A prospective, double-blinded study. The present study consisted in applying Stroke volume variation fluid guided therapy to old patients (65 years old) undergoing radical resection of colon surgery. The aim of this study is to compare the effects of norepinephrine and phenylephrine in treating perioperative hypotension, and to find the safest and most effective vasopressor for elderly.

Fluid management has a great influence on patient's morbidity, mortality and on the overall outcome.Goal directed therapy was proved by multiple studies to be a goal therapy in maintaining a stable hemodynamic state. Nevertheless, as the population worldwide is aging, elderly presenting for surgery are also increasing. This population is known for the multiple comorbidities and physiological changes which makes them prone to dehydration and hypotension. That's when we are going to use either Norepinephrine in one group or Phenylephrine in the other and compare the effects of both vasopressors.

Parameters we are going to use are: Hemodynamic (HR, MAP, CO, CI, SVV, SV); Tissue Oxygenation and perfusion (DO2, VO2, ERO2, CRT, Lactate, blood gas); Urine output.

Inclusion Criteria:

• Operation time should be equal or greater than 2h,
• Patients older than 65 years old,
• ASA Ⅱ or Ⅲ.

Exclusion Criteria:

• Clear arrhythmia,
• Need to apply PEEP,
• Peripheral vascular disease and arterial catheter contraindications.