Transcutaneous (PtcO2) and conjunctival (PcjO2) oxygen tensions and transcutaneous (PtcCO2) carbon dioxide tension were serially measured in 31 critically ill patients. Sixteen patients maintained a normal or greater blood pressure and 15 patients were severely hypotensive (MAP less than 60 mm Hg) or suffered cardiac arrest while in the emergency department. In hemodynamically stable patients, the correlations between PtcO2 and PaO2, PcjO2 and PaO2, and PtcCO2 and PaCO2 were significant, with correlation coefficients (r values) of 0.62, 0.48, and 0.73, respectively. In hemodynamically unstable patients, there was poor correlation between PtcO2, PcjO2, PtcCO2, and arterial blood gas values. In the severely hypotensive patients, however, PtcO2 and PcjO2 were sensitive real-time monitors of peripheral perfusion and tissue oxygen delivery. Increases in PcjO2 and PtcO2 occurred within three minutes in patients who regained a palpable blood pressure from a pulseless baseline. PtcO2 fell one to five minutes after the loss of blood pressure in patients whose clinical condition deteriorated in the emergency department. Decreases in PcjO2 from previously stable levels occurred over three to four minutes before loss of blood pressure. Transcutaneous and conjunctival sensors can be used as continuous monitors of respiratory status in hemodynamically stable patients. In severely hypotensive patients and during cardiopulmonary resuscitation, these sensors no longer accurately reflect arterial blood gases, but act as sensitive real-time monitors of cardiac function and peripheral perfusion. PcjO2 can detect deterioration of clinical state before alterations in blood pressure occur.