It is well known, that patients with circulatory impairment sometimes, but not always, benefit from intravenous fluids. Predicting if a fluid administration will improve circulation is therefore of substantial clinical interest.

Ventilator treatment induces cyclic variation in blood pressure due to interaction between the lungs and the heart. This variation is minor, but its amplitude may be used for guiding fluid administration.

However, this method of using ventilator-induced variation in blood pressure to predict the effect of fluid administration was developed when different settings for ventilator treatment was recommended, compared with today. With today's recommend ventilator treatment, the method is, unfortunately, less reliable.

The investigators will investigate how different ventilator settings influence variation in blood pressure, and the investigators will test if this knowledge allows us to better predict the effect of a fluid administration, by taking the ventilator settings into account.

Inclusion Criteria:

• Scheduled open abdominal surgery.
• Scheduled treatment with Aarhus University Hospital's goal-directed therapy protocol (hemodynamic monitoring).

Exclusion Criteria:

• Left ventricular ejection fraction ≤ 40 %
• Irregular heart rhythm (e.g. atrial fibrillation or frequent ectopic beats)
• Known right ventricular dysfunction (if reported qualitatively in pre-operative assessment or objectively via Tricuspid Annular Plane Systolic Excursion (TAPSE) < 17 mm)
• Pregnancy
• Prior participation in this study

Patients at department of abdominal surgery, Aarhus University Hospital, scheduled for open abdominal surgery with planned use of Aarhus University Hospital's goal-directed therapy protocol (hemodynamic monitoring).