Question: How does bilirubin affect the measurement of StO2?

The results published by others, and our own unpublished results indicate the following:

1. Bilirubin does not appear to affect the InSpectra™ StO₂ System Measurement when present at a physiologically low concentration, not higher than 2 mg/dL.
2. It remains to be established whether bilirubin affects the InSpectra StO₂ Measurement when present at significantly higher concentrations, such as encountered in hyperbilirubinemia. However, it should be noted that high levels of bilirubin, accompanied by excessive red blood cell hemolysis and elevated rates of heme degradation, may affect the InSpectra StO₂ Measurement by elevating StO₂ readings via an increase in tissue blood flow.

Background

Because red blood cells are short-lived and constantly undergo natural degradation, the metabolic degradation of heme and formation of bilirubin are recurring physiological phenomena. Bilirubin is present in the blood in normal physiological concentrations (usually less than 2.5 mg/dL) along with oxygenated (HbO₂) and de-oxygenated (Hb) hemoglobin.

Results published by others and our own unpublished data¹ show that bilirubin at concentrations up to 2 mg/dL absorbs light only in a visible range below 680 nm. This allows one to reasonably conclude that at its normal physiological range, bilirubin does not appear to affect the InSpectra StO₂ Measurement.

Hyperbilirubinemia may present a different story. Hyperbilirunemia occurs when the bilirubin concentration is higher than 2.5 mg/dL. At high concentrations bilirubin causes jaundice, which is reflected in the distinct discoloration of the skin and sclera of the eye. While no results exist showing spectral interference (or lack thereof) with the InSpectra StO₂ Measurement at concentrations higher than 2 mg/dL, other publications may be instructive. The interference of bilirubin with NIRS-derived measurement of cerebral oxygenation (ScO₂) was observed in patients with jaundice who underwent liver transplantation². The absorbance range of ScO₂ is similar to that used by the InSpectra™ StO₂ Tissue Oxygenation Monitor. A spectral interference in a similar absorbance range was also observed when bilirubin and biliverdin at high concentrations were tested together in a mixture with hemoglobin³.

Bilirubin’s influence on patient physiology may also affect the InSpectra StO₂ Measurement. Bilirubin, and its metabolic pre-cursor biliverdin, both have cyto-protective abilities and can minimize oxidative stress⁴. This serves to improve tissue oxygenation, which would be manifested in elevated StO₂ readings. In addition, carbon monoxide, which is generated in the same metabolic pathway as the heme degradation, has been shown to play an important role in regulating vascular tone causing vessel dilatation and increased blood flow⁵. Vessel dilatation and increased blood flow may also lead to elevated StO₂ readings.

References

1. Merrick MF, Pardue HL. Evaluation of absorption and first- and second-derivative spectra for simultaneous quantification of bilirubin and hemoglobin. Clin Chem. 1986;32(4):598-602.

2. Zarr S. Internal HTI Document: Log Book Number 1994, 1997, p.55.

3. Madsen PL, Skak C, Rasmussen A, Secher NH. Interference of cerebral near-infrared oximetry in patients with icterus. Anesth Analg. 2000;90(2):489-493.

4. Samsoondar, J. Indicator of hemolysis. United States Patent, Patent No: US 6,689,612 B2. 2004, 1-58.

5. Durante W. Carbon monoxide and bile pigments: surprising mediators of vascular function. Vascular Medicine. 2002;7(3):195-202.