Spirometry is a specialised test of lung function. It is essential in the diagnosis and follow up of respiratory disorders such as asthma, chronic obstructive pulmonary disease, interstitial lung disease and others.

It is also needed before major surgery in patients with respiratory diseases to assess the preoperative fitness for surgery. Spirometry is also helpful to assess the degree of severity and ventilator disability of lung in certain specific situations.

There are different types of spirometers available. The procedure essentially involves breathing through a mouthpiece into a pipe which is attached to the spirometer. The rate and flow of air being breathed is then measured and used to derive further values. Some of the common values which are measured are:

  1. VC (Vital Capacity): Amount of air inhaled and exhaled after a maximum effort
  2. FVC (forced vital capacity) – the maximum volume of air exhaled after a forced maximum inspiratory effort
  3. FEV1 (Forced expiratory volume in one second) – this is the maximum amount of air exhaled in one second
  4. The ratio of FEV1/FVC – this is important for differentiating obstructive from respiratory disease i.e. lung volume increase vs lung shrinkage respectively.
  5. Mid-flow rates: Flow rates in the middle part of the expiratory curve 

The results of spirometry depend a lot upon patient effort and technician training. So, if any of these two factors are lacking proper results will not be there. A lot of spirometry labs do not do proper diligence to these two factors and as a result wrong diagnosis can be made. Also, it is very important not to depend on the diagnosis provided by the spirometry machine.

It is the doctor’s interpretation which is valid and which should be focused upon. Spirometric parameters vary widely in normal situations according to age, sex, height and weight in different ethnicities. These are therefore better expressed as “% of predicted” rather than in absolute terms.
With spirometry, we can diagnose an ‘obstructive’, ‘restrictive’ or ‘mixed’ pattern suggestive of one or the other disease group.