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 (H₂O) content. We estimated tissue H₂O in severe sepsis patients and healthy controls, compared their difference, and investigated the correlation of tissue H₂O 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 ([HbO₂]), deoxy-Hb ([HbR]), total Hb ([HbT]), and H₂O ([H₂O]) at rest for three consecutive days. Two-sample t-test or Mann-Whitney U test, chi-square test, and generalised estimating equations (GEEs) were used for comparisons. Results: In severe sepsis patients, the [H₂O] in the anterior tibia was higher [mean (standard deviation, 95% confidence interval), 10.57 (3.37, 9.81–11.34) vs. 7.40 (1.89, 6.70–8.11)] and the [HbO₂], [HbT], and tissue Hb oxygen saturation (StO₂) were lower [0.20 (0.01, 0.20–0.20) vs. 0.22 (0.01, 0.22–0.23), 0.42 (0.02, 0.42–0.43) vs. 0.44 (0.02, 0.44–0.45), and 47.25% (1.97%, 46.80–47.70%) vs. 49.88% (1.26%, 49.41–50.35%), respectively] than in healthy controls in first-day measurements. GEE analysis revealed significant differences in [H₂O], [HbO₂], [HbT], and StO₂ between groups over three consecutive days (all P≤0.001). In addition, [HbO₂] and StO₂ levels gradually decreased over time in the patient group. A negative correlation was observed between [H₂O] and [HbO₂] and StO₂, 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 [H₂O] (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 H₂O. Severe sepsis patients have increased tissue H₂O, which responds to changes in arterial blood pressure and affects tissue oxygenation.