Spirometry

- It should be undertaken if lung disease is suspected. It is an important tool in assisting diagnosis, and in monitoring patients with respiratory disease, especially when used in conjunction with other investigations. - Spirometry can identify and differentiate between obstructive and restrictive lung disorders. - People with persistent symptoms of wheeze, breathlessness and a productive cough should be considered for referral for spirometry. In addition, the NICE COPD Guidelines1 (2004) advise that people over 35 years of age who are lifelong heavy smokers should have a spirometry test to identify early disease. Other reasons for referral for spirometry are: 1. To assess the severity or progression of a respiratory disease 2. To assess the therapeutic effect of respiratory medication such as bronchodilators or steroids 3. To assess the risk of surgery 4. To assess the response to allergens 5. To assess the effect of occupational exposure to respiratory irritants such as dust and fumes. The indices most commonly measured in spirometry are Vital Capacity (VC), both as a relaxed blow (RVC) and as a forced blow (FVC), and Forced Expiratory Volume in one second (FEV1). Most spirometers contain computer software to calculate the predicted values and the percentage of predicted that the patient achieves. The predicted value is based on the following patient characteristics: - Age - Height - Gender - Ethnic origin. Readings that are 20% either side of the predicted values are considered to be within the normal range, and healthy individuals should achieve at least 80% of the FEV1 and FVC values predicted for them.

The Vital Capacity (VC) is the maximum volume of air that can be breathed out of the lungs following a maximal inhalation. It is a measure of the size of the lungs and can be measured in two ways: 1. Relaxed Vital Capacity (RVC) or Slow Vital Capacity (SVC). The air is exhaled in a relaxed or slow manner. Because air is exhaled slowly, there are fewer limitations on the flow from narrow airways or damaged lungs. 2. Forced Vital Capacity (FVC) is where the air is exhaled rapidly from full inspiration to full expiration. As the air is being blown forcibly out of the lungs, the positive pressure inside the chest can cause premature closure of the smaller airways. This can cause air to ‘trap’ in the alveoli. This will be seen if the FVC measures less than the SVC. In healthy people, the SVC and the FVC are normally within 200mls of each other, but in people with obstructive lung diseases, such as asthma and COPD, premature closure of the airways may lead to greater differences. It is recommended that both are measured if there is a suspected obstructive lung disease. ****FEV1**** This is the volume of air exhaled in the fi rst second of a forced exhalation from maximum inhalation, using maximum effort. Several factors infl uence the FEV1, including the elastic properties of the lung, the size of the lungs and the caliber and collapsibility of the airway. FEV1 and VC are expressed in absolute values, in litres per minute and as a percentage of the predicted value. ****FEV1 ratio**** This is the ratio between the volumes exhaled in one second (FEV1) to the total of air exhaled, either the SVC or FVC, whichever is greatest. It is an excellent measure of airflow limitation and allows differentiation between obstructive and restrictive lung disease. Measurement ranges from 0.75–0.85 in a normal adult. Anything below 0.7 may indicate obstructive disorders as it takes longer to exhale the air. The FEV1/VC ratio is normal or high in restrictive disorders as the flow of air is not reduced.

1. Obstructive lung disorders Anything that reduces the diameter of the airways will reduce the rate of airfl ow into or out of the lungs. For example: - COPD - Asthma - Bronchiectasis - Cystic fibrosis - Obliterative bronchiolitis - Tumour or foreign body in the airways. - The obstruction is caused by: Bronchospasm Mucus plugging Loss of elastic recoil Release of infl ammatory markers Scarring. - A reduced airway diameter means that air is exhaled at a reduced speed. The definition of obstructive spirometry is a FEV1/FVC ratio <0.7. The FEV1 is usually reduced at <80% of predicted but the FVC may be normal, or slightly reduced. 2. Restrictive lung disorders - Fibrosing lung diseases such as idiopathic fibrosing alveolitis, sarcoidosis and pneumoconiosis - Skeletal deformity such as kyphosis or scoliosis - Pulmonary oedema - Neuromuscular disorders such as muscular dystrophy - Previous surgery, such as pneumonectomy or lobectomy - Malignancy - Obesity. Reduced lung volumes are a result of the inability of the lungs to expand and relax at the rate and completeness the diaphragm and intercostal muscles demand. Restrictive spirometry is indicated by a reduced VC, both slow and forced at <80% of predicted. The FEV1 is also reduced, not because there is any obstruction present, but because predicted values for FEV1 are based on normal volumes of air being present. However, the FEV1/FVC ratio is normal or high. 3. Combined obstruction and restriction Conditions that produce a combined obstructive and restrictive pattern of spirometry include: Severe airflow obstruction in advanced COPD Cystic fibrosis Severe bronchiectasis. This results in reduced VC, especially the FVC, reduced FEV1 and reduced FEV1/FVC ratio.