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. Author manuscript; available in PMC: 2019 Jul 14.
Published in final edited form as: Can J Physiol Pharmacol. 2018 Oct 26;97(3):174–182. doi: 10.1139/cjpp-2018-0376

Fig. 2.

Fig. 2.

Relationship between oxyhemoglobin saturation, arterial partial pressure of oxygen (PaO2), and arterial oxygen content (CaO2). Oxyhemoglobin saturation measured by pulse oximeter (SpO2) has an accuracy of 3% when compared with the true arterial oxyhemoglobin saturation (SaO2) by co-oximetry (1 standard deviation (SD) = 3%; rectangular area represents 1 SD, and thus 68% of values; triangular areas represent 2 SD, so 95% of values). An SpO2 value of 95% can therefore represent an SaO2 between 92% and 98%. This represents a wide range in PaO2, and since adult hemoglobin (Hgb A) has a lower affinity for oxygen, for any given SaO2, the PaO2 is higher in blood containing Hgb A. Note the impact of hemoglobin on oxygen content (CaO2).