Mechanical behaviour of the canine respiratory system at very low lung volumes

We studied the changes in dynamic elastance and resistance of the respiratory system in 6 supine, anaesthetized, paralysed, tracheostomised and open chested dogs. Tracheal pressure (Ptr), tracheal flow (V) and 3 alveolar pressures (Palv by alveolar capsule) were measured continuously for 20 min at 5 levels of positive end expiratory pressure (PEEP) between 0.1 and 0.5 kPa. The lungs were inflated to total lung capacity (TLC) at the start of each recording period. Lung elastance (EL) and resistance (RL) were estimated by fitting the equation Ptr = RLV + ELV + K to the measured data for each breath by multiple linear regression (V = volume, K = constant). Airway resistance (Raw) was obtained from the difference between Ptr and Palv. EL increased progressively in the 20 min following lung inflations. The increase in EL over this time was about 45% of its baseline value at a PEEP of 0.1 kPa compared to an increase of only about 10% at a PEEP of 0.5 kPa. In contrast, RL changed very little over the recording period at all levels of PEEP. At low levels of PEEP Palv often bore no resemblance to Ptr indicating that significant airway obstruction or closure had occurred. These results suggest that the increase in EL at low PEEP was primarily due to the accretion of airspace closure, and that nonlinear tissue mechanical properties were responsible for the lack of change in RL.