Background information respiration | Physiology | Spirometric testing | Overview

Composition of a spirometer

Measurements obtained with a spirometer

Spirometry is a simple lung function test that measures lung volume as a function of time. It can monitor quiet breathing and thereby measure tidal volumes and also trace deep inspirations and expirations.

A spirometer consists of a floating drum put over a chamber, with the drum counterbalanced by a weight. The drum filled with air or oxygen is connected with the mouth of the patient by a tube. If the patient exhales into the spirometer, the drum rises, by inhaling in the drum falls. On a moving sheet of paper an appropriate recording is made.

The following measurements can be obtained with a spirometer:

Tidal volume – the volume of a single breath during quiet breathing, in average about 500 ml

Inspiratory and expiratory reserve volume – volume that can be additionally inhaled (exhaled) after a normal inspiration (expiration), in average about 3000 ml (inspiratory) and 1100 ml (expiratory)

Vital capacity / Forced vital capacity (VC / FVC) – maximum volume of air, which can be exhaled starting at full lung inhalation, in average about 4600 ml

Forced expiratory volume in one second (FEV1) – the volume, that can be exhaled in the first second during a forced expiration, normally 80% of VC

Ratio of FEV1 to VC – the volume, that can be forcibly expelled in one second (FEV1) divided by the total volume (VC), which can be exhaled, normally 0.8

Peak expiratory flow (PEF) – maximum flow generated during expiration performed with maximal force and started after a full inspiration, in average about 10 l/s

Maximum / forced expiratory flow % (MEFx% / FEFx%) – maximum flow generated when x% of the vital capacity has been exhaled (FEFx%) or x% of the FVC remain to be exhaled (MEFx%)

In a healthy population spirometric values vary substantially even after consideration of factors like age, height, gender and ethnic group. Therefore great emphasis has to be put on clinical data and the patient's previous medical history in interpreting spirometric data. The best predicted value for a patient is the personal reference value, the value obtained in a clinically optimal period.

Literature:

Klinke R, Silbernagel S. Lehrbuch der Physiologie. 2001. Thieme: Stuttgart

Shier D, Butler J, Lewis R. Hole's human anatomy and physiology. 2004. McGraw Hill: New York

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