The aim of this study is to compare the accuracy of Pleth Variability Index (Radical 7 device) and of Pulse Pressure Variation (Nexfin device) with the Doppler measurement of stroke volume (esophageal Doppler) to detect the response to a fluid challenge.

Inclusion Criteria:

• patients undergoing major abdominal surgical procedures performed by laparotomy
• patients undergoing major urological surgical procedures performed by laparotomy
• patients undergoing major gynecological surgical procedures performed by laparotomy
• age ≥ 18 years
• signed informed consent

Exclusion Criteria:

• pregnant women
• esophageal disease
• aortic valve disease
• allergy to gelatin
• BMI > 35
• beta blocker treatment, antiarrhythmic treatment, arrhythmia, pace-maker,
• presence of an arterio-venous shunt or of any other vascular disease on upper limb