Practice

Oxygen and inhalers

Respiratory Unit, Aberdeen Royal Infirmary, Aberdeen.

Oxygen

Administering oxygen for chronic obstructive pulmonary disease (COPD) is not without risk and it should be properly prescribed?in terms of flow rate and mode of delivery?like any other drug. Giving high concentrations of oxygen to hypoxaemic patients with hypercapnia can result in individuals losing their hypoxic drive to breathe, with development of CO₂ retention, respiratory acidosis, and even death.

The oxyhaemoglobin dissociation curve showing the relation between partial pressure of oxygen and haemoglobin saturation

However, in acute and chronic ventilatory failure, oxygen supplementation is essential to maintain adequate delivery of oxyhaemoglobin to organs such as the heart, kidneys, and brain. Many patients who are chronically hypoxic are able to cope satisfactorily with an oxygen saturation of arterial blood of around 90%. However, at saturations below this, the oxygen dissociation curve rapidly steepens, and a sharp fall in oxygenated haemoglobin occurs with reduction in oxygen supply to vital organs.

Long term oxygen therapy prolongs survival in hypoxaemic patients with COPD when used for 15 hours/day. (Results from the nocturnal oxygen therapy trial (NOTT) and the MRC trial)

Oxygen during an exacerbation of COPD
During an exacerbation of COPD, give 24% or 28% oxygen via a Venturi facemask to patients with hypercapnia in order to maintain an oxygen saturation > 90%. In patients without hypercapnia, titrate the oxygen concentration upwards to keep the saturation > 90%. Check arterial blood gases at 30-60 minutes later to check for any rise in CO₂. Nasal cannulas deliver less reliable fractions of inspired oxygen than a facemask but allow patients to communicate, eat, and drink more easily.

A concentrator supplies oxygen to patients without the need for gas cylinders

Long term oxygen therapy
Two large trials have shown that using oxygen for at least 15 hours a day improves survival of hypoxaemic patients with COPD. Consider long term oxygen therapy in non-smoking patients with COPD if

• Arterial oxygen partial pressure (PaO₂) is <7.3 kPa on two separate occasions at least three weeks apart during a period of clinical stability or
  
• PaO₂ is 7.3-8.0 kPa and there is evidence of secondary polycythaemia, pulmonary hypertension, peripheral oedema, or nocturnal hypoxaemia.
  

Survival benefits have not been observed in patients who are not sufficiently hypoxaemic or who do not use oxygen for the required length of time each day ( 15 hours). Before arranging long term oxygen therapy, ensure that patients have stopped smoking and are aware of the dangers of naked flames near oxygen supplies. Oxygen is most conveniently and economically supplied by a concentrator, which removes nitrogen and therefore produces oxygen-enriched air. Nasal cannulas are the most practical means of delivering oxygen, although some patients?especially those with troublesome drying of the nasal mucosa?may prefer a Venturi facemask.

Short burst oxygen
Despite maximal inhaled and oral drug treatment, many patients with advanced COPD remain breathless on exertion. Such patients are frequently prescribed oxygen cylinders for use "as required." However, studies of oxygen therapy after exercise have failed to show any consistent effect on breathlessness scores or rate of symptomatic recovery, but it has been shown to reduce the degree of dynamic hyperinflation during recovery from exercise. The potential role of "short burst" oxygen therapy in COPD thus remains controversial, but it should be considered in patients with episodes of severe breathlessness not relieved by other treatments.

A modern portable cylinder without an oxygen conserving device will last for up to four hours with a flow rate of 2 l/min and up to two hours with a flow rate of 4 l/min

Ambulatory oxygen
Patients already receiving long term oxygen therapy can be provided with portable oxygen for use during exercise and activities of daily living outside the home. Its usefulness is limited by the duration of oxygen supply from portable pulse-dose cylinders. In the future, small lightweight cylinders, oxygen conserving devices, and portable liquid oxygen systems may become available. Further studies are needed to establish whether patients who do not fulfil the criteria for provision of long term oxygen therapy but who show significant oxygen desaturation during exercise would also benefit from ambulatory oxygen.

Only a small fraction of drug from a metered dose inhaler reaches the lungs

Air travel and oxygen
The partial pressure of oxygen (PO₂) in the air falls with increasing altitude, which can compound any respiratory difficulties faced by a hypoxaemic patient with COPD. Commercial aircraft are pressurised to a cabin pressure of 8000 feet (2438 metres), at which the PO₂ is roughly equivalent to that of 15% oxygen at sea level. Thus, patients with COPD with adequate oxygen saturation at sea level may find oxygenation falls below desirable levels under aircraft cabin pressure. This desaturation is exacerbated by minimal exercise.

Patients with COPD who are planning to travel by air should have their oxygen saturation measured with a pulse oximeter before flights are booked. This helps determine whether in-flight oxygen will be required. All patients who require in-flight oxygen should inform the relevant airline when booking and should be aware that some airlines charge for this service. The need for oxygen on the ground and while changing flights must also be considered, and many airports can provide wheelchairs for transport to and from aircraft. Advise patients to carry both preventive and reliever inhalers in their hand luggage and that nebulisers may be used on aircraft at the discretion of the cabin crew.

Inhalers

Hand held inhalers are used by most patients to facilitate delivery of drugs to the endobronchial tree. Unfortunately, with all inhalers, a substantial proportion of the drug is deposited in the oropharynx. Some patients?such as those who are elderly, cognitively impaired, or with reduced manual dexterity?have difficulty in using inhaler devices.

Before starting inhaled therapy for COPD patients, instruct them on how to use the inhaler device correctly and, if necessary, switch them to an alternative, more suitable device. Moreover, assessment of inhaler technique should be carried out at every available opportunity, as many patients become less proficient over time. The over-riding factors that determine which type of inhaler should be used include patient preference, ease of use, effectiveness of drug delivery, and cost.

Metered dose inhalers
One of the most common inhaler devices is a pressurised metered dose inhaler. Patients notoriously have difficulty in using such inhalers correctly, which has led to the introduction of a variety of other types of inhaler. Correct use of a metered dose inhaler requires strict adherence to the instructions for use.

Metered dose inhaler plus spacer
A spacer device attached to a metered dose inhaler serves two main functions. Firstly, this avoids problems in coordinating the timing of inhaler actuation and inhalation. Secondly, the speed of delivery of the aerosol into the mouth is slowed, which minimises the "cold freon" effect and in turn results in less drug being deposited in the oropharynx and more in the endobronchial tree. If used correctly, a metered dose inhaler with spacer is at least as effective as any other device for delivering inhaled drugs.

A metered dose inhaler with spacer attached

Different manufacturers make different sizes of spacers and inhalers, but the following principles of use can applied to most types:

• Shake the inhaler and ensure that it fits snugly into the end of the spacer device
  
• Place the spacer mouthpiece in the mouth
  
• Start breathing in and out slowly and gently
  
• Press the inhaler and continue to breath in and out several more times
  
• Wait about 30 seconds before repeating the first four steps
  
• Wipe clean the mouthpiece after use
  
• Clean the spacer at least once a month with soapy water and leave to air dry
  
• Replace the spacer every 12 months or according to the manufacturer's recommendations.
  

Dry powder inhalers
Dry powder inhalers?such as Accuhalers, Turbohalers, Clickhalers, and Twisthalers?are breath activated. They therefore avoid the problem with metered dose inhalers of coordinating inhaler actuation and inhalation. They are also less bulky and more portable. Many patients prefer them to other types of inhaler, but they tend to be more expensive.

Examples of the various spacer devices produced by different manufacturers

Nebulisers
Nebulisers can be driven by oxygen or, for patients with hypercapnic respiratory failure, compressed air. They create a mist of drug particles that are inhaled via a facemask or mouthpiece.

Using a nebuliser is as effective as a metered dose inhaler plus spacer used correctly, and it requires less effort

Determining which COPD patients should be prescribed a nebuliser to deliver short acting ₂ agonists and other drugs is controversial. With the introduction of dry powder inhalers, many more patients are now able to use hand held devices correctly, which means that fewer patients need be considered for domiciliary nebulisers. However, it is reasonable to prescribe a nebuliser for patients who remain symptomatic despite maximal treatment with hand held inhalers, although objective evidence of benefit should generally be demonstrated. Patients using a nebuliser should receive adequate training, and be provided with appropriate servicing and support for their equipment from a designated individual.

Competing interests: GPC has received funding for attending international conferences and honorariums for giving talks from pharmaceutical companies GlaxoSmithKline, Pfizer, and AstraZeneca. JGD has received funding for attending international conferences from GlaxoSmithKline and Novartis; fees for speaking from Boehringer, AstraZeneca, Atlanta, and GlaxoSmithKline; and research funding from Boehringer and GlaxoSmithKline.

The graph of the effects of long term oxygen therapy on survival of hypoxaemic patients was adapted from Gorecka et al. Thorax 1997;52: 674-9[Abstract]