The study is a non-blinded trial with one single intervention in sick newborn infants. The Neosense umbilical catheter will be used instead of the routinely used umbilical catheter on infants where the treating physician has ordered use of an umbilical catheter. The catheter will be connected to the Neosense monitor/measuring unit.

Blood samples for blood gas analysis are collected from the patients according to the clinical routine (every 4th to 6th hour). Oxygen tension data from the Neosense measuring system, from the time points when a blood gas sample was collected, will be recorded. This data will be compared to the oxygen tension value from the blood gas sample collected at the same time point. The blood gas samples will be analysed according to clinical and laboratory routine.

The infants will remain in the study as long as the Neosense measuring system is used

Inclusion Criteria:

Sick newborn infants where the clinical routine indicates use of an umbilical catheter. At least one of the criteria below should be fulfilled:

• The infant needs invasive measurement of blood pressure.
• The infant needs repetitive sampling of blood.
• The infant needs prolonged infusion(s) > 2 days.
• The infant needs infusion of vessel irritating and potentially vessel harming solutions.
• The infant is born extremely preterm (before 28+0 weeks gestational age).
• A newborn infant with severe respiratory disorder, requiring oxygen treatment (more than 40% Fraction of inspired oxygen (FiO2)).
• The infant is undergoing therapeutic hypothermia following asphyxia "oxygen deficiency at birth").
• The infant has a severe infection/sepsis.

Also:

• Signed informed consent form by both parents (or guardians), (if the infant has only one parent (or guardian) one informed consent is required).

Exclusion Criteria:

• Gastroschisis
• Omphalocele
• Peritonitis
• Necrotizing enterocolitis
• Omphalitis
• The infant has a severe infection/sepsis
• Expected MRI investigation within the 7 first days after birth
• The infants birth weight is below 750 g
• The infant is born before week 25+0
• Local vascular compromise in lower limbs or buttocks area, or portal venous hypertension