There is no objective evidence of how long intravenous solutions remain inside venous blood vessels after they have been administered, therefore there is no definite guideline of how to administer them in the preoperative setting. Besides, obese patients represent a particular group of subjects as they theoretically with-hold a constant inflammatory response and that would modify the way solutions behave intravenously, that is how long they remain inside.

Having said this, we wish to describe the way colloid solutions behave in this group of patients by taking serial blood samples in 12 obese patients after a colloid infusion, to calculate plasma dilution curves based on hemoglobin dilution and therefore infer the time it remains intravascularly.

All this in the hope this information will help, in the near future, to establish a more objective way to use these solutions and avoid possible complications due to over-administration.

Inclusion Criteria:

• clinical diagnosis of obesity (IMC >35kg/m2) that will be treated with gastrojejunal laparoscopic bypass

Exclusion Criteria:

• Renal failure KDOQI >3
• Cardiac failure NYHA III-IV
• Sepsis
• Allergy to tetrastarch
• Allergy to any of the anesthetic medication that is to be used in the protocol previously established
• Patients in which vasoactive drugs are used
• Patients in which the blood samples are completed.