Respiratory mechanics determined by flow interruption during passive expiration in cats

We used the interrupter technique to measure the resistance Rinit (equal to the initial change delta Pinit in tracheal pressure divided by flow at interruption) during expiration in six normal anaesthetized-paralyzed cats. By performing interruptions at different points in expiration we found Rinit in each cat to be linearly dependent on flow. By allowing the cats to expire through two different resistances we were also able to demonstrate a volume dependence of Rinit in four of the cats. In addition, we obtained a secondary pressure change delta Pdif in each cat, as the magnitude of the slow change in tracheal pressure in the 2 sec following interruption of flow. delta Pdif was approximately constant over most of the expired volume range, and represented the difference between the static elastic recoil pressure of the respiratory system and the pressure driving flow at any volume during a passive expiration. delta Pdif became larger than delta Pinit towards the end of expiration. Since previously used methods for measuring respiratory system resistance have employed varying combinations of delta Pinit and delta Pdif as the resistive pressure drop, it is clear that measurements of resistance must be made with standard techniques under standard conditions if they are to be compared.