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J Crit Care. 2010 Sep;25(3):542.e9-13. doi: 10.1016/j.jcrc.2010.04.009. Epub 2010 Jul 23.

Pulse oximetry saturation to fraction inspired oxygen ratio as a measure of hypoxia under general anesthesia and the influence of positive end-expiratory pressure.

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  • 1Department of Anesthesiology, University of Michigan Health System, Ann Arbor, MI 48109, USA.



In ICU patients with acute lung injury, the pulse oximetry saturation (Spo(2)) to fraction of inspired oxygen (Fio(2)) (S/F) ratio is a reliable surrogate measure for the P/F (Pao(2)/Fio(2)) ratio. Our goal was to determine the correlation of the S/F to the P/F in a large sample of patients undergoing general anesthesia and the influence of positive end-expiratory pressure (PEEP) on this measure.


We studied adult general anesthetics performed with arterial blood gas analysis. Intraoperative data were collected from an anesthesia information system. The S/F ratios corresponding to P/F ratios of 300 were determined.


A total of 4439 values were collected. Linear correlation between S/F and P/F was identified (r = 0.46; P < .01) with a P/F of 300 corresponding to an S/F of 206. The correlation was stronger in patients with 5 to 9 cm PEEP (r = 0.52; P < .01), more than 9 cm H(2)O PEEP (r = 0.68; P < .01), and a P/F ratio of 300 or less (r = 0.61; P < .01).


The S/F correlates with the P/F in our cohort of patients undergoing general anesthesia, especially those ventilated with PEEP more than 9 cm H(2)O and/or with P/F less than 300. It has use as a noninvasive measure to screen for increased pulmonary dysfunction and to trend oxygenation during a general anesthetic.

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