Oxygen Therapy

The atmosphere contains 20.95% oxygen, the most essential element needed to sustain human life. Oxygen exerts a partial pressure of 159.6 mm Hg at sea level (dry air) and approximately 97 mm Hg (Torr) in arterial blood. The normal range, as measured by arterial blood gas (ABG) analysis, is 80 to 100 mm Hg (Torr). Under normal circumstances, oxygen molecules travel from the atmosphere to the mitochondria at the cellular level, where it is used to produce ATP in a process called aerobic metabolism (oxygen transport mechanisms). The oxygen transport pathway involves the airways and lungs, heart and peripheral vasculature, and the muscle tissue in which the mitochondria reside. Any process that inhibits the transport of oxygen from the atmosphere to the cellular level can cause hypoxemia, decreased patient function, and, ultimately, death.

When oxygen is prescribed, it should be considered a drug. A particular dosage must be ordered by a set prescription. In general, physical therapists do not instruct patients to increase or decrease medication they are taking without a physician’s order. Some practices have standing orders that allow the health care team to change oxygen settings by following a standardized protocol. For example, a prescription may be for 1 L/min of oxygen by nasal cannula at rest titrated up to 6 L/min to keep oxygen saturation above 90%. Although higher-flow oxygen cannulas can deliver increased liter flow, it is more standard in practice to increase titration according to a prescribed protocol.

Patients with cardiovascular and pulmonary impairments frequently need oxygen supplementation.¹ Respiratory care practitioners and the nursing staff are generally responsible for administration of this drug under a physician’s order. Therapists have a variety of methods from which to choose to deliver oxygen in the most effective way (individual methods are discussed later in this chapter). Long-term oxygen therapy for patients with chronic obstructive pulmonary disease (COPD) who are hypoxemic has been reported to improve quality of life and life expectancy.^2–5 However, even though the benefits of long-term oxygen therapy have been established, U.S. congressional mandates for competitive bidding and caps in rental agreement programs have led to cutbacks. As a consequence, the dose of oxygen and its mode of delivery may need to be modified at rest and during activity.⁶ This is an issue of significant concern. As practitioners working with patients requiring oxygen, physical therapists need to advocate for their patients to ensure that they receive appropriate oxygen therapy.

The purpose of oxygen therapy is to treat and prevent hypoxemia, excessive work of breathing, and excessive myocardial work.⁷ Although individual oxygen appliances offer suggested guidelines regarding oxygen administration, the only way to ensure effective delivery from a given device is by blood gas monitoring of PaO₂ (partial pressure of oxygen in the arterial blood) or monitoring hemoglobin saturation by oximetry. Medicare and third-party payers have specific indications for oxygen therapy. These indications may differ and physical therapists need to identify these in the practice setting and provide the documentation required for patients to receive the oxygen they need. Initially, a patient may need oxygen only with higher levels of activity or exercise; that needs to be documented by an exercise test that shows desaturation by oximetry. As a patient’s disease progresses, continuous oxygen may be indicated. Careful monitoring of patients will determine when the need for continuous oxygen occurs. Oxygen therapy is usually administered by one of two methods: low-flow or high-flow systems.

Low-Flow Systems

A low-flow oxygen system is one that is not intended to meet the total oxygen requirements of the patient (does not deliver atmospheric oxygen concentration to the patient). For example, using a normal minute ventilation (minute ventilation = tidal volume [TV] × respiratory rate [RR]) of 8 L/min (500 mL × 16 breaths per minute) with a patient receiving 1 L/min of oxygen, the patient is breathing 8 L/min but the device only provides 1 L/min. Thus 7 L/min are contributed from room air. Each time there is a change in the patient’s breathing pattern, tidal volume, and respiratory rate, the fraction of inspired oxygen (FiO₂) may also change. Ideally, for efficient use of a nasal cannula, the patient should have a normal tidal volume, respiratory rate, and breathing pattern. This is often not the case, but the cannula is used because it is well tolerated by the patient. This method of oxygen delivery is not used when a specific concentration of oxygen is needed.⁸

Nasal Cannulas

The nasal cannulas (also called nasal prongs) are one of the most common low-flow devices encountered because of low expense and high patient compliance (Figure 43-1). This device supplies an FiO₂ of approximately 24% to 40% oxygen with flow rates from 1 L/min to 6 L/min. Flow rates greater than 6 L/min can cause nasal mucosal irritation and drying. The approximate liter flow and resultant FiO₂ values are shown in Table 43-1.

Table 43-1

Approximate Liter Flow and Resultant FiO₂ Values

Flow (L/min)	FiO2 (%)
1	24
2	28
3	32
4	36
5	40
6	44

Adapted from the American College of Sports Medicine: Guidelines for exercise testing and prescription, ed 6, Philadelphia: Lippincott, Williams & Wilkins, 2000.

Nasal cannulas deliver 100% oxygen; however, this percentage significantly lessens as the oxygen mixes with inspired air from the room. The amount of oxygen delivered depends on the flow rate and the ventilatory pattern of the patient. A larger minute volume (TV × RR) would dilute the oxygen at any given flow rate and cause a greater decrease in the percentage delivered to the lungs. In other words, the faster and deeper a patient breathes, the more diluted the oxygen becomes. On the other hand, if a patient has a low minute volume, the oxygen percentage delivered increases.⁷ If precise control of FiO₂ is needed, the nasal cannula should not be used.⁹ In these cases a high-flow system (i.e., a Venturi mask) may be used.

Skin integrity, especially behind the ears, needs to be evaluated because the pressure of the cannula can cause skin breakdown. Patients at times will pull on the cannula or take the oxygen off. It is important to check for skin breakdown behind the ears, especially if patients are not able to communicate.

Mouth-breathing by patients using a nasal cannula typically causes concern from some of the health care team. This concern, however, may be unnecessary. If the nasal passages are unobstructed, then oxygen is able to collect in the oral and nasal cavity (anatomical reservoir). On inspiration, the oxygen collected in this area is drawn into the airways and lungs. If a patient with a nasal cannula is mouth-breathing, the practitioner should ensure the nasal passages are unobstructed. If there is concern that the patient is not receiving adequate oxygen, a blood gas or oxygen saturation measurement should be obtained. If this is not feasible, or if the patient is unable to breathe through the nose, it may be appropriate to switch the patient to a mask.

The patient should be encouraged to breathe through the nose to receive maximum benefit from the nasal cannula; however, mouth breathing does not mean the patient will not receive appropriate oxygen.¹⁰ Clinically, it is found that some patients who mouth-breathe do so because of nasal polyps, sinus congestion, deviated septum, or other physical issues. If the work of breathing is increased when the patient tries to breathe through the nose, it may be counterproductive.

Nonrebreathing Mask

Another low-flow delivery device, the nonrebreathing mask, also contains a bag reservoir, but it can deliver up to 100% oxygen. This mask has valves on the reservoir bag and side vents. This feature prevents ambient air mixing on inspiration and exhaled air mixing on expiration. For this mask to operate effectively, a good seal between the patient’s face and the mask must be achieved. The bag should partially deflate during inspiration (Figure 43-2).

High-Flow Systems

A high-flow oxygen delivery system delivers a specific oxygen concentration (FiO₂) despite the patient’s ventilatory pattern. This system delivers atmospheric oxygen pressure to the patient, which means that the entire volume that is breathed is delivered through the device. A patient requires oxygen delivered at specific FiO₂ of 50% to keep the arterial oxygen and oxygen saturation at a safe level, then a high-flow system is the method of choice. If a patient has CO₂ retention and a hypoxic drive to breathe, a high-flow system with an exact FiO₂ can be used. This is often the case when a patient with COPD who is a CO₂ retainer has pneumonia and goes into respiratory failure. The physician wants to provide adequate oxygen to the patient and avoid having to mechanically ventilate. The ideal choice would be a high-flow device at a lower FiO₂ to prevent CO₂ retention while not reducing the hypoxic drive to breathe. Unfortunately, patient tolerance of the high-flow oxygen-delivery mask, which covers both the nose and mouth, is often less than optimal. Patients who are acutely ill and very dyspneic or those who have decreased cognitive awareness may tolerate the mask, but as these patients begin to improve, they may have a claustrophobic reaction to the mask and the high flow of gas.

Venturi Mask

The Venturi mask is a common method of delivering high-flow oxygen concentrations from 24% to 50%. This mask operates via the Venturi principle, which provides for a mixing of 100% oxygen and entrained ambient air. The oxygen flows though a narrow orifice at a high velocity, causing a subatmospheric pressure. This drop in pressure is what causes the ambient air to be entrained through a port. The size of the port determines the amount of air that is entrained, thus the percentage of oxygen delivered. The Venturi mask has a rotating air entrainment port that allows the health care provider to “dial in” the desired FiO₂ (Figure 43-3). Flow rates of 40 to 80+ L/min provide minute ventilation at rates that are higher than those the patient could breathe without assistance; consequently, the mask provides the entire inspired atmosphere to the patient. This means that regardless of the patient’s respiratory rate, tidal volume, or breathing pattern, the oxygen delivery (FiO₂) will be consistent.

Mechanical aerosol systems also operate via air entrainment, but the mask is connected to the aerosol unit by large-bore tubing to allow a specific FiO₂ to be delivered with high humidity (Figure 43-4). Although drainage bags are usually attached to the tubing to collect condensation, the tubing must be monitored for possible pooling of water, which causes flow obstruction to the patient. If partial obstruction occurs, a mild back pressure results in the tubing, causing less air entrainment. This means that a higher concentration of oxygen results and potentially a higher dosage of oxygen than desired may be delivered to the patient.

Portable Oxygen

As health care continues to move out of the hospital setting and into the home, more health care practitioners will provide care to patients who require oxygen in their homes. When this is the case, it is essential that a sign be posted about the use of oxygen in the home. Patient teaching before hospital discharge should include a discussion of safety concerns—for example, not cooking with a gas stove, not smoking, and allowing anyone to smoke in the home (or anywhere around the patient). In addition, it must be made clear to the patient that oxygen is considered a prescription medication and thus the amount of oxygen delivered should not be changed unless ordered by the doctor.

Oxygen is most commonly provided in the home for those patients with long-term oxygen needs, usually patients with COPD who are hypoxemic. Generally 1 to 2 L/min is prescribed via nasal cannula, but the flow rate will depend on the patient’s need.

Home oxygen-delivery systems include high-pressure oxygen cylinders, low-pressure liquid oxygen,¹⁸ and oxygen concentrators.⁸ Additional options include continuous flow versus demand or pulsed oxygen delivery.^19–21 There are also a number of oxygen-conserving devices from which to choose.²² When a continuous system is changed to either a demand or pulsed system, or when a new oxygen-conserving device is started, the patient must be monitored carefully. Clinical experience has shown that some patients may not tolerate the same liter flow with a pulsed system, and oximetry and perceived exertion must be evaluated when there is any change in oxygen delivery. The oxygen delivery may not be as effective as a continuous flow device when changing to a pulsed oxygen system. Unfortunately, there is no mandate for clinical testing of the devices before they come to market, so documented effectiveness of one versus another in clinical use is not available.²³

Heliox, a combination of 21% oxygen and 79% helium, has been used in acute care for patients who have upper airway obstruction and increased work of breathing (spontaneously breathing patients, as well as those on a ventilator). However, use of heliox has not been widespread because of its cost, which has been estimated to be 13 times that of oxygen. Heliox for portable oxygen and home use has been documented as effective for patients with asthma and COPD. It has been shown to decrease airway resistance and decrease the work of breathing and has also been effective with patients who have increased mucus secretions. Studies have demonstrated an increase in patients’ ability to exercise using heliox; however, it has not been shown to significantly decrease dyspnea.24–26

High-pressure oxygen tanks are not used as frequently as are low-pressure tanks, but they are used in some settings. High-pressure tanks come in various sizes. The larger ones, such as the H and K sizes, are used as a base unit or reservoir. Long oxygen tubing connected to these tanks allows for patient mobility. Smaller tanks (such as E cylinders) are used for portability and can provide up to 3 hours of use depending on the liter flow of oxygen. Some systems allow for the smaller tank to be refilled by transferring gas from the larger reservoir tank. The primary concern with the high-pressure tanks is guarding them against tipping over. They must be well tied and secured because if a tank tips over or falls, the neck that holds the regulator could break. If this happens, the high pressure inside can propel the tank at a very high velocity, causing damage to surrounding structures and posing a danger to anyone around. The tanks are usually tied to a bed, put in a standard box, or secured on a portable wheeled cart. Thus the high-pressure tanks are difficult to move and maneuver, especially up and down stairs, making their use less than optimal for most patients, especially those who wish to be more active. In terms of cost, the high-pressure tanks are less expensive overall, but deliveries and replacements need to occur regularly.

Liquid oxygen, a low-pressure oxygen delivery method, tends to be more convenient than the high-pressure oxygen tanks, especially for active patients on oxygen. The canisters are lightweight and allow patients to be away from the home reservoir for up to 8 hours. The downside is that liquid oxygen costs more and, with heavy use, the reservoirs may need to be filled up to twice a week. If high flows are needed, these units are not recommended.

Oxygen concentrators are commonly used in the home health setting, especially because Medicare offers coverage for these devices (Figure 43-5). They are electrically powered and create oxygen by drawing ambient air across a semipermeable membrane, separating oxygen from nitrogen. They generally operate at 2 L/min and provide FiO₂ 90% oxygen output. Long oxygen tubing, up to 50 feet, allows patients extra mobility. Patients using concentrators should have a back-up device, such as a portable tank, in case of a power outage. In addition, the electrical power company should be notified of any patient who is using oxygen by concentrator so that the patient’s location is designated as a high priority for backup and restoration of power if an outage occurs.