Spirometry and spirometer

What is spirometry and how is it done?

Spirometry measures various lung function parameters such as vital capacity and FEV1. Vital capacity is the maximum amount of air you can breathe in or breathe out. Maximal amount of air that can be exhaled in one second is called FEV1. These are sensitive indices for measuring severity of asthma and airway obstruction.


Spirometry is usually performed using computerized equipment. In this procedure, you will be asked by a technician to breathe into a machine, known as a spirometer, using a mouth piece and a nose-clip. The technician would instruct you on how the breathing maneuver needs to be carried out. You will be asked to take a maximal deep breath (inspiration) after breathing normally a few times. Then, you will be instructed to blow out as hard and as fast as possible and for as long as possible. You may then be asked to breathe in again and repeat the procedure. During this process, a computer calculates your vital capacity, FEV1 and other pulmonary variables.

What are normal spirometry test values?

Unlike normal temperature or normal blood, there is no single normal value for spirometry or lung function testing. A patient is typically assigned expected normal spirometry test values for his/her vital capacity and FEV1. These expected values are also known as predicted values. Your predicted spirometry values are based on your ethnic origin, sex, age and height. These values are read from tables containing such values calibrated and compiled from studying hundreds of normal persons with similar physical traits. For example, a native American man of 30 years and a height of 5ft 8 in would have a predicted or expected vital capacity quite different from a native Indian man of 5ft 6 in. The predicted values differ from country to country, from race to race and are different for males and females.

Spirometry test measurements made on you are called observed values. These values are compared with your predicted values to see if there is any difference. As a rule of thumb, an observed value of vital capacity or FEV1 that is less than 80% of the predicted value is considered abnormal.

What is a personal best spirometry test value?

Personal best spirometry value is achieved by an individual typically when he/she has no impairment/symptoms and the disease is fully under control. Your personal best spirometry value may be above or below your predicted spirometry value (i.e. personal best value expected of person of same ethnic origin, sex, age and height). Predicted spirometry values are used often as general guiding values, but there is always a range that is considered as normal. Note that there is no single value as a normal height but only a range; similarly, predicted spirometry values may have a range. For example, a person’s predicted vital capacity may range from 4 to 4.6 liters.

Personal best spirometry values can be found out only after spirometry has been carried out repeatedly over several weeks after optimal treatment. For an asthma patient, the personal best ultimately becomes the goal to be achieved with treatment.

What do the spirometry measurements indicate?

The physician decides whether the spirometry measurements are normal or abnormal. Usually, in asthma, vital capacity is normal or only a little reduced, especially in moderate or severe cases. The most informative measurement is FEV1. By comparing it with predicted values, a physician can conclude whether there is mild, moderate or severe airways obstruction. This gives an objective measurement of the degree of airway narrowing. Other useful information obtained from spirometry is variability and reversibility of airway narrowing.

If the first set of lung function tests shows abnormal results, the patient is asked to take bronchodilator drugs and the tests are repeated. If the second test shows more than 15-20% improvement in FEV1, the diagnosis of asthma is almost certain.

What are the clinical uses of spirometry?

Spirometry provides an objective measurement of the severity of asthma, something that is not possible by a physician’s examination.

•           It allows an assessment of the effect of treatment – i.e. whether it is adequate or needs to be increased.

•           It allows setting up of an important goal of treatment – i.e. attainment of personal best test values.

•           It allows assessment of the degree of permanent impairment in those patients who have severe asthma and who can never achieve normal lung function.

•           It allows the surgeon to decide whether the patient is fit for general anesthesia and his risk of post-operative complications; this is for those who have to undergo surgical operations for other diseases.

What is the difference between a spirometer and a peak flow meter?

Both are used for lung function testing: sprometers are used in clinical setting while peak flow meters are used in home setting.

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