Fluid therapy is considered a first-line therapy in resuscitation protocols for hemodynamically unstable patients. The administration of fluids usually translates into an increase in Cardiac Output. However, not all patients increase mean blood pressure after fluid administration.

To determine if fluids the administration improves blood pressure, it is necessary to evaluate the dependence of preload, vasomotor tone, and left ventricular stroke volume.

The aim of this study is to confirm the usefulness of dynamic elastance, cardiovascular impedance, cardiac cycle efficiency, and other hemodynamic parameters calculated with the PRAM method as predictors of blood pressure response after fluid administration in open abdominal surgery. This will allow us to make and evaluate a predictive model for the blood pressure response after fluid administration in open abdominal surgery.

Inclusion Criteria:

• Patients ≥ 18 years
• Patients scheduled for open abdominal surgery lasting ≥ 120 minutes under general anesthesia with mechanical ventilation and the use of VT of 8 ml / kg ideal weight and who require serious arterial monitoring.

Exclusion Criteria:

• Patients <18 years.
• Patients> 80 years.
• Pregnant women.
• Urgent surgery.
• ASA >3
• Pathologies that may alter the quality of the arterial signal due to alterations of the dicrotic incisura.
• Poor quality of the arterial pressure wave due to artifacts of the transduction system (resonance and damping).
• Personal history of:
• Cardiac arrhythmia.
• Left ventricular ejection fraction <30%
• Right ventricular dysfunction (peak systolic tricuspid annulus velocity <0.16 m / sec)
• Intracardiac shunt
• Preoperative creatinine> 1.4 mg / dl.
• Dialysis
• Previous treatment with beta-blockers

Patients scheduled for open abdominal surgery, older than 18 years. All sex participates and not gender based.