Conjunctival oxygen tension and its relationship to arterial oxygen tension

Using a miniaturized Clark electrode embedded in a polymethylmethacrylate eyepiece, we measured transconjunctival oxygen tension (PcjO₂) in 5 healthy volunteer subjects at multiple levels of steady-state isocapnic hypoxia, normoxia, and hyperoxia. PcjO₂ was linearly related to arterial oxygen tension (PaO₂) as PaO₂ ranged from 28 to 205 mm Hg (PcjO₂=0.59 PaO₂+0.36 mm Hg;r=0.94; standard error of the estimate=7.09 mm Hg). However, the relationships between PcjO₂ and PaO₂ varied significantly among subjects. Whereas the overall mean ratio of PcjO₂ to PaO₂ was 0.59, the mean ratio for subjects ranged from 0.47 to 0.79 and was significantly different among subjects (P<0.0001). The time response of the electrode to a step change in oxygen tension in vitro was exponential, with a 90% response time of 38 seconds after a lag of 3.7 seconds. The time responses to in vivo changes in oxygen tension were also exponential. From hypoxia to normoxia, 90% response time was 45.0 seconds after a lag of 5.1 seconds; from room air to hypoxia, 90% response time was 72.4 seconds after a lag of 30.3 seconds; from room air to hyperoxia, 90% response time was 87.2 seconds after a lag of 6.8 seconds. We conclude that, although PcjO₂ measured by a miniaturized Clark electrode is linearly related to PaO₂ in healthy subjects, variation in the relationship of PcjO₂ to PaO₂ among individuals will prevent precise estimation of PaO₂ for any individual unless subject-specific calibration is performed.