Clonidin works as a presynaptic agonist of alpha-sympathomimetic receptors of the brain but also in many other tissues. It is part of balanced anesthesia in general anesthesiology as well as a sedastive agent in intensive care units, but also serves as a third-line agent in many psychiatric diseases including ADHD, schizophrenia and PTSD.

The investigators want to show whether clonidin has an procoagulatoric effect on thrombozytes of patients under double anticoagulaotirc treatment with ASS and clopidogrel. The investigators will use a flowcytometric assay to determine activation of thrombozytes when they come into contact with clonidine in vitro as well as Multiplate analyses to determine activation of thrombozytes, either thrombin.induced aggregation, ADP-induced aggregation or arachidonic acid-induced aggregation of thrombozytes.

Inclusion Criteria:

• Elective vascular surgical patients with long-term medication (at least 5 days) with ASS and clopidogrel
• Age above 18 years

Exclusion Criteria:

• Additional anticoagulatoric treatment woth another inhibitor of platelet aggregation
• Surgery within the last two weeks
• Patients with long-term mediaction with omeprazole
• Patients under hemodialysis
• Patients with renal insufficiency (creatinine above 1.5mg/dl, urea above 80mg/dl)
• Patients with hepatic dysfunction (GOT above 30U/I, GPT above 30 U/I, GLDH above 10 U/I)
• Age under 18 yeary
• Non-responder to ASS and / or clopidogrel

patients going under vascular surgery with anticagulation with both ASS and clopidogrel