WO2025083590A1

WO2025083590A1 - Apparatus for monitoring physiological signals

Info

Publication number: WO2025083590A1
Application number: PCT/IB2024/060167
Authority: WO
Inventors: Behzad Aliahmad
Original assignee: Compumedics Medical Innovation Pty Ltd
Current assignee: Compumedics Medical Innovation Pty Ltd
Priority date: 2023-10-19
Filing date: 2024-10-17
Publication date: 2025-04-24
Anticipated expiration: 2026-04-19

Abstract

The present invention relates to the improvement in design of both reflective and transmissive photoplethysmography (PPG) devices to reduce motion artifacts, avoid false and non-physiological baseline shift in the signal, improve signal quality and accuracy of the metrics derived from the PPG. The invention results in an improved accuracy in estimation of all PPG driven parameters, such as blood oxygen saturation (SpO₂), heart rate, heart rate variability, respiration rate, blood pressure and continuous measurement of the arterial pulse wave volume (i.e. peripheral and pulse arterial tonometry). The present invention incorporates the capability to measure and/or track and/or compensate for variations or absolute values of the applied pressure between a sensor and the surface of a biological subject, including all sensors or associated probes.

Description

Apparatus for Monitoring Physiological Signals

Technical Field

The present invention relates to an apparatus for acquiring electrophysiological signals associated with physiological processes, in particular, blood oxygen saturation of haemoglobin (SpO2) measurements.

Background

Photoplethysmography (PPG) is a non-invasive technique that is used for transcutaneous measurement of oxygen saturation of haemoglobin in arterial blood (i.e. SpO₂) and allows for extraction of some valuable information such as heart rate, PPG driven respiration and respiration rate, arterial tone and pulse wave volume. It is a non-invasive technology that relies on two Light Emitting Diodes (LED), one Red and one Near-Infrared (N-IR), plus a photodetector.

The principal application of a photoplethysmogram to measure SpO₂ is based on the principle that the amount of red and near-infrared light absorbed by oxygenated (HbO₂) and deoxygenated (Hb) Haemoglobin differ significantly. Hb absorbs a greater amount of red and a lower amount of N-IR light compared to HbO₂. In other words, Hb reflects lower amount of red and greater amount of N-IR light compared to HbO₂. This characteristic has been used as the basis for development of two types of oximeters known as i) transmissive and ii) reflective. In transmissive oximeters the photo diodes and the detector are placed on opposite sides of the measurement site, such as the fingertips, and the amount of light transmitted through the tissue is detected by the receiver, while in reflective oximeters both the light source and the detector are placed on the same side of the measurement site and the amount of light reflected from the tissue is captured. In both cases, the amount of light absorbed/ reflected fluctuates because of i) changes in arterial blood volume during systolic and diastolic phases of the cardiac cycle and ii) variations in blood oxygen concentration. The relative amount of red to N-IR absorbed is used to differentiate between cardiac cyclic variations and the changes in blood oxygen concentration.

The accuracy of measurements of transmissive and reflective PPG devices is susceptible to motion and change in contact pressure between a sensor and the skin. For the PPG devices to work reliably, a stable contact pressure is required which is not easily achievable as the pressure may change as the patient moves and, in the case where there is high amount of motion. Weak contact pressure can result in severe light leakage and therefore, a weak and noisy PPG signal. In contrast, excessive pressure can result in vascular constriction, temporarily reduce or block blood perfusion and alter the morphology of the PPG signal. In both cases, non- optimal contact pressure changes the morphology of the PPG signals (i.e. both Red and IR) and the relative intensity of Red to N-IR absorbed/reflected, resulting in a significant reduction in the accuracy and baseline of all PPG driven signals, including but not limited to, SpO₂. This presents a problem especially when it comes to continuous monitoring over a long period of time, such as during an overnight sleep study; as the contact pressure may vary by changes in position during sleep. This can introduce a bias in the diagnosis of medical conditions such as cardiopulmonary complications, sleep apnoea, chronic obstructive pulmonary disease (COPD) and heart failure. Non-optimal contact pressure will also lead to heavier contamination of PPG by motion artifacts, reduced signal to noise ratio, PPG baseline wandering and unreliable or erroneous heart rate measurements. What is needed is a better design for PPG devices to minimise these problems and provide improved measured signals.

Summary of the Invention

The present invention provides apparatus with improved measurement of biological signals, in particular, photoplethysmographic measurements, of a subject. In one aspect, the invention provides apparatus for monitoring physiological signals of a subject, comprising of a flexible substrate for application to the subject’s skin, including at least one sensor for monitoring the physiological signals of the subject, a sensor for measuring oxygen concentration of blood, and a floating or sprung optical sensor to mechanically regulate the contact pressure and ensure vascular constrictions due to application of the sensor is always controlled, limited, and/or minimised. The invention advantageously provides a mechanism for measuring the pressure of the flexible substrate against the skin, or any limb or other surface part of the body. Preferably, the flexible substrate comprises of a flexible pressure sensitive material. Preferably, the invention incorporates an algorithm for automatic and realtime optimisation of the signals and adjustment to derived metrics according to the measured contact pressure. Preferably, the invention incorporates the capability to measure and/or track and/or compensate for variations or absolute values of the applied pressure between a sensor and the surface of a subject, including all sensors or associated probes. Preferably, one or more applied sensor(s) or probe(s) can be deployed to achieve said functionality. Preferably, the measures or outcomes from the sensor can be modified, adapted, compensated, or computed in anyway considering the "applied pressure between a sensor and the surface of a subject' as part of a related algorithm or any form of related computation.

Brief Description of the Figures

FIG.1 is side view of an embodiment of the invention incorporating the inventive reflective pulse oximeter equipped with pressure sensitive material or sensor.

FIG.2 is a side view of an embodiment of invention incorporating the inventive transmissive finger clip pulse oximeter equipped with pressure sensitive material or sensor, showing a finger being inserted into the clip.

Detailed Description of the Invention

The present invention relates to the improvement in design of both reflective and transmissive photoplethysmography (PPG) devices to reduce motion artifacts, avoid false and non-physiological baseline shift in the signal, improve signal quality and accuracy of the metrics derived from the PPG. The invention results in an improved accuracy in estimation of all PPG driven parameters, such as blood oxygen saturation (SpO₂), heart rate, heart rate variability, respiration rate, blood pressure, and continuous measurement of the arterial pulse wave volume (i.e. peripheral and pulse arterial tonometry). The present invention incorporates the capability to measure and/or track and/or compensate for variations or absolute values of the applied pressure between a sensor and the surface of a biological subject, including all sensors or associated probes.

The invention most advantageously introduces a mechanism for the pulse oximeters to continuously monitor the contact pressure between the skin and the sensor and adjust/optimise the signals according to the contact pressure. The invention incorporates a floating or sprung optical sensor to mechanically regulate the contact pressure and ensure vascular constrictions due to application of the sensor is always controlled, limited, and/or minimised. Advantageously, the invention provides a mechanism for measuring the pressure of the flexible substrate against the skin, or any limb or other surface part of the body. The invention may incorporate an algorithm for automatic and real-time optimisation of the signals and adjustment to derived metrics according to the measured contact pressure. The invention may include the ability for measures or outcomes from the sensor to be modified, adapted, compensated, or computed in anyway considering the applied pressure between a sensor and the surface of a biological subject as part of a related algorithm or any form of related computation.

The invention may assist in differentiation between cases with naturally low perfusion and blood flow restriction (i.e. vascular constriction) due to increased contact pressure. Advantageously, the invention can assist in differentiation between genuine low SpO2 baseline (i.e., are a result of conditions such as but not limited to Chronic obstructive pulmonary disease) and artifactual low baseline because of non-optimal contact pressure between skin and the sensor.

This invention includes applications beyond the translucent body sites such as finger, toe, and earlobe where only transmissive optical sensors can be used. It expands the option/choices for greater choices of measuring sites and opens number of opportunities for development of wearable devices such as smart watches, forehead, and chest-worn PPG monitors with clinical grade level of accuracy.

The invention preferably incorporates the application of a flexible pressure sensitive material such as “Velostat” or “Linqsatat” and/or an electronic pressure sensor, to engage with a spring-loaded optical sensor. The optical sensor is mounted on a spring of known type and specifications which can be made in any form such as a coil or folded metal, to regulate the contact pressure against any body part. Addition of a presser sensor (or pressure sensitive material) to the spring-loaded optical sensor allows for the real-time measurement of a pressure signal that relates to the integrity of the connection between the skin and the sensor.

The figures illustrate preferred embodiments of the invention. Other embodiments within the scope of the claims are possible. Referring now to FIG 1 is an embodiment of the reflective PPG device according to the invention, which incorporates a pulse oximetry function is shown on the skin 1 (i.e. measurement site). Depending on the location of the measurement site, it incorporates either a strap or adhesive patch 2 to stay in place. The transmitter 3 and receiver 10 both are mounted on a rigid substrate 15 which itself is connected to the main rigid substrate 5 via two flexible substrates 9 and 18. These two substrates carry electrical signals from 15 to 16 and then to 5, which has all the electronic components 4, via the third flexible substrate 19. The pressure sensitive material or sensor 7 is placed between the two rigid substrates 5 and 16 to sense the contact pressure. A spiral spring 6 with a metal bar 12 to house the spring is placed between substrates 15 and 16. This mechanism allows for the sensors and lenses move inside the plastic housing 8. This provides a capacity to absorb and release energy, add cushioning and help regulating contact force based on the shape of the measurement site. The transceivers 3 and 10 are covered by the plastic case 15 that has two cut-outs positioned on top of each of the sensors. These two cut-outs hold optical lenses 11 and 17 to carry light with a U-shaped optical isolator 13 in the middle, to block light leakage between the transmitter 3 and the receiver 10. This isolator has a U-shape to house the metal bar 12 when the spring 6 is compressed.

Referring now to FIG 2, the drawing provides a further example of the embodiment of the invention as a transmissive PPG with finger clip probe showing a finger 1 being inserted into the clip. It has top 4 and bottom 9 housings which are interconnected with spring 7. The bottom housing 9 houses the receiver 11 is fitted on the rigid substrate 10 mounted on the pressure sensitive material 8. The top housing 4 houses the transmitter 3 mounted on the rigid substrate 5. All electronic components 2 are mounted on the other side of a rigid substrate 5. Inside the device opening there is the silicon cushion 6 to take the shape of the finger 1 and help regulating contact force between finger 1 and the transceivers 3 and 11. This cushion also provides optical isolation and avoids light leaking from 3 to 11.

Claims

1. Apparatus for monitoring physiological signals of a subject, comprising of: a) a flexible substrate for application to the subject’s skin; b) at least one sensor engaged with the flexible substrate for monitoring the physiological signals of the subject; c) a sensor engaged with the flexible substrate for measuring oxygen concentration of blood; and d) a floating or sprung optical sensor engaged with the flexible substrate to mechanically regulate the contact pressure and ensure vascular constrictions due to application of the sensor is always controlled, limited, and/or minimised.

2. The apparatus of claim 1 , where the flexible substrate comprises of a flexible pressure sensitive material.

3. The apparatus of claim 1 or claim 2, further incorporating a microprocessor programmed with an algorithm to calculate automatic and real-time optimisation of the monitored signals and adjust derived metrics according to the measured contact pressure.

4. The apparatus of any of claim 1 to claim 3, further incorporating a microprocessor programmed to calculate and/or track and/or compensate for variations or absolute values of the applied pressure between the sensor and the surface of the subject.