Studies were performed on 44 patients who were monitored continuously with transcutaneous carbon dioxide (PtcCO2) sensors. The patients were monitored intermittently with arterial and mixed venous blood gases and full hemodynamic and oxygen transport data. Twenty of the studies were performed intraoperatively. A total of 411 data sets revealed a correlation coefficient, r, between arterial and transcutaneous PCO2of 0.80 when the patients were not in low flow shock, i.e., cardiac index (CI) > 1.5 L/min μ M2. On the basis of these data, the authors have found the normal arterial-transcutaneous carbon dioxide gradient, ΔCO2, (ΔCO2= PtcCO2– PaCO2) to be 23 ± 11 torr. The PtcCO2monitor was found to be a valuable trend monitor of arterial CO2tensions of adults during adequate cardiac function in the ICU and the operating room.Twenty-four data sets were collected while 3 patients were monitored during severe shock (CI < 1.5 L/min μ M2). PtcCO2trended inversely with changes in CI during shock and did not follow PaCO2(r = −0.85). During shock, ΔCO2= 61 ± 25 torr. The severity of shock could be roughly determined by comparing the PtcCO2values with arterial CO2tensions.