Estimation of respiratory mechanics in dogs with acute lung injury

In mechanically ventilated anesthetized dogs with acute lung injury induced by intravenous infusion of oleic acid (OA), changes in airflow, lung volume, and tracheal (Pao), transpulmonary (Ptp) pressures were measured. Changes in lung mechanics were studied before and after OA infusion at intervals, during an observation period lasting 4-5 hours, by using the interrupter technique and identification technique, and by measuring the pressure-volume (PV) characteristics. The main results are listed as follows: (1) compliance and FRC showed a marked decrease, Pao and Ptp showed a marked increase within 2 hours after OA. (2) airflow resistance estimated by the identification technique showed a marked increase after OA. (3) static PV characteristics of both the lungs and total respiratory system showed similar changes 4 hours after OA, characterized by the presence of an inflexion on the inflation limb, increased hysteresis and a rightward and downward shift of the loop. (4) the trend of changes of dynamic PV characteristics obtained by changing the PEEP value in a stepwise manner, with the ventilation uninterrupted, was similar to that of the static PV curve. It is suggested that an on-line identification technique with one-compartment model to track the slow changes in combination with serial measurements of the dynamic PV characteristics of the total respiratory system would be more appropriate for monitoring lung mechanics of ventilated patients with ARDS.