Sublingual NIRS and reflectance spectrophotometry: new methods to monitor sublingual oxygen availability

Introduction

Sublingual hemodynamic monitoring (with orthogonal polarization spectral imaging) [1] and metabolic monitoring (with capnography) [2] has revealed detailed information about the microcirculatory and cellular dysfunction in sepsis and shock. These techniques, however, give no information about oxygen availability. To this end we hypothesized that NIRS and reflectance spectrophotometry can be used to monitor sublingual oxygen availability in a non-invasive and continuous manner. We tested this hypothesis in patients during cardiac surgery with cardiopulmonary bypass (CPB) as a clinical environment, where large changes in blood flow and oxygen availability occur.

Methods

NIRS and reflectance spectrophotometry were used to measure the microcirculatory haemoglobin oxygen saturation and haemoglobin concentration in deeper layers (0–23 mm; InSpectra^®; Hutchinson Technology, Arnhem, The Netherlands) and superficial layers (0–8 mm; O2C^®; Lea Medizintechnik, Giessen, Germany), respectively. Measurements were made before and just after (<15 min) CPB was initiated.

Results

Group A (deeper layers, n = 9): six males, three females, 72 ± 6 years, BSA 1.83 ± 0.19 m². Group B (superficial layers, n = 11): five males, six females, 70 ± 8 years, BSA 1.81 ± 0.21 m². The microcirculatory haemoglobin oxygen saturation in group A decreased significantly from 91.9 ± 5.0 to 86.8 ± 6.7% (P < 0.01), whereas the microcirculatory haemoglobin oxygen saturation in group B showed a significant increase from 51.3 ± 4.8 to 62.6 ± 6.8% (P < 0.01). In both groups the systemic haemoglobin concentrations decreased significantly after a switch to CPB (from 7.5 ± 1.0 to 5.3 ± 1.1 mmol/l, P < 0.01, respectively, from 7.4 ± 1.3 to 4.6 ± 0.9 mmol/l, P < 0.01). The sublingual haemoglobin concentrations measured by the different techniques in the different layers showed, however, both a decrease in a similar manner.

Conclusion

The deeper layers of sublingual tissue monitored by NIRS showed a significant decrease of oxygen availability, whereas the superficial layers showed a significant increase of oxygen availability. It is suggested that redistribution of oxygen availability during CPB can be monitored in this way. It is concluded that monitoring sublingual oxygen availability by these techniques in combination with orthogonal polarization spectral imaging and sublingual capnography will provide comprehensive and integrative information about the functional state of the sublingual microcirculation.