Reprint & Copyright 9 by Aerospace Medical Association, Washington, DC Conjunctival Oxygen Tension at High Altitude THOMAS H. MADER, M.D., KARL E. FRIEDL, M.A., Ph.D., LAWRENCE C. MOHR, M.D., and WILLIAM N. BERNHARD, M.D. Ophthalmology Service and Department of Clinical Investigation, Madigan Army Medical Center, Tacoma, Washington; Department of Medicine, Walter Reed Army Medical Center, Washington, DC; and Department of Anesthesiology, New York University Medical Center, New York MADER TH, FRIEDL KE, MOHR LC, BERNHARD WN. Conjunctival oxygen tension at high altitude. Aviat. Space Environ. Med. 1987; 58:76-9. Transconjunctival oxygen tension (PciO2) was studied using a hypobaric chamber and during mountaineering excursions. Measurements obtained during acute chamber exposures (15- 20 rain) at sea level, 1829 m (6000 ft), 3048 m (10,000 ft), 4267 m (14,000 ft) and return to sea level were (means • 60.1 • 49.1 • 38.3 • 27.4 • and 61.1 • mm Hg, respectively (n = 13). The ratio of Pcio2 to arterial blood oxygen tension (Pao2) did not change in a consistent manner between sea level and 4267 m; PciO2 was 74 • of Pac~. The 16 subjects participating in the mountaineering phase of the study revealed similar means at sea level and 1829 m (57.4 • and 46.3 • mm Hg respectively), but a smaller decrement was observed at 3048 m (43.0 • mm Hg). The difference between mountain and chamber values may be accounted for by a partial acclimatization to altitude brought about by longer exposure on the mountain excursions. A comparison between PciO2 and transcutaneaus oxygen tension during the chamber study suggests that a greater precision and sensitivity is obtained with measurement of oxygen tension at the conjunctival site. PciO2 measurement is a non-invasive reflection of PaO2 which is suitable for continuous monitoring during hypoxia studies. T HE CONJUNCTIVAL oxygen monitor is a recently developed device which measures oxygen tension at the palpebral conjunctiva. This measurement reflects perfusion and oxygenation in the conjunctival vascular bed. The conjunctival bed is normally supplied by the internal carotid artery via the ophthalmic artery. Transconjunctival oxygen tension (Pci02) has been reported to correlate well with arterial oxygen tension This manuscript was received for review in December 1985. The revised manuscript was accepted for publication in March 1986. Please address reprint requests to: LTC Thomas Mader, MC, USA, Chief, Ophthalmology Service, Madigan Army Medical Center, Tacoma, WA 98431. (PaOz). Nevertheless, Pcio2 is lower than PaO2 and in sea level studies the measured PciO2 is reported to be 71 +4.6% of the Pao2 measurement (1). The conjunctival sensor consists of a miniaturized platinum Clark electrode with an oxygen permeable membrane. The electrode is laminated into the edge of a smooth, hollow-centered polymethylmethacrylate con- former. The conformer is inserted into the conjunctival fornices, placing the electrode in direct contact with the superior temporal conjunctiva (Fig. 1). Oxygen passing through the oxygen permeable membrane is reduced at the electrode, producing an electrical charge and this is displayed on the monitor as a partial pressure of oxygen after correction for measured conjunctival temperature. The monitor requires a two point calibration using zero and the ambient partial pressure of oxygen. We tested the transconjunctival oxygen monitor in high altitude environments, real and simulated, in order to establish normal values for PolO2 at altitudes above sea level. The measurements were also compared to transcutaneous oxygen tension measurements and PaO2 in order to identify any alterations in the relationship between these different measurements. MATERIALS AND METHODS The study consisted of two parts. Part One was performed in a hypobaric chamber and Part Two was performed during two mountaineering excursions. In all cases, voluntary consent was obtained from the subjects prior to the study. Only two climbers in Part Two, citing personal reasons, chose not to take part in the experiments. There were 15 volunteers between the ages of 18 and 36 selected for the hypobaric chamber study. All were healthy and none took any medications. No one had lived above 915 76 Aviation,Space, and Environmental Medicine 9 January, 1987