LONG TERM OXYGEN THERAPY IN ADULTS
Prashant N Chhajed*, Ravi Iyer**
*Cardiopulmonary Transplant Unit, St. Vincentís Hospital; **Department of Medicine, University of Sydney, Sydney, Australia.
Domiciliary oxygen therapy is an effective but potentially an expensive therapy. Previous studies have demonstrated reduced mortality with the use of domiciliary oxygen therapy in patients with chronic obstructive pulmonary disease (COPD)., There is also evidence that it improves right heart failure caused by cor pulmonale, enhances neurophysiological function, and increases exercise tolerance in the performance of day to day activity. Supplementary oxygen is unlikely to contribute usefully to the relief of dyspnoea, heart failure or angina in absence of hypoxaemia. The other conditions likely to benefit from supplemental oxygen therapy include cyanotic congenital heart disease, severe congestive cardiac failure, interstitial lung disease, advanced lung cancer, bronchiectasis or any illness with chronic hypoxaemia.
Patient Assessment for Oxygen Therapy
The nature and the severity of the pulmonary disorder responsible for hypoxaemia needs to be assessed prior to the commencement of oxygen therapy. It should be treated optimally after the assessment of right heart failure and pulmonary hypertension. This includes maximum therapy of airway obstruction, attention to weight and nutrition, exercise rehabilitation programme, control of infection, treatment of cor pulmonale and cessation of smoking. Gas exchange may improve substantially on cessation of cigarette smoking and hence
patients need to be assessed at least one month after having stopped smoking. The degree of hypoxaemia should be assessed once the patientís condition has been stabilised and the drug therapy optimised over a period of four weeks. This is achieved by measurement of arterial blood gases on room air at rest on two occasions and measurements of partial pressure of arterial oxygen (PaO2) or oxygen saturation during sleep, when indicated. The adequacy of relief of hypoxaemia and/or improvement in exercise capacity has to be assessed using appropriate oxygen delivery systems. The patients need to be reassessed after one month by the measurement of PaO2 and partial pressure of arterial carbon dioxide (PaCO2) with and without supplemental oxygen. Oxygen flow rate should be set at the lowest possible needed to maintain a resting PaO2 of 60 mm Hg. It should be increased by 1L/min during exercise and sleep. Some patients may reveal a sustained rise in PaO2 to > 60 mm Hg on room air on follow up assessment. However this may represent the reparative effects of supplementary oxygen and should not be a rationale for stopping therapy.[3,4] This recommendation may change with further evidence.
Continuous Oxygen Therapy (at least 15 hours a day)
This is considered for patients with stable chronic lung disease, especially COPD, with an arterial PaO2 on room air, less than or equal to 55 mm Hg at rest. The factors, which strengthen the case for the therapeutic use of oxygen are polycythaemia (Hb > 17 gm%), clinical or electrocardiographic evidence of pulmonary hypertension and episodes of right failure. Patientswith these complications should be prescribed continuous oxygen therapy if their stable PaO2 is 55-59 mm Hg. It is of utmost benefit in patients with COPD having a PaCO2 of > 45 mm Hg. Patients have been shown to benefit with the increasing use of oxygen for up to 19 hours per day.
Intermittent Oxygen Therapy
Intermittent oxygen therapy may improve exercise capacity in patients with chronic lung diseases. Comparing exercise endurance whilst breathing room air and breathing oxygen (using a treadmill, stationary bicycle or a six-minute walk test) helps in assessment of benefit of oxygen therapy in these patients.
Nocturnal Oxygen Therapy
The clinical importance of isolated nocturnal hypoxaemia in the absence of hypoxaemia during the day or obstructive sleep apnoea has been established in previous studies. The diagnosis of nocturnal hypoxaemia should be considered in patients who have daytime somnolence, polycythaemia or right heart failure, but acceptable arterial blood gas tensions when awake. Nocturnal hypoxaemia should be distinguished from obstructive sleep apnoea. The diagnosis can be established by formal sleep studies.
Continuous positive airway pressure or other ventilatory support may need to be considered to replace or complement oxygen therapy. Pulmonary hypertension has been shown to improve in patients with hypoxaemia during sleep, with nocturnal oxygen therapy at 3L/min over a period of three years. However it did not alter the mortality in comparison with a control group. The current consensus supports nocturnal oxygen therapy in patients with oxygen saturation below 88%. However further studies need to be undertaken to support this hypothesis.
Supplemental oxygen therapy is contraindicated in,
1. Patients with chronic pulmonary disease who are dyspnoeic but do not qualify the blood gas or clinical criteria for supplementary oxygen.
2. Patients, who have not received adequate therapy for their underlying respiratory condition, associated respiratory infection and right heart failure.
3. Patients who continue to smoke (in view of the poor prognosis conferred by smoking, risks involved due to the inflammable properties of oxygen and the evidence of benefit from trials being based on non-smokers).
Supplementary oxygen in patients with increased arterial PaCO2 is known to depress ventilation, increase physiological dead space, and further increase arterial PaCO2. Sedatives, narcotics, alcohol, and other drugs that impair the central regulation of breathing should not be used in patients with hypercapnia receiving oxygen therapy.
OXYGEN DELIVERY SYSTEMS
The various oxygen delivery systems are,
3. Liquid oxygen
4. Conservation devices
The cylinders contain pure oxygen in a compressed state. They deliver 100% oxygen at the outlet and are preferred for intermittent use at home. Cylinders need to be protected from heat, as it causes an increase in their pressure. When oxygen is used for more than 8 hours a day, it is more convenient to use an oxygen concentrator. The use of portable oxygen cylinders may improve exercise tolerance, quality of life and the ability to do simple tasks. Portable cylinders can be refilled at home from a source of liquid oxygen using a special valve, but not from large gas cylinder or an oxygen concentrator.
Oxygen concentrators are electrically driven devices that entrain room air. A molecular sieve removes nitrogen and delivers oxygen at the outlet. They do not store oxygen and hence must run all the time for which oxygen is needed. The concentrators should be placed in a well-ventilated area with adequate tubing and with multiple outlets to increase patient freedom. These units deliver up to 90%-95% oxygen at the outlet at a flow rate of 2L/min. The percentage of oxygen falls with the increase in flow rate. A back up oxygen cylinder is desirable in case of concentrator breakdown or power failure.
Portable concentrators that can run off a 12-volt car battery are available in the United Kingdom.
Liquid Oxygen Systems
Liquid oxygen systems provide the most flexible source of home oxygen. Oxygen has a boiling point of -183oC and 1 litre of liquid oxygen provides 860 litres of oxygen. The liquid oxygen containers are insulated and are at a relatively low pressure. Frost bite or burns can occur by contact with the container or the tubing. The reservoir is used to fill light portable cylinders containing 1 litre of liquid oxygen, which can last up to 8 hours when the oxygen is delivered at 2 litres/min.
Conservation devices are introduced between the oxygen delivery source and the patient. They ensure that oxygen is delivered only during inspiration and not wasted during expiration. These are useful cost and time conserving devices, especially for portable units. Conservation devices switch on the flow by sensing negative pressure at the nares via the nasal cannula. These may not trigger if the patient breathes by his mouth.
Devices for delivering oxygen
Nasal prongs are generally the best way of delivering long-term oxygen as it is convenient whilst eating and talking. Extra-soft nasal prongs may be used for continuous oxygen therapy. Facemasks can be used when there is no danger of carbon dioxide retention as the fraction of inspired oxygen (FiO2) delivered by variable performing masks varies with changing breathing pattern. Transtracheal oxygen delivery has the advantage of allowing substantially lower flow rates. However, care of this relatively invasive delivery system is demanding.
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