The human ear canal: investigation of its suitability for monitoring photoplethysmographs and arterial oxygen saturation

Abstract

For the last two decades, pulse oximetry has been used as a standard procedure for monitoring arterial oxygen saturation (SpO2). However, SpO2 measurements made from extremities such as the finger, ear lobe and toes become susceptible to inaccuracies when peripheral perfusion is compromised. To overcome these limitations, the external auditory canal has been proposed as an alternative monitoring site for estimating SpO2, on the hypothesis that this central site will be better perfused. Therefore, a dual wavelength optoelectronic probe along with a processing system was developed to investigate the suitability of measuring photoplethysmographic (PPG) signals and SpO2 in the human auditory canal. A pilot study was carried out in 15 healthy volunteers to validate the feasibility of measuring PPGs and SpO2 from the ear canal (EC), and comparative studies were performed by acquiring the same signals from the left index finger (LIF) and the right index finger (RIF) in conditions of induced peripheral vasoconstriction (right hand immersion in ice water). Good quality baseline PPG signals with high signal-to-noise ratio were obtained from the EC, the LIF and the RIF sensors. During the ice water immersion, significant differences in the amplitude of the red and infrared PPG signals were observed from the RIF and the LIF sensors. The average drop in amplitude of red and infrared PPG signals from the RIF was 52.7% and 58.3%. Similarly, the LIF PPG signal amplitudes have reduced by 47.52% and 46.8% respectively. In contrast, no significant changes were seen in the red and infrared EC PPG amplitude measurements, which changed by +2.5% and −1.2% respectively. The RIF and LIF pulse oximeters have failed to estimate accurate SpO2 in seven and four volunteers respectively, while the EC pulse oximeter has only failed in one volunteer. These results suggest that the EC may be a suitable site for reliable monitoring of PPGs and SpO2s even in the presence of peripheral vasoconstriction.

Reflectance Photoplethysmography as Noninvasive Monitoring of Tissue Blood Perfusion

“…The ZenPPG is a flexible, dual channel, battery operated PPG processing system developed by the Biomedical Engineering Group at City University London [33], [34]. The instrument permits the acquisition of raw, dual-wavelength (red and infrared) and dual-channel photoplethysmographic signals [33].…”

“…The system allows the full control of light emission and the simultaneous acquisition of raw PPG signals (AC and DC) from the two independent channels. ZenPPG has been recently used in PPG studies carried by the Biomedical Engineering Group at City University London [34]- [36].…”

Investigation of photoplethysmography, laser doppler flowmetry and near infrared spectroscopy during induced thermal stress

“…Pulse oximeters provide a robust measure of arterial oxygen saturation (SpO 2 ) by shining light at two different wavelengths into the vascular tissue, and measuring the changes in absorbed or reflected light occurring during the cardiac cycle [1]. The changes in light absorbances at both the wavelengths, during arterial pulsations are measured as a voltage signal known as photoplethysmograph (PPG).…”

Investigation of peripheral photoplethysmographic morphology changes induced during a hand-elevation study

“…It has been shown that this variable is affected by many cardiovascular responses such as vasoconstriction due to a cold pressor test [11] or vasodilation due to reactive hyperemia [3], etc. However, it is difficult to compare systolic amplitudes between subjects or between measurement sites because absorption of light is dependent on local factors (such as skin pigmentation, sensor location etc.…”