Spirometry - Pulmonary Function Test (PFT)

Spirometry is a test that is used to assess the lung function in an individual. It is useful in diagnosing common respiratory diseases like asthma and COPD. Spirometry is also used for pre-operative assessment in smokers and patients with respiratory symptoms. During spirometry the subject is asked to take deep breaths and blow as fast as possible into a machine called as spirometer. The machine calculates the volume of air that you had blown along with the time required. Some of the following measures are available from the spirometry.

1)Forced Vital Capacity (FVC) – the volume of air that was forcibly expired after maximal inspiration. 
2)Forced expiratory volume in one second (FEV1) – the volume of air exhaled in the first second.
3)FEV1/FVC – Ratio of the air exhaled in the first second to the total air exhaled.

Other parameters that can be assessed include post bronchodilator reversibility of airway obstruction and diffusion studies. To calculate the post bronchodilator reversibility, nebulized or inhaled salbutomol is used. Salbutomol is a beta agonist that dilates the airways. Post salbutamol administration the spirometry is repeated and improvements in FVC and FEV1 are noted. A significant increase in FEV1 suggests that the airway obstruction is reversible and amenable to medications. The reversibility is generally seen in asthma. In contrast patients with COPD demonstrate poor reversibility after salbutamol administration. In advanced spirometers the diffusion of carbonmonoxide (DLCO) can be estimated. In diseases affecting the interstitium of lung, the diffusion of oxygen across the alveolar capillary membrane is impaired. Diffusion studies are useful in patients with interstitial lung diseases. 

Pre procedure Instructions
1)If possible, it is advisable to skip the dose of inhaler several hours prior to the spirometric assessment. You should skip only if your underlying lung condition is stable and no harm is expected by skipping the dose. Do inform the spirometric technician about the inhalers you use and the last dose taken by you. 
2)Do stop smoking prior to spirometry
3)No allergy testing should be done prior to spirometry

Contraindications
Spirometry is safe in healthy individuals but the need to perform maximally can affect certain groups of individuals. Some of the contraindications of spirometry are listed below
1)Recent Myocardial Infarction (heart attack)
2)Recent thoracic or abdominal surgery
3)Eye surgery
4)Vascular aneurysm in any part of body
5)Cardiac disease that can worsen with exertion
6)Hemoptysis
7)Pneumothorax
8)Chest and abdominal pain
9)Acute COPD or asthma exacerbation

Performing spirometry
Generally the subject is seated and asked to blow into the spirometer as fast as and as hard as possible after a maximal inspiration. The spirometric values are effort dependent. The role of spirometric technician is very important as he/she need to coax the subject to perform at maximal effort. Generally the test is carried out three times to check for reproducibility.

Interpretation
Normal Spirometry – What is normal?
It is a usual practise to compare the lung function values of an individual subject obtained by spirometry against normal values for that subject predicted using a regression equation. The lung function varies with height, gender, age, ethnicity and these variables are incorporated into the prediction equations.  Ethnic variations in lung function have been extensively documented. For example the normal lung function values of lndians are lower than whites (Caucasians) of similar height. Hence using Caucasian prediction equations as a standard normal for indian subjects will lead to erroneous misinterpretations of lung function. It is important that the prediction equations are derived from the same ethnic population as the subjects who are tested. For Indian subjects a prediction equation developed at PGI, Chandigarh by Prof. Dr. S.K. Jindal can be used.

Abnormal spirometry
If the spirometry is not normal, then based on the flow volume loop , two types of patterns can be recognized; obstructive pattern and restrictive pattern. In obstructive pattern, there is obstruction to the exhalation of air with air getting trapped inside the lungs. The lungs are characteristically hyperinflated. The forced expiratory volume in the first second is low (low FEV1) with a prolonged expiratory time seen as coving of the descending limb of the flow volume loop. FEV1/FVC is reduced. Obstructive spirometry is seen in asthma and COPD. To differentiate between asthma and COPD, the post bronchodilator reversibility is measured. Post bronchodilator improvement in FEV1 is generally seen in asthma and rarely in COPD.  In restrictive pattern, the lung volumes are lower, the flow volume loop is smaller, the FVC is low and hence FEV1/FVC can be greater than 80%. The low lung volume can be secondary to external compression as in obesity or due to fibrosis as in interstitial lung disease. Maximal Expiratory Flow (25-75) can be used in the assessment of small airway disease. In some patients both the obstructive and restrictive elements are seen.

Limitation
The residual volume of the lung, the amount of air remaining in the lung after maximal expiration cannot be estimated by spirometry. Since residual lung volume cannot be measured the total lung capacity of the subject cannot be calculated.  To calculate the total lung volume other methods like the helium dilution method and body plethysmography are used.

Infection control
There is a slight risk of respiratory tract infection from the previous subjects who undertook the study. Possibility of cross infection can be reduced by using disposable mouthpieces and disinfecting the spirometers regularly.