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Measured and calculated SvO2: do they alter clinical decisions?


Blood gas analysis (BGA) and PA oximetry catheters (PAOC) used to determine mixed venous oxygen saturation (SvO2) are based on fundamentally different technologies and thus they often produce discrepant values [1]. Directly measured SvO2 by the PAOC is the criterion standard against which calculation of SvO2 from PvO2 by BGA is judged.


We investigated the accuracy of SvO2 determination between BGA (AVL 995-Hb) and PAOC (Opticath, PA Catheter P 7110, Abbot) in 61 critically ill ICU patients. We had 244 couples' of SvO2 values simultaneously determined by the two different technologies.


Results, descriptive statistics and correlation coefficients are shown the Table. The difference between measured and calculated SvO2 was statistically significant (P < 0.000).



Calculation of SvO2 using BGA technology is always higher than PAOC SvO2 direct measurement by 1.6%. Although this difference is statistically significant (P < 0.00) the correlation between the two methods is quite high (r = 0.828, P < 0.01). BGA significantly overestimates SvO2 in comparison to PAOC. These results suggest that calculated SvO2 may affect therapeutic decisions in comparison to directly measured SvO2 because the slope of the oxyhemoglobin dissociation curve is very steep in the usual SvO2 range and thus small changes in the determination of PvO2 will result in relatively large changes in calculated saturation [1]. Also, minor calculated hemoglobin saturation differences in this steep part of the curve represent major differences in hemoglobin O2 carrying capacity.


  1. Bowton D, Scuderi P: Monitoring of mixed venous oxygenation. In Principles and Practice of Intensive Care Monitoring, Chapter 19. Edited by T Martin. McGraw-Hill, Inc, 1998, 303-315.

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Myrianthefs, P., Ladakis, C., Fildissis, G. et al. Measured and calculated SvO2: do they alter clinical decisions?. Crit Care 5 (Suppl 1), P156 (2001).

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