H. C. Pereira

Pulse pressure waveform estimation using distension profiling with contactless optical probe

The pulse pressure waveform has, for long, been known as a fundamental biomedical signal and its analysis is recognized as a non-invasive, simple, and resourceful technique for the assessment of arterial vessels condition observed in several diseases. In the current paper, waveforms from non-invasive optical probe that measures carotid artery distension profiles are compared with the waveforms of the pulse pressure acquired by intra-arterial catheter invasive measurement in the ascending aorta. Measurements were performed in a study population of 16 patients who had undergone cardiac catheterization. The hemodynamic parameters: area under the curve (AUC), the area during systole (AS) and the area during diastole (AD), their ratio (AD/AS) and the ejection time index (ETI), from invasive and non-invasive measurements were compared. The results show that the pressure waveforms obtained by the two methods are similar, with 13% of mean value of the root mean square error (RMSE). Moreover, the correlation coefficient demonstrates the strong correlation. The comparison between the AUCs allows the assessment of the differences between the phases of the cardiac cycle. In the systolic period the waveforms are almost equal, evidencing greatest clinical relevance during this period. Slight differences are found in diastole, probably due to the structural arterial differences. The optical probe has lower variability than the invasive system (13% vs 16%). This study validates the capability of acquiring the arterial pulse waveform with a non-invasive method, using a non-contact optical probe at the carotid site with residual differences from the aortic invasive measurements.

Empirical mode decomposition for self-mixing Doppler signals of hemodynamic optical probes.

A new type of optical probe based on laser Doppler self-mixing technology, for a truly non-contact measurement in a single location, and extraction of the temporal features of the distension wave in the arterial wall, was developed. The monitoring of temporal features allows the assessment of cardiovascular function when measurement is carried out at the carotid artery. An algorithm based on the short-time Fourier transform and empirical mode decomposition was applied to the test setup self-mixing signals for the determination of waveform features, with an accuracy of a few milliseconds and a root mean square error less than 3 ms. In vivo testing signals show great consistency in the measured pulse pressure waveform.

Visible and infrared optical probes for hemodynamic parameters assessment

Four optical probes were developed to measure the arterial distension waveform generated by the ventricular contraction and assess clinically relevant information. The pressure wave propagates through the arterial tree and can be measured in the peripheral arteries. The probes make use of two distinct photo-detectors: planar and avalanche photodiodes. Independently, two different light sources were tested: visible and infrared light. Performance of the probes was evaluated in a test setup that simulates the fatty deposits commonly seen in the obese, between skin and the artery. The probes show good overall performance in the test setup with less than 8% root mean square error (RMSE). However, the probes lit with IR sources show better results for the more extreme cases, with a better resolution in the waveform, higher definition of notable points and higher SNR when compared to the visible source signals. In vivo, the IR probes allow easier waveform detection, even more relevant with the increasing of the deposit structures.

Comparison of Low-Cost and Noninvasive Optical Sensors for Cardiovascular Monitoring

New optical probes are developed for carotid distention waveform measurements, in order to assess the risk of cardiovascular diseases. The probes make use of two distinct photodetectors: planar and avalanche photodiodes. Their performance is compared for visible and infrared (IR) light wavelengths. The test setup designed for the evaluation of the probes simulates the fatty deposits commonly seen in the obese people, between skin and the artery. The performed tests show that the attenuation of the signal is lower for the IR light, with higher penetration and better resolution in the captured distension waveform, with higher definition in morphological features on the wave and higher signal-to-noise ratio when compared to the visible source signals. The probes show good overall performance in the test setup with a root mean square error lower than 8%. In vivo, the IR probes allow easier waveform detection, even more relevant with the increasing deposit structures.

Programmable Test Bench for Hemodynamic Studies

The non-invasive assessment of hemodynamic parameters has been a permanent challenge posed to the scientific community. The literature shows many contributions to this quest expressed as algorithms dedicated to revealing some of its characteristics and as new probes or electronics, featuring some enhanced instrumental capability that can improve their insight.

Optically Isolated Current Source

This work addresses two different issues associated with the design voltage controlled current sources (VCSS): galvanic isolation between the load and the control voltage and loss of performance due to operational amplifier common mode rejection ratio (CMRR) degradation with frequency.

Non-contact Pulse Wave Velocity Assessment Using Optical Methods

The clinical relevance of pulse wave velocity (PWV), as an indicator of cardiac risk associated to arterial stiffness, has gained clinical relevance over the last years. Optic sensors are an attractive instrumental solution for this type of measurement due to their truly non-contact operation capability, which has the potential of an interference free measurement. The nature of the optically originated signals, however, poses new challenges to the designer, either at the probe design level as at the signal processing required to extract the timing information that yields PWV. In this work we describe the construction of two prototype optical probes and discuss their evaluation using three algorithms for pulse transit time (PTT) evaluation. Results, obtained in a dedicated test bench, that is also described, demonstrate the possibility of measuring pulse transit times as short as 1ms with less than 1% error.

Assessment of the Pulse Wave Variability for a New Non-invasive Device

The main motivation of this work was to provide a valid contribution for the assessment of the cardiovascular condition by the analysis of several Arterial Pressure Waveform (APW) parameters collected by a new non-invasive device. Three sets of recordings for the carotid pressure waveform at left and right carotid arteries were performed, under standardized conditions, in 20 volunteers by three trained operators. The mean of the inter-operator differences were higher for the right artery, comparatively to the left artery. In this case, an Augmentation Index (AIx) value of -2.31 ± 7.29 % and a Systolic Wave Transit Time (SWTT) value of -12.94 ± 31.46 ms were observed, which are higher than the left measurements, 0.94 ± 7.52 % and -2.96 ± 22.67 ms, respectively. Intra-operator differences were calculated for each of the three sets of measurements and showed good reproducibility. The pulse-by-pulse variability analysis gives very good markers for the Left Ventricular Ejection Time (LVET), Dicrotic Wave Amplitude (DWA), Reflection Wave Amplitude (RWA), Coefficient of Variation (CV) < 10 %, and satisfactory values for the AIx (CV< 30 %). The SWTT and Reflected Wave Transit Time (RWTT) also presented satisfactory results (10 % < CV < 30 %). Results demonstrated the reproducibility of the parameter, being a simple and non-invasive device, that can be used to assess central hemodynamics.

Metabolic.Care: Development and characterization of a new thermographic platform for diabetic foot detection

Summary form only given. Foot complications due to diabetes - diabetic foot disease - constitute an important economic burden on healthcare systems and impose a loss of quality of life for the patients, leading to limb-threatening (amputation) in most circumstances. Over the last years, several studies have demonstrated that there is a correlation between increased temperature and diabetic foot condition and that temperature augmentation may be detected at a reversible phase of the disease. Conventional non-invasive methods to assess foot temperature include physical palpation or infrared (IR) thermometry. However, the increase in temperature is typically too slight to be detected manually while taking foot temperature with a thermometer is very time consuming and provides only discrete values. The ideal instrument should provide fast temperature readings of the entire foot in one measurement procedure. Although a few technologies are available (liquid crystal thermography, IR cameras, etc.), none has been adopted in clinical practice up till now. This paper proposes a novel thermographic platform for diabetic foot detection based on a thermochromic liquid crystal (TLC) sheet and a modified A3 scanner that acquires images from the TLC sheet. Both elements are enclosed in a metallic and ergonomic structure that allows their physical support. The image is obtained when the patient, in a sitting position, places his feet on the TLC sheet and is subsequently analysed using dedicated image processing algorithms. The significant advantage of this system over the state of the art is the capability to obtain several subsequent images during dynamic changes in skin temperature, making the necessary human intervention minimal. Furthermore, the system is connected to an application hosted on the eVida web platform that allows data uploading, processing and the creation of alarms. The thermographic platform was characterized in a bench test, using two novel Peltier-based calibration systems, and with the feet of a healthy subject. Aspects related to differential thermal resolution of the TLC sheet, contact time between the calibration system/foot with the TLC sheet, the cooling process of the platform and image resolution were tested. The new platform demonstrated a very good performance on the bench and in vivo tests and results showed that it was possible to detect thermal differences between 1o C - 1.6o C (bench tests). Although in vivo studies will be extended to a more significant number of subjects to differentiate between healthy/non-healthy groups, this thermographic platform appears to be a promising and easy to use alternative to detect and monitor diabetic foot condition in a medical environment.

Submicron Surface Vibration Profiling Using Doppler Self-Mixing Techniques

Doppler self-mixing laser probing techniques are often used for vibration measurement with very high accuracy. A novel optoelectronic probe solution is proposed, based on off-the-shelf components, with a direct reflection optical scheme for contactless characterization of the target’s movement. This probe was tested with two test bench apparatus that enhance its precision performance, with a linear actuator at low frequency (35 µm, 5–60 Hz), and its dynamics, with disc shaped transducers for small amplitude and high frequency (0.6 µm, 100–2500 Hz). The results, obtained from well-established signal processing methods for self-mixing Doppler signals, allowed the evaluation of vibration velocity and amplitudes with an average error of less than 10%. The impedance spectrum of piezoelectric (PZ) disc target revealed a maximum of impedance (around 1 kHz) for minimal Doppler shift. A bidimensional scan over the PZ disc surface allowed the categorization of the vibration mode (0, 1) and explained its deflection directions. The feasibility of a laser vibrometer based on self-mixing principles and supported by tailored electronics able to accurately measure submicron displacements was, thus, successfully demonstrated.