Short Time Low PEEP Challenge and Mini Fluid Challenge

Hemodynamic and ventilatory parameters will be recorded at five time points (T1 - T5). After baseline meausrements (T1), additional 5 cmH2O PEEP will be applied to patients for 30 seconds (short time low peep challenge, SLPC). Prior to PEEP lowering T2 measurement will be performed. T3 measurement will be performed one minute after PEEP is decreased to its initial value and will be recorded as the second baseline. Thereafter, 100 ml isotonic saline will be infused over one minute (MFC). T4 measurement will be performed one minute after MFC is completed,. Lastly, T5 measurement will be performed three minutes after additional 400 ml of isotonic saline is infused within 10 minutes to complete 500 ml of fluid loading.