Pre-processing of the photoplethysmography (PPG) signal plays an important role in the analysis of the pulse wave signal. The task of pre-processing is to remove noise from the PPG signal, as well as ...to transmit the signal without any distortions for further analysis. The integrity of the pulse waveform is essential since many cardiovascular parameters are calculated from it using morphological analysis. Digital filters with infinite impulse response (IIR) are widely used in the processing of PPG signals. However, such filters tend to change the pulse waveform. The aim of this work is to quantify the PPG signal distortions that occur during IIR filtering in order to select a most suitable filter and its parameters. To do this, we collected raw finger PPG signals from 20 healthy volunteers and processed them by 5 main digital IIR filters (Butterworth, Bessel, Elliptic, Chebyshev type I and type II) with varying parameters. The upper cutoff frequency varied from 2 to 10 Hz and the filter order-from 2nd to 6th. To assess distortions of the pulse waveform, we used the following indices: skewness signal quality index (S
), reflection index (RI) and ejection time compensated (ETc). It was found that a decrease in the upper cutoff frequency leads to damping of the dicrotic notch and a phase shift of the pulse wave signal. The minimal distortions of a PPG signal are observed when using Butterworth, Bessel and Elliptic filters of the 2nd order. Therefore, we can recommend these filters for use in applications aimed at morphological analysis of finger PPG waveforms of healthy subjects.
Laser-based medical techniques for evaluating blood flow (BF), such as laser Doppler flowmetry, laser speckle contrast imaging, etc., are known, but expensive and have some disadvantages. Recently, ...we have proposed a new technique—incoherent optical fluctuation flowmetry (IOFF), which is realized using a LED-based optical probe. This work aims to theoretically study the dependence of BF registered by IOFF on the source-detector distance (SDD) in the probe. For this purpose, we developed a three-layer optical model of skin and used Monte Carlo (MC) simulations of light propagation. All computations were performed for a wavelength of 810 nm and several SDDs from 1 to 14 mm. MC results showed that the BF depends nonlinearly on the SDD. Herewith, the BF is strongly correlated with the mean photon path length in a tissue (R = 0.92). Thus, flowmeters with different SDDs can give different BF values on the same patient. Based on the study results, to standardize BF measurements, it has been justified that BF magnitudes measured should be normalized to the exponential function of the SDD in the used optical probe in the form of 1 − exp(−b·SDD), where b is a constant.
This study aimed to examine the mechanisms of the photoplethysmography (PPG) signal formation using Monte Carlo simulations of light transport in biological tissues and experimental observations. ...Based on a three-layer skin model in backscattering geometry, we sequentially simulated volumetric blood changes and the aggregation/disaggregation of erythrocytes in the dermal layer and estimated their contribution to the registered PPG signal. The calculations were conducted for two wavelengths: 525 nm and 810 nm. For green light, absorption predominates over scattering in the formation of a PPG signal, whereas, for near-infrared light, scattering prevails over absorption. This theoretical result was verified using the Modified Beer–Lambert law and clinical in vivo PPG data of seven healthy subjects. Changes in the size of the scatterers during erythrocyte aggregation and disaggregation can significantly contribute to the PPG signal at near-infrared light. Thus, for the green waveband, the classical volumetric model can be considered dominant in the PPG signal formation. In contrast, for the near-infrared range, both volumetric and aggregation effects must be considered as being approximately equal.
Optical non-invasive diagnostic methods and equipment are used today in many medical disciplines. However, there is still no generally accepted and unifying engineering theory of such systems. Today, ...they are developed most empirically and do not always have the desired effectiveness in clinics. Among reasons for their insufficient clinical efficiency, we can claim the limited set of measured parameters, the poorly substantiated technical design parameters, and the lack of metrological certification, which all together lead to large uncertainties and inaccuracies in diagnostic data. The purpose of this study is to develop a new instrument for non-invasive optical oximetry by means of substantiating and creating amore informative tissue oximeter with an enhanced number of measured parameters and equipped with the basic metrological tools—imitational measures. The combination of two related optical diagnostic techniques—a tissue oximetry, including a cerebral one, and a fluctuation flowmetry on a single hardware platform—was used. Theoretical modeling of light transport in tissues was applied to substantiate the main technical design parameters of the device. For each measuring channel, relevant imitation measures for metrological verification and adjustment have been proposed. Some common principles for the operation of such equipment are described in the article, as well.
Introduction
The development of new highly accurate, inexpensive and accessible methods for the detection of lower-extremity peripheral artery disease (LE-PAD) in diabetic patients is required. The ...aim of this study was to evaluate the accuracy of a new incoherent optical fluctuation flowmetry (IOFF) method in detecting legs with hemodynamically significant stenoses compared to ankle brachial index (ABI) and transcutaneous oximetry (TcPO2) in patients with diabetes mellitus (DM).
Materials and methods
Patients were recruited into 2 groups. Group 1 included patients with DM without LE-PAD and/or diabetic foot syndrome; Group 2 included patients with DM and LE-PAD. All patients underwent the following measurements: ultrasound (reference method), ABI, TcPO2, and the new IOFF method.
Results
The new IOFF method showed a sensitivity of 79.5% and a specificity of 89.8% in detecting limbs with hemodynamically significant stenosis (AUC 0.890, CI 0.822–0.957). TcpO2 allows the diagnosis of LE-PAD with 69.2% sensitivity and 86.2% specificity (AUC 0.817, CI 0.723–0.911). Using a standard ABI cut-off of less than 0.9, the sensitivity and specificity for this parameter were 34.5% and 89.7%, respectively. Increasing the diagnostic cut-off of the ABI on the study group to 0.99 improved sensitivity to 84.6% and specificity to 78% (AUC,0.824 CI 0.732–0.915).
Conclusions
The new IOFF technique has demonstrated high sensitivity and specificity in the detection of LE-PAD in patients with DM. The high accuracy, rapid measurement, and potential availability suggest that the new IOFF method has a high potential for clinical application in the detection of PAD.
The aim of the present study was to evaluate changes in cutaneous blood flow detected by incoherent optical fluctuation flowmetry (IOFF) in diabetic patients with diagnosed microangiopathies. The ...study includes 52 patients with type 2 diabetes mellitus (12 males, 40 females, median age 57 (51; 60) years). All patients were examined using the IOFF method using functional tests: occlusion test on the index finger, local heat test on the dorsum of the foot, and local heat test on the big toe. The patients were divided into 2 groups: 13 patients with both diabetic retinopathy and diabetic nephropathy (Group 2, severe microcirculatory disorders) and 39 patients with only 1 microangiopathic complication or without any (Group 1). Local thermal hyperaemia (LTH2_1) was higher in Group 1 than in Group 2 (p = 0.011). Post-occlusive reactive hyperaemia (PORH_2) was higher in Group 2 than in Group 1 (p = 0.009). No changes were detected on the big toe. When PORH_2 and LTH2_1 were used in combination, the area under the ROC curve (95% CI) was 0.733 (0.62 to 0.846). Thus, the IOFF method has the potential to accurately detect blood flow abnormalities associated with diabetic microangiopathies.
Noise in the differential two-channel scheme of a classic laser Doppler flowmetry (LDF) instrument was studied. Formation of false spectral components in the output signal due to beating of ...electrical signals in the differential amplifier was found out. The improved block-diagram of the flowmeter was developed allowing to reduce the noise.
Incoherent Optical Fluctuation Flowmetry (IOFF) is an optical technique for assessing tissue blood flow (BF). This technique is based on the spectral analysis of low-frequency fluctuations in the ...registered optical signal caused by blood volume changes inside a tissue. The technique can be realized using an optical probe with LEDs. The aim of this study was to evaluate the signal-to-noise ratio (SNR) in IOFF as a function of the distance between a LED and a detector. For this, we developed an experimental setup with a variable source-detector separation. The measurements were carried out on 5 healthy volunteers at the wavelength of 810 and at distances of 4, 6, 8 and 10 mm. To obtain a useful signal, we used the spectral approach to remove noise. The best signals were obtained at a distance of 10 mm (SNR median is 63). Good quality signals were also obtained at distances of 6 and 8 mm (SNR medians are 49 and 55, respectively). It was found that calculated BF is directly proportional to the source-detector separation. As one of the options for standardization of measurements, it was proposed to normalize the output value of BF to a distance.
In this paper, we propose a method for measuring a pulse wave velocity (PWV) along the entire body of the patient with the use of a standard tonometer cuff and a photoplethysmography (PPG) sensor ...attached to a toe. This approach makes it possible to calculate PWV simultaneously with the measurement of a blood pressure (BP), which takes several minutes. To test the developed method, 2 contrasting groups of subjects were recruited: group 1 (n=15) - healthy volunteers; group 2 (n=14) - patients with diabetes mellitus (DM). Healthy volunteers had the lowest median PWV value: 5.055 4.427; 5.295 m/s, whereas in the group 2, the median PWV was significantly higher than in the group 1: 5.905 5.612; 6.215 m/s, p<0.001. The proposed method for measuring PWV correlates with clinical data and allows a rapid integral assessment of vascular stiffness in both the trunk and lower extremities.
