Lung Function Tests in Asthma in Children

Introduction

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Asthma is a common childhood condition which can be treated very effectively allowing normal lifestyles for affected children. As medication is expensive and may have side effects, it is important that the diagnosis should be correct, and that treatment is ideally tailored to the needs of each individual child. Lungfunction tests are useful aids to this process.

Lung function tests can be used to measure:

• mechanical characteristics of the lungs
• flow of air through the airways
• volume of air in the lungs
• adequacy of gas exchange between the body and the environment.

In asthma lung function tests are usually used to measure air now and lung volumes. These measurements, if done properly a reliable and objective means of assessing the diagnosis and treatment of asthma.

Lung function tests have been used for adults with asthma over many years, but there is an increasing realisation that these tests should be used routinely in children as well.

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Lung function tests are investigations used to measure lung functions. Many aspects of lung function at all ages can be assessed. Some of these test require sophisticated equipment and highly trained staff. There are however simple tests which are relevant to children with asthma.

The tests that are most commonly used in children include:

• Peak expiratory flow rate (see “A Guide to Peakflow in Childhood Asthma”)
  • The child takes as deep a breath as possible and then blows out as hard as possible
  • The equipment used measures the highest now of air that is achieved during the blowing out phase
  • This test is not very sensitive. However the equipment required is cheap and easily portable. It is thus the ideal way to measure lung functions at home and during normal daily ctivities
• Forced expiratory manoeuvre
  The child takes as deep a breath as possible and then blows out as hard as possible until the lungs feel completely empty. The equipment measures the flow of air throughout this manoeuvre. From this the following parameters are usually calculated: �
  • forced vital capacity (FVC) or the volume of air that can be blown out from the end of a deep breath in
  • forced expiratory volume in 1 second (FEV1) or the volume of air that can be blown out from the end of a deep breath in within 1 second
  • the ratio of FEV1/FVC
  • forced expiratory flow between 25 and 75% of expired volume (FEF25-75)� This is the average rate of air flow over the period when 25% to 75% of the FVC is being blown out.

  This test is more sensitive, however the equipment required is more expensive and not very portable. It is the ideal test for doctors consulting rooms, emergency departments at hospitals etc.

• Oxygen saturation
  • This is a reflection of the amount of oxygen which is being carried in the blood, and is measured using a small probe which is usually attached (painlessly strapped or clipped on) to a finger or toe
  • This test has been shown to be very usefull in the assessment of acute severe episode
  • Patients who have low oxygen saturations should be admitted for emergency care, while those with high oxygen saturations can usually be safely sent home with appropriate medication
  • The equipment is expensive and is thus again not practical for home use, but very useful in centres where emergency medical care is offered

These tests can be used on their own, but also in association with other interventions. Many other tests are available, but not often of use in the diagnosis and management of asthma.

Why do lung function tests?

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• Lung function tests are able to show abnormalities that patients may not be aware
• The lungs of all normal children are able to function under a wide variety of circumstances, and also have what is known as functional reserve (or capacity which is not usually used, but may be used when necessary)
• One consequence is that symptoms such as cough and breathlessness only become apparent when lung function is substantially below normal
• Lung function tests provide an objective way of diagnosing asthma, monitoring responses to treatment, and monitoring lung over long periods

• To diagnose asthma
  While the diagnosis asthma may be clear cut, there are occasions when it is difficult to be sure that symptoms are related to asthma and lung function tests are usefull to clarify the diagnosis. There are different ways in which lung function tests can be used to diagnose asthma by demonstrating:
  • obstruction to air now (forced expiratory manoeuvre)
  • excessive variation in lung function over the course of a day (Peak Flow manoeuvre)
  • response to a bronchodilator. The airway obstruction of asthma is usually reversible and improvement after administration of a bronchodilator may be diagnostic of asthma deterioration following exercise of administration of methacholine or histamine.

• To monitor treatment
  Once treatment has been initiated lung function tests can be used to demonstrate:
  • decreased airway obstruction
  • minimisation of lung function variability ‘
  • decreased sensitivity of the airways to challenges such as exercise or irritants such as methacholine histamine
  • deterioration in lung function before patients are aware of symptoms

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• International guidelines have been established to ensure that results obtained from lung function tests are reliable and reproducible
• Only equipment and methods which comply with these guideline should be used
• Extensive training is not required to perform these simple lung function tests, but it is found that reliable and reproducible results are only obtained when people who have some experience with testing children undertake the tests.

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• It is difficult to do lung function tests in children until they are 5 or 6 years of age, although younger children may be able to perform test adequately
• Tests should be done when:
  • at the time of diagnosis of the respiratory problems (when age allows)
  • there is an exacerbation of symptoms
  • patients visit their doctors to review treatment
  • whenever there is a review of treatment
• Tests such as peak now measurement can be done at home and during normal daily activities and may provide information about airway responses which is very difficult to obtain by any other means
• Following exacerbation’s of symptoms peak now and forced expiratory manoeuvres provide the information that is essential for appropriate adjustment of treatment
• Patients who are unstable should use lung function tests such as peak expiratory flow rates to provide early warning of any deterioration in asthma control.

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• The tests mentioned above are not dangerous
• Sometimes mild coughing and wheezing can be precipitated by repeated vigorous attempts at lung function tests. These can usually be cleared rapidly and completely by the administration of a bronchodilator (reliever)
• Allergen challenge tests used to be used and these may be dangerous as lung function may be adversely affected hours after the test. Allergen challenge tests should not be done routinely, and if done at all they should only be done under close supervision by people with specialised training.

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Copyright: Allergy Society of South Africa.
Written by Dr Andrew Argent
Endorsed by ALLSA.

ALLSA P.O. Box 88 Observatory, 7935 Cape Town, R.S.A.

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