Generalized Transfer Function Research Articles

Validation of non-invasive derivation of the central aortic pressure waveform from fingertip photoplethysmography using a novel selective transfer function method

Introduction: Central aortic pressure waveform analyses can provide clinically relevant information beyond conventional brachial blood pressure (BP) assessment. This waveform can be reproduced non-invasively through application of a general transfer function (GTF) on a peripheral waveform, as conventionally performed by applanation tonometry. Photoplethysmography (PPG) is an alternate approach, however, differences in measurement site and modality demand the use of a transfer function (TF) specific for those differences. This study aims to compare central aortic waveform features generated from radial tonometry (reference) using a proprietary GTF with a central aortic waveform and its features generated from a simultaneous fingertip PPG measurement using a selective method where one of three different TFs is chosen based on the input signal harmonic profile. Methods: Brachial BP was measured in triplicate under resting conditions and was used for subsequent calibration. Multiple simultaneous radial tonometry (SphygmoCor CvMS) and fingertip PPG measurements were then performed in individual participants (n=21, 10 females, age: 39±16 years). Measurements were converted into central aortic waveforms with their respective TFs. Twenty central aortic pressure waveform parameters were compared through correlation analysis, Bland-Altman plots, and a repeated measure mixed-effects ANOVA model. Central aortic waveform shape was compared using the root-mean-squared error (RMSE). Results: Correlation (r) of PPG-derived parameters with radially tonometry derived central aortic parameters were high ranging from 0.79 to 0.99. Mean differences of pressure-related parameters were within 1.3 mmHg and differences of time-related parameters ranged from –2.2 to 3.4%. Whilst some parameters were statistically different these differences are not physiologically meaningful. Central aortic waveform shape had an average RMSE of 1.8±0.9%. Conclusion: Fingertip PPG derived central aortic waveform parameters using a novel selective TF were comparable to central aortic waveform features derived from radial tonometry using a previously validated GTF.

A new methodology for determining the central pressure waveform from peripheral measurement using Fourier-based machine learning

Radial applanation tonometry is a well-established technique for hemodynamic monitoring and is becoming popular in affordable non-invasive wearable healthcare electronics. To assess the central aortic pressure using radial-based measurements, there is an essential need to develop mathematical approaches to estimate the central pressure waveform. In this study, we propose a new Fourier-based machine learning (F-ML) methodology to transfer non-invasive radial pressure measurements to the central pressure waveform. To test the method, collection of tonometry recordings of the radial and carotid pressure measurements are used from the Framingham Heart Study (2640 individuals, 55 % women) with mean (range) age of 66 (40–91) years. Method-derived estimates are significantly correlated with the measured ones for three major features of the pressure waveform (systolic blood pressure, r=0.97, p < 0.001; diastolic blood pressure, r=0.99, p < 0.001; and mean blood pressure, r=0.99, p < 0.001). In all cases, the Bland-Altman analysis shows negligible bias in the estimations and error is bounded to 5.4 mmHg. Findings also suggest that the F-ML approach reconstructs the shape of the central pressure waveform accurately with the average normalized root mean square error of 5.5 % in the testing population which is blinded to all stages of machine learning development. The results show that the F-ML transfer function outperforms the conventional generalized transfer function, particularly in terms of reconstructing the shape of the central pressure waveform morphology. The proposed F-ML transfer function can provide accurate estimates for the central pressure waveform, and ultimately expand the usage of non-invasive devices for central hemodynamic assessment.

The influence of perceived discrimination on aortic hemodynamics in young Black adults

Black adults experience the greatest prevalence of cardiovascular disease (CVD) risk factors and higher CVD mortality of any racial or ethnic group in the United States. The chronic stress from experiencing racial discrimination may exert a weathering effect that contributes to cardiovascular health inequities observed among Black adults. Prior evidence has suggested that Black adults experience greater aortic stiffness as compared with white adults. However, there is limited information regarding perceived racial discrimination and aortic hemodynamics in young adults. Purpose/Hypothesis: The purpose of this study was to determine whether perceived racial discrimination was associated with worse aortic hemodynamics in young Black adults. Methods: Black adults aged 18-25 (n=18, 44% female) volunteered to participate in this study. Participants arrived at the lab at 7:00AM in a fasted state. Following 30 minutes of rest, central pressure waveforms were derived from the radial artery waveform acquired via applanation tonometry and the use of the generalized transfer function. We report aortic augmentation pressure (AP, mmHg), augmentation index (AIx, %), augmentation index normalized to a heart rate of 75bpm (AIx75), round trip transit time (Tr, ms), and the subendocardial viability ratio (SEVR, %), also known as the Buckberg Index. Peak oxygen consumption (VO2 peak) was measured via Indirect calorimetry during a graded treadmill test. Participants completed a questionnaire that assessed personal-level and group-level discrimination. Partial correlations (Pearson’s r controlled for VO2 peak) were used to assess the association between perceived racial discrimination and aortic hemodynamics. Results: Perceived group-level discrimination was positively associated with AP (r = 0.571, p = 0.026), AIx (r = 0.580, p = 0.023), and AIx75 (r = 0.571, p = 0.026), signifying worse central hemodynamics that are indicative of increased left ventricular systolic load. However, perceived personal-level discrimination was not associated with aortic hemodynamics (ps&gt;0.05). VO2peak was positively associated with Tr (r = 0.511, p = 0.043), signifying a later return of the reflected wave (i.e, better) to the heart to facilitate coronary perfusion during diastole. VO2 peak was also inversely associated with AIx75 (r = -0.657, p = 0.006). Conclusion: These preliminary data indicate that young Black adults who report higher group-level discrimination have an increased left ventricular systolic afterload in the aorta. Future studies investigating coping strategies or the effects of increased fitness to improve central hemodynamics in Black adults are warranted. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Vascular Dysfunction and Inflammation in Liver Transplant Recipients

INTRODUCTION Liver transplant (LT) recipients have an increased risk of developing cardiovascular disease (CVD) after transplant. The physiological mechanisms driving this risk are not fully understood. Vascular endothelial dysfunction is the initial pathophysiological step in the development of atherosclerosis. The objective of this study was to characterize vascular endothelial function in LT recipients. It was hypothesized that compared to matched healthy controls (HC), LT recipients would have impaired vascular function. METHODS. Conduit artery endothelial function was measured by flow-mediated dilation of the brachial artery (FMDBA) with duplex ultrasound in 40 LT patients (18 male, 22 female; mean ± SD: Age, 56±11 years) and 10 age matched HC (4 male, 6 female; Age 51±16 years). Central aortic blood pressures were derived from radial arterial tonometry and the use of the generalized transfer function. A venous blood sample was obtained for nuclear magnetic resonance spectroscopy assessment of the inflammation biomarker GlycA, and high-density lipoprotein (HDL) particle subspecies. A novel metabolic inflammation index was derived from plasma GlycA and HDL subparticle concentrations. RESULTS. LT recipients had higher peripheral (Median [IQR]: HC vs. LT, 119 [111-136] vs. 135 [124-147] mmHg, p=0.01) and central aortic (113 [102-126] vs. 124 [116-139] mmHg, p=0.03) systolic blood pressures. LT recipients had impaired vascular endothelial function (HC vs. LT, FMDBA: 6.08 [5.01-7.61] vs. 4.54 [3.31-6.24] %, p = 0.04). In LT recipients, the inflammation index (55 [45-62] AU) was negatively associated with vascular endothelial function ( r=-0.4, p=0.03). CONCLUSIONS. Despite optimal pharmacological intervention, LT recipients had higher systolic blood pressures and worse vascular endothelial dysfunction compared to HC. In LT recipients, a higher inflammation index was associated with worse endothelial function, suggesting that chronic low-grade inflammation may partly contribute to endothelial dysfunction in this patient population. These findings provide support for the vascular endothelium as a potential therapeutic target to improve cardiovascular health in LT recipients. Supported by CTSA award No. UM1TR004360 from the National Center for Advancing Translational Sciences. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Assessing the Accuracy of Systolic Aortic Pressure Estimation From a Brachial Cuff Alone: A Comparison With Radial Tonometry.

A novel method for estimating central systolic aortic pressure (cSAP) has emerged, relying solely on the peripheral mean (MBP) and diastolic (DBP) blood pressures. We aimed to assess the accuracy of this Direct Central Blood Pressure estimation using cuff alone (DCBPcuff = MBP2/DBP) in comparison to the use of a generalized transfer function to derive cSAP from radial tonometry (cSAPtono). This retrospective analysis involved the International Database of Central Arterial Properties for Risk Stratification (IDCARS) data (Aparicio et al., Am J Hypertens 2022). The dataset encompassed 10,930 subjects from 13 longitudinal cohort studies worldwide (54.8% women; median age 46.0 years; office hypertension: 40.1%; treated: 61.0%), documenting cSAPtono via SphygmoCor calibrated against brachial systolic BP (SBP) and DBP. Our analysis focused on aggregate group data from 12/13 studies (89% patients) where a full BP dataset was available. A 35% form factor was used to estimate MBP = (DBP + (0.35 × (SBP-DBP)), from which DCBPcuff was derived. The predefined acceptable error for cSAPtono estimation was set at ≤ 5 mm Hg. The cSAPtono values ranged from 103.8-127.0 mm Hg (n = 12). The error between DCBPcuff and cSAPtono was 0.2 ± 1.4 mm Hg, with no influence of the mean. Errors ranged from -1.8 to 2.9 mm Hg across studies. No significant difference in errors was observed between BP measurements obtained via oscillometry (n = 9) vs. auscultation (n = 3) (P = 0.50). Using published aggregate group data and a 35% form factor, DCBPcuff demonstrated remarkable accuracy in estimating cSAPtono, regardless of the BP measurement technique. However, given that individual BP values were unavailable, further documentation is required to establish DCBPcuff's precision.

Structured interpolation for multivariate transfer functions of quadratic-bilinear systems

High-dimensional/high-fidelity nonlinear dynamical systems appear naturally when the goal is to accurately model real-world phenomena. Many physical properties are thereby encoded in the internal differential structure of these resulting large-scale nonlinear systems. The high dimensionality of the dynamics causes computational bottlenecks, especially when these large-scale systems need to be simulated for a variety of situations such as different forcing terms. This motivates model reduction where the goal is to replace the full-order dynamics with accurate reduced-order surrogates. Interpolation-based model reduction has been proven to be an effective tool for the construction of cheap-to-evaluate surrogate models that preserve the internal structure in the case of weak nonlinearities. In this paper, we consider the construction of multivariate interpolants in frequency domain for structured quadratic-bilinear systems. We propose definitions for structured variants of the symmetric subsystem and generalized transfer functions of quadratic-bilinear systems and provide conditions for structure-preserving interpolation by projection. The theoretical results are illustrated using two numerical examples including the simulation of molecular dynamics in crystal structures.

Ascending aortic impedance in young endurance athletes: a time-resolved phase-contrast MRI study.

Ventricular-vascular coupling in endurance athletes remains incompletely understood. The purpose of this study was to determine the ascending aortic impedance in endurance athletes and explore its associations with traditional cardiovascular measurements. In 15 young male endurance runners and 19 young healthy men, time-resolved (CINE) two-dimensional (2-D) phase-contrast MRI quantified the ascending aortic flow while the pressure waveform was simultaneously collected via a generalized transfer function. The aortic impedance modulus and phase were calculated in the frequency domain while characteristic impedance (ZcF) was calculated by averaging moduli between the 4th and 8th heart rate (HR) harmonics. Stroke volume (SV), left ventricular (LV) morphometry, double product, aortic compliance, and total peripheral resistance (TPR) were also measured. Endurance athletes had higher SV, slower HR, greater LV end-diastolic volume and mass, and lower double product than sedentary participants (all P < 0.05). ZcF was significantly lower in athletes than in sedentary participants (73.3 ± 19.2 vs. 93.4 ± 19.0 dyn·s/cm5, P = 0.005). Furthermore, ZcF was negatively correlated with SV (r = -0.691) and aortic compliance (r = -0.601) but was positively correlated with double product (r = 0.445) and TPR (r = 0.458; all P < 0.05). Multivariate analysis revealed that ZcF was the strongest predictor of SV followed by TPR and HR (adjusted R2 = 0.788, P < 0.001). Therefore, our findings collectively suggest that LV afterload quantified by aortic ZcF is significantly lower in endurance athletes than in sedentary adults. The lower pulsatile LV afterload may contribute to greater SV in endurance athletes.NEW & NOTEWORTHY This is the first study to investigate aortic impedance with the noninvasive, simultaneous recordings of aortic pressure using SphygmoCor XCEL and flow using phase-contrast MRI. We found that the characteristic impedance (Zc) is significantly lower in endurance athletes than sedentary adults, is the strongest predictor of stroke volume (SV), and is inversely associated with aortic compliance. These findings suggest that aortic impedance is a key determinant of the ventricular-vascular coupling adapted to long-term training in endurance athletes.

Overcoming the challenges of performing metasurface-based general spatial transfer functions

The concept of performing mathematical operations with metasurfaces has been suggested by Silva et al (Science 343, 160 (2014)). However, their proposed structure in implementing any transfer function (corresponding to any mathematical operator) for various input signals faces limitations. To tackle this issue, in this study, four different scenarios are proposed on their metasurface-based structure to generalize in a way that can implement each spatial transfer function. To evaluate the performance of the presented scenarios, seven different transfer functions are simulated to encompass a wide range of mathematical operators in the spatial domain. The implementations are based on the Fourier approach. Simulation results based on the finite element method closely match the desired values. From the results of this study, it can be seen that the third and fourth scenarios provide better accuracy. For example, when the fifth transfer function is performed by the basic structure and the fourth scenario, the normalized root mean square error, decreases from the value of 0.235 to the value of 0.0348, respectively. Furthermore, a tunable structure is achievable using the third scenario to produce different operators on the same structure. The realization of these scenarios is possible by using nanostructure-based metasurfaces.

Global dataset of soil organic carbon in tidal marshes

Tidal marshes store large amounts of organic carbon in their soils. Field data quantifying soil organic carbon (SOC) stocks provide an important resource for researchers, natural resource managers, and policy-makers working towards the protection, restoration, and valuation of these ecosystems. We collated a global dataset of tidal marsh soil organic carbon (MarSOC) from 99 studies that includes location, soil depth, site name, dry bulk density, SOC, and/or soil organic matter (SOM). The MarSOC dataset includes 17,454 data points from 2,329 unique locations, and 29 countries. We generated a general transfer function for the conversion of SOM to SOC. Using this data we estimated a median (± median absolute deviation) value of 79.2 ± 38.1 Mg SOC ha−1 in the top 30 cm and 231 ± 134 Mg SOC ha−1 in the top 1 m of tidal marsh soils globally. This data can serve as a basis for future work, and may contribute to incorporation of tidal marsh ecosystems into climate change mitigation and adaptation strategies and policies.

A generalized canine transfer function accurately reconstructs central aortic pressure waveforms to enable enhanced pulse wave analysis

Routine preclinical blood pressure evaluation is an important risk assessment tool. Although proximal aortic pressure is most relevant for key target organs, abdominal aortic pressures are more commonly recorded. Pulse pressure amplification and waveform distortion in abdominal waveforms make it inappropriate for central hemodynamic analytical methods without the use of a mathematical transfer function. Clinical transfer functions have been developed to estimate ascending aortic waveforms from brachial or radial artery waveforms in humans, but no preclinical analogues exist. The aim of this study was to develop a canine-specific transfer function to reconstruct thoracic aortic pressure waveforms from abdominal aortic data to enable the application of central hemodynamic analytical methods. Simultaneous abdominal and thoracic blood pressures were recorded from seven conscious, male beagle dogs administered 3 well-characterized pharmacologic standards and animals were appointed to a training (n = 3) or validation (n = 4) group at baseline and during dosing. A generalized transfer function was developed from the training group data and evaluated for its ability to synthesize thoracic pressure waves in the training and validation groups. Select hemodynamic parameters were evaluated in measured and synthesized thoracic data. There was a high degree of correlation between measured and synthesized thoracic parameters (r2 = 0.74–0.99). There was no difference between indices computed from synthesized or actual thoracic waveforms at baseline or after administration of pharmacologic standards. This work demonstrates that a generalized preclinical transfer function can reproduce thoracic pressure waves across a range of hemodynamic responses thus enabling the application of central hemodynamic analytical methods.

General Closed-Form Transfer Function Expressions for Fast Filter Bank

The existing literature focuses on the applications of fast filter bank due to its excellent frequency responses with low complexity. However, the topic is not addressed related to the general transfer function expressions of the corresponding subfilters for a specific channel. To do this, in this paper, general closed-form transfer function expressions for fast filter bank are derived. Firstly, the cascaded structure of fast filter bank is modelled by a binary tree, with which the index of the subfilter at each stage within the channel can be determined. Then the transfer functions for the two outputs of a subfilter are expressed in a unified form. Finally, the general closed-form transfer functions for the channel and its corresponding subfilters are obtained by variables replacement if the prototype lowpass filters for the stages are given. Analytical results and simulations verify the general expressions. With such closed-form expressions lend themselves easily to analysis and direct computation of the transfer functions and the frequency responses without the structure graph.

MODELS OF FIRE EXTINGUISHING WHEN FLAMMABLE LIQUID COMBUSTION

The process of extinguishing a class B fire with sprayed water is described by the differential equation of heat conduction with boundary conditions of the second kind. The solution of this differential equation in a dimensionless form allows to make a transition to the operative form of representation of the mathematical model of the quenching process - to the transfer function. The peculiarity of such a fire transfer function when extinguishing it with sprayed water is its irrationality, which causes difficulties in its use. One of the ways out of this situation is to switch to an equivalent representation of an irrational function of a complex variable in the form of a small-rational function of such a variable. Such a transition is carried out with the help of the minimax approximation using the Remez algorithm. Determination of the maximum order of the characteristic polynomial of the fire transfer function is carried out using the Hurwitz stability criterion. It is shown that for the transfer function of class B fire, the order of the Hurwitz polynomials does not exceed four, the approximation error does not exceed 3.6% using the method of undetermined coefficients, the transfer functions of the fire when they are extinguished with sprayed water are constructed in the form of a superposition of the transfer functions of aperiodic links. Numerical values of the parameters of such transfer functions were obtained. For the case when the intensity of the sprayed water supply is described by the Heaviside function, expressions describing the temperature of the surface of the burning liquid were constructed. It is noted that these expressions correspond to the expressions that describe the dynamic properties of class B fire in the time domain. It is shown that the maximum value of the relative error when using such transfer functions does not exceed 4.0%, if the general transfer function represents the superposition of the transfer functions of two aperiodic links. It is noted that when using the Padé approximation, the transfer function in the form of the transfer functions of eight aperiodic links corresponds to this value of the divergence error. Keywords: flammable liquid, fire extinguishing, sprayed water, transfer function.

Non-Invasive Estimation of Central Systolic Blood Pressure by Radial Tonometry: A Simplified Approach.

Central systolic blood pressure (cSBP) provides valuable clinical and physiological information. A recent invasive study showed that cSBP can be reliably estimated from mean (MBP) and diastolic (DBP) blood pressure. In this non-invasive study, we compared cSBP calculated using a Direct Central Blood Pressure estimation (DCBP = MBP2/DBP) with cSBP estimated by radial tonometry. Consecutive patients referred for cardiovascular assessment and prevention were prospectively included. Using applanation tonometry with SphygmoCor device, cSBP was estimated using an inbuilt generalized transfer function derived from radial pressure waveform, which was calibrated to oscillometric brachial SBP and DBP. The time-averaged MBP was calculated from the radial pulse waveform. The minimum acceptable error (DCBP-cSBP) was set at ≤5 (mean) and ≤8 mmHg (SD). We included 160 patients (58 years, 54%men). The cSBP was 123.1 ± 18.3 mmHg (range 86-181 mmHg). The (DCBP-cSBP) error was -1.4 ± 4.9 mmHg. There was a linear relationship between cSBP and DCBP (R2 = 0.93). Forty-seven patients (29%) had cSBP values ≥ 130 mmHg, and a DCBP value > 126 mmHg exhibited a sensitivity of 91.5% and specificity of 94.7% in discriminating this threshold (Youden index = 0.86; AUC = 0.965). Using the DCBP formula, radial tonometry allows for the robust estimation of cSBP without the need for a generalized transfer function. This finding may have implications for risk stratification.

Comparison of effects of peripheral vasculature on tonometric radial pulse and cuff-based brachial pulse waveform as used in estimation of central aortic pressures.

Aortic pressure estimation requires reliable peripheral pulse waveform acquisition. The peripheral waveform can change with local vascular effects that can be independent of aortic pressure. This study quantifies the effects of peripheral vasculature changes on radial and brachial waveforms. In 20 subjects (37± 15 years, 7 female), brachial volumetric displacement (cuff-based) and radial tonometry waveforms were simultaneously measured whilst a cuff around the hand on the same arm was inflated to induce transmural pressures of -60, -30, -15, 0, 15 and 30 mmHg, altering local peripheral resistance and compliance by graded arterial wall unloading. Aortic blood pressure (BP), augmentation index (AIx) and ejection duration were calculated from the measurements using a generalized transfer function. The parameters under unloaded conditions were compared to baseline measurements. Brachial systolic and diastolic BP did not change throughout the experiment. Altering peripheral resistance and compliance did not significantly change calculated aortic BP values, although changes were nominally greater for radial (maximum +8±1 mmHg) compared to brachial (maximum +2±1 mmHg) waveforms. AIx at 0 mmHg transmural pressure (maximum arterial wall unloading) was higher when derived from radial waveforms (+24±3%, p<0.001) but not when derived from brachial waveforms. Localized changes in peripheral resistance and compliance affect tonometer acquired radial waveforms but not volumetric displacement acquired brachial pressure waveforms, as judged by computed central aortic augmentation pressure parameters. This suggests aortic pressure estimation from the brachial cuff waveform is less sensitive to peripheral vasculature disturbances that alter the peripheral arterial pulse morphology.

From peripheral finger-derived pulse waveforms to aortic pressure waveform features: an application of a generalized transfer function.

Aortic (central) pressure features are associated with cardiovascular complications and can be algorithmically derived from non-invasive peripheral arterial waveforms. This has conventionally been performed with a pressure waveform (i.e., tonometry or oscillometry) rather than with the optical-based sensor (photoplethysmography (PPG)) that is predominantly used in wearable health devices. Extraction of aortic features from a peripheral PPG waveform has yet to be investigated. This study aims to compare aortic features extracted from peripheral arterial waveforms acquired with different sensor modalities using the same transfer function. Radial tonometry (reference), finger volume-clamped PPG (Peňáz) and fingertip PPG waveforms were measured in participants (n=29, 36±16 years, 15 female) under baseline conditions. Waveforms were converted into an aortic pressure waveform using the transfer function. Waveform features were extracted from the converted waveform. Extracted features were compared with correlation plots and a Bland-Altman analysis. Aortic pressure features extracted from a finger using the Peňáz technique were comparable to radial tonometry derived features. Aortic features extracted from a fingertip waveform were more variable in comparison to radial tonometry-derived features. Aortic (central) pressure waveform features contain valuable haemodynamic information and have the capacity to be easily and conveniently implemented in wearable health devices. Future use of these features in wearable health devices incorporating PPG requires the development, and/or, optimization of a unique transfer function to more accurately represent the aortic pressure waveform for cardiovascular assessment.Clinical Relevance- Aortic pressure features might be used in wearable health devices following the development of a unique transfer function for optical-transduced peripheral vascular signals.

A study on transfer function to estimate the central aortic blood pressure waveform

Noninvasive measurement of the central aortic blood pressure has become an important technique that has generated a lot of interest around the medicine industry because it can estimate the central aortic blood pressure waveform without inserting a pressure-sensing catheter into the ascending aorta. The accuracy of noninvasive estimation of aortic hemodynamics and cardiac contractility is still debatable in noninvasive measurement of the central aortic blood methods. The objective of this project is to investigate the transfer functions available for converting radial blood pressure waveforms to central aortic blood pressure waveforms. In this project, three different transfer functions will be investigated which are The Generalized Transfer Function, N-Point Moving Average and Adaptive Transfer Function in order to recommend the effective method in terms of accuracy. The study methodology will be conducted through the software MATLAB R2021b. The investigation of these three methods will be conducted with data collected from virtual subjects from the HaeMod database. The Generalized transfer function, N-Point Moving Average and Adaptive Transfer Function will be coded, evaluated, and analyzed as part of the methodology. The determination of the most effective method among the three transfer functions will be attributed according to output comparison of Systolic peak differences, the RMSE and the MAPE between the estimated central aortic blood pressure and the measured central aortic blood pressure.

Attenuated pulsatile transition to the cerebral vasculature during high-intensity interval exercise in young healthy men.

What is the central question of this study? High-intensity interval exercise (HIIE) is recommended for its favourable haemodynamic stimulation, but excessive haemodynamic fluctuations may stress the brain: is the cerebral vasculature protected against exaggerated systemic blood flow fluctuation during HIIE? What is the main finding and its importance? Time- and frequency-domain indices of aortic-cerebral pulsatile transition were lowered during HIIE. The findings suggest that the arterial system to the cerebral vasculature may attenuate pulsatile transition during HIIE as a defence mechanism against pulsatile fluctuation for the cerebral vasculature. High-intensity interval exercise (HIIE) is recommended because it provides favourable haemodynamic stimulation, but excessive haemodynamic fluctuations may be an adverse impact on the brain. We tested whether the cerebral vasculature is protected against systemic blood flow fluctuation during HIIE. Fourteen healthy men (age 24 ± 2 years) underwent four 4-min exercises at 80-90% of maximal workload (Wmax ) interspaced by 3-min active rest at 50-60% Wmax . Transcranial Doppler measured middle cerebral artery blood velocity (CBV). Systemic haemodynamics (Modelflow) and aortic pressure (AoP, general transfer function) were estimated from an invasively recorded brachial arterial pressure waveform. Using transfer function analysis, gain and phase between AoP and CBV (0.39-10.0Hz) were calculated. Stroke volume, aortic pulse pressure and pulsatile CBV increased during exercise (time effect: P<0.0001 for all), but a time-domain index of aortic-cerebral pulsatile transition (pulsatile CBV/pulsatile AoP) decreased throughout the exercise bouts (time effect: P<0.0001). Furthermore, transfer function gain reduced, and phase increased throughout the exercise bouts (time effect: P<0.0001 for both), suggesting the attenuation and delay of pulsatile transition. The cerebral vascular conductance index (mean CBV/mean arterial pressure; time effect: P=0.296), an inverse index of cerebral vascular tone, did not change even though systemic vascular conductance increased during exercise (time effect: P<0.0001). The arterial system to the cerebral vasculature may attenuate pulsatile transition during HIIE as a defence mechanism against pulsatile fluctuation for the cerebral vasculature.

Central Hemodynamic Response to Activation of the Vestibular Sympathetic Reflex with Head Down Rotation in Healthy Young Adults.

The purpose of this study was to assess the effect of vestibular-sympathetic activation on estimated measures of central (aortic) hemodynamic load in young adults. Methods: 31 participants (n=14 female, 17 male) underwent radial tonometry and popliteal Doppler-ultrasound in the prone position with the head in a neutral position and during 10 minutes of head-down rotation (HDR) as a means of activating the vestibular sympathetic reflex. Subjective orthostatic intolerance was assessed utilizing a 10-item orthostatic hypotension questionnaire. Results: There was a reduction in brachial systolic BP during HDR (111±10 vs. 109±9 mmHg, p<0.05), but there was no change in aortic systolic BP (100±9 vs. 99±8 mmHg, p>0.05). Aortic augmentation index (-5±11 vs. -12±12%, p<0.05) and reservoir pressure (28±8 vs. 26±8mmHg, p<0.05) were reduced with a concomitant reduction in popliteal conductance (5.6±0.7 vs. 4.5±0.7 ml/min∙mmHg, p<0.05). Change in aortic systolic BP was associated with subjective orthostatic intolerance score (r = -0.39, p<0.05). Conclusions: Activation of the vestibular sympathetic reflex via HDR resulted in slight reductions in brachial BP concomitant with preservation of aortic BP. Despite peripheral vascular constriction during HDR, there was a reduction in pressure from wave reflections and reservoir pressure. Finally, there was an association between change in aortic systolic BP during HDR and orthostatic intolerance score suggesting that individuals that cannot defend against drops in aortic BP during vestibular sympathetic reflex activation may be more likely to experience higher subjective symptomatology of orthostatic intolerance.

An examination of complex fractional order physical phenomena in IOPD controller design

This research focuses on the fractional complex order plant (FCOP). The significant contribution is the role of complex plant models in system stability and robustness and associated physical phenomena. A general transfer function is studied in the paper. Other plant models may be built with this structure since the FCOP is a general mathematical form covering integer order plant (IOP) and fractional order plant (FOP). Using the equations produced with the proposed technique and the recommended integer order proportional derivative (IOPD controller, physical changes in integer, fractional and complex coefficients, and orders are observed within this paper. Analysis of the plant controlled with an IOPD controller is done by applying an integrator to reveal the differences. The effects of the parameters are discussed together with the visuals, supported by simulations. The aim is to tune the controller parameters to achieve the phase and specifications as the researcher desired. It is observed that the integrator greatly takes part in reducing the steady‐state error. The IOP with the integrator showed the lowest steady‐state error, and also, the settling and overshoot time were enhanced. Increase in the phase margin also caused an increase in the phase crossover frequency. It is also observed that the fractional order affected the phase crossover frequency comparing with the IOP, and the complex order modification also had an effect comparing to the fractional order version. The complex order of the system is considered with its conjugate components in the imaginary part thus, the results are found separately for each case.

Personalized aortic pressure waveform estimation from brachial pressure waveform using an adaptive transfer function

Background and objectiveThe aortic pressure waveform (APW) provides reliable information for the diagnosis of cardiovascular disease. APW is often measured using a generalized transfer function (GTF) applied to the peripheral pressure waveform acquired noninvasively, to avoid the significant risks of invasive APW acquisition. However, the GTF ignores various physiological conditions, which affects the accuracy of the estimated APW. To solve this problem, this study utilized an adaptive transfer function (ATF) combined with a tube-load model to achieve personalized and accurate estimation of APW from the brachial pressure waveform (BPW). MethodsThe proposed method was validated using APWs and BPWs from 34 patients. The ATF was defined using a tube-load model in which pulse transit time and reflection coefficients were determined from, respectively, the diastolic-exponential-pressure-decay of the APW and a piece-wise constant approximation. The root-mean-square-error of overall morphology, mean absolute errors of common hemodynamic indices (systolic blood pressure, diastolic blood pressure and pulse pressure) were used to evaluate the ATF. ResultsThe proposed ATF performed better in estimating diastolic blood pressure and pulse pressure (1.63 versus 1.94 mmHg, and 2.37 versus 3.10 mmHg, respectively, both P < 0.10), and produced similar errors in overall morphology and systolic blood pressure (3.91 versus 4.24 mmHg, and 2.83 versus 2.91 mmHg, respectively, both P > 0.10) compared to GTF. ConclusionUnlike the GTF which uses fixed parameters trained on existing clinical datasets, the proposed method can achieve personalized estimation of APW. Hence, it provides accurate pulsatile hemodynamic measures for the evaluation of cardiovascular function.