Observation of natural flexural pulse waves in retinal and carotid arteries for wall elasticity estimation

Direct extraction of respiratory information from pulse waves using a finger-inspired flexible pressure sensor system

The long-term monitoring of respiratory status is crucial for the prevention and diagnosis of respiratory diseases. However, existing continuous respiratory monitoring devices are typically bulky and require either chest strapping or proximity to the nasal area, which compromises user comfort and may disrupt the monitoring process. To overcome these challenges, we have developed a flexible, attachable, lightweight, and miniaturized system designed for extended wear on the wrist. This system incorporates signal acquisition circuitry, a mobile client, and a deep neural network, facilitating long-term respiratory monitoring. Specifically, we fabricated a highly sensitive (11,847.24 kPa-1) flexible pressure sensor using a screen printing process, which is capable of functioning beyond 70,000 cycles. Additionally, we engineered a bidirectional long short-term memory (BiLSTM) neural network, enhanced with a residual module, to classify various respiratory states including slow, normal, fast, and simulated breathing. The system achieved a dataset classification accuracy exceeding 99.5%. We have successfully demonstrated a stable, cost-effective, and durable respiratory sensor system that can quantitatively collect and store respiratory data for individuals and groups. This system holds potential for everyday monitoring of physiological signals and healthcare applications.

Toward quantitative X-ray elastography of coronary arteries using flexural pulse waves

Dynamic elastography uses an imaging system to visualize the propagation of elastic waves, the speed of which is directly related to the elasticity felt by palpation. Very few studies have focused on X-ray elastography because of the technical challenges it poses: a planar image of an integration volume at a very slow sampling rate. We demonstrate that tracking a slow elastic wave guided along a one-dimensional structure could provide a possible solution. The recently discovered flexural pulse wave, which is naturally generated by heartbeats and propagates along arteries, is the perfect candidate for X-ray elastography. As it reflects the cardiovascular health of patients, arterial elasticity is a biomarker of high clinical interest. We first validate the method by measuring the elasticity in artery phantoms using X-ray. We then move on to data obtained in vivo on coronary arteries during a routine angiography examination. During coronary angiography, a catheter is used to inject an X-ray contrast dye into the patient's aorta. X-rays are then taken as the dye spreads through the coronary arteries. It shows the movement of the coronary arteries for a few seconds and provides an opportunity to follow the natural flexural pulse waves. The obtained Young's moduli for two patients are E = 38 ± 30 kPa and E = 38 ± 28 kPa, respectively. These preliminary results are expected to pave the way for X-ray elastography.

Feasibility of a method for measurement of retinal pulse-propagated wave velocity in humans

Blood flow regulation has been shown to be compromised in common ocular diseases, such as diabetic retinopathy and glaucoma. The capacity of the retinal vessels to regulate blood flow can potentially serve as an oculomics biomarker for evaluating ocular and systemic diseases. Pulse-propagated intravascular pressure waves cause deformations of the vessel walls, thus offering a means to interrogate vascular compliance. The purpose of the current study is to report a method for measuring retinal pulse-propagated wave velocity (rPWV) based on spectral analysis of pulsatile intensity waveforms in human circumpapillary retinal vasculature. Arterial and venous rPWV values, as well as inter-subject variabilities of rPWV in non-diabetic and diabetic subjects, are reported. Preliminary results demonstrated the feasibility of this method for measuring rPWV and its potential for assessment of vascular plasticity in response to blood flow changes due to ocular and systemic diseases.

Instrumenting Carotid Sonography Biomarkers and Polygenic Risk Score As a Novel Screening Approach for Retinal Detachment

Purpose

Retinal detachment (RD) is a vision-threatening condition that manifests silently before abrupt disease onset; thus, most of the RD at-risk individuals are left unchecked until the first RD attack.

Methods

To establish an RD risk-informing system for a broader population, we utilized carotid ultrasonography (CUS) biometrics, RD polygenic risk score (PRSRD), and clinical covariates (COVs) to assess RD risk predisposition factors. First, a backpropagation logistic regression model identified RD-associated CUS biomarkers and further incorporated them as a multivariable RD-risk nomogram. Next, a PRSRD model was established with the selected single-nucleotide polymorphisms (SNPs) curated as high functional expression candidates in the retina single-cell RNA datasets. Finally, a three-component RD prediction model (CUS, PRSRD, and COVs) was assembled by logistic cumulative analysis.

Results

Demographic analysis reported hypertension (HTN) status was associated with RD (odds ratio [OR] = 1.601). The CUS regression model revealed that the minimum flow of the right internal carotid artery (ICA-Qmin; OR = 1.04) and the time-averaged maximum velocity of the right common carotid artery (CCA-TAMAX; OR = 1.03) were associated with increased RD risk. Notably, genome-wide association studies (GWAS) identified three significant SNPs (IGFBPL1 rs117248428, OR = 1.63; CELF2 rs56168975, OR = 1.72; and PAX6 rs11825821, OR = 1.61; P < 5.00 × 10-6) that are highly expressed at the RD border of the retinal pigment epithelium and choroid. Finally, the three-component model demonstrated state-of-the-art RD prediction (AUCHTN+ = 0.95, AUCHTN- = 0.93).

Conclusions

Based on instrumenting CUS images and genetic PRSRD, we are proposing a screening method for RD at-risk patients.

Translational Relevance

Results from this study demonstrated the combination of CUS and GWAS as a cost-effective, population-wide screening framework for identifying RD at-risk individuals.

Silk Fibroin Hydrogel for Pulse Waveform Precise and Continuous Perception

Precise and continuous monitoring of blood pressure and cardiac function is of great importance for early diagnosis and timely treatment of cardiovascular diseases. The common tests rely on on-site diagnosis and bulky equipments, hindering early diagnosis. The emerging hydrogels have gained considerable attention in skin bioelectronics by virtue of the similarities to biological tissues and versatility in mechanical, electrical, and biofunctional engineering. However, hydrogels should overcome intrinsic issues such as poor mechanical strength, easy dehydration and freezing, weak adhesiveness and self-recovery, severely limiting their precision and reliability in practical applications. Here, silk fibroin hydrogels are developed as resistive sensors for pulse waveform monitoring. The silk fibroin hydrogel is simultaneously transparent, extremely stretchable, extra tough, adhesive, printable, and environmentally endurable. The silk fibroin hydrogel is also conductive with high sensitivity, short self-healing time, highly repeatable and reliable response, meeting the requirements for wearable sensors for continuous monitoring. The sensors with silk fibroin hydrogel present high-quality and stable waveforms of radical and brachial pulses with high precision and rich features, providing physiological signals of blood pressure and cardiac function. The sensors are promising for personalized health management, daily monitoring and timely diagnosis.

Calculation of vessel pulse wave velocities in retinal vein segments within the optic disc centre

The carotid-femoral pulse wave velocity (PWV) method is used clinically to determine degrees of stiffness and other indices of disease. It is believed PWV measurement in retinal vessels may allow early detection of diseases. In this paper we present a new non-invasive method for estimating PWVs in retinal vein segments close to the optic disc centre, based on the measurement of blood column pulsation in retinal veins (reflective of vessel wall pulsation), using modified photoplethysmography (PPG). An optic disc (OD) PPG video is acquired spanning three cardiac cycles for a fixed ophthalmodynamometric force. The green colour channel frames are extracted, cropped and aligned. A harmonic regression model is fitted to each pixel intensity time series along the vein centreline from the centre to the periphery of the OD. The phase of the first harmonic is plotted against centreline distance. A least squares line is fitted between the first local maximum phase and first local minimum phase and its slope used to compute PWV. Five left eye inferior hemi-retinal veins from five healthy subjects were analysed. Velocities were calculated for several induced intraocular pressures ranging from a mean baseline of 14 mmHg (SD 5) to 56 mmHg in steps of approximately 5 mmHg. The median PWV over all pressure steps and subjects was 20.77 mm/s (IQR 29.27). The experimental results show that pulse wave propagation direction was opposite to flow in this initial venous segment.

Flexural pulse wave velocity in blood vessels

Arteriosclerosis is a major risk factor for cardiovascular disease and results in arterial vessel stiffening. Velocity estimation of the pulse wave sent by the heart and propagating into the arteries is a widely accepted biomarker. This symmetrical pulse wave propagates at a speed which is related to the Young's modulus through the Moens Korteweg (MK) equation. Recently, an antisymmetric flexural wave has been observed in vivo. Unlike the symmetrical wave, it is highly dispersive. This property offers promising applications for monitoring arterial stiffness and early detection of atheromatous plaque. However, as far as it is known, no equivalent of the MK equation exists for flexural pulse waves. To bridge this gap, a beam based theory was developed, and approximate analytical solutions were reached. An experiment in soft polymer artery phantoms was built to observe the dispersion of flexural waves. A good agreement was found between the analytical expression derived from beam theory and experiments. Moreover, numerical simulations validated wave speed dependence on the elastic and geometric parameters at low frequencies. Clinical applications, such as arterial age estimation and arterial pressure measurement, are foreseen.

The fundamental mechanisms of the Korotkoff sounds generation

Blood pressure measurement is the most widely performed clinical exam to predict mortality risk. The gold standard for its noninvasive assessment is the auscultatory method, which relies on listening to the so-called "Korotkoff sounds" in a stethoscope placed at the outlet of a pneumatic arm cuff. However, more than a century after their discovery, the origin of these sounds is still debated, which implies a number of clinical limitations. We imaged the Korotkoff sound generation in vivo at thousands of images per second using ultrafast ultrasound. We showed with both experience and theory that Korotkoff sounds are paradoxically not sound waves emerging from the brachial artery but rather shear vibrations conveyed in surrounding tissues by the nonlinear pulse wave propagation. When these shear vibrations reached the stethoscope, they were synchronous, correlated, and comparable in intensity with the Korotkoff sounds. Understanding this mechanism could ultimately improve blood pressure measurement and provide additional understanding of arterial mechanical properties.