Prediction of the Effect of Fluid Administration Using Arterial Pressure and Ventilator Data During Abdominal Surgery

"Before a planned fluid administration, the investigators will apply a series of 10 ventilator settings for 30 seconds each.~The settings are the following combinations of respiratory rate (RR) and tidal volume (TV):~RR (min\^-1), TV (ml/kg predicted body weight)~31, 6~31, 8~24, 6~24, 8~17, 6~17, 8~10, 4~10, 6~10, 8~10, 10~(the order of the respiratory rates: 17 to 31, will be randomized. 10/min will always be last. Tidal volume is always applied from lowest to highest for each respiratory rate)."