Flow was measured with a heated pneumotachograph (Hans Rudolph Pneumotachometer, model 3700; Hans Rudolph Inc, Kansas City, Mo) inserted via cones between the endotracheal tube and the Y-piece of the ventilator and connected to a differential pressure transducer (Validyne MP 45, ±2.5 cm H20; Validyne Co, Northridge, Calif). The response of the pneumotachograph was linear over the experimental range of flows. Volume was obtained by electronic integration of the flow signal (Validyne FV 156-871).

Airway pressure (Pao) was measured at the mouth via a side hole proximal to the endotracheal tube by means of a pressure transducer (Validyne MP 45, ±88 cm H20). As previously described,” changes in end-expiratory lung volume (AEELV) were obtained using the sum signal of respiratory inductive plethysmography (RIP) (Respigraph, NIMS; Miami Beach, Fla) on the DC coupled mode. The RIP was calibrated against the pneumotachograph with the semiquantitative single-position calibration method. The calibration was validated and accepted only if the error was within 2 percent.
All signals were recorded on an 8-channel pen recorder (Mingo-graph; Siemens, Solna, Sweden) at a paper speed of 10 mm/s.