Background: Pulse oximeters continuously monitor arterial oxygen saturation. Continuous\nmonitoring of venous oxygen saturation (SvO2) would enable real-time assessment\nof tissue oxygen extraction (O2E) and perfusion changes leading to improved\ndiagnosis of clinical conditions, such as sepsis.\nMethods: This study presents the proof of concept of a novel pulse oximeter method\nthat utilises the compliance difference between arteries and veins to induce artificial\nrespiration-like modulations to the peripheral vasculature. These modulations make the\nvenous blood pulsatile, which are then detected by a pulse oximeter sensor. The resulting\nphotoplethysmograph (PPG) signals from the pulse oximeter are processed and\nanalysed to develop a calibration model to estimate regional venous oxygen saturation\n(SpvO2), in parallel to arterial oxygen saturation estimation (SpaO2). A clinical study with\nhealthy adult volunteers (n = 8) was conducted to assess peripheral SvO2 using this\npulse oximeter method. A range of physiologically realistic SvO2 values were induced\nusing arm lift and vascular occlusion tests. Gold standard, arterial and venous blood\ngas measurements were used as reference measurements. Modulation ratios related to\narterial and venous systems were determined using a frequency domain analysis of the\nPPG signals.\nResults: A strong, linear correlation (r2 = 0.95) was found between estimated venous\nmodulation ratio (RVen) and measured SvO2, providing a calibration curve relating\nmeasured RVen to venous oxygen saturation. There is a significant difference in gradient\nbetween the SpvO2 estimation model (SpvO2 = 111 âË?â?? 40.6*R) and the empirical SpaO2\nestimation model (SpaO2 = 110 âË?â?? 25*R), which yields the expected arterial-venous differences.\nMedian venous and arterial oxygen saturation accuracies of paired measurements\nbetween pulse oximeter estimated and gold standard measurements were 0.29\nand 0.65%, respectively, showing good accuracy of the pulse oximeter system.\nConclusions: The main outcome of this study is the proof of concept validation of\na novel pulse oximeter sensor and calibration model to assess peripheral SvO2, and\nthus O2E, using the method used in this study. Further validation, improvement, and\napplication of this model can aid in clinical diagnosis of microcirculation failures due to\nalterations in oxygen extraction.
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