Transfer Function Analysis Research Articles

Menstrual cycle and oral contraceptives influence cerebrovascular dynamics during hypercapnia.

Women experience fluctuating orthostatic intolerance during the menstrual cycle, suggesting sex hormones may influence cerebral blood flow. Young (aged 18–30) healthy women, either taking oral contraceptives (OC; n = 14) or not taking OC (NOC; n = 12), were administered hypercapnic gas (5%) for 5 min in the low hormone (LH; placebo pill) and high hormone (HH; active pill) menstrual phases. Hemodynamic and cerebrovascular variables were continuously measured. Cerebral blood velocity changes were monitored using transcranial doppler ultrasound of the middle cerebral artery to determine cerebrovascular reactivity. Cerebral autoregulation was assessed using steady‐state analysis (static cerebral autoregulation) and transfer function analysis (dynamic cerebral autoregulation; dCA). In response to hypercapnia, menstrual phase did not influence static cardiovascular or cerebrovascular responses (all p > 0.07); however, OC users had a greater increase of mean middle cerebral artery blood velocity compared to NOC (NOC‐LH 12 ± 6 cm/s vs. NOC‐HH 16 ± 9 cm/s; OC‐LH 18 ± 5 cm/s vs. OC‐HH 17 ± 11 cm/s; p = 0.048). In all women, hypercapnia improved high frequency (HF) and very low frequency (VLF) cerebral autoregulation (decreased nGain; p = 0.002 and <0.001, respectively), whereas low frequency (LF) Phase decreased in NOC‐HH (p = 0.001) and OC‐LH (p = 0.004). Therefore, endogenous sex hormones reduce LF dCA during hypercapnia in the HH menstrual phase. In contrast, pharmaceutical sex hormones (OC use) have no acute influence (HH menstrual phase) yet elicit a chronic attenuation of LF dCA (LH menstrual phase) during hypercapnia.

Invited Tutorial: Analog Matrix Computing With Crosspoint Resistive Memory Arrays

Matrix computation is ubiquitous in modern scientific and engineering fields. Due to the high computational complexity in conventional digital computers, matrix computation represents a heavy workload in many data-intensive applications, e.g., machine learning, scientific computing, and wireless communications. For fast, efficient matrix computations, analog computing with resistive memory arrays has been proven to be a promising solution. In this Tutorial, we present analog matrix computing (AMC) circuits based on crosspoint resistive memory arrays. AMC circuits are able to carry out basic matrix computations, including matrix multiplication, matrix inversion, pseudoinverse and eigenvector computation, all with one single operation. We describe the main design principles of the AMC circuits, such as local/global or negative/positive feedback configurations, with/without external inputs. Mapping strategies for matrices containing negative values will be presented. The underlying requirements for circuit stability will be described via the transfer function analysis, which also defines time complexity of the circuits towards steady-state results. Lastly, typical applications, challenges, and future trends of AMC circuits will be discussed.

Pitfalls to avoid in nonlinear perturbation analysis of structured population models

AbstractDespite the ubiquity of nonlinear functional relationships in nature we tend to characterize mechanisms in science using more tractable linear functions. In demographic modeling, transfer function analysis is used to calculate the nonlinear response of population growth rate to a theoretical perturbation of one or more matrix elements. This elegant approach is not yet popular in ecology. Inconveniently, using transfer function without care can produce erroneous results without warning. We used a large matrix projection model database to explore the potential pitfalls to be avoided in using transfer function analysis. We asked a fundamental population control question, what matrix element perturbation would be needed to reach a minimum goal of replacement population growth? We then tracked instances in which transfer function yields erroneous output and explored these cases in detail to measure how frequently it occurs. We developed a phylogenetically‐corrected mixed effects logistic regression model in a Bayesian framework to test the effect of species traits and the identity of matrix elements on the probability that transfer function yields errors. We found in 16% of cases the transfer function yielded erroneous outcomes. These errors were more likely when perturbing demographic stasis and also for shrubs more than any other life form. Errors in transfer function analysis were often due to perturbing matrix elements beyond their biological limits, even when this is still mathematically correct. To use transfer function analysis properly in demographic modeling and avoid erroneous results, input must be carefully selected to include only a biologically admissible set of perturbations.

Comparison of flame response characteristics between Non‐premixed and premixed flames under acoustic excitation

A comparative study was conducted on the response characteristics according to the increase in velocity perturbation intensity (u'/u) under acoustic excitation conditions of non-premixed and premixed flames. In addition, the flame dynamic characteristics and flame structure according to acoustic excitation were analyzed, and the response characteristics were compared according to the two flame types through the flame transfer function (FTF) and flame height perturbation analysis. In a non-premixed flame, the number of waveforms (undulations) on the flame surface increased according to the forcing frequency in the flame structure according to the acoustic excitation. On the other hand, as the velocity perturbation intensity increased, the flame height decreased. Thus, in the low forcing frequency range, the magnitude of the flame fluctuation increases, and in the high forcing frequency, the magnitude of the flame fluctuation tends to decrease. Therefore, the Strouhal number and FTF gain are nonlinear properties, and the nonlinear effect depends on the magnitude of the gain and the critical Strouhal number. In a premixed flame, various flame structures are shown according to acoustic excitation. The average flame height does not change much with increasing (u'/u¯). The correlation between the Strouhal number and FTF gain is linear. The flame height perturbation parameters were introduced to examine the flame dynamic characteristics according to the acoustic excitation, and non-premixed and premixed flames were analyzed. As a result, it shows the trend and similarity of the FTF gain, suggesting its potential as a parameter for simple analysis of the heat-release rate. In the correlation between the Strouhal number and FTF gain, the non-premixed flame has a nonlinear characteristic and agrees with the analytical study. On the other hand, the premixed flame is not consistent with analytic studies when the Strouhal number is greater than 1. The reason for various flame structures is to consider parameters such as the unusual flame structure and laminar flame speed by external perturbation.

Surface Wind Responses to Mesoscale Sea Surface Temperature over Western Boundary Current Regions Assessed by Spectral Transfer Functions

Abstract Satellite observations have revealed that mesoscale sea surface temperature (SST) perturbations can exert distinct influence on sea surface wind by modifying the overlying atmospheric boundary layer. Recently, spectral transfer functions have been shown to be useful to elucidate the wind response features. Spectral transfer functions can represent spatially lagged responses, their horizontal scale dependence, and background wind speed dependence. By adopting the transfer function analysis, the present study explores seasonality and regional differences in the wind response over the major western boundary current regions. Transfer functions estimated from satellite observations are found to be largely consistent among seasons and regions, suggesting that the underlying dominant dynamics are ubiquitous. Nevertheless, the wind response exhibits statistically significant seasonal and regional differences depending on background wind speed. When background wind is stronger (weaker) than 8.5 m s−1, the wind response is stronger (weaker) in winter than in summer. The Agulhas Retroflection region exhibits stronger wind response typically by 30% than the Gulf Stream and Kuroshio Extension regions. Although observed wind distributions are reasonably reconstructed from the transfer functions and observed SST, surface wind convergence zones along the Gulf Stream and Kuroshio Extension are underrepresented. The state-of-the-art atmospheric reanalysis and regional model represent well the structure of the transfer functions in the wavenumber space. The amplitude is, however, underestimated by typically 30%. The transfer function analysis can be adapted to many other atmospheric responses besides sea surface wind, and thus provide new insights into the climatic role of the mesoscale air–sea coupling.

Significance of low-level control to string stability under adaptive cruise control: Algorithms, theory and experiments

Commercial adaptive cruise control (ACC) systems are bi-level: an upper-level planner decides the target trajectory and the low-level system executes it. Existing literature on ACCs mostly focus on the planner algorithms or the actuator delay, while the transition process between them, e.g. the low-level control design and its impact are often ignored. This paper tries to fill this gap by digging into the codebase of a recent open-source self-driving system, Openpilot (OP), Comma.ai, from which we extract and formulate the algorithms at both the upper and lower levels. For linear ACCs, the paper extends the transfer function analysis from planners only to full control loops and investigates the impact of slow/fast low-level control on the overall string stability (SS). For MPC ACCs, it studies their planning characteristics based its optimization objectives and approximates the low-level impact using an ODE approach.We find that low-level control has a significant impact on the overall SS of ACCs: (i) slow low-level control undermines SS under small frequencies and improves SS given large frequencies for linear systems, (ii) MPC features a varying gain throughout an oscillation, where the fast low-level control typically results in a ‘fast-slow’ changing process of the MPC gain, which benefits the SS, whereas the slow low-level control leads to a ‘slow-fast’ varying gain which undermines the SS, (iii) slow low-level control are common as they arise from comfort-oriented control gains, from a ”weak” actuator performance or both, and (iv) the SS is very sensitive to the integral gain under slow low-level control for both PI and PIF controllers. Overall, the study recommends fast low-level control for ensuring vehicular SS to reduce traffic congestion, considering that large congestion waves usually feature both small frequencies and large amplitudes, although slow controllers could perform even better provided a short and small leader perturbation. The findings of this paper are verified both numerically and experimentally. For the first time in the literature we implement custom ACC algorithms on market cars, and achieve SS on open roads with a random leader by only tuning the low-level controllers. The source code is shared at https://github.com/HaoZhouGT/openpilot to support on-road experiments of arbitrary car-following models, which may be of interest to other studies.

Spatial analysis of acoustic noise transfer function with a human-body phantom in a clinical MRI scanner.

The acoustic noise in magnetic resonance imaging (MRI) potentially depends on the measurement position and presence of a patient inside the scanner bore. To analyze the spatial characteristics of the acoustic noise by using the gradient-pulse-to-acoustic-noise transfer function (GPAN-TF) with and without a human-body phantom on the examination table. Acoustic noise waveforms were acquired at 80 and 110 measurement positions with and without a phantom. The GPAN-TFs µPa/(mT/m) in the coils were calculated by deconvolution. The phantom effect on the spatial distribution of the acoustic noise was assessed using the peak sound pressure levels (SPLs), mean values, peak values, and peak frequencies of the GPAN-TFs. The peak SPLs in all positions for the X-, Y-, and Z-gradient coils were increased by 11.1 dB, 1.4 dB, and 6.1 dB, respectively, compared with the peak SPL of the magnetic isocenter. The maximum peak SPLs among all positions of the X-, Y-, and Z-gradient coils with the phantom were increased by 4.9 dB, 7.4 dB, and 6.9 dB, respectively, relative to those without the phantom. However, the peak SPLs decreased at some positions with the phantom placed on the table (X-gradient coil = 4.6 dB, Y-gradient coil = 5.0 dB, Z-gradient coil = 8.4 dB). The most common peak frequencies were in the range of 2000-3000 Hz. "Hotspot" areas with and without the phantom were associated with acoustic noise sources in the clinical MRI scanner and were enhanced by the phantom's presence.

Point-Counterpoint: Transfer function analysis of dynamic cerebral autoregulation: To band or not to band?

Transfer function analysis (TFA) is the most frequently adopted method for assessing dynamic cerebral autoregulation (CA) with continuously recorded arterial blood pressure (ABP) and cerebral blood flow velocity (CBFV). Conventionally, values of autoregulatory metrics (e.g., gain and phase) derived from TFA are averaged within three frequency bands separated by cut-off frequencies at 0.07 Hz and 0.20 Hz, respectively, to represent the efficiency of dynamic CA. However, this is of increasing concerns, as there remains no solid evidence for choosing these specific cut-off frequencies, and the rigid adoption of these bands can stifle further developments in TFA of dynamic CA. In this 'Point-Counterpoint' mini-review, we provide evidence against the fixed banding, indicate possible alternatives, and call for awareness of the risk of the 'one-size-fits-all' banding becoming dogmatic. We conclude that we need to remain open to the multiple possibilities offered by TFA to realize its full potential in studies of human dynamic CA.

Cerebral Augmentation Effect Induced by External Counterpulsation Is Not Related to Impaired Dynamic Cerebral Autoregulation in Ischemic Stroke.

Background and PurposeDynamic cerebral autoregulation is impaired after ischemic stroke. External counterpulsation (ECP) augments the cerebral blood flow of patients with ischemic stroke by elevation of blood pressure (BP). We aimed to investigate if cerebral augmentation effects during ECP were associated with impaired dynamic cerebral autoregulation in patients after acute ischemic stroke.MethodsForty patients with unilateral ischemic stroke and large artery atherosclerosis in the anterior circulation territory within 7 days from symptom onset and eighteen healthy controls were recruited. We monitored changes in mean flow velocity over both middle cerebral arteries (MCA) by transcranial Doppler (TCD) before, during, and immediately after ECP. Cerebral augmentation index was MCA mean flow velocity increase in percentage during ECP compared with baseline to evaluate the augmentation effects of ECP. Spontaneous arterial BP and cerebral blood flow velocity in both bilateral MCAs were recorded using a servo-controlled plethysmograph and TCD, respectively. Transfer function analysis was used to derive the autoregulatory parameters, including phase difference (PD), and gain.ResultsThe cerebral augmentation index in patients with stroke was significantly higher on both the ipsilateral and contralateral sides than that in controls, while the PD in patients with stroke was significantly lower on both sides than those in controls (all P < 0.05). The cerebral augmentation index did not correlate with PD and gain on either the ipsilateral or contralateral side of patients with stroke or in controls (all P > 0.05). The cerebral augmentation index of patients with stroke was significantly related to mean BP change on the ipsilateral side (R2 = 0.108, P = 0.038).ConclusionThe degree of ECP-induced cerebral augmentation effects as measured by the cerebral augmentation index did not correlate with the magnitude of impaired dynamic cerebral autoregulation.

Cerebral autoregulation across the menstrual cycle in eumenorrheic women.

There is emerging evidence that ovarian hormones play a significant role in the lower stroke incidence observed in pre‐menopausal women compared with men. However, the role of ovarian hormones in cerebrovascular regulation remains to be elucidated. We examined the blood pressure‐cerebral blood flow relationship (cerebral autoregulation) across the menstrual cycle in eumenorrheic women (n = 12; mean ± SD: age, 31 ± 7 years). Participants completed sit‐to‐stand and Valsalva maneuvers (VM, mouth pressure of 40 mmHg for 15 s) during the early follicular (EF), late follicular (LF), and mid‐luteal (ML) menstrual cycle phases, confirmed by serum measurement of progesterone and 17β‐estradiol. Middle cerebral artery blood velocity (MCAv), arterial blood pressure and partial pressure of end‐tidal carbon dioxide were measured. Cerebral autoregulation was assessed by transfer function analysis during spontaneous blood pressure oscillations, rate of regulation (RoR) during sit‐to‐stand maneuvers, and Tieck’s autoregulatory index during VM phases II and IV (AI‐II and AI‐IV, respectively). Resting mean MCAv (MCAvmean), blood pressure, and cerebral autoregulation were unchanged across the menstrual cycle (all p > 0.12). RoR tended to be different (EF, 0.25 ± 0.06; LF; 0.19 ± 0.04; ML, 0.18 ± 0.12 sec−1; p = 0.07) and demonstrated a negative relationship with 17β‐estradiol (R2 = 0.26, p = 0.02). No changes in AI‐II (EF, 1.95 ± 1.20; LF, 1.67 ± 0.77 and ML, 1.20 ± 0.55) or AI‐IV (EF, 1.35 ± 0.21; LF, 1.27 ± 0.26 and ML, 1.20 ± 0.2) were observed (p = 0.25 and 0.37, respectively). Although, a significant interaction effect (p = 0.02) was observed for the VM MCAvmean response. These data indicate that the menstrual cycle has limited impact on cerebrovascular autoregulation, but individual differences should be considered.

Does oscillation size matter? Impact of added resistance on the cerebral pressure-flow Relationship in females and males.

Sinusoidal squat‐stand maneuvers (SSM) without resistance have been shown to produce ~30–50 mmHg swings in mean arterial pressure which are largely buffered in the brain via dynamic cerebral autoregulation (dCA). This study aimed to further elucidate how this regulatory mechanism is affected during SSM with added resistance (~20% bodyweight). Twenty‐five participants (sex/gender: 13 females/12 males) completed two bouts of 5‐min SSM for both bodyweight and resistance conditions (10% bodyweight in each arm) at frequencies of 0.05 Hz (20‐s squat/stand cycles) and 0.10 Hz (10‐s squat/stand cycles). Middle and posterior cerebral artery (MCA/PCA) cerebral blood velocities were indexed with transcranial Doppler ultrasound. Beat‐to‐beat blood pressure (BP) was quantified via finger photoplesmography. Transfer function analysis was employed to quantify dCA in both cerebral arteries across the cardiac cycle (diastole, mean, and systole). Two‐by‐two Analysis of Variance with generalized eta squared effect sizes were utilized to determine differences between resistance vs. bodyweight squats and between sexes/genders. Absolute mean and diastolic BP were elevated during the resistance squats (p < 0.001); however, only the BP point‐estimate power spectrum densities were augmented at 0.10 Hz (p < 0.048). No differences were noted for phase and gain metrics between bodyweight and resistance SSM (p > 0.067); however, females displayed attenuated systolic regulation (p < 0.003). Despite augmented systemic BP during resistance SSM, the brain was effective at buffering the additional stress to mitigate overperfusion/pressure. Females displayed less dCA regulation within the systolic aspect of the cardiac cycle, which may be associated with physiological underpinnings related to various clinical conditions/presentations.

Middle‐Aged and Older Adults with Greater White Matter Hyperintensity Burden Display Attenuated Blood Pressure and Cerebral Blood Velocity Responses During Orthostatic Tilt

Cerebral autoregulation (CA) is the process by which the brain maintains adequate blood flow despite sudden changes to perfusion pressure. White matter hyperintensities (WMH) are associated with increased risk of cerebrovascular disease and dementia. This study investigated whether dynamic CA is reduced in cognitively unimpaired middle‐aged and older adults. Additionally, we evaluated dynamic CA and cerebrovascular responses at rest and in response to orthostatic tilt. We hypothesized that CA would be impaired during orthostatic tilt and that diminished blood pressure and cerebral artery blood velocity changes would be associated with greater WMH fraction. We studied 33 middle‐aged and older adults (age = 63 ± 4 y). Intracranial volume (ICV) and WMH lesion volume were measured using a T1 and FLAIR scan, respectively, on a 3T MRI scanner. WMH fraction was calculated as the cube root of (WMH lesion volume/ICV) *100. CA was measured using middle cerebral artery blood velocity (MCAv) from transcranial Doppler ultrasound and beat‐to‐beat mean arterial pressure (MAP) from a finger cuff. Five minutes of data were collected at rest and during 60º orthostatic tilt. Transfer function analysis was used to quantify dynamic CA and reported as gain and phase at low frequency (LF) (0.07‐0.20 Hz). MCAv, MAP and cerebrovascular resistance index (CVRi; calculated as MAP/MCAv) were used to quantify the cerebrovascular response to orthostatic tilt. LF gain was greater at baseline compared to 60º tilt (0.52 cm/s/mmHg vs 0.46 cm/s/mmHg, p &lt; 0.05); however, there were no differences in LF phase between baseline and 60º tilt (29.70 degrees vs 29.42 degrees, p = 0.95). During 60º tilt, MAP and MCAv decreased compared to baseline (MAP: 106 mmHg vs 101 mmHg, p = 0.02; MCAv: 59 cm/s vs 53 cm/s, p &lt; .01). CVRi was higher at 60º tilt compared with baseline (1.90 mmHg/cm/s vs 2.02 mmHg/cm/s, p &lt; 0.01). There were significant inverse associations between the percent change from baseline to 60º tilt in MAP (r = ‐0.4, p = 0.03), MCAv (r = ‐0.39, p = 0.03), but not CVRi (r = 0.13, p = 0.50), and WMH fraction. There were no associations at baseline or 60º tilt between gain (Baseline: r = ‐0.18, p = 0.33; 60º: r = ‐0.31, p = 0.09) or phase (Baseline: r = 0.07, p = 0.71; 60º: r = ‐0.29, p = 0.12) and WMH fraction. Our results indicate that dynamic CA is preserved in cognitively unimpaired adults during tilt. However, individuals with diminished MAP and MCAv responses to orthostatic tilt displayed greater WMH fraction. Future studies are needed to investigate the relationship between blood pressure and cerebral blood flow in individuals with cognitive impairment to further understand the role of cerebral blood flow regulation in the development of cognitive decline.

Resistance Breathing and Sympathetic Nerve Activity During Simulated Hemorrhage in Humans

BackgroundResistance breathing amplifies the respiratory pump during inspiration, so may be an effective intervention for treatment of hemorrhagic injuries. In animal studies of actual hemorrhage, and human studies of simulated hemorrhage, resistance breathing protects arterial pressure, and improves tolerance to this stress. Anecdotally, resistance breathing also increases the coupling between sympathetic nerve activity and arterial pressure. The impact of resistance breathing on overall sympathetic nerve activity, however, has not been examined. We tested the hypothesis that resistance breathing increases sympathetic nerve activity during simulated hemorrhage in healthy humans.MethodsLower body negative pressure (LBNP) was used to simulate hemorrhage in five human subjects (3M, 2F; 29.2 ± 6.8 y). Two experiments were conducted (randomized order): 1) a control condition in which LBNP was applied at 3 mmHg/min until the onset of presyncope, and 2) a resistance breathing condition in which the same LBNP protocol was used, but subjects breathed through a resistance device (set at ‐7 cm.H2O) during the final stages of the protocol. Arterial pressure and muscle sympathetic nerve activity (MSNA) of the radial nerve were monitored continuously. Bursts frequency (bursts/min) and burst incidence (burst/ 100 heart beats) were used to quantify MSNA. Coupling between diastolic arterial pressure (DAP) and MSNA was assessed by transfer function analysis coherence within the low frequency range (0.04‐0.15 Hz). Two‐way repeated measures ANOVAs were conducted for assessment of responses in the control and resistance breathing conditions, between baseline and at matched time points late in the LBNP protocol.ResultsWhile LBNP induced increases in both MSNA burst frequency (P=0.003) and burst incidence (P=0.06), there was no effect of resistance breathing on MSNA for either index during LBNP (control, 57.9 ± 25.9 bursts/min vs. resistance breathing, 50.6 ± 21.7 bursts/min, P=0.99; control, 55.6 ± 25.6 b/100 heart beats vs. resistance breathing, 42.3 ± 18.3 b/100 heart beats, P=0.42). Additionally, there was no effect of resistance breathing on DAP (control, 73.2 ± 9.9 mmHg vs. resistance breathing, 72.8 ± 4.4 mmHg; P=0.99), or coherence between MSNA and DAP during LBNP (control, 0.53 ± 0.21 vs. resistance breathing, 0.69 ± 0.17; P=0.46).ConclusionContrary to our hypothesis, resistance breathing had no effect on sympathetic nerve activity during LBNP. A limitation of this study is the low sample size (N=5), and high variability of MSNA. Future investigations with a larger sample size are needed to determine if respiratory dynamics can influence the coupling between MSNA and arterial pressure.

Real‐Time dynamics of renal autoregulatory responses to BP changes in conscious, normotensive and hypertensive rat strains

Renal autoregulatory (AR) mechanisms are believed to insulate RBF and GFR from the continuously fluctuating systemic pressures and protect the glomerular capillaries from barotrauma by preventing glomerular transmission of systemic hypertension. However, renal AR has usually only been assessed by the degree to which RBF is restored to baseline after step changes in BP are imposed, generally under anesthesia. AR indices (% changes in RBF / % changes in BP) are used to quantitate the magnitude of the AR compensation. However, such methods do not provide an assessment of the real time dynamics of AR responses to the spontaneously occurring BP fluctuations in real time. We have recently described novel analytic methods to characterize both the magnitude and the kinetics of such AR responses to BP fluctuations in conscious Sprague‐Dawley rats with intact (controls) and impaired AR (3/4 nephrectomy) and/or calcium channel blockade (JASN 31(2):324‐336, 2020). Essentially, AR indices were calculated for adjacent segments of varying lengths (0.5 – 20 sec) which exhibited a MAP difference of at least 5 mmHg. These studies indicated a much greater rapidity of the normal AR response than observed using standard methods under anesthesia with the bulk of AR compensation (&gt;50%) occurring within 2.5 sec with complete compensation occurring more slowly by ~10 sec. This rapid initial and likely myogenic component of the response was noted to be significantly slower in states of impaired renal AR with &lt;40% of AR compensation being achieved by 5 sec. To assess the generalizability of these pattern of AR responses and interpretations of rapid myogenic protection, similar AR analysis were performed on concurrent BP and RBF recordings of 1‐4 hr duration that had been obtained in conscious chronically instrumented hypertensive and normotensive rat strains in our laboratory for transfer function analysis in control SHR, SHRsp, Dahl S (DS) and R (DR) rats, and Sprague‐Dawley (SD) rats over the past 20 years (Fig. 1). The pattern of AR responses and compensation is remarkably similar in all rat strains except that the responses are even faster and stronger in hypertensive rats with ~80% compensation being achieved by 2.5 sec (Fig. 2). To further examine the impact of ambient BP on AR responses, AI were separately calculated when the ambient MAP was within 30 mmHg below (low BP) or above (high BP) the average MAP during the recordings for each strain. There was a clear trend for the AR responses to be stronger at higher ambient MAP, although statistical significance was not achieved in all strains because of the scatter in individual animal data (Fig. 3). These data suggest that in addition to genetic factors, ambient BP per se may modulate the dynamics of protective renal AR responses in conscious rats.

Dynamic Cerebral Autoregulation is Intact in Chronic Kidney Disease

IntroductionChronic Kidney Disease (CKD) patients experience an elevated risk for cerebrovascular disease and neurocognitive dysfunction. One mechanism that may contribute to this heightened risk is an impairment in dynamic cerebral autoregulation; the ability of cerebral vessels to stabilize changes in cerebral blood flow during fluctuations in arterial pressure. We compared dynamic cerebral autoregulation between patients with CKD stages III‐IV, and controls (CON) without CKD matched for age, race, sex, and anti‐hypertensive medication use. We hypothesized that dynamic cerebral autoregulation would be impaired in CKD compared to CON.Methods12 CKD participants (age=63±3y, eGFR=40±3 mL/min/1.73m2), and 16 CON participants (age=58±2y), performed 2, 5‐minute bouts of repeated sit‐to‐stand maneuvers at 0.05Hz (10‐s sit, 10‐s stand) and 0.10Hz (5‐s sit, 5‐s stand) which correspond to the frequency ranges in which cerebral autoregulation is operant. The order of protocols was randomized and separated by a 5‐minute rest period. Heart rate (via ECG), arterial pressure (via finger photoplethysmography), middle cerebral artery velocity (MCAv; via transcranial Doppler ultrasonography), and end‐tidal carbon dioxide (etCO2) were measured continuously throughout both protocols. Dynamic cerebral autoregulation was characterized by performing a transfer function analysis on the mean arterial pressure‐middle cerebral artery velocity (MAP‐MCAv) relationship to derive coherence, phase, and gain. MAP‐MCAv coherence, phase, and gain were compared between groups during each of the 2 maneuvers via unpaired, 2‐tailed T‐tests.ResultsAs expected, MAP‐MCAv coherence approached 1.0 during both maneuvers (≥0.8 during 0.05Hz and ≥0.6 during 0.10Hz) with no differences between groups (P≥0.60) supporting reliability of phase and gain measures. We observed no group differences in MAP‐MCAv phase or gain during the 0.05Hz maneuver (Phase: CKD=1.4±0.09, CON=1.2±0.07, P=0.14; Gain: CKD=0.70±0.07, CON=0.75±0.05, P=0.59) suggesting no impairment in the myogenic component of cerebral autoregulation in CKD. Not all participants were able to perform the 0.10Hz maneuver due to musculoskeletal limitations, thus N=6 CKD and N=12 CON during 0.10Hz repeated sit‐to‐stand. Similarly, we observed no group differences in phase or gain measurements during the 0.10Hz maneuver (Phase: CKD=1.4±0.12, CON=1.3±0.09, P=0.35; Gain: CKD=0.69±0.08, CON=0.88±0.06, P=0.11) suggesting no impairment in the neurogenic component of cerebral autoregulation in CKD.DiscussionContrary to our hypothesis, we observed no impairment in cerebral autoregulation in CKD stages III‐IV. These findings suggest that other mechanisms likely contribute to the increased cerebrovascular disease risk observed in CKD. Future work should explore the role of other cerebrovascular regulatory mechanisms such as cerebrovascular CO2 reactivity and neurovascular coupling as potential mediators of the increased cerebrovascular disease risk exhibited in CKD.

The impact of age, sex, cardio-respiratory fitness, and cardiovascular disease risk on dynamic cerebral autoregulation and baroreflex sensitivity

BackgroundHumans display an age-related decline in cerebral blood flow and increase in blood pressure (BP), but changes in the underlying control mechanisms across the lifespan are less well understood. We aimed to; (1) examine the impact of age, sex, cardiovascular disease (CVD) risk, and cardio-respiratory fitness on dynamic cerebral autoregulation and cardiac baroreflex sensitivity, and (2) explore the relationships between dynamic cerebral autoregulation (dCA) and cardiac baroreflex sensitivity (cBRS).Methods206 participants aged 18–70 years were stratified into age categories. Cerebral blood flow velocity was measured using transcranial Doppler ultrasound. Repeated squat-stand manoeuvres were performed (0.10 Hz), and transfer function analysis was used to assess dCA and cBRS. Multivariable linear regression was used to examine the influence of age, sex, CVD risk, and cardio-respiratory fitness on dCA and cBRS. Linear models determined the relationship between dCA and cBRS.ResultsAge, sex, CVD risk, and cardio-respiratory fitness did not impact dCA normalised gain, phase, or coherence with minimal change in all models (P > 0.05). cBRS gain was attenuated with age when adjusted for sex and CVD risk (young–older; β = − 2.86 P < 0.001) along with cBRS phase (young–older; β = − 0.44, P < 0.001). There was no correlation between dCA normalised gain and phase with either parameter of cBRS.ConclusionAgeing was associated with a decreased cBRS, but dCA appears to remain unchanged. Additionally, our data suggest that sex, CVD risk, and cardio-respiratory fitness have little effect.

Frequency-domain analysis of fNIRS fluctuations induced by rhythmic mental arithmetic.

Functional near‐infrared spectroscopy (fNIRS) is an increasingly used technology for imaging neural correlates of cognitive processes. However, fNIRS signals are commonly impaired by task‐evoked and spontaneous hemodynamic oscillations of non‐cerebral origin, a major challenge in fNIRS research. In an attempt to isolate the task‐evoked cortical response, we investigated the coupling between hemodynamic changes arising from superficial and deep layers during mental effort. For this aim, we applied a rhythmic mental arithmetic task to induce cyclic hemodynamic fluctuations suitable for effective frequency‐resolved measurements. Twenty university students aged 18–25 years (eight males) underwent the task while hemodynamic changes were monitored in the forehead using a newly developed NIRS device, capable of multi‐channel and multi‐distance recordings. We found significant task‐related fluctuations for oxy‐ and deoxy‐hemoglobin, highly coherent across shallow and deep tissue layers, corroborating the strong influence of surface hemodynamics on deep fNIRS signals. Importantly, after removing such surface contamination by linear regression, we show that the frontopolar cortex response to a mental math task follows an unusual inverse oxygenation pattern. We confirm this finding by applying for the first time an alternative method to estimate the neural signal, based on transfer function analysis and phasor algebra. Altogether, our results demonstrate the feasibility of using a rhythmic mental task to impose an oscillatory state useful to separate true brain functional responses from those of non‐cerebral origin. This separation appears to be essential for a better understanding of fNIRS data and to assess more precisely the dynamics of the neuro‐visceral link.

Linear instability of a premixed slot flame: Flame transfer function and resolvent analysis

The response to forcing of a 2D laminar premixed slot flame is investigated by means of linear analysis, based on the compressible flow equations with a two-step reaction scheme for methane combustion. The flame transfer function (FTF) is computed from this linear model, in excellent agreement with reference nonlinear calculations. The input-output gain between externally applied forcing and the global heat release rate response is computed, and peaks in the gain are related to intrinsic thermoacoustic (ITA) modes. The receptivity of the flame to arbitrary flow forcing is characterised by the resulting amplitude of global heat release rate fluctuations. Linear resolvent analysis is used to identify optimal forcing structures and their associated flame response, leading to a discussion of the dominant mechanisms for the amplification of flow perturbations, which trigger flame oscillations. These seem to involve a resonance with ITA instability modes.

Active control of road noise considering the vibro-acoustic transfer path of a passenger car

This paper presents an efficient approach to select the reference sensor position in active control of road noise by considering the vibro-acoustic transfer path of a passenger car. To ensure sufficient noise attenuation in active road noise control, the reference signals should be correlated with vehicle interior noise signals. To achieve this aim, the vibro-acoustic transfer path delivers promising options. An appropriate number of reference signals have generally been obtained by principal component analysis. Then reference sensor positions have been determined by evaluating a large number of candidate locations on a car from a coherence perspective. However, this task is time-consuming and inefficient in the actual vehicle development process. Furthermore, in practice, it is challenging to obtain a highly coherent reference signal for road noise with broadband noise characteristics without understanding the characteristics of the road noise transmitted to the interior of the vehicle. In this study, a structural transfer path analysis was conducted to derive the primary vibro-acoustic paths of structural road noise, and the optimum positions of the reference sensors were determined through the analysis of path contribution and vibro-acoustic transfer function. A practical active noise control system was implemented with the remote microphone technique based on a vehicle audio system to validate the noise attenuation performance. The experiments demonstrated an average attenuation of 3.4 dB at the driver and rear right-hand-side seat positions for up to 400 Hz frequencies. This approach is expected to increase the efficiency in developing active road noise control systems based on adaptive feedforward control systems.

Impaired Cerebral Autoregulation in Parkinson's Disease: An Orthostatic Hypotension Analysis.

Orthostatic hypotension (OH) is an early non-motor manifestation of Parkinson's disease (PD). However, the underlying mechanism of hemodynamic changes in patients with PD and OH remains unclear. This study aimed to investigate the dynamic cerebral autoregulation changes in patients with PD with OH. Ninety patients with PD and 20 age- and sex-matched healthy controls (HCs) were recruited. The patients' non-invasive blood pressure (BP) and cerebral blood flow velocity were simultaneously recorded at supine and orthostatic positions during the active standing test (AST). Transfer function analysis was used to determine autoregulatory parameters including gain [i.e., damping effect of dynamic cerebral autoregulation (dCA) on the magnitude of BP oscillation] and phase difference (i.e., the time delay of the cerebral blood flow response to BP). Sixteen patients (17.8%) in the PD population were diagnosed with OH (PD-OH). The AST results were normal for 74 patients (82.2%) (PD-NOR). In the supine position, the PD-OH group had a lower phase degree than the PD-NOR group (50.3 ± 23.4 vs. 72.6 ± 32.2 vs. 68.9 ± 12.1, p = 0.020); however, no significant difference was found upon comparing with the HC group. In the orthostatic position, the normalized gain was significantly higher for the symptomatic OH group than for the asymptomatic OH group and HC group (1.50 ± 0.58 vs. 0.97 ± 0.29 vs. 1.10 ± 0.31, p = 0.019). A symptomatic OH in the PD population indicates an impaired cerebral autoregulation ability in the orthostatic position. Cerebral autoregulation tends to be impaired in the supine position in the OH population.