Venous Component Separation From a Pulse Oximeter Signal using Blind Source Separation with Ultrasound Imaging as Reference

Abstract

Intradialytic hypotension (IDH) is a major drawback of haemodialysis and occurs in more than every fifth dialysis treatment. There is a need for real-time measurement, with an ability to detect IDH before it occurs. Photoplethysmography (PPG) may provide sufficient information about hemodynamic status to prevent IDH. In particular, the venous component may be useful.
This thesis aims to investigate suitable signal separation methods for venous component extraction, and the wavelengths for optimal venous signal extraction. Furthermore, the effect of different body positions and measurement location-combinations are examined. Moreover, the thesis aims to answer if a better signal can be provided using additional wavelengths as a complement to the two conventional wavelengths used in standard pulse oximeters (I.e., red and infrared). Lastly, potential applications of the venous component are searched for in literature, apart from blood volume dynamics.
For signal separation, principal component analysis (PCA) was, with regards to morphology, found to give similar results as the second-order blind identification (SOBI) for venous extraction. Optimal wavelengths for venous component were established to be somewhere between 500-1000 nm, based on the limitations. The lower limit is due to bad signal to noise ratio and shallow penetration depth, whereas the higher limit was due to water absorption. Regarding penetration depth and absorption, another subject to consider is the effect of melanin and other chromophores absorbing differently for various wavelengths. Based on literature findings and LED-availability this thesis continued to investigate venous component extraction using a customized sensor, provided by Baxter. The sensor used the wavelengths 515 nm (blue green), 577 nm (yellow green), 660 nm (red) and 940 nm (infrared). For reference, ultrasound measurements of nearby blood vessels were conducted simultaneous to PPG-measurements using ultrasound equipment provided by the Department of Biomedical Engineering at LTH. Combinations of PPG and ultrasound reference were done in three different setups: PPG measurements of the thumb, forehead, and neck. Respectively, ultrasound reference was acquired from wrist, external jugularis and internal jugularis. The components from PPG were compared with the ultrasound reference using correlation and visual inspection. Likewise, comparison of separation using only the two conventional versus all four of the available sensor channels was done utilizing correlation. Additionally, a comparison of the ratios of the principal component eigenvalues were done and summarized.
No concluding results could be shown from comparing correlation with ultrasound reference using two conventional wavelengths versus four. Ultrasound images proved to be unexpectedly difficult to obtain and no satisfactory setup of PPG- and ultrasound measurement could be found. At times, signal separation on the PPG-signals did indeed reveal components with venous-like pulsations which correlated well with the ultrasound reference signal, but these results were never consistently acquired. Extraction of the venous component using PPG could be useful as a non-invasive option for Peripheral Intravenous Analysis. Moreover, it might also be used as an estimator of Central Venous Pressure if the measurement location is on the neck.

Popular Abstract

SEPARATING THE VENOUS COMPONENT FROM A PULSE OXIMETER SIGNAL

A severe drop of blood pressure occurs in up to every fifth dialysis treatment. This troublesome complication is often accompanied with vomiting and fainting and may lead brain damage or even death. No robust method to predict this occurrence exists at the time of writing. One approach to solve this could be to monitor the venous system using optical measurements of the tissue.

It is known that certain mechanisms in the vascular system comes into play during hypotension, especially so in the venous part. Our work aimed to establish a connection between changes in venous pressure and optical measurements of the tissue. Because of the complex mix of blood vessels in the body, absorption of light in the cardiovascular system was investigated. The acquired data was treated mathematically as an attempt to separate the signals that are thought to originate from a mix of arteries and veins.

With the use of two signal separation techniques (PCA and SOBI), the estimated separated source signals were compared with an ultrasound reference signal. At times, a venous-like signal was extracted that also correlated well with the reference signal. This indicates that it may be possible to separate venous and arterial signals from a mixed optical signal.

The extraction of the venous component can be useful for non-invasively approximating the central venous pressure and as an alternative to peripheral intravenous analysis. This may in turn provide an easily acquirable parameter that can be used to predict and prevent severe drops of blood pressure during dialysis treatment.

As this was an experimental study, further research is required in this area. One avenue for practical devices could be the use of wearable technology, such as smartwatch solutions, in-ear devices or optics glued to the fingernail.

• author: Walter, Björn and Piiroinen, Stefan
• supervisor: Frida Sandberg; Mattias Holmer; Tobias Erlöv; Ioan Tabus; Alpo Värri; Matilda Träff
• organization: Department of Biomedical Engineering
• alternative title: SEPARATING THE VENOUS COMPONENT FROM A PULSE OXIMETER SIGNAL with Ultrasound Imaging as Reference
• course: BMEM01 20211
• year: 2021
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: Photoplethysmogram, PPG, Ultrasound, Blind Source Separation, PCA, SOBI, venous component
• language: English
• additional info: 2021-08
• id: 9054342
• date added to LUP: 2021-06-21 14:32:31
• date last changed: 2021-06-21 14:32:31