Toward Noninvasive Simultaneous Arterial and Venous Oxygenation Monitoring with an Optical E-Tattoo

While noninvasive arterial blood oxygenation (SaO2) is easily estimated using peripheral pulse oximeters, noninvasive venous blood oxygenation (SvO2) monitoring is still a critical unmet medical need. Critical conditions that lead to inefficient extraction of oxygen from the blood, such as sepsis or shock, can only be detected by analyzing the oxygen content of the venous blood. A fundamental assumption of photoplethysmography (PPG) and pulse oximetry is that only the arterial blood induces a pulsatile or alternating current (AC) signal. As a result, by analyzing only the pulsatile absorptivity changes of the tissue, arterial blood oxygenation alone will be isolated. However, this assumption may not hold outside of peripheral vasculature. Recent work has suggested that a venous pulse can be measured with a reflectance PPG probe placed over the jugular vein. A significant obstacle for simultaneous SaO2 and SvO2 extraction is the anatomic proximity of the artery and vein, thus leading to crosstalk. We characterize this crosstalk with simulation, in vitro, and in vivo experiments. We have devised a soft wearable optical e-tattoo sensor that simultaneously measures the arterial and venous pulses and used it to characterize how the PPG waveform morphology changes when the PPG sensor is shifted laterally over the carotid artery and jugular vein. We hypothesize that the venous and arterial pulses can be used to simultaneously extract SaO2 and SvO2, thus enabling noninvasive oxygen consumption estimation. This work offers a potential solution for minimizing the crosstalk through spatial filtering.