Oxygen should be considered a drug with clear guidelines and indications for its use. Oxygen is used for short-term hospitalized patients and as long-term therapy in patients with chronic lung disease.

Oxygen is costly therapy and hence a rational understanding regarding its use is required. In the United States, home oxygen therapy is provided to approximately 1 million Medicare recipients costing nearly $2 billion dollars per year.^1,2

Short-term Oxygen Therapy in the Acute Care Setting

The most common indication for oxygen therapy in the acute setting is arterial hypoxemia with a PaO₂ of less than 60 mm Hg. Ventilation perfusion mismatch is the most common cause of arterial hypoxemia. Ventilation and blood flow are mismatched in various regions of the lung and all gas transfer becomes inefficient. Alveolar hypoventilation occurs when the volume of fresh gas going to alveoli (alveolar ventilation) is reduced. This is commonly caused by diseases outside the lung such as CNS insults, drug overdose, thoracic cage abnormalities, and upper airway obstruction and is often easily corrected with supplemental oxygen. Shunts, either in the form of extreme ventilation perfusion mismatch or anatomic right-to-left shunts, are often less responsive to administration of supplemental oxygen. A shunt occurs when blood reaches the arterial system without passing through ventilated regions of the lung. The role of diffusion impairment as a cause of hypoxemia is controversial, although it is thought to play a role in exercise oxygen desaturation seen in advanced interstitial lung disease.

A PaO₂ of 60 mm Hg is often set as a reasonable goal in the initial treatment of arterial hypoxemia. Accepted indications for short-term oxygen therapy include acute hypoxemia, cardiac and respiratory arrest and low cardiac output with metabolic acidosis. Questionable indications for which supplemental oxygen is used clinically, but for which there is little supporting data, include uncomplicated acute myocardial infarction, dyspnea without hypoxemia, sickle cell crisis and angina.

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Long-term Oxygen Therapy

Long term oxygen therapy (LTOT) is delivered to reduce long-term complications of chronic hypoxemia, particularly cor pulmonale. Hypoxemia induces physiologic responses that aim to maintain adequate oxygen delivery to tissues. These include increased heart rate and stroke volume, pulmonary vasoconstriction and increases in erythropoietin and hemoglobin concentration. Oxygen supplementation in patients with chronic hypoxemia has been shown to improve survival, pulmonary hemodynamics, exercise capacity and neuropsychological performance.^3,4,5,6

There were two landmark trials of LTOT in the 1980s- the British Medical Research Council (MRC) Working Party Trial and the American Nocturnal Oxygen Therapy Trial (NOTT).^7,8 The MRC trial compared COPD patients receiving oxygen for 15 hours/day with controls receiving no oxygen. The NOTT trial compared continuous daily oxygen (17.7 hours/day) with overnight oxygen use (average 12 hours/day). The main outcome in both trials was improved survival in those patients receiving oxygen for at least 15 hours/day, although this improved survival was not seen in the MRC trial until after a year of oxygen therapy. The NOTT trial showed a reduced exercise Pulmonary Arterial Pressure (PAP) after 6 months of continuous or nocturnal oxygen therapy. The MRC trial failed to show a fall in mean PAP with LTOT. The reason for the improved survival with LTOT is not clear.

Although LTOT is often prescribed in patients with pulmonary infiltrative or vascular disease, COPD is the disease for which LTOT is most commonly prescribed and the disease in which the original studies were completed.

Indications for home oxygen therapy

The indications for home oxygen therapy are based on the original NOTT and MRC oxygen trials but for Medicare requirements include:

Group I Requires annual recertification

• A resting arterial blood gas oxygen content equal to or below 55 mmhg or
• A resting or ambulatory oxygen saturation of 88% or below that is correctable with oxygen therapy

Group II Requires recertification and testing within 90 Days

• A resting arterial blood gas oxygen content of 56-59 mmHg or
• A resting oxygen saturation of 89%.
• Additionally Group II requirements include:
  • Dependent edema suggesting congestive heart failure or
  • Pulmonary hypertension or cor pulmonale, demonstrated by echocardiogram or ekg (p wave >3 mm in standard lead II, III, or AVF or
  • Erthrocythemia with HCT >56%

Indications for ambulatory oxygen

The indications for ambulatory oxygen are poorly researched. Ambulatory oxygen may be suitable for:

• Patients on LTOT who are mobile.
• Those with exercise desaturation and who show an improvement in exercise capacity or dyspnea with oxygen. Oxygen should be titrated to limit exercise desaturation, aiming for an oxygen saturation of >90%.

Indications for LTOT in non COPD patients

Studies similar to the NOTT and MRC are not available in non COPD patients with chronic hypoxemia. But extending the use of LTOT for patients with resting hypoxemia from other cardiopulmonary conditions such as interstitial lung diseases, cystic fibrosis, pulmonary arterial hypertension, chronic cardiac disease and neuromuscular diseases causing ventilatory failure.

Long-term oxygen therapy is also sometimes prescribed for the dyspnea of lung cancer or other causes of disabling dyspnea due to terminal disease.

Nocturnal oxygen therapy

Patients who develop significant decreases in arterial oxygen during sleep may also benefit from chronic oxygen administration. These include patients with primary sleep disordered breathing (obstructive sleep apnea or obesity hypoventilation syndrome) and patients with chronic lung disease with nocturnal desaturation. In patients with primary sleep disorders, oxygen therapy may need to be given in conjunction  with continuous positive airway pressure(CPAP) or other invasive or noninvasive ventilator support for the treatment of hypercarbia.

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Techniques of Oxygen Administration

A variety of delivery systems are available for short-term oxygen administration which vary in complexity, expense and precision of oxygen delivery. Oxygen delivery devices are considered either low-flow or high-flow appliances.

Low-flow oxygen devices

These provide a fraction of the patient’s minute ventilatory requirement as pure oxygen. The remainder of the ventilatory requirement is filled by addition of entrained room air. Flows supplied through these devices are low, usually less than 6L/min. Since small fluctuations in each tidal volume lead to variations in the amount of entrained room air, these devices cannot deliver constant inspired oxygen concentrations. Nasal cannulae, simple mask and oxygen conserving reservoir cannulae are the most widely used devices for delivery of low flow oxygen. These are simple, inexpensive, easy to use and well tolerated.

Nasal cannulae

Low-flow nasal cannulae set to deliver oxygen at flows between 1-6L/min lead to an FiO₂ between 0.24 and 0.44. Flows above 6L/min do not significantly increase FiO₂above 0.44. These higher flows may result in drying of mucous membranes.

Oxygen masks

Simple plastic oxygen masks which cover the nose and mouth are capable of delivering oxygen concentrations up to 30-60%. Depending on mask size, these devices provide a self-contained reservoir of 100 to 200 ml of additional gas facilitating increase in achievable FiO₂ above 0.44. They require a flow of oxygen of 5-6L/min to avoid CO₂ accumulation within the mask.

Conserving reservoir cannulae

Oxygen conserving cannulae are available in a mustache or pendant format and have the same limitations as other low flow-devices but may allow considerable oxygen flow reductions. This is accomplished by oxygen being stored and inhaled through a reservoir chamber, thereby allowing improved oxygenation at lower liter flows. Estimates for oxygen savings ratio versus continuous flow standard cannulae are 1.5:1 to 4:1 savings. These savings vary with liter flow with the greatest savings at lower liter flows. The limitations are the devices are larger and may be considered unsightly.

High-flow oxygen devices

Oxygen appliances that meet or exceed the patient’s inspiratory demands are considered high-flow systems. These devices are not generally used in the home care setting because of the prohibitive higher liter flow required. These devices include venturi mask, partial and non-rebreather mask and high-flow cannulae or mask.

Venturi mask

The venturi mask meets or exceeds patient inspiratory demand by using the Bernoulli principle, varying liter flow and air entrainment port size to provide 30-50 total liters of air and oxygen flow to provide a 24-50% FiO₂. Venturi masks are used when more precise amounts of FiO₂ are desired.

Partial and no-rebreathing mask

The partial and non-rebreather mask can provide 65-95% FiO₂ depending on the liter flow and configuration of exhalation valves. In non-rebreathing circuits, the inspiratory gas is not made up of any portion of the exhaled volume and the only inhaled CO₂ is entrained from ambient room air. Rebreathing is avoided through use of one way valves to sequester expired from inspired gas.

High-flow cannulae or mask

High flow systems offer liter flows in excess of 30l/min and offer a variety of Fi0₂ levels. The purported benefit is that higher liter flows flush out dead space thereby potentially decreasing carbon dioxide levels. Mucosal drying with higher flow rates is a concern, but new advances have improved delivery and tolerance to high flows. These systems like other high flow options are not yet available for home use.

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Oxygen in the Home

LTOT is provided in the home via oxygen concentrator, liquid oxygen or oxygen cylinders. Current technology and reimbursement practices have changed the availability and provision of home LTOT. The choice of which system is best for each patient is determined by the oxygen requirements, liter flow and duration, the available systems and insurance reimbursement.

Home oxygen concentrators

Home oxygen concentrators are compressors that use a molecular sieve material to remove the nitrogen from room air and provide oxygen concentrations of 85-97% pure oxygen. These concentrators can provide liter flows from 0.5l/min to 10l/min. Recent advances have allowed concentrators to become more portable, functioning on internal batteries, automobile adaptors or standard electricity. Portable oxygen concentrators are approved for airline travel.

Liquid oxygen

The primary benefit of liquid oxygen is that a large amount of oxygen can be supplied in a relatively small container because one liter of liquid oxygen offers approximately 860 liters of gaseous oxygen. This occurs because liquid O₂ is stored at about -300°F and when heated transforms to gaseous oxygen. The systems designed to store home liquid oxygen are Thermos-like containers with capacities of 20-40 liters for home storage units to 0.5-1.0 liter for portable units.

The advantage of liquid oxygen is availability of compact portable units that offer a wide range of liter flow and duration. A disadvantage is that while the storage containers are efficient they are not perfect and heat does cause wasteful dissipation of oxygen. The units are also quite costly and require regular home deliveries.

Oxygen cylinders

Oxygen cylinders are the primary source for home oxygen portable systems. The advantages of oxygen cylinders include the fact that they come in a variety of sizes, do not waste oxygen, they can have their duration increased with the use of a conserving device and can provide high liter flows. The disadvantages include often cumbersome size, limited duration, need for frequent refills and the high storage pressure of the gas (often up to 2000 psi).

Oxygen conserving devices

Oxygen conserving devices allow all types of oxygen systems to last longer increasing patient independence. Oxygen conservation can be accomplished through the interface or through the delivering apparatus.

Oxygen conservation can be obtained through the interface with oxygen conserving cannulae as described earlier or through a transtracheal oxygen catheter.⁹ A transtracheal oxygen catheter is a small catheter that is surgically placed into the trachea, bypassing the upper airways and delivering oxygen directly into the lungs just above the carina. Advantages include lower liter flows and much touted improved aesthetics. Disadvantages include requiring the mandatory outpatient surgical procedure, potential for infection, tracheal irritation or mucus accumulation.

Additional conserving devices are external and sense and deliver oxygen via pulse dose delivery, thus delivering oxygen on demand. The advantages of these delivery systems is that they increase the durations of tanks and portable oxygen concentrators 30% to 60%.¹⁰ The noise created by the pulse of oxygen, limited liter flow (up to 6lm) and inability to maintain adequate oxygen saturationin some patients are all considered disadvantages of oxygen conserving devices.

Summary

In conclusion, oxygen therapy is a widely used therapeutic agent with clear guidelines and indications for its use. While the landmark studies which define its use were performed in COPD patients, it is also widely used in hypoxemic states from other advanced cardiopulmonary disorders. There are a wide variety of devices for oxygen administration and these continue to undergo refinement.

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