Blood copeptin will be measured during the routine treatment of neonates, children and adolescents on the Intensive Care Unit of the University Children's Hospital Zurich at different time points (admission, 12, 24, 48, 96, 168 hours after admission). These values will be primarily analysed for their variability and their association with arterial hypotension. Blood samples will be drawn together with otherwise medically indicated blood withdrawals to avoid extra harm. Further, copeptin values will be compared to clinical and vital parameters, all of them open-label available during clinical routine. Copeptin's predictive value for patients' outcome will be analysed as secondary outcome.

Inclusion Criteria:

• Age: first day of life until 18th birthday.
• Ability of the care taker or the adolescent (if≥14 years of age) to understand verbal and written instructions and informed consent in German.

Exclusion Criteria:

• Care taker or adolescent (if ≥14 years of age) unwilling to give written informed consent.
• Care taker or adolescent (if ≥14 years of age) not understanding German and without a family member able to translate.
• Adolescent (if ≥14 years of age) unwilling to give written informed consent following sedation < 24 hours.
• Care takers of long-term sedated (>24 hours) adolescents (if ≥14 years of age) unwilling to give written informed consent or not present within 24 hours.

This is a study designed to explore Copeptin values in all kinds of critically ill paediatric patients. Patients with normal blood pressures will allow calculating Copeptin reference values for the ones requiring cardiovascular support. Therefore, all patients, irrespective of their medical condition (e.g. medical or surgical, mildly or severely affected) and their age (neonates to adolescents) admitted to the Intensive Care Unit of the University's Children Hospital Zurich will be eligible, as long as they fulfil the mentioned inclusion criteria.