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

Abstract

INTRODUCTION: Clinical accuracy of pulse oximeters (giving Spo₂) is routinely tested down to an Sao₂ of 70%, but lower oxygen saturations are often experienced during hypobaric hypoxia. Cerebral (Sco₂) and peripheral tissue (Sto₂) oxygen saturations can be measured using near infra-red spectroscopy. In a project simulating oxygen system failure during high altitude-high opening parachuting (HAHO), Sao₂, Spo₂, Sco₂, and forearm Sto₂ were measured. The aim of the present analysis was to explore the agreement between Sao₂ and the three noninvasive measurements of hypoxemia (Spo₂, Sco₂, and Sto₂).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. Sao₂ was compared with Spo₂, Sco₂, and Sto₂ in scatterplots and Bland-Altman plots, calculating bias and limits of agreement (LOA).RESULTS: The bias ± LOA were: Sao₂ vs. Spo₂: -5.8% ± 16, Sao₂ vs. Sco₂: -3.4% ± 11, and Sao₂ vs. Sto₂: 17% ± 30. The bias for Sao₂ vs. Spo₂ 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 Sao₂ and Spo₂. Sao₂ and Sco₂ agreed better, whereas Sao₂ and forearm Sto₂ had wide LOA. The agreement between Sao₂ and Spo₂ improved when correcting for the underestimation of Spo₂ at low values. There is a poor agreement between Spo₂ and the gold standard Sao₂ 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.