**1. Introduction**

The functional state and balance in regulation mechanisms of the cardiovascular system are some of the main factors determining the robustness in a living organism. The evolutionary development of vertebrates in a field of gravitational attraction has led to

**Citation:** Fedorovich, A.A.; Loktionova, Y.I.; Zharkikh, E.V.; Mikhailova, M.A.; Popova, J.A.; Suvorov, A.V.; Zherebtsov, E.A. Body Position Affects Capillary Blood Flow Regulation Measured with Wearable Blood Flow Sensors. *Diagnostics* **2021**, *11*, 436. https://doi.org/10.3390/ diagnostics11030436

Academic Editor: Xavier Muñoz-Berbel

Received: 31 January 2021 Accepted: 24 February 2021 Published: 4 March 2021

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series of adaptations in the blood supply system. Changes in hemodynamic parameters depending on the position of the body significantly assist the homeostasis in the limbs and essentially in the sufficiency of blood supply for the brain [1]. For the last two decades, laser Doppler perfusion monitoring has become an established technique capable of providing useful diagnostic information about parameters of regulation of the skin blood perfusion. The recent emergence of laser Doppler flowmetry (LDF) as a wearable device has allowed for detailed assessments of individual adaptive capabilities of the blood flow circulation system and can be of particular interest for diagnostics and sport medicine. The next decade is likely to witness a considerable rise of novel optical sensor technologies in wearable sensors, not only to be used as fitness trackers, but to provide clinicians with diagnostic information with better sensitivity and specificity.

Earlier studies have shown significant variability of the cardiovascular system's parameters (in particular, heart rate and blood pressure) associated with posture and body position [2–4]. It has also been shown that parameters of gender and age significantly affect the reactivity of blood flow in response to postural change [5]. It is known that different anatomical parts of the human body demonstrate differences in the blood flow regulation mechanisms [6]. In that respect, regional variability of the effects in microcirculation should also be taken into account. Previous studies reported that the low-frequency mechanisms of microcirculation regulation (endothelial, neurogenic and myogenic) measured with the LDF technique differ between the arm and leg regions under thermoneutral conditions [7,8]. It has also been shown that regulation of microcirculation differs in the leg and forearm under local heating [8]. The regions of glabrous and nonglabrous skin are also reported to have different responses of different types in the parameters of blood perfusion under the type of functional loading [9].

In the study of I. Tikhonova et al., it was found that a postural test (change of supine position to sitting) did not influence the forearm skin blood flow oscillations; they noted a remarkable increase in the respiratory flow and a decrease in the cardiac oscillations in the blood microcirculation in the skin of the legs [10]. The work [11] presents the results of a study of the effects of body position on oxygen consumption (*VO*2) and hemodynamics. It was found that the heart rate, the blood pressure and the product of velocity pressure and oxygen consumption were highest in the sitting position compared to the lying position, and lowest in the lying position on the left side. Narayanan et al. [12] published results on a study of changes in the parameters of blood pressure (BP) and the speed of cerebral blood flow (CBFV) when changing the body position from sitting to vertical in young and old people. It is noted that in young people the linear relationship between blood pressure and the blood flow rate of the middle cerebral artery in stationary sitting conditions changes with orthostatic stress in a wide range of physiological frequencies. Nevertheless, the effects in the parameters of skin microcirculation during changes of the posture and body position were not studied comprehensively, so a lack of systematically conducted research can be identified in this area.

Multi-point measurements using recently developed wearable laser Doppler flowmetry devices [13] can be effectively used for simultaneous recordings of blood perfusion signals from arbitrary anatomical skin sites, thereby providing grea<sup>t</sup> potential for finding multiple applications in physiological measurements and medial diagnostics in the near future. One of the challenges that can be mitigated by the use of the distributed measuring system is the difficulty of the high spatial heterogeneity of the LDF signal. Recently, the prototypes of the measuring system have been validated by authors of the work demonstrating the effectiveness of using laser wearable Doppler analyzers for measurements of the parameters of skin blood microcirculation. It has been demonstrated that the sensitivity of the wearable 850 nm VCSEL-basedblood perfusion sensors is sufficient to reliably register physiological changes in skin blood perfusion [14–16], including high coherence of blood flow oscillation in the contralateral limbs of healthy volunteers in the basal state and during functional tests [17].

The wireless LDF sensors have been tested in the realm of pre-clinical trials in healthy volunteers of different ages and patients with type 2 diabetes [14,16,18,19]. Additionally, the dynamical changes in the blood perfusion evaluated by LDF and laser speckle contrast imaging techniques were compared, demonstrating that both techniques can be used for the recording of the blood perfusion oscillations [20].

Thus, the use of wearable LDF sensors is promising for both health monitoring, and for evaluating the effectiveness of treatment and monitoring its dynamics. While the multipoint recordings of blood perfusion have demonstrated grea<sup>t</sup> promise for the diagnostics of vascular complications, there is a significant gap of knowledge on the effects of the body position during measurements, which introduce systematic impact and additional variability to the recorded signals, which requires accurate systematic studies for the main cases such as measurements while standing upright and in supine position.

Thus far, to the best of our knowledge, no one has systematically studied the effects of postural changes on the skin blood flow by use of wearable LDF sensors as a prime measuring technique. A review identified only one study that used a miniaturized LDF device for the measurements of hemodynamic changes in response to changes of body position [21]. The authors reported a decrease in earlobe microcirculation in response to the squat–standing and the footstool standing tests synchronized with the decrease in blood pressure in subjects. Nevertheless, the mentioned research lacks systematic studies of the effects taking place during the transition of body position from lying supine to standing upright. Thus, the overall aim of this work was to study the reaction of the microcirculation system in skin to changes in body position using the newest wireless wearable measuring platform for the multi-point blood perfusion recordings.

#### **2. Material and Methods**

The technique of laser Doppler flowmetry (LDF) measurements with a prototyping system consisting of 6 wireless compact sensors manufactured by SPE "LAZMA" Ltd. (Moscow, Russia) has been applied in this study for the registration of the skin blood perfusion. The LDF method is based on the coherent techniques with the analysis of the laser radiation scattered by moving red blood cells in the living tissue. The output signal of blood perfusion with the LDF method (Figure 1) is a time sequence of an integral parameter that depends on the speed of red blood cells and their concentration in the diagnosed volume.

**Figure 1.** Representative trace of laser Doppler flowmetry (LDF) recordings by the employed measuring system (**a**), the wavelet analysis of the LDF signal with the highlighted frequency ranges for E—endothelial (e, 0.095–0.021 Hz), N—neurogenic (n, 0.021–0.052 Hz), M—myogenic (m, 0.052–0.145 Hz), R—respiratory (r, 0.145–0.6 Hz) and C—cardiac (c, 0.6–2 Hz) regions of blood flow modulation (**b**).

The distributed measuring system has built-in channels for recording microcirculation blood flow and allows for simultaneous measurements at multiple points of the human body. Every measuring device of the system employs compact VCSELs with an

emission wavelength of 850 nm and the power of output of the laser radiation of about 1 mW. Apart from the blood flow measurements, the analyzers were also equipped with a built-in accelerometer to monitor and eliminate possible motion artifacts and a skin temperature sensor.

The object of the study was a cohort of 10 conditionally healthy male volunteers, whose average age was 44 ± 12 years, height 177 ± 6 cm, weight 77 ± 6 kg, BMI 24.5 ± 1.9. All participants were staff testers of the Institute of Biomedical Problems of the Russian Academy of Sciences (IBMP RAS); twice a year they undergo a comprehensive clinical examination for admission to participate in the physiological studies. The main areas of scientific activity of the Institute are research in the fields of space biology, physiology and medicine, which is the reason for the high requirements for the physiological state of the testers. The IBMP RAS Biomedical Ethics Commission has approved the experimental studies, min number 483 dated 3 August 2018, following the rules of the Declaration of Helsinki of 1975, revised in 2013. All volunteers signed informed consent prior to the study. The LDF sensors were located at 6 points on the body: 2 devices were fixed on the forehead above the eyebrows; 2 on the distal third of the outer surface of the forearm (each arm), 2–3 cm proximal to the wrist joint; and 2-in the distal third of the shins along the anterior surface of the tibia, 10 cm proximal to the medial malleolus.

The studies were carried out in a laboratory with a maintained microclimate (air temperature +23 ± 1 ◦C; humidity 40–60%) in the morning (from 09:00 to 12:00). The studies were carried out in the same order on all subjects (Figure 2)—(1) horizontal position; (2) orthostasis ( +75◦); (3) head-down position of the body ( <sup>−</sup>15◦, Trendelenburg position). The study was carried out on a turntable, which was developed and manufactured by the Special Design Bureau of the Institute of Biomedical Problems of the Russian Academy of Sciences. The table has a mechanical drive that allows one to change and fix the angle of inclination of the surface with a step size of 5◦ in the range from −30◦ to +90◦ with a maximum speed of position change of up to 20 ◦/s. The table is equipped with a leg rest, and chest and knee safety belts. Transfer of subjects from horizontal position to orthostasis took 7–10 s, from orthostasis to Trendelenburg position—9–12 s. Cutaneous perfusion was recorded for 10 min at each body position. The adaptation of the subjects to the horizontal position lasted 10–15 min, during which time the sensors were fixed and the research equipment was adjusted. During the transition to orthostasis and the Trendelenburg position, the registration of cutaneous perfusion began after 2 min of adaptation to the new body position. Immediately before the change in body position, hemodynamic parameters were recorded with an automatic tonometer "OMRON M10-IT" (OMRON HEALTHCARE Co, Ltd., Kyoto, Japan) on the right hand, due to the design features of the turntable—a technical "pocket" for placing additional research equipment is located on the right. The temperature of the skin in each area of the study was monitored continuously throughout the entire study by built-in thermal sensors. The protocol was used for recordings in three body positions for each volunteer (Figure 2): (a) horizontal body position; (b) vertical position of the body (head at the top); (c) head tilted down (15° from the horizontal, Trendelenburg position).

The applied combination of the Trendelenburg position and orthostatic probe makes it possible to characterize the functional reserve of the blood circulatory system for the volunteers, and to correlate the adaptations of peripheral hemodynamics to the body position changes. The Trendelenburg position is known to be an effective method to change cerebral perfusion, and to fill and stretch the upper central veins and the external jugular vein [22].

The measuring procedure was composed of several stages. One basal recording of blood perfusion for every body position took 10 min; then the blood pressure was measured. Thus, for each spatial position, three pairs of measurements were recorded at the corresponding symmetrical points on the forehead, wrists and shins.

**Figure 2.** The measurements have been conducted and compared for three distinct body positions on a tilt table: (**A**) supine; (**B**) upright; (**C**) tilted with the feet elevated above the head and an inclination of body axis of 15 °(Trendelenburg position).

The amplitude–frequency characteristics of the skin perfusion oscillations were calculated using the mathematical apparatus of the wavelet transform. The wavelet spectrum of the signal was calculated according to the following expression:

$$\mathcal{W}(s,\tau) = \frac{1}{\sqrt{s}} \int\_{-\infty}^{\infty} x(t) \psi^\* \left( \frac{t-\tau}{s} \right) \tag{1}$$

where *x*(*t*) is a sample of the signal, *τ* is time index, *s* is scaling factor, \* means complex conjugation. As a core wavelet, Morlet wavelet function *ψ*(*t*) = *e*2*πit* · *e*<sup>−</sup>*<sup>t</sup>*2/*<sup>σ</sup>* was choosen with decay parameter *σ* = 1.

The time-averaged amplitude of vasomotions was assessed by the maximum values (Ai) in the corresponding frequency ranges for endothelial (e, 0.095–0.021 Hz), neurogenic (n, 0.021–0.052 Hz), myogenic (m, 0.052–0.145 Hz), respiratory (r, 0.145–0.6 Hz) and cardiac (c, 0.6–2 Hz) regions of blood flow modulation [23] (Figure 1b). The level of cutaneous perfusion (Im) and the amplitude of the units of modulation of microcirculation (Ai) were assessed as quantitative parameters measured in arbitrary (perfusion) units (p.u.). The wavelet analysis has been implemented in the MATLAB software environment. The LDF signals in this particular study were not a subject of pre-processing or filtering before the analysis. The statistical analysis was performed in Origin Pro 2019b (vers. 9.65) software. Due to the limited size of the sample, a non-parametric Mann–Whitney U test was used for the check of statistical significance of differences.

## **3. Results**

#### *3.1. Measurements Conducted on Wrists*

The results of the amplitude analysis of cutaneous perfusion in the skin of the wrists are shown in Figure 3.

From the data obtained, it can be seen that during the transition from the horizontal position to orthostasis, the level of cutaneous perfusion has an insignificant tendency to decrease, which is accompanied by significant decreases in the amplitude of cardiac oscillations in blood flow and the amplitude of vasomotions of all tone-forming mechanisms of microcirculation—endothelial, neurogenic and myogenic. During the transition from orthostasis to the Trendelenburg position, the level of skin perfusion significantly increased, which was accompanied by significant increases in the amplitudes of cardiac and myogenic oscillations. There were no significant differences between the Trendelenburg position and the horizontal position for any of the analyzed parameters. The level of perfusion and the amplitude of the cardiac fluctuations both have a clear tendency to increase, but we did not find significant differences.

**Figure 3.** Analysis of average blood perfusion parameters on the wrists for three tested body positions: supine, upright and tilted (Trendelenburg position): (**a**) average blood perfusion; (**b**) cardiac oscillations; (**c**) respiratory oscillations; (**d**) endotelial oscillations; (**e**) neurogenic oscillations; (**f**) myogenic oscillations (\* the significance of a difference between values was confirmed with *p* < 0.05 using the the Mann–Whitney test).

#### *3.2. Measurements on Lower Legs*

Figure 4 demonstrates the distribution of the studied parameters during measurements in the lower third of shin.

The parameters of microcirculatory blood flow demonstrated a significant decrease in the average level of tissue perfusion and the amplitude of cardiac oscillations during the transition to orthostasis. During the change from orthostasis to the Trendelenburg position, these parameters significantly increased and were comparable with those measured in the horizontal position. In contrast to the measurements conducted in forearms, the functional state of the tone-forming mechanisms of microcirculation modulation in shins (parameters Ae, An and Am) did not demonstrate any significant changes during all three stages of the study.

**Figure 4.** Analysed blood perfusion parameters measured on the shins for three body positions: supine, upright and tilted (Trendelenburg position): (**a**) average blood perfusion; (**b**) cardiac oscillations; (**c**) respiratory oscillations; (**d**) endotelial oscillations; (**e**) neurogenic oscillations; (**f**) miogenic oscillations (\* the significance of a difference between values was confirmed with *p* < 0.05 using the Mann–Whitney test)

#### *3.3. Measurements on the Forehead*

Figure 5 shows the results of the measurements of the cutaneous blood perfusion dynamics on the forehead.

During measurements on the forehead, we did not register any significant changes in the index of microcirculation caused by postural changes. With unchanged tissue perfusion, however, significant increases in the amplitudes of neurogenic and myogenic oscillations were recorded when changing from a supine to an upright position, which is the opposite of the results obtained in the shins. Despite the presence of a tendency towards moderate increases in the amplitudes of endothelial and neurogenic oscillations in the Trendelenburg position, these changes did not reach statistically significant levels.

#### *3.4. Blood Pressure and Heart Rate*

Before each stage of the study, for every tested subject the parameters of blood pressure and heart rate were recorded via the right arm. The results of these measurements are shown in Figure 6.

**Figure 5.** Analysis of blood perfusion parameters in the skin of the brow of the forehead for three tested body positions: supine, upright and tilted (Trendelenburg position): (**a**) average blood perfusion; (**b**) cardiac oscillations; (**c**) respiratory oscillations; (**d**) endotelial oscillations; (**e**) neurogenic oscillations; (**f**) miogenic oscillations (\* the significance of a difference between values was confirmed with *p* < 0.05 using the Mann–Whitney test).

**Figure 6.** Analysis of parameters of blood pressure and hear rate measured in the tested body positions: supine, upright and tilted (Trendelenburg position): (**a**) systolic blood pressure; (**b**) diastolic blood pressure; (**c**) heart rate (\* the significance of a difference between the values was confirmed with *p* < 0.05 using the Mann–Whitney test).

From the data obtained, it can be seen that in the position of orthostasis, significantly higher values of diastolic pressure and heart rate were observed compared to the supine and the Trendelenburg positions. At the same time, the values of systolic blood pressure did not undergo significant changes during postural changes.
