Typical Number in Hospital: 3 Cost Bands: 4 References: 3,6,9

This measures the flow rate of gases during breathing. The breath is passed through a short tube (Fleisch tube) in which there is a fine mesh which presents a small resistance to the flow. The resulting pressure drop across the mesh is in proportion to the flow rate. The pressure drop is very small (e.g. 2 mmHg) and so the measuring circuit must be of high quality and produce very little drift with time. A differential pressure transducer is normally used.

The advantage which this device has over the mechanical spirometer is that the patient under investigation can continue to breathe fresh air through the transducer while the measurements are taken. The volume in each breath and the cumulative volume can be found by electronic integration of the flow rate. Problems exist because inspired and expired volumes and gas mixtures are not the same, and so the pressure drop will not be the same for equal flow in each direction. Also, water vapour in the expired gases may condense on the mesh unless heating is applied, or some other anti-condensation measure is used.

The pneumotachograph may be used in lung function analysis, or during artificial ventilation of the lungs. For routine work in each of these applications simpler devices are normally used, such as the spirometer, dry gas meter, and the Wright's respirometer or Wright's respiration meter.

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