Transfer Function Gain Research Articles

The effects of acoustic disturbances and mechanical vibration on laminar premixed flames: A comparative study

The energy conversion between the combustion process and acoustic waves is very complex in high-performance propulsion systems, which may trigger large-amplitude combustion instabilities, potentially leading to severe vibration or structural damage. Both acoustic disturbances and mechanical vibration are found to be able to cause the combustion instability, but their differences on the influence of laminar premixed flames are still unknown. In this paper, a premixed methane flame burner was disturbed by either a loudspeaker or an electromagnetic vibration exciter, and the effects of their forcing frequencies and amplitudes on the dynamic responses of the premixed methane flame were evaluated. Specifically, the unsteady heat release rate was measured by a photomultiplier tube, and the flame dynamic behaviors were captured by a high-speed camera with image intensifier. Furthermore, the acoustic disturbances upstream of the flame was measured by the two-microphone method, and the structure vibration was measured by an accelerometer. Then, the flame transfer functions were obtained at various equivalence ratios and mean flow velocities. The results demonstrated that the premixed flame exhibited obvious band-pass filtering characteristics when subjected to external forcing, whether through a loudspeaker or an electromagnetic vibration exciter. In addition, increasing either acoustic or structure excitation amplitude to a large extent can make the flame respond in a nonlinear manner. However, when the flame was subjected to mechanical vibration, the peak frequency corresponding to the maximum gain of the flame transfer function had minimal sensitivity to variations in equivalence ratios and mean flow velocities. In contrast, the acoustic disturbances were more likely to induce the nonlinear flame responses and flame front wrinkling.

Analytical modeling of thermoacoustic instability influences in gas turbine combustors: A detailed parameter sensitivity analysis

In this study, the effects of various system parameters on combustion instability were numerically identified through a network-based thermoacoustic model using the concept of a transfer function. The acoustic transfer function was derived from the wave equation and various conservation laws in a two-duct system consisting of a nozzle and combustion chamber that is a simplified form of a gas turbine combustor. From the current numerical approach, the major parameters affecting the system acoustics, such as the duct length, area ratio, sound speed ratio, and Mach number, as well as the inlet and outlet acoustic boundary conditions, were defined explicitly, and a method for intuitively examining their influence was presented. The gain (nf) of the flame transfer function was noted to have a complex effect in combination with factors such as temperature ratio and area ratio, and the time delay (τf) was one main dominant factor that determined the main characteristics of instability, resulting in mode transition as well as changes in frequency and growth rate of the instability.

The impact of a single high-fat meal on indices of cerebrovascular function & health

High-fat diets contribute to elevated risk for cardiovascular, cerebrovascular, metabolic, and neurocognitive diseases with reduced vascular health being a contributing factor. It is well documented that a single high-fat meal (HFM) reduces peripheral vascular function. Despite the elevated risk for/prevalence of cerebrovascular and neurocognitive disease with a high-fat diet, the impact of a single HFM on cerebrovascular function/health is less known. Cerebrovascular pulsatility index (PI) (measure of cerebral vascular stiffness) and cerebral autoregulation (ability of the cerebral vasculature to maintain constant blood flow during changes in arterial blood pressure) are indices of cerebral vascular function/health that are impaired in populations with elevated disease risk. To our knowledge, the impact of a HFM on these indices have not been studied. Accordingly, this study tested the hypothesis that a single HFM will blunt cerebrovascular function/health as indexed by increased PI and transfer function gain. Methods: Fifteen individuals (13 male; age: 28±11yr; BMI: 24±4 kg/m²) participated. Measures were made before and 2hr post-consumption of a HFM (990 kcal, 55g fat, 89g CHO, 35 g protein, 2120 mg sodium) which was purchased from McDonald’s. The HFM consisted of the following: 1 Sausage Biscuit, 1 Egg McMuffn, 2 hash browns, and 1 bottle of water. Continuous measures of beat-to-beat blood pressure and middle cerebral artery blood velocity (MCAV) were obtained during a minimum of 6-min quiet supine rest. Pulsatility index (measure of cerebrovascular stiffness) was calculated as MCAVsystolic − MCAVdiastolic / MCAVmean. Cerebral autoregulation was calculated as transfer function gain between spontaneous beat-to-beat changes in mean blood pressure and MCAVmean. Results: Cerebrovascular stiffness (PI) was increased following the HFM (pre: 0.81±0.12, post: 0.92±0.15; p<0.001). Cerebrovascular autoregulation indexed as transfer function gain was elevated following the HFM in the very low, low, and high frequency ranges ( p=0.01 for all). Pulsatility index and transfer function gain were correlated in the low frequency range before (R=0.77, p<0.01) and after (R=0.64, p=0.02) the HFM. Conclusion: These preliminary data indicate that a single HFM reduces indices of cerebral vascular function/health. Lastly, there is a positive relationship between PI and transfer function gain. Future work will continue to build upon these findings in a larger data set and will consider the impact of long-term exposure to a traditional HFM diet. Funding: The University of Texas at Arlington College of Nursing and Health Innovation and Institutional Start up Funds to Dr. Brothers. 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.

Experimental investigation on the effects of transverse injection distribution scheme on dual flame dynamics subjected to flow disturbances

Combustion instability has become a major problem in low-emission annular gas turbine combustors. It is of practical importance to investigate the effects of transverse nozzle distribution scheme on multi-flame dynamics subjected to external acoustic perturbations. In this study, the responses of heat release rate perturbations from dual propane/air laminar premixed flames to axial acoustic perturbations have been experimentally measured. Specifically, the velocity disturbance upstream the flame base has been measured by two-microphone method and the flame heat release rate perturbation is obtained by the photomultiplier tube with CH* filter lens. The dynamic evolution of dual flames is captured by the high-speed camera and then analyzed with the proper orthogonal decomposition (POD) method. Results indicate that there is a critical distribution distance, where the peak gain of the dual flame transfer function is the largest. And the direct injection dual flame transfer function can behave as the low-pass filter. Furthermore, the dual flame transfer function behaves nonlinearly with the increase of the injection angle. The dual flame transfer function for large injection angle can behave as the hybrid mode of the low-pass filter and the band-pass filter. The POD analysis reveals that the coupling effect of gas thermal expansion and stretch effect would contribute to the interaction between dual flame front dynamics. The transverse injection distribution scheme (dual flame distance and injection angle) has a significant effect on the dual flame front dynamics, which in turn affects the flame heat release rate disturbance. The bulk mean flow velocity can enhance the interaction effect between dual flame front dynamics.

Experimental study on conical flame transfer functions considering velocity profiles

This paper investigates the linear thermoacoustic dynamic response of laminar premixed stoichiometric propane/air flames under different oncoming flow velocity profiles. Experiments focused on four nozzle thickness-to-diameter ratios: 0.75, 1.5, 2.5 and 3.75, and three different mean bulk velocities: 1.1m/s, 1.2m/s and 1.3m/s. As the nozzle thickness-to-diameter ratio increases, the velocity profile transitions from a developing velocity profile to a fully developed velocity profile. The flame transfer function (FTF) gain exceeds unity at low frequencies. Notably, the effect of the velocity profile is mainly manifested in the moderate frequency range where the FTF gain decreases. When the velocity profile approaches the resonance frequency of the corresponding small orifice, the decline in FTF gain becomes less pronounced. For fully developed velocity profiles, the decrease in mean bulk velocity leads to an increase in FTF gain at low frequencies, but a more rapid decrease at higher frequencies. Additionally, this paper also investigates segmented flame transfer functions (segFTFs) at various axial positions along the flame. With the increase in frequency, the effect of flame front disturbance on FTF changes from the overall flame displacement caused by flame root stand-off distance, to the effect of flame front wrinkle counteracting effect. The different velocity profiles significantly impact the flame dynamic response, thereby affecting the flame heat release rate and consequently the FTF.

Control of Intrinsic ThermoAcoustic instabilities for methane and hydrogen air flames: DNS and network analysis

Replacing methane by hydrogen in premixed burners is a typical decarbonation scenario. This is usually done by ensuring a constant flame speed, i.e. by using ultra-lean H2 mixtures. This change can modify the thermoacoustic properties of burners. This study focuses on one class of thermoacoustic instabilities, Intrinsic ThermoAcoustic (ITA) modes, for flames stabilized on a diaphragm. An acoustic network approach is built to construct an ITA stability criterion (called ITAS criteria) for a three-duct configuration and a fully analytical expression is obtained in the limit of thin diaphragms. It shows that instability will grow when the Flame Transfer Function (FTF) gain n−π, located at the frequency at which u′-q′ phase ϕ=−π, is greater than a critical gain nc expressed by the ITAS criteria. Two-dimensional DNS, with anechoic terminations, of methane–air and hydrogen–air flames stabilized on a diaphragm are simulated with similar laminar flame speed sL0 and theoretical flame length, with an equivalence ratio of 0.73 and 0.4 respectively. The theory developed allows to analyze the differences with these two strong unstable ITA modes. For ultra-lean hydrogen flame, preferential diffusion near the diaphragm lips and tip opening reduces the flame time delay and shifts the ITA frequency towards higher values compared to methane. The assessment of two new control strategies is possible with the help of the analytical network model by increasing nc. A first strategy involves the use of preheated leaner mixtures: increasing Tu and reducing ϕ while keeping the flame speed sL0 constant. A second strategy consists of modifying the combustor geometry by changing the section ratio S2/S0, between plenum and combustion chamber. Both strategies successfully mitigate the hydrogen–air ITA modes and seems promising strategies for methane cases despite the fact that instabilities are not fully mitigated for this fuel at the conditions of interest in this study.

Dynamic Cerebral Autoregulation After Carotid Endarterectomy.

Studies have shown that dynamic cerebral autoregulation (dCA) is impaired in patients with severe internal carotid artery (ICA) stenosis and that carotid endarterectomy (CEA) may improve dCA in these patients. However, the time course of dCA changes in patients after CEA remains unclear. Therefore, this study aimed to investigate the effects of CEA on the dCA in patients with carotid artery stenosis at different time points. This prospective study enrolled 44 patients (19 symptomatic stenosis patients and 25 asymptomatic stenosis patients) who underwent CEA and 44 age- and sex-matched controls. In the CEA group, the patients underwent dCA measurements at baseline, within 3 days, and 1 month after CEA. Transfer function parameters, phase difference (PD), and gain were used to quantify dCA. Changes in dCA before and after CEA were analyzed in detail. The bilateral PD of the patients before CEA was significantly lower than that of the control group. This damage did not improve within 3 days after surgery. One month after surgery, the PD on the affected side of the patients significantly improved compared with before surgery and reached the level of the control group. The PD of affected side across time points in symptomatic and asymptomatic stenosis patients is consistent with that in all patients. The dCA level did not improve immediately after CEA but significantly improved 1 month after surgery. This suggests that the occurrence of stroke should be considered in the acute period after CEA surgery, and its preventive effect on stroke may be effective after 1 month. We found the dCA level did not improve immediately after CEA but significantly improved 1 month after surgury. This suggests that the occuttencce of stroke and surgical complications (such as cerebral hyperperfusion syndrome) associated with impaired dCA in the acute phase after CEA surgery should be of particular concern.

Feedback Stability of Generalized Positive Real and Negative Imaginary Systems

This paper derives necessary and sufficient conditions for robust stability of a feedback interconnection of linear time-invariant (LTI) systems that exhibit positive realness on finite nonzero frequencies (excluding pole locations) through a weighting function, i.e. a multiplier. This class of LTI systems importantly includes the well-studied negative imaginary systems without poles at the origin as a subset. Under the assumption that the instantaneous gain of the loop transfer function is zero, it is shown that the aforementioned feedback interconnection is stable if and only if the static (a.k.a. DC) loop gain is less than unity. This condition is identical to the one that guarantees feedback stability of negative imaginary systems, but is applicable to a much wider class of systems beyond negative imaginariness. Our proof is based entirely on the multivariable Nyquist stability criterion — it does not make use of realisations of the transfer functions. Comparisons with integral quadratic constraint based robust stability results are also made.

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.

Coupling reduction in an active highly integrated Tx phased array antenna with scanning capability

This paper presents a novel method for analyzing and reducing the total coupled power in the Active Highly Integrated Phased Array Antennas (AIPAA), incorporating both the nonlinear impacts of the power amplifier (PA) transfer functions and the normalized scattering matrix. The proposed method introduces a new degree of freedom to reduce the coupling level utilizing two key factors: (i) the balance between the PAs’ transfer function gain and the coupled power difference to the PAs’ gate and drain/antenna ports, and (ii) the out-of-phase sum based on PAs’ transfer function phase response. The proposed method is theoretically demonstrated by accounting for all tones and also is subsequently simplified for the main tone. The method is practically validated by applying the method to a 1 × 5 AIPAA structure to reduce the total coupled power at its drain/antenna port. The results demonstrate a coupling reduction of −5 dB as an average with a maximum reduction of −20 dB, over a scan coverage of ±30∘ and 10% fractional bandwidth at 22 GHz for the center cell.

Effectiveness of butylphthalide on cerebral autoregulation in ischemic stroke patients with large artery atherosclerosis (EBCAS study): A randomized, controlled, multicenter trial.

Finding appropriate drugs to improve cerebral autoregulation (CA) in patients with acute ischemic stroke (AIS) is necessary to improve prognosis. We aimed to investigate the effect of butylphthalide on CA in patients with AIS. In this randomized controlled trial, 99 patients were 2:1 randomized to butylphthalide or placebo group. The butylphthalide group received intravenous infusion with a preconfigured butylphthalide-sodium chloride solution for 14 days and an oral butylphthalide capsule for additional 76 days. The placebo group synchronously received an intravenous infusion of 100 mL 0.9% saline and an oral butylphthalide simulation capsule. The transfer function parameter, phase difference (PD), and gain were used to quantify CA. The primary outcomes were CA levels on the affected side on day 14 and day 90. Eighty patients completed the follow-up (52 in the butylphthalide group and 28 in the placebo group). The PD of the affected side on 14 days or discharge and on 90 days was higher in the butylphthalide group than in the placebo group. The differences in safety outcomes were not significant. Therefore, butylphthalide treatment for 90 days can significantly improve CA in patients with AIS.Trial registration: ClinicalTrial.gov: NCT03413202.

A Christmas Tree-Shaped Quadband Compact Antenna with Triple Slots for Various Wireless Technologies

A compact, coplanar waveguide (CPW)-fed multiband antenna is developed and analyzed in this paper for quad-band applications. Christmas tree-like monopole antennae with three symmetric slots are designed on a low-cost FR4 epoxy substrate with the overall dimensions of 0.312λ 1 × 0.364λ 1 × 0.0083λ 1 where λ 1 is the wavelength corresponding to the lower cut-off frequency. Three rectangular-shaped slots of different dimensions are etched on the three triangular-shaped stacked patches to enhance the current carrying path due to which the proposed antenna resonates at the required frequency bands. Different antenna parameters such as reflection coefficient (S 11), gain, radiation efficiency, surface current, radiation pattern, group delay and magnitude response of transfer function (S 21) are analyzed for inspecting the antenna performance. The quad-band antenna offers measured bandwidths of 3.1–3.9 GHz (22.86%), 5.46–5.98 GHz (9.1%) 9.76–11.74 GHz (18.42%) and 17.55–19.25 GHz (9.24%). The antenna shows nearly omnidirectional radiation patterns in all resonating bands with the highest gain of 4.5 dBi and measured radiation efficiency greater than 70% in all the bands. The antenna also offers stable group delay response and linear S 21 response in its operational spectrums. All these characteristics ensure the antenna’s applicability in IEEE 802.16 WiMAX band, 5.5 GHz Wi-Fi band, 5.8 GHz ISM band, terrestrial broadband, radiolocation and earth explorer satellite applications.

Dynamic accentuated antagonism of heart rate control during different levels of vagal nerve stimulation intensity in rats.

Accentuated antagonism refers to a phenomenon in which the vagal effect on heart rate (HR) is augmented by background sympathetic tone. The dynamic aspect of accentuated antagonism remains to be elucidated during different levels of vagal nerve stimulation (VNS) intensity. We performed VNS on anesthetized rats (n = 8) according to a binary white noise signal with a switching interval of 500 ms at three different stimulation rates (low-intensity: 0-10 Hz, moderate-intensity: 0-20 Hz, and high-intensity: 0-40 Hz). The transfer function from VNS to HR was estimated with and without concomitant tonic sympathetic nerve stimulation (SNS) at 5 Hz. The asymptotic low-frequency (LF) gain (in beats/min/Hz) of the transfer function increased with SNS regardless of the VNS rate [low-intensity: 3.93 ± 0.70 vs. 5.82 ± 0.65 (P = 0.021), moderate-intensity: 3.87 ± 0.62 vs. 5.36 ± 0.53 (P = 0.018), high-intensity: 4.77 ± 0.85 vs. 7.39 ± 1.36 (P = 0.011)]. Moreover, SNS slightly increased the ratio of high-frequency (HF) gain to the LF gain. These effects of SNS were canceled by the pretreatment of ivabradine, an inhibitor of hyperpolarization-activated cyclic nucleotide-gated channels, in another group of rats (n = 6). Although background sympathetic tone antagonizes the vagal effect on mean HR, it enables finer HR control by increasing the dynamic gain of the vagal HR transfer function regardless of VNS intensity. When interpreting the HF component of HR variability, the augmenting effect from background sympathetic tone needs to be considered.

Analysis of Lewis number effects on dynamic response of laminar premixed flames

The dynamic response of premixed flames with different fuel Lewis numbers is crucial to understand for the design of safe combustion chambers with wide operating range. Due to preferential diffusion effects associated with fuel Lewis number for lean mixtures, a premixed flame responds differently to flame stretch. These responses could manifest in the flame’s response to small perturbations. In order to investigate this effect, in this study, lean premixed hydrogen, methane and propane flames at the same adiabatic unstretched flame speed are studied using numerical simulations. Steady unperturbed flames show significant differences in flame stand-off distances, flame height, flame stabilization limits, burner temperature and recirculation zone length. Flames are perturbed using a step function excitation and noticeable changes in the gain and phase of the flame transfer function are found even when the flame burning velocity and the inlet flow speed are kept the same. Maximum gain of the response is found to increase with the mixture Lewis number and is shown to be directly dependent on the flame tip burning strong or weak. Phase difference at low frequencies is found to depend on the average of the time scales of the recirculation vortex which in turn depends on the flame burning stronger or weaker under the influence of positive stretch at the flame base, axial convective wave and perturbation travelling along the flame front. Time for the flame to settle to a new position after the step excitation is found to correlate well with the sum of time scales of vortex, flame tangential speed and flame tip speed. Overall, it is found that hydrogen flame responds the fastest to the perturbations and exhibits smaller increase in gain compared to methane and propane.

The reciprocal relationship between cardiac baroreceptor sensitivity and cerebral autoregulation during simulated hemorrhage in humans.

A reciprocal relationship between the baroreflex and cerebral autoregulation (CA) has been demonstrated at rest and in response to acute hypotension. We hypothesized that the reciprocal relationship between cardiac baroreflex sensitivity (BRS) and CA would be maintained during sustained central hypovolemia induced by lower body negative pressure (LBNP), and that the strength of this relationship would be greater in subjects with higher tolerance to this stress. Healthy young adults (n=51; 23F/28M) completed a LBNP protocol to presyncope. Subjects were classified as high tolerant (HT; completion of -60mmHg LBNP stage, ≥20-min) or low tolerant (LT; did not complete -60mmHg LBNP stage, <20-min). R-R intervals (RRI), systolic arterial pressure (SAP), mean arterial pressure (MAP), and middle cerebral artery velocity (MCAv) were measured continuously. Cardiac BRS was calculated in the time domain (ΔHR/ΔSAP) and frequency domain (RRI-SAP low frequency (LF) transfer function gain), and CA was calculated in the time domain (ΔMCAv/ΔMAP) and frequency domain (MAP-mean MCAv LF transfer function gain). There was a moderate relationship between cardiac BRS and CA for the group of 51 subjects in both the time (R=-0.54, P<0.0001) and frequency (R=0.61, P<0.001) domains; there was a stronger relationship in the HT group (R=0.73) compared to the LT group (R=0.31) in the frequency domain (P=0.08), but no difference between groups in the time domain (HT: R=-0.73 vs. LT: R=-0.63; P=0.27). These findings suggest that an interaction between BRS and CA may be an important compensatory mechanism that contributes to tolerance to simulated hemorrhage in young healthy adults.

Linear and nonlinear identification of the carotid sinus baroreflex in the very low-frequency range.

Since the arterial baroreflex system is classified as an immediate control system, the focus has been on analyzing its dynamic characteristics in the frequency range between 0.01 and 1 Hz. Although the dynamic characteristics in the frequency range below 0.01 Hz are not expected to be large, actual experimental data are scant. The aim was to identify the dynamic characteristics of the carotid sinus baroreflex in the frequency range down to 0.001 Hz. The carotid sinus baroreceptor regions were isolated from the systemic circulation, and carotid sinus pressure (CSP) was changed every 10 s according to Gaussian white noise with a mean of 120 mmHg and standard deviation of 20 mmHg for 90 min in anesthetized Wistar‐Kyoto rats (n = 8). The dynamic gain of the linear transfer function relating CSP to arterial pressure (AP) at 0.001 Hz tended to be greater than that at 0.01 Hz (1.060 ± 0.197 vs. 0.625 ± 0.067, p = 0.080), suggesting that baroreflex control was largely maintained at 0.001 Hz. Regarding nonlinear analysis, a second‐order Uryson model predicted AP with a higher R 2 value (0.645 ± 0.053) than a linear model (R 2 = 0.543 ± 0.057, p = 0.025) or a second‐order Volterra model (R 2 = 0.589 ± 0.055, p = 0.045) in testing data. These pieces of information may be used to create baroreflex models that can add a component of autonomic control to a cardiovascular digital twin for predicting acute hemodynamic responses to treatments and tailoring individual treatment strategies.

The Stress Hormone ACTH Causes a Prolonged Reduction in Heart Rate Variability

Hypoglycemia is a complex stressor that activates multiple hormonal systems and the sympathetic nervous system. Hypoglycemia also impairs autonomic control of blood pressure. It is possible that stress hormones produced during hypoglycemia (such as ACTH) play a role in the attenuation of autonomic function after recurrent hypoglycemia known as “cardiovascular hypoglycemia‐associated autonomic failure”. Prior work from this placebo‐controlled cross‐over study has shown that ACTH infusion reduces Modified Oxford baroreflex gain and that the reduction persists to the following day. The purpose of this analysis was to determine whether measures of heart rate variability (HRV) demonstrate a similar pattern. In the original study, men and women aged 18 – 55 years (n = 22) completed 2 study visits (ACTH or placebo in random order) with a 1–3‐month washout period between. A study visit consisted of a 3‐day/2‐night inpatient stay with the infusion occurring on day 2. Standard HRV measures from 3 minutes of 0.1 Hz breathing (6 breaths/minute) were collected on each of the 3 days. Overall HRV (time or frequency domain), as well as low frequency HRV, were reduced on the infusion day and the day after by ACTH, with no effect on blood pressure variability. Low‐frequency transfer function gain was similarly reduced, but only on day 3. In addition, these frequency domain gains were significantly correlated with the Modified Oxford derived gains. These results suggests that the ACTH impairment of autonomic control of blood pressure is reflected in HRV measures.

Effect of Hyperinsulinemia on Cerebral Autoregulation and Myogenic Control of Cerebral Blood Flow in Healthy Young Adults

ObjectiveHyperinsulinemia has marked vasodilatory effects within the skeletal muscle vasculature in healthy young adults; however, the magnitude of insulin‐stimulated cerebrovascular vasodilation is less clear. Differences between circulations may be due to a greater importance of the cerebral circulation to maintain a constant level of blood flow (i.e., cerebral autoregulation) during hyperinsulinemia. We hypothesized cerebral autoregulation would increase during hyperinsulinemia. Autoregulatory mechanisms, such as myogenic control, play important roles in mediating cerebral vasomotor responses. Thus, we further sought to examine the contribution of the myogenic mechanism to the control of cerebral blood flow during hyperinsulinemia.MethodsMiddle cerebral artery blood velocity (MCAv) and femoral artery blood velocity (FAV) were measured with Doppler ultrasound and femoral artery blood flow (FBF) was calculated in 17 healthy adults (11M/6F, 28±2 yrs, 24±1 kg/m2) at baseline and following a hyperinsulinemic (40 mU/m2body surface area/min), euglycemic infusion. Finger plethysmography was used to assess mean blood pressure (MBP). Normalized transfer function gain between changes in MBP and MCAv were estimated in the 0.02‐0.07 Hz (very low frequency, VLF) and 0.07–0.20 Hz (low frequency, LF) ranges using the Welch method. Interactions between MBP and MCAv or FAV were determined using cross‐correlation analyses.ResultsIntravenous insulin infusion increased plasma insulin concentrations (6±1 to 50±3 μIU/mL, p&lt;0.01) with blood glucose maintained at baseline levels (p=0.33). There was a significant increase in FBF (121±15 to 151±19 mL/min, p=0.03) during hyperinsulinemia, with no change in MCAv (61±4 to 59±4 cm/s, p=0.28) or MBP (86±2 to 89±2 mmHg, p=0.10). There was no effect of hyperinsulinemia on normalized transfer function gain in the VLF (p=0.25) or LF ranges (p=0.46). Hyperinsulinemia had no effect on the association between MBP and MCAv (p=0.15) or FAV (p=0.82).ConclusionsHyperinsulinemia in healthy young adults elicits peripheral vasodilation, but no change in MCAv. Contrary to our hypothesis, the sensitivity of the cerebrovascular circulation to changes in MBP (i.e., cerebral autoregulation) is not altered during systemic hyperinsulinemia in healthy adults. Furthermore, the contribution of the myogenic mechanism to the control of blood flow in the peripheral and cerebral circulations is unaffected by hyperinsulinemia. Whether these results translate to disease states (e.g., diabetes) and the implications for cerebrovascular health warrant further exploration.

Sympathetic transduction of blood pressure during graded lower body negative pressure in young healthy adults.

Sympathetic transduction of blood pressure (BP) is correlated negatively with resting muscle sympathetic nerve activity (MSNA) in cross-sectional data, but the acute effects of increasing MSNA are unclear. Sixteen (4 female) healthy adults (26 ± 3 years) underwent continuous measurement of heart rate, BP, and MSNA at rest and during graded lower body negative pressure (LBNP) at -10, -20, and -30 mmHg. Sympathetic transduction of BP was quantified in the time (signal averaging) and frequency (MSNA-BP gain) domains. The proportions of MSNA bursts firing within each tertile of BP were calculated. As expected, LBNP increased MSNA burst frequency (P < 0.01) and burst amplitude (P < 0.02), although the proportions of MSNA bursts firing across each BP tertile remained stable (all P > 0.44). The MSNA-diastolic BP low-frequency transfer function gain (P = 0.25) was unchanged during LBNP; the spectral coherence was increased (P = 0.03). Signal-averaged sympathetic transduction of diastolic BP was unchanged (from 2.1 ± 1.0 at rest to 2.4 ± 1.5, 2.2 ± 1.3, and 2.3 ± 1.4 mmHg; P = 0.43) during LBNP, but diastolic BP responses following nonburst cardiac cycles progressively decreased (from -0.8 ± 0.4 at rest to -1.0 ± 0.6, -1.2 ± 0.6, and -1.6 ± 0.9 mmHg; P < 0.01). As a result, the difference between MSNA burst and nonburst diastolic BP responses was increased (from 2.9 ± 1.4 at rest to 3.4 ± 1.9, 3.4 ± 1.9, and 3.9 ± 2.1 mmHg; P < 0.01). In conclusion, acute increases in MSNA using LBNP did not alter traditional signal-averaged or frequency-domain measures of sympathetic transduction of BP or the proportion of MSNA bursts firing at different BP levels. The factors that determine changes in the firing of MSNA bursts relative to oscillations in BP require further investigation.

Sedentary behavior is associated with reduced cardiovagal baroreflex sensitivity in healthy adults.

Sedentary behavior is an established risk factor for cardiovascular disease; however, it remains unclear whether sedentary behavior is associated with the deterioration of arterial blood pressure regulation. The purpose of this study was to determine the association between the time spent in sedentary behavior and cardiovagal baroreflex sensitivity (cBRS) in healthy adults. We investigated the cross-sectional relationship between sedentary time and cBRS in 179 adults aged 22-81 years. Sedentary time was objectively measured using a triaxial accelerometer. cBRS was evaluated by the transfer function gain of beat-by-beat changes in systolic blood pressure and the R-R interval during 5 min of spontaneous resting. Glycemic, lipidemic, and vascular risk factors were measured as potential covariates of cBRS and sedentary behavior. Men had a longer sedentary time and lower cBRS than women (p = 0.001). In a simple correlation analysis, older age was negatively associated with cBRS and positively associated with sedentary time, but sedentary time was not correlated with cBRS. However, after adjustment for age and sex, a longer sedentary time was associated with a lower cBRS. Multiple linear regression analysis showed that sedentary time was independently associated with lower cBRS with adjustment for covariates (β = -0.325, p = 0.002). A significant association was also confirmed when the analysis was performed separately in the younger and older groups. This finding suggests that high sedentary behavior may have an adverse effect on arterial blood pressure regulation.