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.