Effects of hemoglobin concentration, skull thickness, and the area of the cerebrospinal fluid layer on near-infrared spectroscopy measurements

Background: Previous studies documented that near-infrared spectroscopy values were affected by factors related to optical path length, such as hemoglobin concentration, the differential path length factor, skull thickness (t-skull), and die area of the cerebrospinal fluid layer (a-CSFL). lately, the NIRO-100 (Hamamatsu Photonics, Hamamatsu, Japan) has provided a tissue oxygen index (TOI) that theoretically is not supposed to be affected by optical path length. Therefore, die authors hypothesized that TOI is not influenced by the above-described individual factors. Methods: Cardiac surgical or neurosurgical 103 patients (65 men and 39 women; aged 63 +/- 14 yr) were studied. TOI and regional cerebral oxygen saturation (rSO(2)) (INVOS 4100; Somanetics, Troy, MI) were measured sequentially on patients in a resting state. The t-skull and a-CSFL were calculated using computed tomographic image slices of the head corresponding with the position of near-infrared spectroscopy sensors. The effects of these two factors, hemoglobin concentration and mean arterial pressure, on TOI and rSO(2) values were evaluated by linear regression analysis. Results: Simple linear regression analysis showed that mean arterial pressure (r = 0.27, P = 0.008), t-skull (r = 0.22, P = 0.034), a-CSFL (0.26, P = 0.012), and hemoglobin concentration (r = 0.42, P < 0.0001) were significant determinants of rSO(2). Multiple linear regression analysis showed that hemoglobin concentration (r = 0.34, P < 0.001), a-CSFL (r = -0.252, P = 0.012), and t-skull (r = 0.22, P = 0.037) were significant determinants of rSO(2). On the other hand, simple and multiple linear regression analysis showed that there was no significant determinant of TOI. Conclusion: rSO(2) values were affected by hemoglobin concentration, a-CSFL, and t-skull, but TOI values were not affected by individual factors.