Mechanical properties of rat lung during prolonged hypercapnia

Young adult rats were exposed to short-term (2 or 4 h) and prolonged (1 day to 3 wk) 5% CO2 to determine whether changes in lung mechanical properties might contribute to ventilatory adaptation. Anaesthetized rats were tracheotomized for measurement of functional residual capacity (FRC) and respiratory rates (chest intact), and for pressure-volume (PV) curves (chest open). Total lung capacity, minimal volume, lung compliance, and lung wet weight-to-dry weight ratio did not change significantly (P greater than 0.05) from air control values with CO2 exposure. From 4 h to 3 wk of hypercapnia, FRC was increased significantly (P less than 0.01) by about 25%; some of this rise could be associated with dynamic factors such as increased breathing frequency. Lung PV curves showed a small (0.5 cmH2O) left shift early in the short exposure period; this reversed into a slight (0.5 cmH2O) right shift after prolonged CO2. It is unlikely that these relatively small changes in FRC and lung recoil could contribute significantly to the previously reported long-term ventilatory adaptation to CO2.