Many people are referred for lung function tests for a wide variety of reasons. These could include investigations into shortness of breath, assessment of inhaler therapy, monitoring of the progression of conditions such as COPD, or one of many other reasons. A brief outline of the common pulmonary function tests follows, which will give some basic information on the tests performed and what each test measures.

It is important to follow any instructions you may have received with your appointment letter regarding preparation for the test, particularly concerning the use of your inhalers or other medications before you are tested. If you do not follow these instructions, it may not be possible to complete your tests, and this may mean a second visit at a later date.

Spirometry

Spirometry is normally the first lung function test performed in the laboratory. Spirometry measures how much and how quickly you can move air in and out of your lungs and is therefore useful in assessing airway function. The patient is attached to a recording devices called a spirometer via a mouthpiece and is instructed to perform a couple of normal breaths before taking a full breath in and blowing out as hard and as fast as possible for as long as possible.

From this test the following lung function values are obtained:

• Forced Vital Capacity (FVC) - the amount of air you can forcibly breathe out after a full breath in.
• Force Expiratory Volume in 1 Second (FEV₁) - the amount of air you can forcibly breathe out in one second.
• Peak Expiratory Flow (PEF) - measures the highest flow rate reached during a forced breath out.
• FEV₁/FVC Ratio - the percentage of the FVC exhaled in one second.

All lung function results are expressed as a percentage of the predicted values. Predicted values are calculated from population studies to give expected values for a person of similar height, age, sex and race.

A low FEV1 with a reduced FEV1/FVC ratio may indicate an obstructive lung disorder.

A low FVC and FEV1 but with a normal FEV1/FVC ratio may indicate a restrictive lung disorder

Transfer Factor/Diffusion Capacity (DLCO)

The transfer factor measures how effectively oxygen crosses from the lung to the blood and is therefore useful in investigations of the function of the lung tissues.

At the end of a full breath out, a valve is opened and the patient is instructed to perform a full inspiration of a special gas that contains carbon monoxide in such small amounts that it is not harmful. Carbon Monoxide is used as it has very similar properties to oxygen. The patient holds the breath for a a few  seconds and then breathes out again fully. The gas analysers compare the concentrations of exhaled gas to the known inhaled gas and calculate the level of diffusion that has occurred.

Lung volumes

Even after the patient performs a full breath out, there is still a volume of air left in the lungs to keep them inflated. This volume is called the Residual Volume (RV). Calculating the RV allows us to measure the Total Lung Capacity (TLC). It is important to measure the RV and TLC in certain circumstances.

In obstructive lung diseases, the RV can be much higher than normal. This is known as air trapping. In restrictive lung diseases the TLC is lower than normal. The RV and TLC cannot be measured directly by spirometry but is measured indirectly by three different methods.

• Body plethysmography - the patient is seated inside an airtight box and performs normal breaths. At the end of a normal breath, a shutter is closed and the patient is instructed to perform panting maneuvers against the closed shutter. The pressures changes that occur within the box are used to calculate the RV indirectly.
• Nitrogen Washout Method - the patient breathes normally until a stable breathing pattern is observed. At the end of a normal breath, a valve is opened and the patient begins breathing 100% oxygen. The oxygen replaces the nitrogen in the lungs until the nitrogen concentration reaches 1.5%. The volume of oxygen used and the time the test takes is used to calculate the RV and then the TLC.
• Helium Dilution Method -At the end of a normal breath, a valve is opened and the patient begins breathing 9-14% Helium until the concentration of helium in the lungs is in equilibrium with the helium in the spirometer. The concentration of helium at equilibrium and the change in helium concentration are used to calculate the RV and TLC.

Provocation Tests

Exercise Provocation Test

Exercise Provocation test is used in the diagnosis of exercise-induced asthma. To begin with the patient performs a base-line spirometry. The patient then exercises on a treadmill at 80% of their predicted maximum heart rate for 6-8 minutes at a speed of 3-5 mph and incline 10-12%. The patient performs spirometry at 1, 3, 5, 10, 15, 30 minutes post exercise. A drop in the patient’s FEV₁ of greater than 12% is considered a positive test. Once the test is finished a bronchodilator is given to the patient to help their FEV₁ return to the base-line value.

Airway Challenge Testing

Airway Challenge Testing assesses the sensitivity of the airways. The test may be done using mannitol, histamine or methacholine. Either one of these substances are administered to the patient and may induce bronchospasm in sensitive airways.

To begin the test, the patient performs a base-line spirometry. Then the first (lowest) dose is administered to the patient and spirometry is repeated immediately. The above steps are repeated with a series of higher concentrations. Depending on the test used, a drop in the FEV₁ of 15-20% is considered significant. The lower the concentration at which the drop occurs, the greater the sensitivity of the airways. A short-acting bronchodilator is administered at the end of the test to bring the FEV₁ back to the base-line value.

Exercise Testing

Cardiopulmonary Exercise Test (CPET)

Patients experiencing unexplained shortness of breath on exertion may be referred for a cardiopulmonary exercise test. CPETs are normally performed on a stationary bike or treadmill. The patient is connected to a mouthpiece and flow sensor to record ventilation during exercise while a 12-lead ECG, blood pressure cuff and oxygen saturation monitor cardiovascular responses. The test aims to exercise the patient to their maximum within 8-12 minutes. By monitoring the responses of the patients heart, lungs and muscles to exercise, we can determine whether there is a cardiovascular or respiratory limitation or both to exercise.

Six-minute walk test

The six-minute walk test is commonly used in pulmonary rehabilitation. The aim of the test is for the patient to walk for as far as possible in six minutes between two cones positioned about 30 metres apart. The distance traveled is recorded along with the patient’s perceived exertion using the Borg Scale. The patient’s oxygen saturation is continuously recorded and the test may be performed with oxygen to assess the patient’s oxygen treatment.

Incremental shuttle walk test (ISWT)

The ISWT is a field test that simulates a cardiopulmonary exercise test. The patient walks between two cones to a set of bleeps played on a tape. The pace is initially slow but speeds up every minute as the bleeps get closer together. The test ends when the patient is unable to reach the cones before the bleeps. The number of shuttles walk is recorded and this can be used to prescribe the intensity of exercise.The test must be performed at least twice as there is a learning effect.

Allergy Testing

Skin Allergy Testing

A skin allergy test tests the sensitivity of a patient to common allergens such as grass pollen and dust mite. During the test, a drop of each allergen solution is placed on the skin on the underside of the forearm or on the back in very young patients. A single use sterile lancet is used to pierce the skin underneath the solution. After 15-20 minutes the skin is checked for any reactions. The allergen may cause a reddening of the skin and produce a blister-like bubble on the skin surface called a weal. A positive reaction is taken as a weal diameter of greater than 3mm.

Sleep Studies

Patients with sleep breathing disorders such as obstructive sleep apnoea may be referred to the laboratory for sleep studies.  Sleep studies can range from overnight oximetry, measuring the oxygen saturation continuously during the night, to polysomnography.

Polysomnography

Polysomnography (PSG) is a comprehensive test that measures many physiological parameters during sleep such as brain electrical activity, eye and chest muscle movements, airflow at the nose and mouth, chest and abdominal movements, periodic leg movements, heart rate and oxygen saturation.

Patients that are diagnosed with obstructive sleep apnoea may be treated with a continuous positive airway pressure (CPAP) machine. Respiratory scientists may help in the selection and fitting of CPAP masks and also measure the response to therapy using PSG with CPAP.