Long-term oxygen therapy improves survival in hypoxemic patients with COPD. Systems for home delivery include oxygen concentrators, compressed gas cylinders, and liquid oxygen reservoirs.
Long-term oxygen therapy is widely accepted as the standard treatment for chronic hypoxemia caused by chronic obstructive pulmonary disease (COPD) and other disorders such as interstitial lung disease. Long-term oxygen therapy will reverse secondary polycythemia, increase body weight, alleviate right heart failure due to cor pulmonale, improve cardiac function, enhance neurophysiological function, improve skeletal-muscle metabolism, boost exercise performance, reduce dyspnea, and enable the patient to perform activities of daily living.1 These changes are known to promote a higher quality of life for the patient and family.2
Long-term oxygen therapy improves survival in hypoxemic patients with COPD. The British Medical Research Council study,3 which compared hypoxemic patients receiving oxygen for 15 hours per day (including sleeping hours) with patients not receiving oxygen, demonstrated a significant reduction in mortality. The US National Heart, Lung and Blood Institute’s Nocturnal Oxygen Therapy Trial4 compared hypoxemic patients who averaged 12 or 19 hours per day of oxygen therapy; significant mortality reduction was noted for continuous users.
While the mechanisms of action responsible for these reductions in mortality are still unknown, it appears that pulmonary hemodynamics are involved. Chronic hypoxemia tends to increase pulmonary artery pressures, yet when the hypoxemia is reversed through oxygen administration, the pulmonary artery pressures lower toward normal levels.5,6 This reduction in the pressure load reduces cardiac work and improves oxygen delivery to the tissues.7
Simply correcting hypoxemia, however, may not prevent tissue hypoxia. Clinicians must be concerned about other physiologic systems’ impact on oxygen transport and delivery to metabolically active tissues. Accordingly, lung function should be optimized, infection should be controlled, and congestive heart failure should be treated. Further, hemoglobin deficiencies must be corrected and optimization of cardiac output must be ensured before oxygen transport by the circulatory system can be maximized.
Some clinicians are unduly concerned that oxygen supplementation will lead to respiratory drive depression, hypercapnia, and respiratory acidosis. Consequently, these clinicians may prescribe oxygen inadequately. While carbon dioxide retention does occur, it is often caused by ventilation/perfusion mismatching rather than by respiratory center depression, and it rarely leads to respiratory acidosis. Thus, the therapeutic priority must always be the correction of hypoxemia. Guidelines for supplemental oxygen administration have been presented in great detail.8
Three types of stationary sources supply supplemental oxygen for home use: oxygen concentrators, compressed gas cylinders, and liquid oxygen reservoirs. Portable oxygen systems are available for patients who desire to remain ambulatory (and as backup in the event of primary source failure). Over the past three decades, technological advances have refined source and delivery systems. Large cylinders for home use now have been replaced by oxygen concentrators or liquid systems. Portable systems have undergone a transformation from continuous-flow systems that waste oxygen to storage reservoir cannulae and pneumatic or electronic pulsing devices. Steel E cylinders are being replaced by lighter aluminum or composite cylinders. Because portable storage devices can be coupled with oxygen-conserving technologies, the storage reservoir size can be reduced.
Stationary Systems
The E cylinder is perhaps the most widely distributed and best known. It is routinely used as a backup system for concentrators and liquid oxygen in the event of a system failure. At a flow rate of 2 L/min, the E cylinder will last for about 5.5 hours. This time can be extended, however, through the use of oxygen-conserving technologies. The E cylinder as a primary stationary oxygen source is not practical due to its low storage capacity and overall cost.
Oxygen concentrators extract oxygen from room air using a molecular sieve. Concentrators are preferred by the HME community for their convenience, low cost, and long interval between service calls. Design innovations and use of new space-age materials have improved concentrator power consumption, minimized noise and heat buildup, enhanced safety and efficiency of operation, and lowered overall costs. Some users still find them noisy, however, and they do add to the patient’s monthly electricity costs. For those in remote areas where electricity provision is unreliable, a concentrator may not be the best choice. Most concentrators produce sufficient flows to cover the typical oxygen prescription of 2 to 3 L/min. For patients who require flow rates of more than 5 L/min, there are specialized concentrators that can deliver up to 10 L/min.
Today’s oxygen concentrators have been engineered to embody advances in safety. Motors are now double insulated, the electronics are better isolated and more dependable, and cases can withstand high impact without major damage. Concentrators are now equipped with warning systems that alert the user if there is a potential malfunction. Many concentrators come with oxygen-sensing technologies that continually sample the unit’s output and alert the user to concentrations below target values. Air filtration technologies have improved the quality of air entering the system, extending the life of the compressor and molecular sieve.
Recently, a new generation of concentrators capable of transfilling small, lightweight cylinders was introduced with great success. These systems couple the advantages of an oxygen concentrator with an endless supply of portable oxygen for the ambulatory patient. When first introduced, the systems were sometimes criticized as unsafe due to potential overpressurization of the portable cylinders, but these concerns were addressed. While these systems require additional maintenance, their service record has been extremely good, and having a transfilling system in the home obviates extra service calls.
Lighter-weight concentrator systems have been introduced. One market entry weighs about 10 kg and uses power from the standard 12-V accessory outlet of a car or motor home. It is capable of delivering oxygen flows of 0.5 to 2 L/min. Another highly portable concentrator uses batteries as its power source and weighs just under 4.5 kg. Coupled with an oxygen conserver, this system is capable of delivering the pulsed equivalent of 1 to 5 L/min of oxygen. Battery life is limited to 45 minutes, however, so it generally uses 12-V automotive power or AC power.
Liquid systems have been around for many years and are the most efficient means for storing oxygen. When liquid oxygen is used, 1 L can provide 860 L of oxygen gas, supplying enough oxygen to last 5 days for patients requiring flows of 2 L/min. Liquid oxygen is easily transfilled from stationary reservoirs to portable containers, making it ideal for the ambulatory patient. Recently, liquid modules have incorporated a conserving device, thus creating a highly portable, lightweight system. The disadvantages of liquid oxygen include its higher cost, the tendency of liquid oxygen to evaporate when not being used, and the rare incidence of freezing injuries. Patients must live within the delivery range of the oxygen supplier. For rural patients living outside that radius, the cost of delivery may be prohibitive.
Oxygen-Delivery Devices
There are more than 800,000 patients now receiving long-term oxygen supplementation in the United States. Most of them receive their oxygen via standard nasal cannula. Although simple and inexpensive, the standard nasal cannula is an inefficient delivery system. The wastefulness of the system is derived from the physiology of respiration. During a normal respiratory cycle, about one fourth to one third of the time is spent in inhalation; the rest of the cycle is exhalation. Because the standard cannula delivers oxygen throughout the entire respiratory cycle, much of that oxygen is wasted. In addition, the last half of inhalation does not reach the alveoli; consequently, up to five sixths of continuously flowing oxygen is lost to the atmosphere. This physiological realization, coupled with economic forces and the need for smaller and lighter systems, has led to the development of conserver technologies.
Reservoir cannulae work by storing oxygen in small reservoirs during exhalation and then releasing that stored oxygen at the beginning of the next inhalation. Two variations of the reservoir cannula are available; one stores oxygen in a reservoir below the nose and one uses a small reservoir at chest level. These cannulae have reduced the use of oxygen by 50% to 75%, compared with standard nasal cannulae, while maintaining the patient’s oxygen saturation at comparable values.9,10 They may not be as efficient for mouth breathers, and some individuals find their appearance objectionable. In the event of reservoir failure, the patient can simply use the cannula as a standard flow cannula until it can be replaced.
Transtracheal oxygen (TTO) devices, first introduced by Heimlich,11 deliver oxygen directly through a small catheter inserted into the trachea. Placement of the catheter requires a skilled, experienced physician, good patient selection, and comprehensive support from a nurse and RT.12 Education is the key to success: patients must learn to clean, maintain, and care for the catheter and stoma. Because of these requirements, the relative number of patients receiving transtracheal oxygen therapy tends to be limited, unfortunately.
Transtracheal catheters are more efficient when used in conjunction with a demand-flow device. Transtracheal delivery is known to improve adherence to therapy while conserving oxygen. Patients receiving transtracheal oxygen have improved exercise tolerance, and it has been used with success in patients who have refractory hypoxemia.13
The disadvantages of transtracheal oxygen therapy are its high cost for the minor surgery and medical follow-up required, the risk of local infection, mucous plugging, and the need for constant cleaning. Not all patients are suitable candidates for this delivery method. Contraindications include delayed healing, ie, diabetes mellitus, connective tissue disease, or severe obesity. Oxygen savings using transtracheal catheters are on the order of 30% to 60%, and these figures improve when a demand flow device is used.
Three different oxygen-conserving technologies are available today: electronic devices, pneumatic devices, and liquid oxygen with a conserver. Electronic devices sense the onset of inspiration using very sensitive pressure transducers and then trigger release of a small bolus of 100% oxygen. This small bolus enters during the first few milliseconds of an inhalation, so the enriched oxygen participates in alveolar gas exchange. The result is a 50% to 85% reduction in oxygen use while oxygen saturation is maintained at levels equivalent to those of standard nasal-cannula delivery.14-16
Metering of oxygen using conserver technologies varies from device to device. Some devices deliver a small bolus with each breath, and the volume of the bolus is increased at higher settings. Another approach is to deliver a substantial bolus of enriched oxygen with each breath and regulate the number of breaths during which oxygen is delivered. Thus, the oxygen can be added to every third breath, every other breath, or every breath. Breaths that are not accompanied by a bolus of enriched oxygen are accompanied by room air (21% oxygen).
Pneumatic systems use a specialized regulator that senses inspiratory flow and delivers an oxygen pulse. These systems do not require a battery. They have an advantage in that they weigh less, but their disadvantage is that they are less efficient in delivering oxygen.
Liquid systems coupled with conserver technologies have become popular. This reflects their small size and weight, as well as the potential release of large amounts of gaseous oxygen from a small reservoir. These systems rely on the ability to transfill liquid oxygen from a source reservoir. Like all liquid systems, they continually bleed off a small amount of oxygen, are prone to freezing, and are somewhat expensive to purchase initially. Further, their overall reliability and product lifespans may be limited when these units are placed into everyday home use.
The settings used for most systems are calculated to resemble liter-flow deliveries, as compared with standard-flow oxygen administration. Each manufacturer has its own protocol to approximate FIO2 with continuous flow at the same number setting to achieve equivalent SaO2. To ensure that patients are adequately oxygenated, they must be evaluated while using the device of choice and under the work or exercise conditions commonly encountered (or slightly greater than commonly encountered, to ensure adequate oxygenation during the most demanding situations that patient may experience). Delivery setting changes will be required based on metabolic demands. These systems should not be used for nocturnal oxygen delivery.
Summary
Emerging technologies, competition for market share, consumer education, medical economics, and the need of patients to remain active are the forces driving the refinement of oxygen-delivery systems. The ultimate winner should be the consumer. While the primary medical goal of oxygen delivery is to correct hypoxemia, other goals, including maintenance of an active lifestyle and/or improvement in quality of life, are equally important. Eventually, we will learn how to intervene effectively in the disease process and develop strategies to repair the lung. Until that time, improvements in oxygen-delivery devices will address the changing needs of our patients.
Rick Carter, PhD, MBA, is professor of public health, Georgia Southern University, Statesboro. Brian Tiep, MD, is medical director of the Respiratory Disease Management Institute and associate professor of family medicine at Western University of Health Sciences, Pomona, Calif.
References
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