Variations in near-infrared spectroscopy-derived oxygen downslope during a vascular occlusion test in critically ill patients: relationship with outcome

Introduction

Near-infrared spectroscopy (NIRS) with a vascular occlusion test (VOT) can be used to extrapolate information regarding the tissue oxygen extraction rate. We explored the meaning of variations in tissue oxygen saturation downslope (StO₂down) during a VOT in critically ill patients.

Methods

In this prospective observational study, NIRS (thenar eminence) was applied every day in 93 patients admitted to the ICU. A VOT was performed using a 40% StO₂ target. The slope of the desaturation curve was assessed separately for the first part (StO₂ down1) and the last part (StO₂ down2) of the curve and the difference between Down2 - Down1 was calculated.

Results

No significant differences were seen in StO₂ Down1 or Down2 between ICU survivors (n = 76) and ICU nonsurvivors (n = 17) over the first 10 days in the ICU, while Down2 - Down1 was higher in ICU nonsurvivors (Figure 1). Patients in the upper quartile of mean Down2 - Down1 showed the highest 90-day mortality (P = 0.014).

Figure 1

Conclusion

ICU nonsurvivors tended to show a flattening in the last part of the desaturation curve during a VOT, suggesting a reduced tissue oxygen extraction. This may depend on microvascular dysfunction and/or cellular hypometabolic status.