Noninvasive measurement of conjunctival PCO2 with a fiberoptic sensor

A miniaturized fiberoptic PCO2 probe permits direct measurement of tissue PCO2 on nonkeratinized tissue surfaces without the heating effects produced by transcutaneous PCO2 sensors. This study of anesthetized dogs compared PCO2 measured at the palpebral conjunctiva (PcjCO2) with PaCO2 and mixed venous (PVCO2) measurements during normovolemic normotension and hypovolemic hypotension. During the control period, the average PcjCO2 was 3 +/- 1 (SEM) torr greater than PaCO2 (r = 0.98) when the latter ranged from 20 to 80 torr. There was a close association (r = 0.94) between PcjCO2 and PaCO2 when the cardiac index (CI) was greater than 2.0 L/min X m2. However, as CI decreased below this value, PcjCO2 and PaCO2 were less well correlated (r = 0.69). PcjCO2 was closely associated with PVCO2 at all stages of the experiment, both above (r = 0.95) and below (r = 0.82) a CI of 2 L/min X m2. Fiberoptic conjunctival PCO2 monitoring seems promising as a noninvasive measure of physiologic status.