In critically ill patients, arterial blood gas analysis is the gold standard for evaluating systemic oxygenation and carbon dioxide partial pressure. A new miniaturized carbon dioxide tension Pco₂-Spo₂ single sensor (TOSCA, Linde Medical Sensors AG, Basel, Switzerland) continuously and noninvasively (transcutaneously) monitors both Paco₂ and oxygen saturation by pulse oximetry (Spo₂). The present study was designed to investigate the usability and the accuracy of this device in critically ill patients.