Positive central-mixed venous oxygen saturation gradients: high oxygen saturation in the inferior vena cava confirms high splanchnic oxygen extraction

Introduction

Central venous oxygen saturation (ScvO₂) is increasingly used as a surrogate for mixed venous oxygen saturation (SvO₂). On average, there is a positive gradient between ScvO₂ and SvO₂ that has been explained by the low inferior vena cava saturation (SivcO₂). We aimed to clarify the dynamics and associations between different venous saturations in an experimental setting of porcine peritonitis.

Methods

Thirty-two anaesthetized pigs (40.3 ± 3.8 kg (mean ± SD)) were randomly assigned (n = 8 per group) to a nonseptic control group or one of three septic groups in which the pigs were observed for 6, 12 or 24 hours. Thereafter, resuscitation was performed for 48 hours. The pulmonary artery, superior vena cava and inferior vena cava (IVC) were catheterized. The catheter for IVC measurements was placed 5 cm below the diaphragm. SvO₂, ScvO₂ and SivcO₂ were measured at 12-hour intervals starting at study baseline. Differences between saturations at different time points were tested with a t test for paired measurements.

Results

One hundred and ninety-two (136 in septic and 56 in control animals) simultaneous measurements of SvO₂, ScvO₂ and SivcO₂ were analysed. Mean SvO₂ was 58.7 ± 7.2%, ScvO₂ 61.5 ± 8.3% and SivcO₂ 66.7 ± 8.5%. Dynamics of the saturations throughout the study are presented in Figure 1. ScvO₂ was numerically higher than SvO₂ in 133 (69.3%) of all measurements. In 122 of these 133 measurements (91.7%), SivcO₂ exceeded SvO₂ as well.

Figure 1

Dynamics of mixed venous, superior and inferior vena cava saturations. §Difference between SvO₂ and ScvO₂, P < 0.05. *Difference between SvO₂ and SivcO₂, P < 0.005.

Conclusions

In most of the measurements, both ScvO₂ and SivcO₂ were higher than SvO₂. Our results suggest a high oxygen extraction of splanchnic organs as the reason for positive ScvO₂-SvO₂ gradients.