Effects of lung volume on lung and chest wall mechanics in rats

To investigate the effect of lung volume on chest wall and lung mechanics i n the rats, we measured the impedance (Z) under closed- and open-chest cond itions at various positive end-expiratory pressures (0-0.9 kPa) by using a computer-controlled small-animal ventilator (T. F. Schuessler and J. H. T. Bates. IEEE Trans. Biomed. Eng. 42: 860-866, 1995) that we have developed f or determining accurately the respiratory Z in small animals. The Z of tota l respiratory system and lungs was measured with small-volume oscillations between 0.25 and 9.125 Hz. The measured Z was fitted to a model that featur ed a constant-phase tissue compartment (with dissipation and elastance char acterized by constants G and H, respectively) and a constant airway resista nce (Z. Hantos, B. Daroczy, B. Suki, S. Nagy, and J. J. Fredberg. J. Appl. Physiol. 72: 168-178, 1992). We matched the lung volume between the closed- and open-chest conditions by using the quasi-static pressure-volume relati onship of the lungs to calculate Z as a function of lung volume. Resistance decreased with lung volume and was not significantly different between tot al respiratory system and lungs. However, G and H of the respiratory system were significantly higher than those of the lungs, me conclude that chest wall in rats has a significant influence on tissue mechanics of the total r espiratory system.