FUNCTIONAL RESIDUAL CAPACITY (FRC) ANALYSER

This device may be part of a general lung function analysis system or it may be an independent instrument. There are three techniques in use to establish the residual capacity of the lungs as follows:

1. By nitrogen washout in which the patient is connected to a spirometer at the end of expiration, and draws in a gas mixture containing no nitrogen (pure oxygen is often used). The patient makes several breaths in and out of the spirometer, exhaling oxygen, carbon dioxide, nitrogen and water vapour. The eventual proportion of nitrogen in the breathed gases can be used to estimate the original end- expiratory volume of the lungs and airways (FRC). Nitrogen is measured by sampling off-line.

2. A quantity of a tracer gas which is not absorbed in the lungs (e.g. helium) may be introduced into the inspired air and the concentration achieved in the expired air measured. This will allow an estimation of functional residual capacity. The patient may breath into a spirometer to allow proper mixing of the helium, in which case the volume of the spirometer must be taken into account in the calculation. The helium concentration may be determined by using radioactive helium (see Helium FRC analyser), or by use of a katharometer.

3. Alternatively a total body plethysmograph (TBP) may be used in which the patient is enclosed in a chamber (body box) so that changes in the volume, flow or pressure of the breathed gases, and changes in volume of the whole body may be estimated.

Content and Design Copyright 2000 Dr. Malcolm C Brown.  See Title Page for more details