Arterial Oxygen Saturation, Pulse Oximetry, and Cerebral and Tissue Oximetry in Hypobaric Hypoxia.

Clinical accuracy of pulse oximeters (giving So₂) is routinely tested down to an So₂ of 70%, but lower oxygen saturations are often experienced during hypobaric hypoxia. Cerebral (So₂) and peripheral tissue (So₂) oxygen saturations can be measured using near infra-red spectroscopy. In a project simulating oxygen system failure during high altitude-high opening parachuting (HAHO), So₂, So₂, So₂, and forearm So₂ were measured. The aim of the present analysis was to explore the agreement between So₂ and the three noninvasive measurements of hypoxemia (So₂, So₂, and So₂). 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 (1000 ft · min) to ground level simulating a HAHO parachute flight. So₂ was compared with So₂, So₂, and So₂ in scatterplots and Bland-Altman plots, calculating bias and limits of agreement (LOA). The bias ± LOA were: So₂ vs. So₂: -5.8% ± 16, So₂ vs. So₂: -3.4% ± 11, and So₂ vs. So₂: 17% ± 30. The bias for So₂ vs. So₂ was dependent on the range of values, and correcting for this with a sloped bias line reduced the LOA to ± 8.2%. There were wide limits of agreement between So₂ and So₂. So₂ and So₂ agreed better, whereas So₂ and forearm So₂ had wide LOA. The agreement between So₂ and So₂ improved when correcting for the underestimation of So₂ at low values. There is a poor agreement between So₂ and the gold standard So₂ during extreme hypobaric hypoxemia..