Pulmonary Function Tests
Many people are referred by their GP or hospital consultant for lung function, also called
pulmonary function tests, for a wide variety of reasons they are commonly referred to as PFT’s.
These reasons could include:
- Investigations into cause of shortness of breath, cough, wheeze, frequent chest infections or other symptoms related to the lungs.
- Assessment of how patients are responding to their respiratory inhalers or tablets.
- Therefore it is common practice to have an initial assessment followed by another test in two or three months
- Monitoring of the progression of conditions such as COPD, asthma, lung fibrosis, to assess if the patient is fit for an upcoming surgery and other reasons that your doctor will explain to you.
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.
A brief outline of the common PFT’s follows, which will give some basic information on the tests performed and what each test measures.
Spirometry is normally the first lung function test performed in the laboratory, it 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 seals their lips around small plastic cylinder which is connected to a spirometer 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 followed by a sharp breath back in again. The test is repeated at least three times and a maximum of eight times.
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.
The Spirometry test is increasingly being provided at primary care level, for example at GP surgeries, as spirometry is an essential tool for assessing and monitoring asthma and COPD, the most common lung diseases. It would be worth checking if your GP provides this test as it may save you a long wait for the test and a trip to the hospital.
The remaining tests can only be carried out in a hospital based Pulmonary Laboratory.
Transfer Factor/Diffusion Capacity (DLCO)
The DLCO measures how effectively oxygen crosses from the lung to the bloodstream and is therefore useful in investigations into disorders related to the function of the lung tissues. Certain lung diseases affect gas exchange in your lungs. For example, gas transfer will be reduced emphysema and pulmonary fibrosis.
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 mixture. It does contain carbon monoxide but 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 maximum of 10 seconds and then breathes out again fully. The gas analyzers compare the concentrations of exhaled gas to the known inhaled gas and calculate the level of diffusion that has occurred.
This test will be performed a minimum of two times, up to a maximum of 5 times, and you will be given a minimum of 4 minutes rest between each test.
Even after the patient empties their lungs completely, 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 patients.
In obstructive lung diseases such as COPD, 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 in a Pulmonary function lab by three different methods.
- Body plethysmography -
- the patient is seated inside a large airtight Perspex (transparent) booth and performs normal breaths. At the end of a normal breath, a shutter is closed in the mouthpiece 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. If you feel claustrophobic in the booth, you can open the door from the inside and exit at any time.
- 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 -
- 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.
Exercise Provocation Test
Exercise Provocation test is used in the diagnosis of exercise- induced asthma. To begin with the patient performs a Spirometry test. 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 after stopping exercise. A drop in the patient’s FEV 1 of greater than 12% is considered a positive test. Once the test is finished a bronchodilator is given to the patient to help their FEV 1 return to the pre-test values.
Airway Challenge Testing
Airway Challenge Testing assesses the sensitivity of the airways. The test may be done using a substance called Mannitol, Histamine or Methacholine depending on the instruction of your respiratory consultant. Either one of these substances are administered to the patient and may induce bronchospasm (a feeling of chest tightness) in sensitive airways. To begin the test, the patient performs a Spirometry test. Then the first (lowest) dose is administered to the patient using a nebulizer or dry powder device 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 FEV 1 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 FEV 1 back to the baseline value.
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 patient’s heart, lungs and muscles to exercise, we can determine whether there is a cardiovascular or respiratory limitation or both to exercise or if the patient is simply deconditioned (unfit).
Six-minute walk test
The six-minute walk test is commonly used in a pulmonary rehabilitation programme. 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 meters 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 using a small wrist watch type device on the patient’s wrist and finger. The test may also be performed while the patient breaths supplemental oxygen to assess the patient’s oxygen requirement.
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.
Skin Allergy Testing
A skin allergy test tests the sensitivity of a patient to common allergens such as grass pollen,house dust mite and animal . These allergens may be contributing to the patient’s symptoms. 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 (the sensation is just like a tiny scratch and does not hurt). 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.
Patients with sleep breathing disorders such as obstructive sleep apnoea may be referred to the pulmonary function laboratory for sleep studies.
The patient is sent home with a wrist watch type device which measures the oxygen saturation continuously during the night. The patient returns the device the following day for analysis.
Limited Sleep Study:
The patient attends a hospital (local lab policy applies) appointment to be instructed how to attach the sleep measuring equipment. They are usually sent home and asked to wear the equipment overnight then return the next morning with the equipment for analysis.
(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. This advanced sleep study is only carried in a few specialized sleep centres and requires overnight stay in the hospital.
Patients that are diagnosed with obstructive sleep apnoea may be treated with a continuous
positive airway pressure (CPAP) machine. Respiratory physiologists liaise with respiratory nurse
specialists to help in the selection and fitting of CPAP masks and also measure the response to
therapy using PSG with CPAP.