SPIROMETER/SPIROGRAPH

Typical Number in Hospital: 3

Cost Bands: 2

References: 2,6

These measure the volumes of gases breathed in or out. They are usually displacement (bell) devices, a bellows, or a small turbine device with gears to drive a pointer. The displacement type consists of an inverted closed cylinder with its open end immersed in water. The expired gases are fed into the cylinder through a tube opening just above the water level inside the cylinder. As more gas is fed in, the cylinder must rise. The cylinder is usually linked to a pen which writes on a rotating drum (kymograph). Thus respiratory gas volume movements can be recorded on a graph against time.

Such devices are used in lung-function studies in the respiratory laboratory, and they may also be used by anaesthetists and cardiologists. An alternative method of spirometry is to replace the cylinder and water by a large soft bellows, one side of which is coupled to a pen.

In the lung-function laboratory it is used routinely to measure the vital capacity (VC) and the forced expiratory volume (FEV). The vital capacity is simply the volume which can be breathed out after maximum inspiration and the FEV1 is the volume which can be breathed out in 1 second. For accurate records a correction must be made to take account of the different temperature of the gas in the lungs and in the spirometer. Special tables are available to make this correction. Spirometers are also used for tests involving continuous breathing such as residual volume (RV) and transfer factor (TL) measurements, but in conjunction with other apparatus.

An anaesthetist is likely to use a spirometer to monitor the tidal volume or the minute volume of a patient connected to a lung ventilator, and so the above types would be unsuitable. He may therefore use a device which produces an electronic integration of gas velocity such as the Wright's respiration monitor, an anemometer which is calibrated in volume flow, or the ventilator may have a spirometer bellows which indicates tidal volume by the distance between the two end states. The normal ventilator bellows gives some indication of tidal volume but errors are possible due to leaks, compression of gas due to the action of the ventilator, changes in gas composition in the lungs, fresh gas flow, etc. This form of spirometry can only be conducted accurately in the expiratory part of the circuit.

Spirometers are also used in the calculation of cardiac output (litres of blood/minute) using the Fick method which requires a calculation of oxygen consumption. This is achieved using a spirometer in conjunction with a carbon dioxide absorber. The oxygen consumption is the reduction in volume/minute of the quantity of gas in the spirometer.

Ultrasonic spirometers also exist which detect gas flow from ultrasound transit time up and down the tube. Another type exists which counts vortices produced by the gas flow past baffles in the transducer.