Perfusion Changes at the Forehead Measured by Photoplethysmography during a Head-Down Tilt Protocol

Abstract

Photoplethysmography (PPG) signals from the forehead can be used in pulse oximetry as they are less affected by vasoconstriction compared to fingers. However, the increase in venous blood caused by the positioning of the patient can deteriorate the signals and cause erroneous estimations of the arterial oxygen saturation. To date, there is no method to measure this venous presence under the PPG sensor. This study investigates the feasibility of using PPG signals from the forehead in an effort to estimate relative changes in haemoglobin concentrations that could reveal these posture-induced changes. Two identical reflectance PPG sensors were placed on two different positions on the forehead (above the eyebrow and on top of a large vein) in 16 healthy volunteers during a head-down tilt protocol. Relative changes in oxygenated ( Δ HbO 2 ), reduced ( Δ HHb) and total ( Δ tHb) haemoglobin were estimated from the PPG signals and the trends were compared with reference Near Infrared Spectroscopy (NIRS) measurements. Also, the signals from the two PPG sensors were analysed in order to reveal any difference due to the positioning of the sensor. Δ HbO 2 , Δ HHb and Δ tHb estimated from the forehead PPGs trended well with the same parameters from the reference NIRS. However, placing the sensor over a large vasculature reduces trending against NIRS, introduces biases as well as increases the variability of the changes in Δ HHb. Forehead PPG signals can be used to measure perfusion changes to reveal venous pooling induced by the positioning of the subject. Placing the sensor above the eyebrow and away from large vasculature avoids biases and large variability in the measurements.

Sources of Inaccuracy in Photoplethysmography for Continuous Cardiovascular Monitoring

“…As respiration occurs, the venous system is more compliant to smaller changes in pressure as opposed to the less distensible arterial system [ 118 ]. Shifts in blood volume in the venous system due to changes in respiratory behavior will cause a corresponding change to the baseline amplitude of the PPG signal as blood volume increases and decreases [ 120 ]. Changes in thoracic volume and pressure cause an alternating pressure gradient in the venous vascular system [ 118 ].…”

“…While this relationship is largely maintained over the body, PPG measurements at different body sites can result in stronger relative contributions from the venous system, such as the forearm versus the finger. The finger is arterially dominated while the forearm has a larger venous component [ 120 ]. Thus, the venous system can potentially add noise to a PPG due to vein pulsations and the variation of the amplitude of those pulsations across the body.…”

“…Beyond contact pressure, the respiratory component has been eliminated via adaptive and non-adaptive filters allowing signal from approximately 0.5 to 4.0 Hz [ 133 ]. Additionally, previous works discuss variable-frequency complex demodulation, continuous wavelet transformation, and autoregressive modeling to decouple this signal from the arterial component of the PPG [ 22 , 120 , 127 , 132 ].…”

First Evaluation of a Newly Constructed Underwater Pulse Oximeter for Use in Breath-Holding Activities

“…Applying pressure on the sensor mechanically collapses the venous circulation and reduces the venous contribution to the signal without affecting the arterial component ( Shelley et al, 2005 ; Agashe et al, 2006 ). However, to date there is no standard measure of the actual pressure applied, and too much pressure may also deteriorate the arterial component ( Abey et al, 2019 ). We tried to solve this issue by visually inspecting the real-time PPG signals when the sensors were mounted, and when we obtained a strong signal ( Figure 3 ), we assumed that the applied pressure was adequate.…”

Wearable Systems for Home Monitoring Healthcare: The Photoplethysmography Success Pros and Cons

“…Continuous monitoring of both venous and artery contributions to the PPG waveform indicates a strong correlation between the diastolic peak in the plethysmograph and peaks in the venous pulse at the peripheral locations. Moreover, the noninvasive venous pressure measurement can help diagnose clinically relevant conditions such as congestive heart failure or valvular heart disease [ 11 , 12 ].…”