A noninvasive tool to recognize early shock would improve outcome by providing prompt recognition of tissue ischemia and precise resuscitation endpoint. The skin is the first tissue bed to vasoconstrict in shock states. Studies have demonstrated that transcutaneous partial pressure of oxygen (PtCO2) increases with higher FiO2 in nonshock states as arterial pressure of oxygen (PaO2) increases, but in shock situations, PtCO2 mirrors changes in cardiac output and oxygen delivery with minimum response to increasing FiO2 and PaO2. This study examined the relationship of hemodynamic variables and the degree of PtCO2 response to FiO2 of 1.0 (identified as the "oxygen challenge test") to mortality and organ failure. This prospective observational study examined 38 patients requiring at least 24 h of cardiac output monitoring for shock resuscitation in the Surgical Intensive Care Unit. Patients were resuscitated to the standard protocol of blood pressure, urine output, oxygen delivery (DO2), and mixed venous O2 (SvO2). Seventy-nine percent of the patients (30/38) with a mean age of 59 +/- 21 years had septic shock or severe sepsis with a 26% mortality (10/38). Measurements included hemodynamic variables, PtCO2, and outcome (mortality and organ failure). In this study, the ability of PtCO2 value to increase by 21 mmHg on a FiO2 of 1.0, at 24 h of resuscitation, divided survivors from nonsurvivors, P <.001. The PtCO2 response to FiO2 may provide an additional noninvasive method of detecting early shock as well as a specific endpoint of resuscitation.