Investigation of respiratory physiology
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Monitoring of lung function has been used for many years to establish baseline respiratory function, measure progress and treatment efficacy, and to assess thresholds for initiation of treatment options e.g. lung transplant referral. Traditionally, forced expiratory manoeuvres have been the measure of choice. More recently, other measures of air trapping e.g. Lung Clearance Index have been studied. These measures are not currently available in New Zealand, and their precise role in guiding treatment decisions is not established. To be useful or acceptable, lung function testing must be performed and interpreted according to correct technical standards such as the American Thoracic Society Standards.
Spirometry is the most commonly performed formal lung function testing and is useful for identifying onset or progression of airway obstruction or the progression of restrictive lung disease. A range of parameters are reported; however the most useful are the forced expiratory volume in one second (FEV1) and the forced vital capacity (FVC). The ratio of the FEV1/FVC is useful for identifying the presence of obstructive lung disease.
The usefulness of spirometry is limited by the technical performance of the test and by natural variability in lung function testing results:
- The person performing the procedure must be adequately trained to perform the test correctly and must have the skills to work well with children.
- The patient's age, gender and height must be entered correctly
- An appropriate reference range must be selected for the generation of predicted values. The use of reference ranges must be consistent within and between patients. Appropriate references for ethnicity should be used if ethnicity may be a significant factor.
- The spirometer must be set up correctly including filters and calibration
- The test must be performed correctly with an initial forceful effort and an adequate duration of exhalation
- The test must be repeated with limited variability to ensure the result is consistent and correct
- Hygiene and safety standards must be adhered to
There are several reference equations which relate a specific absolute value for FEV1 or FVC to a normal population range. Current best practice is to use the ERS Global Lung Function Initiative (GLI) reference equations or a similar range such as NHANES III.
Representation of results
Traditionally, spirometry results are expressed as the percent predicted of the mean value for the reference range being used. For adults and older children the normal range is 80-120% of the mean value. However as this percentage range is not constant across age ranges, it is preferable to represent results as a z-score which is a measure of the variability of results at each age range. The lower limit of normal for lung function is set at the 5th centile which is a z-score of -1.645. Results below this are reported as abnormal.
Changes in lung function are generally reported as changes in the percent predicted value. This is reported as a relative rather than an absolute or percentage point change i.e. a change in FEV1 from 50 to 55 percent predicted would be reported as a 10% change. FEV1 is the most repeatable and reliable measure and is the most commonly reported outcome for clinical and research studies.
It is important to recognize that lung function test results are reported in relation to the average (or 'predicted') values for someone of that age, gender and height. Just as we are all a variable build or shoe size compared to average, so our lung function is also a bit variable compared to average. A result of 95 % predicted means that the child's lung function is 95% of average and does not mean that the child has lost 5% of lung function. However, individuals do tend to maintain or track their lung function relative to average so if a child's lung function changes from 110% to 95% then this may represent an important change. Likewise a one off result of 85% may be normal for that patient.
Can we assess significant change from morbidity or treatment by Z-score?
Our ability to detect changes in lung function is limited by the natural variability of the test. At the time of testing the child will perform at least 3 tests and the variability between those three results needs to be less than 5%. Therefore, any change of less than 5% is regarded as not significant. Within any one year a patient's results may vary by up to 10%. Therefore, any change of 10% or more is reported as significant as this is outside normal variability. However, it is important to remember that a change smaller than this may actually represent a clinically significant change in the child's health.
There are a number of other tests of lung function that can be performed. These give us more information about the child's lungs but they may be time-consuming or inconvenient to perform so may be done less often.
Measurement of the child's lung volumes can give useful information about the development of any restrictive lung disease and about gas trapping (increased residual volume) and airway resistance. Performing lung volume tests requires access to a plethysmography cabinet (or 'body box') which may not be available in all centres. The test is harder to perform and children need to be at least 7 years old before performing this test. Lung volumes are often measured as part of the annual review but generally not more often than this.
Sometimes it can be helpful to test if a child is responsive to bronchodilators such as salbutamol. This will help determine if extra inhalers should be prescribed. The test is conducted by performing initial spirometry and then giving the child 6 puffs of salbutamol via a spacer. The spirometry is then repeated to see if the child has a significant response. The test is a bit variable and children with cystic fibrosis may have different results when they are well compared to when they are sick. Some patients don't appear to respond when tested this way but still report that they definitely have less wheeze and shortness of breath after bronchodilator treatment. Bronchodilator testing does not diagnose asthma and does not predict whether a patient will respond to an inhaled corticosteroid such as a flixotide.
Hypertonic Saline Challenge and Inhaled Antibiotic Challenges
Some inhaled medication such as hypertonic saline and inhaled antibiotics can make children with cystic fibrosis wheezy and short of breath. If the child is old enough they should be tested to see if they have a significant drop in their lung function after inhaling these medicines before they start using them regularly at home. The test involves conducting spirometry, nebulising the antibiotic or hypertonic saline (for hypertonic saline the patient is treated with salbutamol first). A doctor should be present in case the patient develops severe wheeze. Spirometry is repeated 10 minutes and 30 minutes after the nebulisation. If the FEV1 drops by 15% or more then the medication should not be administered in the future.
Diffusing capacity of lung for carbon monoxide (DLco), also called "Transfer factor"
Transfer factor or DLco measurement may be helpful in measuring how well gases such as oxygen are transferred from the lungs into the blood stream. This may be helpful in measuring the effects of lung fibrosis on lung disease. However, the results are affected by a range of factors such as airway obstruction, air trapping, anaemia, pulmonary hypertension and pulmonary haemorrhage so are usually not helpful in children with cystic fibrosis (CF).
Cardio-pulmonary Exercise Testing
Cardiopulmonary exercise testing (CPET) is useful to measure a patient's peak oxygen uptake, performance and overall exercise capacity. It can be helpful in determining if a patient's ability to exercise is limited by their overall fitness, by lung disease or by their cardiac output. The test requires the patient to exercise at their maximal level which can be uncomfortable for the patient. The test is performed in an exercise laboratory on a treadmill or exercise bike. The test requires special equipment and specialist interpretation and is not widely available. Other than as a means to monitor maintenance or decline of activity there is no established role for assessing ṼO2max to guide care of children with cystic fibrosis.
Sub-Maximal Exercise Testing
Because maximal exercise testing is uncomfortable for the patient and requires special infrastructure, several sub-maximal tests have been developed to measure patient's progress and fitness. Examples are the six minute walk test (6MWT) and various step tests (performed using an aerobic stepping platform). These are often conducted by physiotherapists rather than in a respiratory physiology laboratory. The distance the patient walks (or the number of steps they can take) is recorded and also their oxygen saturation, pulse rate and level of breathlessness (Borg score). In order to be valid the tests must be conducted under strict conditions or the results will not be accurate. The results of these tests may not directly contribute to management, however exercise and fitness are increasingly being recognised as important aspects of health in CF and so the use of these tests may increase in the future.
Children with CF may have lower oxygen levels in sleep than healthy children. This may be a marker of more severe disease or an indication for extra treatment such as home oxygen or other respiratory support. The test may be conducted at home or in the hospital and consists of the child wearing a simple probe on the finger or toe while oxygen levels are recorded. The result is then printed out and reviewed by a doctor. Care must be taken that the settings on the oximeter such as averaging time are appropriate and that alarm limits have been set appropriately as it is very disturbing and frustrating for caregivers to have the oximeter alarming through the night. Oximetry is generally recognised as an approximation of the patient's true level of oxygen in the blood, the PaO2 which is measured by arterial blood gas measurement. Oximetry has the advantage of providing a result without disturbing the patient's sleep and provides a recording of the trend throughout the whole night which may be more useful than a single measurement.
Blood Gas Measurement by Blood Test
While oximetry is useful to measure oxygen levels, carbon dioxide (CO2) levels are a better measure of ventilation and may be important to measure. This is generally measured in children by a capillary blood gas or "finger prick". This needs to be immediately analysed in the laboratory and so this test is only performed in hospital. CO2 levels will not be affected until the patient has decreased oxygen levels so is only necessary once oximetry becomes abnormal.
Arterial blood gases are commonly measured in adults as a more accurate measure of blood oxygen levels that oximetry. However, arterial blood gas measurement is painful and very difficult in children so is rarely if ever performed.
Polysomnography or "Sleep Study"
Polysomnography (PSG) sleep studies measure many aspects of a child's breathing in sleep. This may be useful when considering extra respiratory support such as non-invasive ventilation. The test may also be used if a child has other normal diseases of childhood such as obstructive sleep apnoea (OSA) due to large adenoids or tonsils. PSG involves attaching leads or stickers to the scalp to measure sleep, stickers on the chest to measure the heart, cannulae in the nose to measure airflow, belts around the stomach and chest to measure respiratory effort, a lead to measure CO2 and an oximeter to measure oxygen levels. While PSG is not painful it is time consuming and uncomfortable. It may be conducted in the home but is usually conducted in the hospital or a sleep laboratory. It this therefore only rarely performed in children with CF.
- Mayer O, Allen JL. Lung function testing in school age children with cystic fibrosis. In: Allen JL, Panitch HB, Rubenstein R (Eds). Cystic fibrosis. New York NY, Informa Healthcare USA. 2010.
- Quanjer PH et al. Eur Respir J 2012;40:1324
- NHANES: National Health and Nutritional Survey. Centre for Disease Control and Prevention, Atlanta, GA.
Document last reviewed: April 2017
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