Rationale: The interaction between macro and microcirculation remains uncertain. Microvascular alterations can occur when systemic hemodynamic parameters are within an acceptable range. Perfusion changes and microvascular alterations may play an important role in anastomotic healing and the onset of anastomotic leakage after gastrointestinal surgery. Nowadays, assessment of bowel perfusion is macroscopically performed by the surgeon prior to anastomosis creation. However, local oxygen delivery may still be compromised as little is known about microcirculatory alterations of the bowel during colorectal surgery. Dark Field (DF) imaging is a technique using a stroboscopic light-emitting diode ring-based imaging modality incorporated in a hand-held device, which illuminates an area of interest and provides high contrast dynamic images of the microvasculature. DF-imaging enables to visualize the bowel's microcirculation.

Objective: To describe the human gastrointestinal microcirculation during gastrointestinal surgery under general anesthesia and to observe whether there is a correlation between bowel microcirculation and systemic hemodynamic parameters.

Study design: A prospective, single center, observational, clinical, pilot study.

Study population: 70 patients undergoing elective, gastrointestinal surgery during which the gastrointestinal tract is accessible for DF-imaging.

Main study parameters/endpoints: To describe human gastrointestinal microcirculation on both the serosal and mucosal side of the bowel during gastrointestinal surgery under general anesthesia. Main parameter: Microvascular perfusion is quantified using the Microvascular Flow Index (MFI).

Nature and extent of the burden and risks associated with participation, benefit and group relatedness: The extend of burden and risk associated with participation is negligible. Using DF imaging on the bowel is a non-invasive technique requiring a minimal amount of time as is described in the study procedure. Previous studies did not show any safety concerns. Measuring will be performed under sterile conditions and the occurrence of tissue damage is highly unlikely. Patients are under general anesthesia and will thus not experience any inconvenience.

Inclusion Criteria:

• All patients aged >18 scheduled for elective, gastrointestinal surgery (as described above) with signed informed consent.

Exclusion Criteria:

• • Age <18 years;

  • Atrial fibrillation (because of possible interference with FloTrac™/Vigileo™ cardiac output monitor);
  • Left ventricular ejection fraction ≤30%;
  • Serious pulmonary disease (resting pO2 <90% at room air);
  • Renal failure (clearance <30 ml/min as calculated using the Modification of Diet in Renal Disease formula);
  • Liver failure;
  • No signed informed consent.

70 patients undergoing elective, gastrointestinal surgery. Surgical procedures included will be:

• All forms of open colorectal surgery;
• Pylorus Persevering PancreaticoDuodenectomy (PPPD);
• Other procedures requiring small bowel resection for cancer;
• Laparoscopic procedures with extracorporeal creation of the bowel anastomosis. These procedures provide unique access to different parts of the bowel. The PPPD procedure provides access to the mucosal and serosal side of the jejunum. Colorectal surgery provides access to both the serosal and mucosal side of the colon and the terminal ileum is sometimes also accessible. We expect to include around 2 patients per week.