INTRODUCTION: Clinical accuracy of pulse oximeters (giving S_po₂) is routinely tested down to an S_ao₂ of 70%, but lower oxygen saturations are often experienced during hypobaric hypoxia. Cerebral (S_co₂) and peripheral tissue (S_to₂) oxygen saturations can be measured using near infra-red spectroscopy. In a project simulating oxygen system failure during high altitude-high opening parachuting (HAHO), S_ao₂, S_po₂, S_co₂, and forearm S_to₂ were measured. The aim of the present analysis was to explore the agreement between S_ao₂ and the three noninvasive measurements of hypoxemia (S_po₂, S_co₂, and S_to₂).METHODS: Healthy volunteers from the Norwegian Special Operations Commando were studied in a hypobaric chamber as supplemental oxygen was removed at 301 hPa ambient pressure (30,000 ft) and recompressed at 25 hPa · min^-1 (1000 ft · min^-1) to ground level simulating a HAHO parachute flight. S_ao₂ was compared with S_po₂, S_co₂, and S_to₂ in scatterplots and Bland-Altman plots, calculating bias and limits of agreement (LOA).RESULTS: The bias ± LOA were: S_ao₂ vs. S_po₂: −5.8% ± 16, S_ao₂ vs. S_co₂: −3.4% ± 11, and S_ao₂ vs. S_to₂: 17% ± 30. The bias for S_ao₂ vs. S_po₂ was dependent on the range of values, and correcting for this with a sloped bias line reduced the LOA to ± 8.2%.DISCUSSION: There were wide limits of agreement between S_ao₂ and S_po₂. S_ao₂ and S_co₂ agreed better, whereas S_ao₂ and forearm S_to₂ had wide LOA. The agreement between S_ao₂ and S_po₂ improved when correcting for the underestimation of S_po₂ at low values. There is a poor agreement between S_po₂ and the gold standard S_ao₂ during extreme hypobaric hypoxemia.Ottestad W, Kåsin JI, Høiseth LØ. Arterial oxygen saturation, pulse oximetery, and cerebral and tissue oximetry in hypobaric hypoxia. Aerospace Med Hum Perform. 2018; 89(12):1045–1049.