Mesenteric Traction Syndrome During Upper Gastrointestinal Surgery

15 arterial blood samples (pre-, intra- and one day postoperatively); no more than 180ml in total over two days.

Measurement of microcirculation with Laser Speckle Contrast Imaging (non-touch setup with no side effects); continuous measurement of the microvascular blood flow on the forehead up to 60 minutes starting one minute before the surgical procedure.

"At three different time points the participants will be head down tilted in 20 degrees.~End of surgery (A) One hour postoperatively (B) 18 hours postoperatively (C)"