Department of Electrical Engineering, Eindhoven University of Technology, 5612AZ Eindhoven, the Netherlands.
Physiol Meas. 2019 Apr 3;40(3):035005. doi: 10.1088/1361-6579/ab051a.
Remote pulse oximetry in visible light (VIS) is a relevant application of photoplethysmography (PPG). However, wavelengths penetrate at different depths and VIS-based pulse oximetry may not guarantee robustness to physiological variations of the skin properties. This paper shows how a simple manoeuver like a posture change can hamper the accuracy of a method relying on red and the less penetrating green wavelengths.
Stationary subjects were measured under normoxic conditions while sitting and in the supine position. For each recording, we extracted remote PPG signals from forehead video recordings of 31 healthy adults at the red and green camera channels. The resulting normalized PPG-amplitudes, and its ratio, red-over-green (RoG), were compared between postures. The observed RoG changes were translated into estimates for arterial blood oxygen saturation (SpO, %) errors by means of Monte Carlo simulations of the skin tissue. Simulations were also used to compare the calibratability errors of SpO in VIS against the conventional red-IR wavelengths.
RoG differs significantly between postures (RoG: sitting, 0.100 [Formula: see text] 0.025; supine, 0.123 [Formula: see text] 0.033), mediated by PPG-amplitude changes in green. The posture interference in RoG may be mitigated by an offset correction. Without this correction, we estimated that the observed change in RoG causes SpO errors >[Formula: see text]. Analogous simulations involving red-IR wavelengths indicate SpO errors <[Formula: see text].
Our results show that the calibrations for remote pulse oximetry in VIS require the specification of a fixed measurement position. Future work could be aimed at controlling for posture in measurements.
可见光（VIS）中的远程光电容积描记法（PPG）是光体积描记法（PPG）的一个相关应用。然而，不同的波长会穿透不同的深度，VIS 为基础的脉搏血氧饱和度仪可能无法保证对皮肤特性的生理变化具有稳健性。本文展示了一个简单的动作，如姿势改变，如何妨碍依赖于红色和穿透性较低的绿色波长的方法的准确性。
在正常氧合条件下，让静止的受试者保持坐姿和仰卧位进行测量。对于每个记录，我们从 31 名健康成年人的额部视频记录中提取红色和绿色相机通道的远程 PPG 信号。比较了两种姿势下的归一化 PPG 幅度及其比值，红色与绿色（RoG）。通过对皮肤组织进行蒙特卡罗模拟，将观察到的 RoG 变化转化为动脉血氧饱和度（SpO，%）的估计误差。模拟还用于比较 VIS 中的 SpO 与传统的红-IR 波长的校准误差。
RoG 在姿势之间有显著差异（RoG：坐姿，0.100 [公式：见文本] 0.025；仰卧位，0.123 [公式：见文本] 0.033），这是由绿光 PPG 幅度的变化介导的。RoG 中的姿势干扰可以通过偏移校正来缓解。如果没有这个校正，我们估计观察到的 RoG 变化会导致 SpO 误差 >[公式：见文本]。涉及红-IR 波长的类似模拟表明 SpO 误差 <[公式：见文本]。
我们的结果表明，VIS 中远程脉搏血氧饱和度仪的校准需要指定一个固定的测量位置。未来的工作可以旨在控制测量中的姿势。
