Spirometry is the most commonly used lung function test in the management of respiratory disease. It is an effective tool in diagnosing lung disease and measuring the response to treatment in a variety of patients. Respiratory disorders can cause changes within the lungs and airways, Spirometry tests are valuable in detecting abnormalities and therefore changes within the lungs. To understand the procedure, it is useful to understand the anatomy and physiology of the respiratory system. Spirometry usually identifies one of four patterns:

• normal;
  
• an obstructive pattern (asthma, COPD);
  
• a restrictive pattern (fibrosing alveolitis, scoliosis);
  
• a combined obstructive/restrictive pattern.

There are numerous different spirometer machines on the market. Some machines will automatically calculate the predicted values and others require manual calculations done by the practitioner. Spirometry is a good tool for measuring the effectiveness of inhaled and exhaled breath. A dynamic lung function test will measure the amount (volume) and speed (flow) of the air during forced inspiratory and expiratory breath. A spirometer measures a variety of parameters. The most commonly used tests are shown below.

• Forced vital capacity (FVC): This is the maximum volume of air which can be expelled from the lungs during a long forced and complete expiration (maximum out-breath) from a position of full inspiration (maximum in-breath).
  
• Forced expiratory volume in 1 second (FEV1): This measures the maximum volume of air which can be expelled from the lungs in the first second of forced expiration (out-breath).
  
• Peak expiratory flow (PEF): This is the maximum flow achieved from a short forced expiration (out-breath). It is commonly called a peak flow (see Chapter 28).
  
• Forced vital capacity/Forced expiratory volume in 1 second (FVC/FEV1): The ratio between FVC and FEV1 expressed as a percentage.

All equipment must be clean and a bacterial filter used for each patient to avoid cross-contamination of equipment. First, prepare the patient and spirometer for the test. It is essential to obtain the patient’s consent and ensure that they are aware of the procedure and its purpose. Record the patient’s gender, most recent height and weight and ethnicity correctly into the spirometer. (Normal values for lung volumes are based on these aspects.) The British Thoracic Society (BTS) guidelines recommend that the patient is sitting during the test as forced expiratory manoeuvres may make the patient feel faint.

To obtain a forced spirometry test:

• Ensure the patient is wearing nose clips to prevent air leaks from the nose.
  
• Ask the patient to breathe in as fully as possible.
  
• Ensure they seal lips and teeth tightly around the mouthpiece.
  
• Make them blow out forcibly, as hard and as fast as possible until all air is expelled from lungs.
  
• Patients are advised not to lean forward during the test.

A minimum of three attempts is required and the result reported should be the highest value achieved from three successful attempts. Consider the patient’s capability and age to accurately perform the test. The BTS guidelines recommend health care professionals assess technique on an individual basis and only to begin teaching spirometry technique for children above 5 years of age.

The NICE guidelines recommend that all practitioners should be competent in the interpretation of the results of spirometry and should have received appropriate training and keep their skills up to date.

Lung function results are reported in absolute values in litres. Using the best of the three consistent readings, identify the predicted values according to weight, height and gender. Compare the reading from the patient with the prediction chart. Some machines interpret the results in data format automatically.

If airways are narrowed, then the amount of air which is expelled (breathed out) is reduced. Therefore, the FEV1 and the ratio of FEV1/FVC will be lower than normal/predicted. Lung volumes of 80–120% of predicted values are considered to be within normal limits.