Characterization of tissue oxygen saturation and the vascular occlusion test: influence of measurement sites, probe sizes and deflation thresholds

Introduction: Tissue oxygen saturation (StO(2)) and the vascular occlusion test (VOT) can identify tissue hypoperfusion in trauma and sepsis. However, the technique is neither standardized nor uses the same monitoring site. We hypothesized that baseline and VOT StO(2) would be different in the forearm (F) and thenar eminence (TH) and that different minimal StO(2) values during the VOT would result in different reoxygenation rates (ReO2). Methods: StO(2) and its change during the VOT were simultaneously measured in the F and TH, with 15 mm and 25 mm probes, using the 325 InSpectra monitor in 18 healthy, adult volunteers. Two VOTs were done to a threshold thenar StO2 of 40% interchanging the 15 mm and 25 mm probes between sites. Two additional VOTs were done to thresholds of 50% and 30%. Baseline StO(2) (BaseO(2)), the deoxygenation rate (DeO(2)) and ReO2 (%O-2/minute) were compared between sites, probes and thresholds. Results are presented as the median (interquartile range), P-value. Results: BaseO(2), DeO(2), ReO2, area under the curve and hyperemia duration values were different when comparing TH vs. F and 15 mm vs. 25 mm probes. ReO2 was different between different thresholds for the TH and 15 mm probes. TH15 (mm) vs. F-15 (mm): BaseO(2), 90.4 (85.2, 93.5) vs. 85.2 (80.7, 90.2), P = 0.031; DO2, -12.1 (-16.2, -11.3) vs. -8.5 (-10.3, -7.8), P = 0.011; ReO2, 297.2 (213.7, 328.6), P < 0.0001; 15 mm vs. 25 mm probe: BaseO(2), 97.2 (89.4, 94.7) vs. 87.3 (81.7, 90.9), P = 0.016; DeO(2), -18.0 (-24.1, -14.8) vs. -9.9 (-15.3, -6.5), P < 0.0001; and ReO2, 401.6 (331.7, 543.2) vs. 160.5 (132.3, 366.9), P = 0.012, respectively. TH15 (mm) vs. TH25 (mm): BaseO(2), P = 0.020; DeO(2), P < 0.0001; and ReO2, P < 0.0001. Threshold StO(2) values (15 mm probe only): ReO2, P = 0.003; DeO(2), P = 0.60. ReO2 at 40% and 50% StO(2) thresholds, P = 0.01. Conclusion: BaseO(2), DeO(2) and ReO2 were different when measured in different anatomical sites (F and TH) and with different probe sizes, and ReO2 was different with differing VOT release StO(2) threshold values. Thus, standardization of the site, probe and VOT challenge need to be stipulated when reporting data.