Respiratory laboratory

Pulmonary Function Tests (PFT’s) are breathing tests that assess how well your lungs work .These tests determine how much air your lungs can hold, how quickly you can move air in and out of your lungs and how well your lungs put oxygen into and remove carbon dioxide from your blood.

Patients are referred for PFT’s for a number of reasons; investigations into shortness of breath, assessment of inhaler therapy, monitoring of the progression of a pulmonary disease/condition, measuring the severity of a pulmonary disease/condition, measuring the effectiveness of therapeutic intervention and medication and pre-operative assessments where a patient is evaluated prior to surgery.

PFT’s are dependent on patient effort and cooperation. These tests are only performed on children old enough to comprehend and follow the instructions (typically about 4-5 years old). Tests are not suitable for patients who are unconscious, heavily sedated, or have limitations that would interfere with respiratory efforts

PFT’s are performed by Respiratory Scientists that work within a multidisciplinary team. ‘A Respiratory Scientist is an expert in the technical and clinical knowledge that is required to both make a measurement of the physiology of the patient and to provide a diagnosis based on these measurements, as part of the Clinical Team

Usually the first test performed in the Laboratory. It is a basic test to asses the airway mechanics of the lung. It measures how quickly you can move air in and out of your lung. 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. This is repeated at least 3 times.

The following values are measured:

• 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 secon

All pulmonary 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, gender, age and sometimes race and weight.

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

A reversibility test or a post bronchodilator test can also be performed after spirometry to assess the reversibility of a particular condition and how responsive a patient is to bronchodilator medication.

This test measures the Total Lung Capacity (TLC). When a patient breaths out fully there is still a volume of air left in the lungs to keep them inflated. This volume is called the Residual Volume (RV). By calculating the RV the TLC can be measured.

The RV and TLC cannot be measured directly by spirometry but can be measured indirectly by the following techniques.

• Body Plethysmography _ the patient is seated inside an airtight box and instructed to breathe normally. At the end of a normal breath, a shutter is closed and the patient is instructed to pant against the shutter. The shutter reopens and the patient will be instructed to take a full breath in followed by a full breath out. The pressure changes that occur within the box are used to calculate the RV indirectly.
• Nitrogen Washout Method – the patient is instructed to breathe normally, a valve will open and the patient begins to breathe 100% oxygen. The oxygen is used to replace the nitrogen in the lungs. The amount of oxygen used and the time this test takes is used to calculate the RV and then the TL

This test is a gas diffusion test that is used to investigate the functions of the lung tissues. It measures how efficiently oxygen moves from the lungs into the blood stream. The patient is instructed to take a full inspiration of a known gas mixture. The patient must hold their breath for 10 seconds before breathing out fully. Comparisons are made of the concentrations of the exhaled gas to the known inhaled gas mixture and from this the level of diffusion that has occurred can be calculated.

The gas mixture contains Carbon Monoxide (CO) but in very small amounts that is not harmful. CO is used because it has very similar properties to oxygen.

A skin allergy test is used to determine the sensitivity of a patient to common allergens such as dust mite and grass pollen. A skin prick test is the most commonly used method with very little risks involved.The test area is the underside of the forearm or on the back in very young patients. A drop of each allergen solution and 2 controls, a negative control (normal saline) and a positive control (histamine) is placed on the skin using a pipette. A single use sterile lancet is used to pierce the skin allowing access of the allergen. After 15-20 minutes the test area 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 that are referred for a skin allergy test should not have active eczema or be on anti-histamine medication.

Exercise Provocation test is used in the diagnosis of exercise-induced asthma. The patient performs a base-line spirometry then exercises on a treadmill at 80% of their predicted maximum heart rate for 6 or 8 minutes at a speed of 4.8-9km/hr and an incline 10-12%. After exercising the patient then performs spirometry at 1, 3, 5, 10, 15, 30 minutes. A drop in the patient’s FEV11 of greater than 12% is considered a positive test. Once the test is finished a short-acting bronchodilator is given to the patient to help their FEV1 return to the base-line value.

Overnight pulse oximetry is a non-invasive indirect method used to measure and record the oxygen saturation (how much oxygen a person’s blood is carrying) and heart rate continuously during the night. Depending on the age and size of the patient a finger or foot probe is used which this is attached to a portable pulse oximeter monitor.

PCO2 Studies is a non-invasive indirect method used to measure and record CO2 in the blood. It is commonly used during sleep to detect hyperventilation. A sensor is attached to a transcutaneous monitor is either placed on the forehead or cheek, or in older children an attachment clip is used on the ear.