Can near infrared spectroscopy measure the effect of pressure on oxygenation of sacral soft tissue?

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To test whether near infrared spectroscopy (NIRS) is applicable for the examination of the influence of external pressure on oxygenation of the soft tissues in the sacral area. Tissue oxygenation was measured in 33 healthy volunteers in the prone position. A NIRS probe was positioned over the sacrum and external pressure was applied in 10 mmHg increments, from 20 mmHg to 200 mmHg and then decreased. At each level, tissue oxygen saturation (StO2) was measured. To test reproducibility, the protocol was repeated in six volunteers, in whom the thickness of the soft-tissue envelope at different levels of external pressure was assessed using ultrasound. There was wide variability in StO2 courses between the 33 subjects, with a non-linear relationship between pressure and StO2. The only consistent finding was that the StO2 was significantly higher after decreasing pressure than at the initial pressure of 20 mmHg, which is indicative of reactive hyperaemia. Despite the application of high external pressures, reasonable tissue oxygenation was maintained in 19 of 33 subjects. Reproducibility of the measurements was poor. Comparison of soft-tissue thickness with corresponding StO2 values showed that, with increasing pressure, the percentage decrease in tissue thickness was higher than the decrease in tissue oxygenation. This study confirms that NIRS is not useful for assessing tissue oxygenation in pressure ulcer research due to unacceptable inter-individual variability and poor reproducibility of measurements.