Spirometry is the method to measure lung volumes and capacities. Simple instrument used for this purpose is called spirometer. Modified spirometer is known as respirometer. Nowadays plethysmograph is also used to measure lung volumes and capacities.


Spirometer is made up of metal and it contains two cham bers namely outer chamber and inner chamber. Outer chamber is called the water chamber because it is filled with water. A floating drum is immersed in the water in an inverted position. Drum is counter balanced by a weight. Weight is attached to the

top of the inverted drum by means of string or chain. A pen with ink is attached to the counter weight. Pen is made to write on a calibrated paper, which is fixed to a recording device.

Inner chamber is inverted and has a small hole at the top. A long metal tube passes through the inner this tube reaches the top portion of the inner chamber. Then the tube passes through a hole at the top of inner chamber and penetrates into outer water chamber above the level of water. A rubber tube is connected to the outer end of the metal tube. At the other end of this rubber tube, a mouthpiece is attached. Subject respires through this mouthpiece by closing the nose with a nose clip. When the subject breathes with spirometer, during expiration, drum moves up and the counter weight comes

down. Reverse of this occurs when the subject breathes the air from the spirometer, i.e. during inspiration. Upward and downward movements of the counter weight are recorded in the form of a graph. Upward deflection of the curve in the graph shows inspiration and the downward deflection denotes expiration. Spirometer is used only for a single breath. Repeated cycles of respiration cannot be recorded by using this instrument because carbon dioxide accumulates in the spirometer and oxygen or fresh air cannot be provided to the subject.


Respirometer is the modified spirometer. It has provision for removal of carbon dioxide and supply of oxygen. Carbon dioxide is removed by placing soda lime inside the instrument. Oxygen is supplied to the

instrument from the oxygen cylinder, by a suitable valve system. Oxygen is filled in the inverted drum above water level and the subject can breathe in and out with instrument for about 6 minutes and recording can be done continuously.


Spirogram is the graphical record of lung volumes and capacities using spirometer. Upward deflection of the spirogram denotes inspiration and the downward curve indicates expiration. In order to determine

the lung volumes and capacities, following four levels are to be noted in spirogram:

1. Normal end expiratory level

2. Normal end inspiratory level

3. Maximum expiratory level

4. Maximum inspiratory level.


Computerized spirometer is the solid state electronic equipment. It does not contain a drum or water

chamber. Subject has to respire into a sophisticated transducer, which is connected to the instrument by

means of a cable.

Disadvantages of Spirometry

By using simple spirometer, respirometer or computerized spirometer, not all the lung volumes and lung

capacities can be measured. Volume, which cannot be measured by spirometry, is the residual volume. Capacities, which include residual volume also cannot be measured. Capacities that include residual volume are functional residual capacity and total lung capacity. Volume and capacities, which cannot be measured by spirometry, are measured by nitrogen washout technique or helium dilution technique or by body plethysmograph.


Plethysmography is a technique used to measure all the lung volumes and capacitie

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