The goal of this clinical trial is to learn more about intestinal regional oxygen saturation measurements made with near-infrared spectroscopy and bladder pressure measurements in infants without risk of intraabdominal hypertension. The main question it aims to answer is if - in comparison to bladder pressure - the regional intestinal oxygen saturation measured with near-infrared spectroscopy is stable in the muscle-relaxed, intubated patients and the awake and non-sedated patient. In case of participation the bladder pressure and the regional intestinal oxygen saturation (measured with near-infrared spectroscopy) will each be measured once intraoperatively and once postoperatively. Patients included in this study will be undergoing an operation which necessitates muscle-relaxation, as well as an indwelling urinary catheter during the operation and for a short-time thereafter for other reasons than this study.