A comparison between the isovolume and the end-inflation occlusion methods for measurement of lung mechanics in rats.

The isovolume method is one of the most used techniques to assess the mechanical properties of the lungs in ordinary challenge tests and in toxicological studies. It has been considered that isovolume lung resistance (RL) could assess central airway calibre in such tests, whereas dynamic elastance (EL,dyn) would reflect the elastic properties of lung parenchyma. However, peripheral phenomena, such as Pendelluft and stress relaxation, can interfere with RL measurements, especially in disease. In order to investigate the contribution of such phenomena to isovolume RL and EL,dyn, we compared the results provided by isovolume technique with those given by the end-inflation occlusion method (EIOM) in 12 normal Wistar rats. We found similar values for total lung resistance (RL,max) and isovolume RL. Homogeneous resistance (RL,min), which describes airway calibre, comprised about 30% of RL in the tidal volume range. Values of EL,dyn calculated by the EIOM were approximately 20% greater than the corresponding value measured during spontaneous breathing. Our data indicate that isovolume RL is significantly affected by tissue forces and cannot be interpreted as a pure index of airway size.