Critically ill patients of all species often have blood flow and acid/base problems. In small patients such as cats and human infants, the amount of blood available for safe removal to monitor acid/base status is very limited. Therefore, skin CO2 and O2 monitoring is frequently used in critically ill human infants to trend blood flow and acid/base status. In cats, obtaining such information currently requires taking blood samples, which can prove more detrimental (increases in patient stress and decreases in patient red blood cell mass) than justifiable by the potential information gain. Thus, this study is to determine if skin CO2 and O2 monitoring is feasible and potentially clinically useful in critically ill cats. Skin CO2 and O2 monitoring is non-invasive and continuous and could prove very useful to monitor critically ill cats without causing stress or discomfort.