Measurement theory

The usefulness and validity of any model of lung mechanics rest entirely on the experimental data upon which it is based. An appreciation for these data and an understanding of how they were obtained are therefore vital for a complete understanding of the model itself.

The experimental data required for the construction of models of lung mechanics usually consist of gas pressures, flows, and volumes. These variables can be measured at a variety of sites around the body. By far the most common measurement site is at the entrance to the airways (this is the nose and mouth in an intact subject, but may be the entrance to the trachea in experimental animals or patients receiving mechanical ventilation). However, other sites have been used, such as at the body surface, inside the esophagus, and even within individual alveoli. Generally speaking, increasing the number of simultaneous measurement sites allows for an increase in the complexity of the possible models that can be identified from the resulting data. Of course, this has to be balanced against practical and ethical considerations.

The measurement of a variable such as pressure occurs in a sequence of steps, as depicted in Fig. 2.1, beginning with the variable itself and ending with the recorded data. First, the pressure is allowed to impinge on a pressure transducer, which is a device that converts the pressure signal into a corresponding voltage signal.