The purpose is to demonstrate that vasoreactivity of patients with septic shock evaluated with dose-response curve is diminished in septic shock and ameliorated by activated protein C (APC).

This amelioration is correlated to decrease of inflammation, decrease of reactive oxygen species (ROS) markers and increase of circulating catecholamines.

Inclusion Criteria:

- Patients with septic shock as determined by standard criteria (including infection and severe infection)

Exclusion Criteria:

• Pregnant women
• Absence of signed informed consent. Due to gravity of medical situation of patients, inclusion will be possible after informed consent of a family member. As soon as possible, an informed consent will be obtained by patient
• Contraindication to Xigris: evolutive internal bleeding , intracranial pathology, neoplasia or brain involvement, concomitant heparin therapy >= 15 IU/kg/h, known hemorrhagic diathesis except acute coagulopathy subsequent to sepsis, severe chronic liver disease, platelet count < 30000 x 10^6/L, high bleeding risk, known hypersensibility to drotrecogin alfa (activated), one of excipients or bovine thrombin