Abstract
BackgroundPulse oximeters continuously monitor arterial oxygen saturation. Continuous monitoring of venous oxygen saturation (SvO2) would enable real-time assessment of tissue oxygen extraction (O2E) and perfusion changes leading to improved diagnosis of clinical conditions, such as sepsis.MethodsThis study presents the proof of concept of a novel pulse oximeter method that utilises the compliance difference between arteries and veins to induce artificial respiration-like modulations to the peripheral vasculature. These modulations make the venous blood pulsatile, which are then detected by a pulse oximeter sensor. The resulting photoplethysmograph (PPG) signals from the pulse oximeter are processed and analysed to develop a calibration model to estimate regional venous oxygen saturation (SpvO2), in parallel to arterial oxygen saturation estimation (SpaO2). A clinical study with healthy adult volunteers (n = 8) was conducted to assess peripheral SvO2 using this pulse oximeter method. A range of physiologically realistic SvO2 values were induced using arm lift and vascular occlusion tests. Gold standard, arterial and venous blood gas measurements were used as reference measurements. Modulation ratios related to arterial and venous systems were determined using a frequency domain analysis of the PPG signals.ResultsA strong, linear correlation (r2 = 0.95) was found between estimated venous modulation ratio (RVen) and measured SvO2, providing a calibration curve relating measured RVen to venous oxygen saturation. There is a significant difference in gradient between the SpvO2 estimation model (SpvO2 = 111 − 40.6*R) and the empirical SpaO2 estimation model (SpaO2 = 110 − 25*R), which yields the expected arterial-venous differences. Median venous and arterial oxygen saturation accuracies of paired measurements between pulse oximeter estimated and gold standard measurements were 0.29 and 0.65%, respectively, showing good accuracy of the pulse oximeter system.ConclusionsThe main outcome of this study is the proof of concept validation of a novel pulse oximeter sensor and calibration model to assess peripheral SvO2, and thus O2E, using the method used in this study. Further validation, improvement, and application of this model can aid in clinical diagnosis of microcirculation failures due to alterations in oxygen extraction.
Highlights
Pulse oximeters continuously monitor arterial oxygen saturation
This study presents the initial proof of concept for a novel pulse oximeter method to assess peripheral custom pulse oximeter estimated SvO2 (SpvO2), in addition to custom pulse oximeter estimated SaO2 (SpaO2), leveraging this arterial-venous compliance difference
The gradient (−40.5%) of this model is different to the gradient (−25.0%) of the empirical SpaO2 estimation model commonly used in pulse oximeter estimation [25] and shown on Fig. 7 for comparison
Summary
Pulse oximeters continuously monitor arterial oxygen saturation. Continuous monitoring of venous oxygen saturation (SvO2) would enable real-time assessment of tissue oxygen extraction (O2E) and perfusion changes leading to improved diagnosis of clinical conditions, such as sepsis. Arterial oxygen saturation (SaO2) and venous oxygen saturation (SvO2) are the two basic parameters used to assess the oxygen delivery process and monitor oxygen extraction (O2E). These two parameters are important for the analysis of whole body oxygen circulation. SaO2 can be non-invasively and reliably estimated by pulse oximeters (SpaO2), using photoplethysmograph (PPG) signals produced by an optical sensor, typically mounted on a finger, toe, or ear-lobe, to detect blood volume changes. Conventional pulse oximetry relies on the pulsatile nature of arterial blood and differential absorption of oxyhaemoglobin and de-oxyhaemoglobin at red (RD) and infrared (IR) wavelengths to estimate S paO2 [4, 5]
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