreather Mask
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Advantages and DisadvantagesADVANTAGES DISADVANTAGES
• Higher oxygen concentration delivered than by nasal cannula, simple face mask, and partial rebreather mask
• Inspired oxygen is not mixed with room air
• Can only be used with a spontaneously breathing patient
• Not tolerated well in severely dyspneic patients
• Can be uncomfortable
• Difficult to hear the patient speaking when the device is in place
• Must be removed at meals
• Mask must fit snugly on the patient ’s face to prevent room air from mixing with oxygen inhaled from the reservoir bag
• May cause skin irritation