ACCP-NHLBI National Conference on Oxygen Therapy: Oxygen Delivery Systems
Methods of Administration
Low-flow Oxygen Systems
Low-flow oxygen systems supply oxygen at flow rates that are less than the patients inspiratory demand. The FIo2 provided may be either high or low. Nasal catheters and cannulae: Nasal catheters and cannulae are the most commonly used devices for administering low-flow oxygen and usually provide from 24 to 50 percent oxygen with flow rates up to 6 L/min.
The FIo2 in the trachea varies with changes in oxygen flow rate, inspiratory flow and minute ventilation. Nasal catheters and cannulae have the advantage of being simple, relatively comfortable and inexpensive. They can provide continuous oxygen during eating, drinking and airway care. Some patients may develop local irritation, dermatitis and drying of the mucous membranes if these devices are used for prolonged periods of time, particularly at high flow rates.
Simple Os masks: Simple oxygen masks deliver approximately 35-50 percent oxygen with flow rates of 5 L/min or greater. Since these masks have a reservoir of 100-200 ml, there is a potential for carbon dioxide rebreathing. In order to prevent carbon dioxide rebreathing, it is recommended that flow rates of at least 5 L/min be used. Because of these relatively high flow rates, simple oxygen masks are generally not suitable to deliver a low FIo2 (less than 30-35 percent oxygen). A disadvantage of all oxygen masks is that they must be removed when patients are eating, drinking, or receiving facial or airway care provided by My Canadian Pharmacy.
Masks with reservoir bag: Disposable nonrebreathing and partial-rebreathing oxygen masks are capable of delivering a high FIo2 (greater than 50 percent) with a low oxygen flow rate, so long as the flow is adequate to maintain partial inflation of the reservoir bag throughout inspiration. Partial-rebreathing oxygen masks provide an FIo2 of 0.6; in contrast, approximately 60 percent oxygen and nonrebreathing masks may be able to deliver a higher FIo2. Under usual clinical circumstances the differences between the disposable par-tial-rebreathing and nonrebreathing masks appear to be minimal with relatively small differences in die FIo2 provided. In addition, the FIo2 may vary with changes in minute ventilation and inspiratory flow rate. Because of the high FIo2, there is a potential risk of absorption atelectasis and oxygen toxicity associated with the use of these masks.
High-flow Oxygen Systems
High-flow oxygen systems provide flow rates that are high enough to completely satisfy the patients inspiratory demand, either by entrainment of ambient air or by a high flow of gas. These systems can provide either a high or low FIo2 at the trachea.
Jet-mixing “Venturi” masks: Jet mixing masks which are designed to deliver 24-28 percent oxygen are usually capable of providing an adequate flow rate so that the FIo2 at the trachea is equal to that of the mask. In order to assure that this is so, jet mixing masks which provide 24 percent oxygen must be operated at a minimum flow rate of 4 L/min, and those which provide 28 percent oxygen should be operated at a minimum flow rate of 6 L/min. Masks which deliver greater than 30 percent oxygen frequently do not provide a constant or predictable Flo2 at the trachea because of inadequate total flow rates. When 50 percent oxygen or greater is administered by the jet mixing masks which are currently available, the oxygen tension at the trachea may be much lower than that of the mask and may result in miscalculation of the alveolar-arterial oxygen gradient and possibly misdirected therapeutic interventions.
Reservoir nebulizers/humidifiers: Reservoir nebulizers and humidifiers with aerosol masks, face tents, continuous positive airway pressure (CPAP) masks, T-tubes or tracheostomy collars can provide both supplemental oxygen and increased water vapor or mist. The indications for mist or humidity therapy are quite different from those for oxygen therapy and, therefore, the two modalities should be considered separately. At times, it is desirable to combine therapy, as in patients with tracheal tubes or CPAP masks. High flow rates, usually in excess of 40 L/min, or in-line reservoirs are necessary to assure a constant FIo2 at the trachea. The FIo2 may vary because of back pressure when jet nebulizers are utilized.
Oxygen bubble humidifiers: Currently there is no subjective or objective evidence that routine humidification of oxygen is necessary at flow rates of 1-4 L/min when environmental humidity is adequate. Elimination of unnecessary humidification of oxygen can result in substantial savings.
In-Hospiial Oxygen Systems
Although most hospitals have centralized bulk oxygen systems which deliver 100 percent oxygen at outlets, oxygen concentrators are now being developed for use as a central supply source and these are currently being utilized in some hospitals. Concentrators are incapable of providing 100 percent oxygen, but usually deliver greater than 90 percent oxygen.
Home, Domiciliary and Portable Oxygen Systems
There are three major sources of supplemental oxygen for home or domiciliary usage: oxygen cylinders, liquid oxygen and oxygen concentrators. Oxygen is usually administered by nasal cannulae; however, at times, jet mixing masks or other devices may be prescribed. The cost of home oxygen commonly ranges from $300-$500 per month for equipment which can provide 2 L/min continuous flow. There are substantial regional differences in cost and availability of equipment.
The large reservoir tanks which are most commonly used in the home are H or К cylinders. Each cylinder will last approximately 2V% days at 2 L/min continuous flow. Therefore, if H or К cylinders are utilized for continuous oxygen therapy, three refills per week will be required. Smaller D and E cylinders can be used for portability and last 3.5 to 5 hours, respectively, at 2 L/min continuous flow. Aluminum cylinders are available and are considerably lighter in weight compared to cast iron cylinders.
Reservoirs for liquid oxygen contain 40-90 pounds of oxygen and will last approximately 4 to 10 days, when oxygen is administered at 2 L/min continuous flow. At this flow rate they require 3 to 7 refills per month. Portable units commonly in use weigh from 6У2 to 11 lbs and are capable of providing continuous oxygen at 2 L/min for 4 to 8hours. The portable units are designed to be transfilled safely from stationary reservoirs which are maintained in the home or a domiciliary facility. Both the reservoirs and the portable units vent gaseous oxygen at a rate of at least 1 lb per day. They should not be stored in a small closed space when they are not in use.
There are two types of electrically powered oxygen concentrators that provide a constant source of oxygen from ambient air. They utilize either a molecular sieve or a polymeric membrane. Molecular sieves remove nitrogen and water from air, and newer models are capable of delivering 85-90 percent oxygen at flow rates of 1-4 L/min. Polymeric membranes are permeable to both oxygen and water vapor. They deliver 30 or 40 percent oxygen at flow rates of 1-10 L/min. Flow rates of polymeric membrane concentrators are usually adjusted by the manufacturer so that the flow is increased by approximately threefold to equal oxygen delivery from a system which provides 100 percent oxygen. Patients who have become acclimated to high flow rates from membrane concentrators may experience “air hunger” when treated with lower flow rates of 100 percent oxygen.
Proper maintenance of oxygen concentrators, which includes frequent replacement of filters and equipment checks, is essential. When long oxygen tubes are attached to concentrators or cylinders, increased back-pressure may result and a pressure-compensated flowmeter should be utilized to assure accurate oxygen flow rates to the patient. Users and manufacturers should refer to ANSI standard Z79.13-1981 for more detailed information. The cost of electricity, which may be as much as $30 per month in some areas, is not reimbursable to the patient. These units do not provide complete portability. For some patients, the availability of a backup oxygen system may also be necessary in the event of a power failure improved with My Canadian Pharmacy.
Investigative Techniques for Long-term Oxygen Therapy
Transtracheal oxygen catheters and demand oxygen systems are currently being investigated. These techniques have the advantage that they can conserve oxygen and, therefore, allow a reduction in size and weight of portable units and an overall decrease in utilization and cost of home or domiciliary oxygen therapy.
Oxygen for Air Travel
Commercial aircraft are pressurized to maintain atmospheres that are equivalent to altitudes of8,500 ft or less. At these altitudes, patients with chronic hypoxemia may require supplemental oxygen. All commercial airlines require prior notice of air travel and personal oxygen equipment is not allowed. The patients diagnosis and information concerning the oxygen flow rate must be communicated to the carrier before travel. Airlines use various types of equipment to attempt to fulfill the prescription, but precise therapy usually cannot be guaranteed.
Transfiring Oxygen Cylinders at Home
When patients are using compressed gas cylinders or oxygen concentrators, transfilling of small cylinders (such as D or E cylinders) from larger H or K cylinders may provide relatively inexpensive portability. Guidelines which have been established by the Compressed Gas Association (CGA) and the National Fire Protection Association (NFPA) tend to discourage transfilling of high-pressure cylinders in the home. CGA pamphlets P2.5, P2.6, and 56HM (1983) are guidelines which have been developed to encompass transfilling of oxygen cylinders under a wide range of commercial conditions. These guidelines appear to contain undue restrictions for home use. Although this is a controversial issue, further attention should be directed towards the development of equipment and guidelines for safely transfilling oxygen cylinders in the home with specific applicability to the patient and family.