Kristjan Pilt

New Photoplethysmographic Signal Analysis Algorithm for Arterial Stiffness Estimation

The ability to identify premature arterial stiffening is of considerable value in the prevention of cardiovascular diseases. The “ageing index” (AGI), which is calculated from the second derivative photoplethysmographic (SDPPG) waveform, has been used as one method for arterial stiffness estimation and the evaluation of cardiovascular ageing. In this study, the new SDPPG analysis algorithm is proposed with optimal filtering and signal normalization in time. The filter parameters were optimized in order to achieve the minimal standard deviation of AGI, which gives more effective differentiation between the levels of arterial stiffness. As a result, the optimal low-pass filter edge frequency of 6 Hz and transitionband of 1 Hz were found, which facilitates AGI calculation with a standard deviation of 0.06. The study was carried out on 21 healthy subjects and 20 diabetes patients. The linear relationship (r = 0.91) between each subjects age and AGI was found, and a linear model with regression line was constructed. For diabetes patients, the mean AGI value difference from the proposed model y AGI was found to be 0.359. The difference was found between healthy and diabetes patients groups with significance level of P < 0.0005.

Photoplethysmographic signal waveform index for detection of increased arterial stiffness

The aim of this research was to assess the validity of the photoplethysmographic (PPG) waveform index PPGAI for the estimation of increased arterial stiffness. For this purpose, PPG signals were recorded from 24 healthy subjects and from 20 type II diabetes patients. The recorded PPG signals were processed with the analysis algorithm developed and the waveform index PPGAI similar to the augmentation index (AIx) was calculated. As a reference, the aortic AIx was assessed and normalized for a heart rate of 75 bpm (AIx@75) by a SphygmoCor device. A strong correlation (r = 0.85) between the PPGAI and the aortic AIx@75 and a positive correlation of both indices with age were found. Age corrections for the indices PPGAI and AIx@75 as regression models from the signals of healthy subjects were constructed. Both indices revealed a significant difference between the groups of diabetes patients and healthy controls. However, the PPGAI provided the best statistical discrimination for the group of subjects with increased arterial stiffness. The waveform index PPGAI based on the inexpensive PPG technology can be considered as a perspective measure of increased arterial stiffness estimation in clinical screenings.

Microwave effect on diffusion: a possible mechanism for non-thermal effect

Abstract In this study, we assume that microwave radiation affects hydrogen bonding between dipolar water molecules and through that diffusion in water at constant temperature. The experimental study was performed on the setup of two identical reservoirs filled with pure water and 0.9% NaCl solution and connected by a thin tube. Alterations of NaCl concentration in the reservoir initially filled with pure water were measured using the resistance of the solution as an indicator. The applied 450 MHz continuous-wave microwave field had the maximal specific absorption rate of 0.4 W/kg on the connecting tube. The standard deviation of water temperature in the setup was 0.02 °C during an experiment. Our experimental data demonstrated that microwave exposure makes faster the process of diffusion in water. The time required for reduction of initial resistance of the solution by 10% was 1.7 times shorter with microwave. This result is consistent with the proposed mechanism of low-level microwave effect: microwave radiation, rotating dipolar water molecules, causes high-frequency alterations of hydrogen bonds between water molecules, thereby affects its viscosity and makes faster diffusion.

An experimental measurement complex for probable estimation of arterial stiffness

Current work is a part of long term research, which aim is to study the possibilities to diagnose the atherosclerosis in early stadium by using pulse wave velocity and its waveform analysis. The mobile experimental measurement complex is built and technically tested for the long term study in hospital. Measurement complex consists of ten physiological signal recording channels and reference devices: Sphygmocor, Arteriograph, Finapres. The measurements with this complex are planned to carry out during six month on patients with different severity of coronary disease and diabetes.

Second derivative analysis of forehead photoplethysmographic signal in healthy volunteers and diabetes patients

The second derivative analysis has been used to characterize the changes in forehead photoplethysmographic (PPG) signal, which are caused by the stiffness of blood vessels. Before the distinctive wave amplitudes were measured the forehead PPG signals from healthy volunteers and diabetes patients were filtered, two times differentiated, normalized in length, and averaged. The correlation relationships between normalized amplitudes and age were found. The values of normalized amplitudes b/a and d/a were found to be influenced by the stiffness of the blood vessels.

Analogue Step-by-Step DC Component Eliminator for 24-Hour PPG Signal Monitoring

For applications where PPG signal AC component needs to be measured without disturbances in its shape and recorded digitally with high digitalization accuracy, the step- by-step DC component eliminator is developed. This paper describes step-by-step DC component eliminator, which is utilized with analogue comparator and operational amplifier. It allows to record PPG signal without disturbances in its shape in 24-hours PPG signal monitoring system. The experiments with PPG signal have been carried out.

Signal Processing methods for PPG Module to Increase Signal Quality

To estimate blood pressure with using pulse wave transit time method, the PPG and ECG signals have to be measured with high quality. This paper describes a device that improves PPG signal quality, with using different analogue and digital signal processing methods. The device is developed for the 24-hour ambulatory blood pressure monitoring system. Part of the device is designed in hardware and part of it is modelled in MATLAB. The experiments with PPG signal, noises and DC component drift included, have been carried out. As a result, the PPG signal quality has been improved with this device.

The effect of local cold and warm exposure on index finger photoplethysmographic signal waveform

The study was carried out in order to analyse the changes in photoplethysmographic (PPG) signal waveform, which are caused by cold and warm stimulation. The study was carried out on 7 subjects. The right hand was immersed in cold and warm water up to the wrist during the experiment. The PPG signal was registered from right hand index finger. As a reference, the temperatures of index finger and pulse transit time (PTT) values were measured. A correlation of r=-0.70 was found between normalised slope of the PPG signal and the subjects age. The normalised slope increased noticeably due to the cold and decreased due to the warm water stimulation. It was opposite to the expected results with PTTs and the behaviour has to be investigated in future studies.

The Analysis of Finger Photoplethysmographic Waveform in Healthy Volunteers and Diabetes Patients

Six indices has been extracted from finger photoplethysmographic (PPG) signal to characterize the changes in PPG waveform, which are caused by the stiffness of blood vessels. The indices are measured from PPG signal at the locations of the second derivative signal peaks and normalized with PPG signal amplitude. The finger PPG signals from healthy volunteers and diabetes patients were filtered, two times differentiated, normalized in length, and averaged before the second derivative wave peaks were detected and the indices were calculated. The correlation relationships between normalized indices and age were found r = -0.81, r = -0.85, r = 0.87 for indices PPGa, PPGb, and PPGd respectively.

Arterial Pulse Transit Time Dependence on Applied Pressure

Arterial pulse transit time dependence on applied pressure is analyzed experimentally. The pressure was applied on brachial artery. Pulse transit times between left and right hand were compared by calculating the correlation on different applied pressures. In addition the pulse transit time characteristics were analyzed on different pressures. It revealed that pulse transit time is not influenced by applied pressure, when it is lowered to certain level. The level can be located from piezoelectric signal amplitude.