Serial TCCO2 measurements will be compared to PaCO2 levels measured on arterial blood gas (ABG) samples obtained simultaneously on these patients. The sensor will be calibrated and applied to the skin according to manufacturer instructions. Whenever an arterial blood gas is collected for clinical indications, the transcutaneous monitor's reading will be simultaneously recorded in a data log including the patient's initial diagnosis, gestational age, sex, sensor location, birth weight, and body temperature. The skin of each infant will be closely inspected for injury (such as irritation or breakdown) before the skin probe is applied, and after each time that the probe is removed. Application skin sites will also be photographed on entry to and exit from the study, and whenever the probe is removed.