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Relative influence of hypoxemia and anemia on the measurement of central venous oxygen saturation

Introduction

Central venous oxygen saturation (ScvO2) is frequently used as a surrogate measurement of adequacy of perfusion. However, ScvO2 is also affected by arterial oxygen saturation (SpO2), oxygen consumption, and hemoglobin (Hb) according to the formula: ScvO2 = SpO2 - (VO2/Q*1/Hb). The aim of this study was to investigate the relative influence of hypoxemia and anemia on the measurement of ScvO2.

Methods

A database of 700 pairs of arterial and central venous blood gases drawn from 300 patients admitted to the ICU of a university hospital was considered. After assessing for the technical adequacy of sampling (defined as a discrepancy of hematocrit and blood glucose between arterial and venous samples lower than 5%), 462 couples were selected for analysis. Samples were then clustered according to ScvO2: <70% (low), ≥70% (high). SpO2, partial pressure of oxygen (PaO2) and Hb were considered. Venous to arterial difference of partial pressure of CO2 (DpCO2), arterial to venous difference of oxygen content (DavO2) and the oxygen extraction ratio (ER) were also considered as measures of perfusion adequacy. Differences between low and high ScvO2 samples were estimated by Mann–Whitney rank sum test (Sigma Stat, SPSS), accepting P < 0.05 as significant. Data are presented as median (25th–75th percentile).

Results

ScvO2 was 62.5% (56.3–66.2) in the low group (n = 180), 76.7% (73.6–80.7) in the high group (n = 282). In the low group values were: SpO2 95.0% (95.1–98.3), PaO2 72.0 mmHg (59.5–112.5), DpCO2 8 mmHg (6–9), DavO2 4.3 ml/100 ml (3.4–5.2), ER 0.346 (0.310–0.399), Hb 9.9 g/dl (8.9–11.1). In the high group values were: SpO2 98.3% (96.2–99.5), PaO2 106.5 mmHg (81.0–167.0), DpCO2 6 mmHg (4–7), DavO2 2.9 ml/100 ml (2.3–3.6), ER 0.215 (0.185–0.250), Hb 9.9 g/dl (8.7–11.1). As expected, DpCO2, DavO2 and ER were different between high and low ScvO2 groups (P < 0.001). However, while Hb was similar (P = 0.670), SpO2 and PaO2 were significantly lower when ScvO2 was below 70% (P < 0.001). Normalization of ScvO2 to SpO2 (ScvO2/SpO2) allowed one to overcome the effects of hypoxemia. Values were: 0.662 (0.603–0.693) in the low group and 0.794 (0.757–0.828) in the high group (P < 0.001).

Conclusion

When considering ScvO2 as a surrogate measure of perfusion adequacy, it is mandatory to consider the relative effect of hypoxemia. Anemia was less relevant in our case mix.

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Valenza, F., Froio, S., Fagnani, L. et al. Relative influence of hypoxemia and anemia on the measurement of central venous oxygen saturation. Crit Care 11, P304 (2007). https://doi.org/10.1186/cc5464

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Keywords

  • Relative Influence
  • Extraction Ratio
  • High Group
  • Arterial Oxygen Saturation
  • Oxygen Extraction