Irina Mizeva

Assessment of endothelial dysfunction in patients with impaired glucose tolerance during a cold pressor test

The objective of this study is to explore changes in microvascular tone during a contralateral cold pressor test and to compare the results obtained in healthy subjects and in patients with impaired glucose tolerance (IGT) and type 2 diabetes. Low-amplitude fluctuations of skin temperature in the appropriate frequency ranges were used as a characteristic for the mechanism for vascular tone regulation. In total, 13 adults with type 2 diabetes aged 40–67 years and 18 adults with IGT aged 31–60 years participated in this pilot study. The control group included 12 healthy men and women aged 39–60 years. The response to the cold pressor test in patients with type 2 diabetes and with IGT differs essentially from that of healthy subjects in the endothelial frequency range. Endothelial dysfunction occurs in the preclinical stage of diabetes and manifests, in particular, as a disturbance of the endothelial part of vascular tone regulation.

Skin blood flow and temperature oscillations during cold pressor test

We study the relationship between the blood flow and skin temperature variations under a cold pressor test (CPT). The simultaneous laser Doppler flowmetry (LDF) and skin temperature (ST) measurements were carried out for 8 healthy subjects on the skin surface of the distal phalanx of the second (LDF) and third (ST) fingers. The skin blood perfusion decreases stepwise about twice during contralateral CPT for all 8 subjects. The temperature of the finger pad decays monotonically during the test and dropped about 1°C in mean. The power spectral densities of LDF flow and ST variations are also affected by the CPT, but subjects under study demonstrate two different types of reaction. LDF pulsations at the frequency about 0.1 Hz, which corresponds to the myogenic mechanism of vascular tone regulation, decreases in 5 subjects and increases in other 3 subjects. However in all subjects the ST pulsations behave contradictory, namely, the changes in amplitude of blood perfusion and ST pulsations due to cold pressor test are strongly anticorrelated. We discuss possible mechanisms of vascular reaction that can cause the behavior observed.

Analysis of skin blood microflow oscillations in patients with rheumatic diseases

Laser Doppler flowmetry (LDF) has been applied for the assessment of variation in blood microflows in patients with rheumatic diseases and healthy volunteers. Oscillations of peripheral blood microcirculation observed by LDF have been analyzed utilizing a wavelet transform. A higher amplitude of blood microflow oscillations has been observed in a high frequency band (over 0.1 Hz) in patients with rheumatic diseases. Oscillations in the high frequency band decreased in healthy volunteers in response to the cold pressor test, whereas lower frequency pulsations prevailed in patients with rheumatic diseases. A higher perfusion rate at normal conditions was observed in patients, and a weaker response to cold stimulation was observed in healthy volunteers. Analysis of blood microflow oscillations has a high potential for evaluation of mechanisms of blood flow regulation and diagnosis of vascular abnormalities associated with rheumatic diseases.

Quantifying the correlation between photoplethysmography and laser Doppler flowmetry microvascular low-frequency oscillations

Abstract. Photoplethysmography (PPG) and laser Doppler flowmetry (LDF) are two recognized optical techniques that can track low-frequency perfusion changes in microcirculation. The aim of this study was to determine, in healthy subjects, the correlation between the techniques for specific low-frequency bands previously defined for microcirculation. Twelve healthy male subjects (age range 18 to 50 years) were studied, with PPG and LDF signals recorded for 20 min from their right and left index (PPG) and middle (LDF) fingers. Wavelet analysis comprised dividing the low-frequency integral wavelet spectrum (IWS) into five established physiological bands relating to cardiac, respiratory, myogenic, neurogenic, and endothelial activities. The correlation between PPG and LDF was quantified using wavelet correlation analysis and Spearman correlation analysis of the median IWS amplitude. The median wavelet correlation between signals (right-left side average) was 0.45 (cardiac), 0.49 (respiratory), 0.86 (myogenic), 0.91 (neurogenic), and 0.91 (endothelial). The correlation of IWS amplitude values (right-left side average) was statistically significant for the cardiac (ρ=0.64, p<0.05) and endothelial (ρ=0.62, p<0.05) bands. This pilot study has shown good correlation between PPG and LDF for specific physiological frequency bands. In particular, the results suggest that PPG has the potential to be a low-cost replacement for LDF for endothelial activity assessments.

Relationship of oscillating and average components of laser Doppler flowmetry signal

Abstract. Signals from laser Doppler flowmeters widely used in intravital studies of skin blood flow include, along with a slowly varying average component, an oscillating part. However, in most clinical studies, pulsations are usually smoothed by data preprocessing and only the mean blood flow is analyzed. To reveal the relationship between average and oscillating perfusion components measured by a laser Doppler flowmeter, we examined the microvascular response to the contralateral cold pressor test recorded at two different sites of the hand: dorsal part of the arm and finger pad. Such a protocol makes it possible to provide a wide range of perfusion. The average perfusion always decreases during cooling, while the oscillating component demonstrates a differently directed response. The wavelet analysis of laser Doppler flowmetry (LDF) signals shows that the pulsatile component is nonlinearly related to the average perfusion. Under low perfusion, the amplitude of pulsations is proportional to its mean value, but, as perfusion increases, the amplitude of pulsations becomes lower. The type of response is defined by the basal perfusion and the degree of vasoconstriction caused by cooling. Interpretation of the results is complicated by the nonlinear transfer function of the LDF device, the contribution of which is studied using artificial examples.

JOINT INVERSE CASCADE OF MAGNETIC ENERGY AND MAGNETIC HELICITY IN MHD TURBULENCE

We show that oppositely directed fluxes of energy and magnetic helicity coexist in the inertial range in fully developed magnetohydrodynamic (MHD) turbulence with small-scale sources of magnetic helicity. Using a helical shell model of MHD turbulence, we study the high Reynolds number magnetohydrodynamic turbulence for helicity injection at a scale that is much smaller than the scale of energy injection. In a short range of scales larger than the forcing scale of magnetic helicity, a bottleneck-like effect appears, which results in a local reduction of the spectral slope. The slope changes in a domain with a high level of relative magnetic helicity, which determines that part of the magnetic energy related to the helical modes at a given scale. If the relative helicity approaches unity, the spectral slope tends to −3/2. We show that this energy pileup is caused by an inverse cascade of magnetic energy associated with the magnetic helicity. This negative energy flux is the contribution of the pure magnetic-to-magnetic energy transfer, which vanishes in the non-helical limit. In the context of astrophysical dynamos, our results indicate that a large-scale dynamo can be affected by the magnetic helicity generated at small scales. The kinetic helicity, in particular, is not involved in the process at all. An interesting finding is that an inverse cascade of magnetic energy can be provided by a small-scale source of magnetic helicity fluctuations without a mean injection of magnetic helicity. Subject headings: magnetic fields - methods: numerical - MHD - plasmas - turbulence

Spatial heterogeneity in the time and frequency properties of skin perfusion

Pathological alterations of the microcirculatory system can be identified by measuring the temporal and spectral properties of laser Doppler flowmetry (LDF) signals acquired on the skin, and their changes following physiological stimulation. A wide range of stimulation protocols and measurement locations is observed in literature. Researchers often use non-invasive stimulation techniques, such as post-occlusive hyperaemia, cold tests, and local heating. As concerns the stimulation/recording sites, the forearm, fingers, and toes are typically selected to conduct microcirculation studies. However, recent clinical investigations showed that different anatomical sites present dissimilar blood flow patterns. Therefore, studies involving the comparison of LDF data, obtained from various anatomical locations, and thus subjected to the intrinsic heterogeneity of the microcirculation, may be methodologically inaccurate. At the moment, no consensus has been reached upon the optimal measurement location, the stimulation pattern, and the physiological parameters of interest. The aim of this study is to quantitatively characterize the heterogeneity of the peripheral perfusion at different anatomical locations: the index finger, the forearm, and the hallux. The skin microvascular system exhibits a complex vasodilatory response in the temporal domain, upon local heating. This physiological reactive hyperaemia comprises two effects: a fast transient response, correlated to neural activation, named axon reflex, followed by a slower hyperaemic plateau, mediated by the release of nitric oxide. In this work, we compare the vasodilatory reaction to heating at the different sites, based on a parametric representation of the perfusion signal. Moreover, skin blood flow is characterized by several components fluctuating at different time scales. Time-frequency decomposition of LDF signals allows to quantitatively evaluate the relative contribution of known physiological mechanisms to the regulation of the peripheral circulation. For this reason, we analyze the wavelet transform coefficients of LDF signals at baseline, to assess potential spatial heterogeneities of the perfusion power spectra among the aforementioned anatomical locations.

Blood flow oscillations as a signature of microvascular abnormalities

Laser Doppler flowmetry (LDF) was utilized for blood ow measurements. Wavelet analysis was used to identify spectral characteristics of the LDF signal in patients with rheumatic diseases and diabetes mellitus. Baseline measurements were applied for both pathological groups. Blood flow oscillations analyses were performed by means of the wavelet transform. Higher baseline perfusion was observed in both pathological groups in comparison to controls. Differences in the spectral properties between the groups studied were revealed. The results obtained demonstrated that spectral properties of the LDF signal collected in basal conditions may be the signature of microvasculature functional state.

A porous media model of human hand to study the relationship between endothelial function and fingertip temperature oscillation

In this study we adopt the porous media model to simulate heat transfer of hand, in which blood perfusion in tissues through micro vessels is considered as fluid infiltration in porous solid body. Temperature response of fingers to a cooling test is simulated by considering parameter variations of ambient temperature, heat convection and tissue porosity. The results show a faster temperature recovering in fingertips than that in other parts, which is demonstrated in some experiments. Furthermore, a contra-lateral cooling test is simulated to discuss the changing of temperature fluctuation amplitude. By giving a periodical porosity variation of fingers in endothelial frequency, we get the result which shows a favorable agreement with the experiment, implying that the periodic change of porosity can be modeled as the period change of micro vessels. We hope that the proposed porous media model can be extended to apply in the non-invasive blood glucose detection via hand temperature oscillation.