Dr Thomas Crowhurst

Pulmonary function testing

Pulmonary function testing is an essential component of the assessment of most respiratory disorders and can be required on a repeated basis as part of monitoring for response to treatment or disease progression.

Pulmonary function testing is usually organised by a respiratory physician but may also be requested by general practitioners, anaesthetists, surgeons, occupational physicians or indeed any other doctor.

There are many different tests that can be undertaken to assess lung health, with some of the most common explained below.

Spirometry is the most commonly requested pulmonary function test and fundamentally assesses the airways, being particularly important for the evaluation of asthma and chronic obstructive pulmonary disease. It involves measuring airflow at the mouth from full inspiration (a full breath all the way in) to full expiration (completely breathing out). The technique requires a forceful maximal effort from the patient. Spirometry is often performed before and after inhalation of salbutamol (Ventolin / Asmol). Repeated measurements are taken to ensure the validity and accuracy of the results.

Gas transfer (also called ‘diffusing capacity’) seeks to assess how efficiently the lungs can transfer inhaled gas into the bloodstream and is thus a surrogate for their capacity to absorb oxygen. This is a very important test to assess for damage of the lung tissue itself, as can occur in emphysema and interstitial lung disease among many other disorders.

Body plethysmography involves the patient sitting in a special glass chamber and breathing through a spirometer, enabling measurement of lung volumes. This allows determination of total lung capacity (the volume of gas within the lungs) as well as subdivisions of lung volumes, which can be particularly relevant in obstructive lung disease where gas trapping within the lungs can be problematic.

Some respiratory disorders can involve weakness of the diaphragm or other muscles of respiration. The strength of these muscles can be ascertained by measuring maximal inspiratory and expiratory pressure, which involves forceful inspiration and expiration during which a flap briefly blocks the testing tube. This is a specialised test which is only required in specific circumstances.

Two of the most important functions of the lungs are to (a) absorb oxygen for use by the body and (b) excrete carbon dioxide, which is a waste product of normal metabolism. Although oxygen can be roughly measured with a non-invasive oximeter (finger probe), this is not perfectly accurate, and provides no information about the carbon dioxide level. In order to understand your lung condition more completely, it may be necessary for an arterial blood gas (ABG) sample to be taken. This involves placing a small needle in the radial artery in the wrist. An ABG is often required to determine eligibility for home oxygen therapy, or if there is concern about carbon dioxide retention.

One of the best ways to quantify the true impact of lung disease on a patient is to determine his / her ‘functional capacity’, which is how much physical activity he / she can do. This is best achieved via a six minute walk test which, as the name suggests, involves walking for six minutes in a supervised environment while oxygen level and heart rate are monitored. Rests are allowed. Symptom severity is recorded before and after. This test may be needed to determine eligibility for exertional home oxygen therapy and is a helpful objective baseline against which future measurements can be compared.

Although the cabins of commercial airliners are pressurised, passengers are still subjected to the equivalent altitude of around 8000 feet. This can be a problem for patients with significant underlying lung disease. If you have moderate or severe lung disease and you are considering flying, it is prudent first to discuss this with a respiratory physician to determine if further testing is warranted beforehand. Testing involves an arterial blood gas sample performed while breathing normal room air and then another taken after at least 20 minutes of breathing a ‘hypoxic gas mixture’ through a facemask; the hypoxic gas mixture is a combination of oxygen and nitrogen combined to give a fraction of inspired oxygen of 15.1% compared with the usual 21% in normal air, which simulates the effect of breathing at 8000 feet of altitude.

Dr Crowhurst will explain why certain pulmonary function tests will be helpful in your particular situation. He will show you the results of your pulmonary function tests, explain these to you and outline the significance of the findings in your individual circumstances. He will note whether any repeat testing may be helpful. Copies of the results will be provided to you and, with your consent, to any other doctors involved in your care.

Dr Crowhurst reports pulmonary function testing at Respiratory SA in Kent Town, Northern Respiratory in Elizabeth, and at the Lyell McEwin Hospital and Modbury Hospital.