Postoperative patients after gastro-intestinal surgery (e.g. liver, pancreatic, colorectal or hyperthermic intraperitoneal chemotherapy (HIPEC) surgery)

Patients will be scanned in both supine and right lateral decubitus position (RLD)s. If the antrum is visible, it will be judged if the antrum is empty in both positions or the antrum is empty in supine position and fluid apparent in RLD position suggesting a low fluid volume (<1,5 ml/Kg). The third option is fluid apparent in both supine and RLD position, suggesting a higher fluid volume (>1,5ml/Kg) and the last option is solid food in one or both positions.

Practicality of gastric ultrasound will be measured by the length in minutes.

The Dutch-System Usability Scale will be used to assess usability

Acceptability will be measured by the patients' perspective towards undergoing gastric ultrasound during recovery after surgery.

Demand will be measured by indications for gastric ultrasound and the change for successful image acquisition. Indications involve a description of clinical signs that were present during the gastric ultrasound performance. Each attempt for gastric ultrasound will be counted, afterwards adequate images will be counted. Change for succesfull image acquisition wil be calculated by adequate images divided by numbers of attemps. Both values indicate the demand according to its definition

The nurses accuracy and nurses' confidence to perform gastric ultrasound will be assessed. Accuracy will be measured by validating the ultrasound findings independently by expert sonographers and the nurses for their peers, determining inter-rater reliability.