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ABC of oxygenOxygen therapy in chronic lung diseaseP J Rees, F Dudley

Short term oxygen therapy

Oxygen in first aidOxygen can be used in acute situations where there is hypoxiabecause of acute lung injury or reduced ventilation, when thereis underlying chronic lung disease, or when it is important tomaintain oxygen delivery to certain tissues. Maintaining oxygendelivery may help tissue survival when there is cardiac failurewith pulmonary oedema, ischaemic tissue (such as after aninfarction), or a cerebrovascular accident. Oxygen saturationshould be monitored by pulse oximetry. If oxygen saturation islow additional inspired oxygen should be used to maintainsaturation near 100%.

Oxygen can be used empirically in acute situations if apatient is cyanosed or has a raised respiratory rate. Generally itis safe to use a high inspired oxygen concentration as long as

there is no suggestion that the patient has chronic obstructivepulmonary disease. The main danger of acute oxygen is thatcarbon dioxide retention may occur in some patients withchronic obstructive pulmonary disease. If this is suspected theoxygen should be given by a high flow 24% or 28% mask untilarterial blood gases can be measured. Oxygen increases the firehazard at the scene of an accident.

In emergency situations lack of availability of oxygen and anappropriate delivery system may prevent early treatment.Ambulances carry oxygen and paramedical staff are trained touse the equipment in acute situations. Some generalpractitioners also carry small cylinders that can deliver 2 l/minfor two hours, which avoids the need to wait for an ambulance.

Occasional oxygen in chronic lung disease

Breathlessness in chronic lung conditions or in carcinoma ofthe bronchus may be intermittent. It is likely to increase onexertion, and simple everyday tasks such as washing, dressing,or eating may cause problems. Some patients feel that a shortperiod on oxygen before or just after such exertions reducestheir breathlessness. It is more appropriate, however, to use theoxygen while the oxygen demand is increased. For most ofthese tasks, especially eating, nasal cannulae are a moreconvenient form of delivery than face masks.

Use of intermittent oxygen can be made more rational bymeasuring baseline saturation or arterial oxygen levels andassessing the effect of supplementation on the measurementsand on the ability to perform the task. A purer approach is tocompare the effects of supplemental oxygen and air in adouble-blind fashion using patients as their own control.

Relief of exercise related breathlessnessThe addition of oxygen during exercise reduces ventilation at agiven workload in normal subjects and in those with chronicrespiratory disease. This is achieved by reducing the respiratoryrate, which relieves breathlessness and increases exercisetolerance. Studies on exercise in chronic obstructive pulmonarydisease show an increase in endurance of 30-50% with oxygen.Any extra exertion involved in carrying equipment such ascylinders will reduce the benefit. Lightweight liquid oxygensystems are the most useful; heavier cylinders can be wheeledon a trolley.

In severely ill patients occasional use ofoxygen can be based on patients reportedneed and followed up with blood gas

analysis to ensure there is no seriouscarbon dioxide retention

Use of oxygen can increase the ability to do everydaytasks in chronic lung disease, but it is often limited

by availability of suitable portable systems

Paramedics giving oxygen after a skiing accident

Small oxygen cylinders

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Most suitable patients have a Pao2 at rest < 8.5 kPa or adrop in saturation to less than 90% on exercise. The benefit canbe assessed in a walking test. This can be done as a maximumdistance in a given time (conventionally 6 or 12 minutes), amaximum walk before stopping, or a timed shuttle test.Performance improves with familiarity over the first two orthree such tests, and a good positive effect would be the abilityto reach an important everyday goal or a 50% increase inexercise. Providing oxygen during exercise is difficult and

should be assessed carefully. Usually this assessment is carriedout in a respiratory function laboratory.

Oxygen used in this way improves quality of life and theability to perform everyday tasks rather than increasing survival,which needs prolonged use of oxygen every day.

Dangers of carbon dioxide retentionIn chronic lung disease it is important to consider thepossibility of carbon dioxide retention when breathing anincreased fraction of oxygen. One reason for a rise in carbondioxide is that hypercapnic respiratory drive has been bluntedby chronic carbon dioxide retention. Hypoxic drive becomesmore important so that an increase in inspired oxygen

improves hypoxia, reducing ventilation and increasing carbondioxide pressure. In addition, increased inspired oxygen mayovercome local pulmonary vasoconstriction in the lungincreasing wasted perfusion of poorly ventilated lung withgreater mismatch of ventilation and perfusion. When chronicoxygen use is proposed the patients arterial blood gases mustbe measured on and off the planned oxygen regimen.

Long term oxygen therapyChronic hypoxia produces a mixture of permanent andreversible structural changes in the pulmonary vasculature.There is hypertrophy of the muscular media in smallpulmonary arteries, muscularisation of pulmonary arterioles,and fibrosis of the intimal layer. In stable disease initiation ofoxygen therapy causes some pulmonary artery vasodilatation inareas of the lung with poor ventilation; this causes a smallincrease in the ratio of dead space to tidal volume. Minuteventilation at rest is unchanged but is reduced on exercise.

Reversal of the profound hypoxia of severe chronicobstructive pulmonary disease has been found to reducemortality if the oxygen is used for a minimum of 15 hours eachday. Correction of hypoxia may have other benefits such asreducing polycythaemia and reducing or preventingprogression of pulmonary hypertension together with anincrease in exercise tolerance.

What the trials showTwo thirds of the patients on long term oxygen therapy in the

United Kingdom have chronic obstructive pulmonary disease.Most of the evidence for this treatment comes from two majortrials published at the start of the 1980s. In the MedicalResearch Council study published in 1981 there were 87patients with a mean age of 58 years and mean FEV 1 of 0.7.They had modest pulmonary hypertension with a meanpressure of 34 mm Hg, Pao2 of 6.8 kPa, and Paco2 of 7.3 kPa.The active group took oxygen 15 hours daily compared withnone in the control group. Thirty of the 45 in the control groupdied compared with 19 out of 42 in the oxygen group.

The predicted five year survival in the control group wasonly 18% and the average annual percentage mortality risk was30% reflecting the poor prognosis of this condition. The

In chronic lung disease problems withcarbon dioxide retention are unlikely ifthe resting carbon dioxide pressure is

normal

UK criteria for long term oxygen

Pao2 6 kPaFEV1

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pulmonary artery pressure was unchanged in the group onoxygen but rose by a mean of 2.7 mm Hg/year in the controlgroup.

The nocturnal oxygen therapy trial conducted in NorthAmerica recruited 208 patients with a mean age of 66 years,FEV1 of 0.7, and pulmonary artery pressure of 30 mm Hg. Thestudy compared 12 hours overnight oxygen with continuousoxygen (in practice 17.7 hours a day). The mean arterial gaspressures at entry were Pao2 6.8 kPa and Paco2 5.8 kPa. Annual

mortality was 21% in the overnight oxygen group and 11% inthe continuous oxygen group. At six months the pulmonaryartery pressure at rest in the continuous oxygen group hadfallen slightly.

Implications of the studiesThe two trials were not blinded by comparison with aircylinders but are accepted as adequate evidence that oxygenimproves prognosis in chronic obstructive pulmonary disease. Ifoxygen is used for at least 15 hours daily the pulmonary arterypressure may fall and, at least, the expected rise seems to beprevented. The combination of the results of the two trialssuggests that the nearer the use of oxygen is to 24 hours dailythe better the outlook. In addition to the improved prognosisthere are mild neuropsychological benefits and improvementsin quality of life.

The mechanism of improvement in prognosis on oxygenremains unclear. Changes in pulmonary artery pressure are notgreat, and improved survival might be related to reduction inarrhythmias related to hypoxia. The effects of long term oxygenin chronic obstructive pulmonary disease have been extendedto the treatment of other respiratory conditions. This isprobably reasonable, although no real evidence exists.

When hypercapnia is prominent it may be appropriate toconsider a combination of oxygen with non-invasive ventilatorysupport using a face mask and supplementary pressure duringinspiration. Such treatment has proved useful in restrictive lungconditions, particularly those involving the chest wall andrespiratory muscles.

Non-invasive ventilation has been used successfully in acuteexacerbations of chronic obstructive pulmonary diseaseassociated with hypercapnia and acidosis and also in long termhome management. However, its precise role in long termmanagement of chronic obstructive pulmonary disease has notbeen established.

SleepThe slight decrease in ventilation during sleep is associated witha slight drop in Pao2 and rise in Paco2. When the Pao2 is initiallylow disturbances in ventilation, particularly those seen in rapideye movement sleep, result in further periods of desaturation.These dips are typically scattered through the night at intervalsof about two hours and are associated with transitory rises inpulmonary artery pressure. In patients on long term oxygen

therapy the flow at night can be increased by 1 l/min tominimise these dips.

Oxygen has been shown to improve sleep quality and reduceelectrocardiographic abnormalities, and in some countriesnocturnal hypoxia alone is accepted as an indication for longterm oxygen. Nocturnal hypoxia is found in a quarter of patientswith chronic obstructive pulmonary disease with daytime Pao2above 8 kPa, who would not meet most standard criteria for longterm oxygen. It is associated with sustained higher pulmonaryartery pressure and shorter survival. Nocturnal oxygen reducesthe pulmonary artery pressure changes, but the use of overnightoxygen is not yet supported by the quality of survival evidenceestablished for daytime hypoxia.

Time (months)

Cumulative%

survival

604020 50301000

20

30

40

50

60

70

80

90

100O2 24 h/day

O2 15 h/day

O2 12 h/day

Controls (no added O2)

10

Survival in studies of long term oxygen in chronic obstructive pulmonarydisease. The MRC study compared 15 hours with no oxygen and the NorthAmerican oxygen therapy trial compared 12 hours with attemptedcontinuous oxygen

Long term non-invasive ventilatory support can be provided at home with aclose fitting nasal or full face mask

Oxygen saturation drops during sleep, and the dipsare most prominent during rapid eye movement sleepwhen the diaphragmatic contribution to ventilation isreduced. Cycles of rapid eye movement sleep usuallyoccur three to four times a night

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Patients with chronic obstructive pulmonary disease, whoare predominantly male middle aged smokers and oftenoverweight, may also have obstructive sleep apnoea. The lowbaseline saturation will mean greater drops in saturation withapnoea. Patients with both conditions are at particular risk offluid retention and cor pulmonale. Patients with chronicobstructive pulmonary disease who have cor pulmonale despitea daytime Pao2 above 8 kPa should have oxygen saturationmonitored continuously for a night to look for a sleep apnoea

pattern or prolonged desaturation. This can be done at homewith a suitable pulse oximeter.

F Dudley is a general practitioner, Hurley Clinic, London

The ABC of Oxygen is edited by Richard M Leach, consultantphysician, and John Rees, consultant physician, Guys and St ThomassHospital Trust, London.

Key references

Medical Research Council Working Party. Long term domiciliaryoxygen therapy in chronic hypoxic cor pulmonale complicatingchronic bronchitis and emphysema. Lancet1981;i:681-6.

Nocturnal Oxygen Therapy Trial Group. Continuous or nocturnaloxygen therapy in hypoxic chronic obstructive lung disease.Ann Inter n Med 1980;93:391-8.

Mr Wardles fat boy, Joe, in Charles Dickenss PickwickPapershad daytime sleepiness, snoring, andobesitytypical of sleep apnoea

BMJ1998;317:871-4

Life long fr ustrationTea for two and two for tea

Over 20 years ago, when I was reading Shakespeares Romeo andJulietin school, I did not realise that Juliets lament, Whats in aname? that which we call a rose. By any other name would smellas sweet, would also conjure up such bittersweet experiences forme.

I was green and timid when I entered Aberdeen UniversityMedical School straight from cosmopolitan Malaysia, petrifiedthat I would have difficulty following the lectures andunderstanding the patients. Scottish names were new to melikean entirely different language. And as I stumbled over thepronunciation of the names of fellow classmates, they equally haddifficulty over my Chinese name.

Im Lee-Suan Teh, I volunteered again, without much hopethat it would be remembered. Yes, Suan, pronounced like swanand Teh, like the River Tay, I suggested helpfully but hopelessly.

Miss Lee, would you like to comment on . . . . The lecturerlooked at me encouragingly. While I was thinking hard to saysomething relevant at the anatomy, physiology, or biochemistrytutorial, I was also plucking up courage to tell the lecturer firmlyand politely that My surname is not Lee but Teh, spelt TEH butpronounced like the River Tay. Oh sorry, they always answeredknowledgeably, I thought all Chinese surnames came first, likeMao Tze-Tung, chuckling as they said it. Yes, but Ive swapped it

round for all your benefit, I said, looking round for support frommy Scottish classmates. Okay then, Lee Teh, would you like tocomment on . . ., they suggested again. Lee-Suan is my firstname, not Lee. Chinese first names are usually double barrelled,like Tze-Tung of Mao Tze-Tung fame, I replied. And so on.

Dr Suan?No, Dr Teh.Lee-Teh Suan?No, Lee-Suan Teh.

And so on and so for th as my job took me from Aberdeen toPontypridd, Cardiff, Manchester, and now Blackburn.

And it was worse over the telephone. Teh, I said. Silence at theother side or Hay? Pay? Say? Teh, spelt T for Timbuktu, E for

Elizabeth, and H for Harry, I suggested seriously before I learntof the radio codeswhich invariably brought fits of giggles fromthe other side.

I have now been in Blackburn for about two years and just as Ithink that I have overcome this problem, I have once again been

defeated, this time by modern technology. When I startedreceiving letters addressed to Dr The, even after I had diligentlyspelt my name out, Teh, T for Tango, E for Echo, and H forHotel, I realised that things were seriously amiss. At a recentrheumatology meeting the organisers insisted that I had not

booked for it. I have definitely booked for this meeting, I insistedas I stood my ground. After much searching through name

badges, it was found under L as the name badge read DrLee-Suan TheBlackburn Royal Infirmary. To my dismay, Idiscovered that most word processing programmes automaticallycorrect all teh to the as it is the most commonly misspelt word.

My surname is only three letters and it is misspelt so manyways, I lamented one day to a regular patient after theconsultation, needing to get it off my chest. And now computerseverywhere are automatically misspelling it, I continue, gettingmyself even more depressed at the thought of it.

Dont worry, Dr Tea, just have your cup of tea before it gets

cold, she said helpfully, as I sink into depths of despair thinking,Teh, not Tea.

Lee-Suan Teh, consultant rheumatologist, Blackburn

We welcome ar ticles up to 600 words on topics such asA memorable patient, A paper that changed my practice, My mostunfortunate mistake, or any other piece conveying instruction,pathos, or humour. If possible the article should be supplied on adisk. Permission is needed from the patient or a relative if anidentifiable patient is referred to. We also welcome contributionsfor Endpieces, consisting of quotations of up to 80 words (butmost are considerably shorter) from any source, ancient ormodern, which have appealed to the reader.

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