Patients were mechanically ventilated (with a Siemens Servo 900B or 900C ventilator). Initially, the ventilator rate was increased to abolish triggering efforts in order to assess the respiratory system mechanics using the interrupter technique. Pao, V, and V were recorded during stepwise deflations (Hans Rudolph 7200 interrupter valve) of the relaxed respiratory system between end inspiration and relaxation volume. Pressure-volume and pressure-flow plots were constructed, from which the elastic and flow resistive properties of the relaxed respiratory system were derived. The severity of airways obstruction was estimated from the expiratory flow at a system recoil pressure of 6 cm H,0 (VP6). Inspiratory muscle strength was assessed from airway occlusion pressure recordings during maximal voluntary efforts (Pimax); the best of three maneuvers was chosen.
The Recruitment Threshold Measurement: This experimental technique has been described previously in detail. Patients were ventilated with a tidal volume of 10 to 12 ml/kg of body weight (except one patient who, following a pneumonectomy, was ventilated with a tidal volume of 8 ml/kg) and with a duty cycle (Ti/Tr) of 0.20 or 0.25.