End-tidal CO2 measurements in children will be assessed for their accuracy with arterial CO2 measurements.

End-tidal and arterial CO2 measurements will be performed in invasively and non-invasively ventilated critically ill children. The endtidal CO2 values will assessed for their accuracy with respect to different underlying diseases and conditions of the children.

Inclusion Criteria:

• Children on pressure controlled mechanical invasive or non-invasive ventilation on the Paediatric Intensive Care Unit of the University Children's Hospital Zurich
• Newborns with a birthweight of at least 2.0 kg
• Newborns with an age of at least 1 hour (age > 60 minutes)
• Children up to the last day of the 13th year of living
• Ability of care taker or patient to understand verbal and written instructions and the general consent or informed consent in German or English
• Obtained written general or informed consent as documented by signature
• Available arterial line, i.e. a specific catheter inserted in an artery

Exclusion Criteria:

• Care taker or participant unable for linguistic, mental or other reasons or unwilling to understand verbal or written information and to give written informed consent in German or English
• Care taker not available
• Newborns with a birthweight below 2.0 kg
• Newborns younger than 1 hour (age <60 minutes)
• Children with an age of 14 years onwards
• Missing arterial line
• Patients dependent on any other kind of respiratory support that is not compatible with the PcCO2-sensor or where a high leakage in the respiratory circuit makes PcCO2 measurements impossible (nasal mask ventilation, low-flow-/high-flow-ventilation, rebreathing mask, high- frequency-oscillation)
• Patients where the investigators act on the assumption that mechanical ventilation will be discontinued and/or the arterial line will be removed within 6 hours or where only one pair of values (arterial and endtidal CO2) can be compared
• Patients with a cyanotic shunt lesion with a weight of 15 kg or above

Critically ill infants and children under invasive and non-invasive mechanical ventilation, regardless of their underlying acute or chronic diseases, as long as an arterial line for blood gas analyses is in situ.