Increased tissue water in patients with severe sepsis affects tissue oxygenation measured by near-infrared spectroscopy: a prospective, observational case-control study.

Background: Tissue oedema affects tissue perfusion and interferes with the monitoring of tissue oxygenation in patients with severe sepsis. However, the underlying mechanisms remain unclear. We used a wireless near-infrared spectroscopy (NIRS) device that transmits tri-wavelength light to quantify tissue haemoglobin (Hb) and water (HO) content. We estimated tissue HO in severe sepsis patients and healthy controls, compared their difference, and investigated the correlation of tissue HO with systemic haemodynamics and its impact on tissue oxygenation.

Methods: Seventy-seven adult patients with new-onset severe sepsis admitted to the intensive care unit within 72 h and 30 healthy volunteers (controls) were enrolled. The NIRS device was placed on the participant's leg to estimate the relative tissue concentrations of oxy-Hb (), deoxy-Hb (), total Hb (), and HO () at rest for three consecutive days. Two-sample -test or Mann-Whitney U test, chi-square test, and generalised estimating equations (GEEs) were used for comparisons.

Results: In severe sepsis patients, the in the anterior tibia was higher [mean (standard deviation, 95% confidence interval), 10.57 (3.37, 9.81-11.34) 7.40 (1.89, 6.70-8.11)] and the , , and tissue Hb oxygen saturation (StO) were lower [0.20 (0.01, 0.20-0.20) 0.22 (0.01, 0.22-0.23), 0.42 (0.02, 0.42-0.43) 0.44 (0.02, 0.44-0.45), and 47.25% (1.97%, 46.80-47.70%) 49.88% (1.26%, 49.41-50.35%), respectively] than in healthy controls in first-day measurements. GEE analysis revealed significant differences in , , , and StO between groups over three consecutive days (all P≤0.001) In addition, and StO levels gradually decreased over time in the patient group. A negative correlation was observed between and and StO, which became more obvious over time (day 1: r=-0.51 and r=-0.42, respectively; both P<0.01; day 3: r=-0.67 and r=-0.63, respectively, both P<0.01). Systolic arterial pressure was positively related to (r=0.51, P<0.05, on day 1) but was not associated with tissue oxygenation parameters.

Conclusions: NIRS can be used to quantify tissue HO. Severe sepsis patients have increased tissue HO, which responds to changes in arterial blood pressure and affects tissue oxygenation.