The course of arterial oxygen-to-fraction of inspired oxygen pressure ratio (PaO
), arterial carbon dioxide partial pressure (PaCO
) and pH immediately before lung recruitment and during continuous positive-pressure ventilation (CPPV) and continuous external negative-pressure ventilation (CENPV). Measurements were taken at time 0 (5 minutes after the recruitment manoeuvre) immediately after starting the 2-hour ventilatory period of CPPV or CENPV. *P < 0.05 compared to corresponding values at 1 or 2 hours during CPPV.