Haemodynamic changes during neck pressure and suction in seated and supine positions

Repetitive pain experiences modulate feedforward control of hemodynamics and modification by nitrous oxide/oxygen inhalation in humans

Background

Repetitive experiences of certain stresses evoke feedforward cardiovascular responses via central command (CC)--central signals from the higher brain. However, it is unclear whether the anticipatory cardiovascular responses before pain stimulation occur after repetitive pain experiences and how nitrous oxide/oxygen inhalation (N2O), a sedative widely used in dentistry, affects the responses. We tested the hypothesis that the repetitive cold pressor test (CPT) alters the anticipatory cardiovascular responses, which are attenuated by N2O.

Materials and methods

Beat-to-beat systolic (SBP) and diastolic blood pressure (DBP), heart rate (HR), and finger arterial stiffness (β-stiffness) were measured during the 5-min rest, 30-s countdown (CD) before CPT, 2-min CPT, and 3-min recovery (CPT[1st]) in 15 young adults [age, 28 ± 4 years]. The same protocols were repeated randomly with the second CPT (CPT + CC) or placebo test (PLCB + CC).

Results

SBP and DBP increased from baseline in CPT[1st] and CPT + CC under room air (RA) and 40 % N2O, while SBP was lower under N2O than under RA in CPT[1st]. HR in CPT[1st] was lower under N2O than under RA. The change (Δ) in HR was smaller during CPT[1st] than during CPT + CC under N2O, and a similar trend was observed under RA. ΔSBP by CD was lower under N2O than under RA in CPT[1st] but not in CPT + CC. HR increased with CD in CPT + CC but not in CPT[1st] under both RA and N2O. β-stiffness increased by CD regardless of the pain experience, while it was lower under N2O.

Conclusion

Repetitive pain experiences induce a feedforward HR increase. 40 % N2O decreases vascular stiffness, which may attenuate the anticipatory pressor response only when the feedforward HR increase does not exist.

Barosensory vessel mechanics and the vascular sympathetic baroreflex: Impact on blood pressure homeostasis

NEW FINDINGS

What is the topic of this review? We review barosensory vessel mechanics and their role in blood pressure regulation across the lifespan. What advances does it highlight? In young normotensive men, aortic unloading mechanics contribute to the resting operating point of the vascular sympathetic baroreflex; however, with advancing age, this contribution is removed. This suggests that barosensory vessel unloading mechanics are not driving the well-documented age-related increase in resting muscle sympathetic nerve activity.

ABSTRACT

An age-associated increase in arterial blood pressure is evident for apparently healthy humans. This is frequently attributed to stiffening of the central arteries and a concurrent increase in sympathetic outflow, potentially mediated by a reduced ability of the baroreceptive vessels to distend. This is supported, in part, by a reduced mechanical component of the vascular sympathetic baroreflex (i.e., a reduction in distension for a given pressure). Previous characterization of the mechanical component has assessed only carotid artery distension; however, evidence suggests that both the aortic and carotid baroreflexes are integral to blood pressure regulation. In addition, given that baroreceptors are located in the vessel wall, the change in wall tension, comprising diameter, pressure and vessel wall thickness, and the mechanics of this change might provide a better index of the baroreceptor stimulus than the previous method used to characterize the mechanical component that relies on diameter alone. This brief review summarizes the data using this new method of assessing barosensory vessel mechanics and their influence on the vascular sympathetic baroreflex across the lifespan.

Habitual physical activity improves vagal cardiac modulation and carotid baroreflex function in elderly women

The impact of habitual physical activity on vagal-cardiac function and baroreflex sensitivity in elderly women is poorly characterized. This study compared vagal-cardiac modulation and carotid baroreflex (CBR) function in eight physically active (67.6 ± 1.9 years; peak O2 uptake 29.1 ± 2.5 mL/min/kg) versus eight sedentary (67.3 ± 1.8 years; peak O2 uptake 18.6 ± 0.9 mL/min/kg) elderly women. Heart rate (HR) variabilities and maximal changes of HR and mean arterial pressure (MAP) elicited by 5-s pressure pulses between +40 and -80 mmHg applied to the carotid sinus were measured at rest and during carotid baroreceptor unloading effected by -15 mmHg lower-body negative pressure (LBNP). HR variability was greater in active than sedentary women in both low (0.998 ± 0.286 versus 0.255 ± 0.063 bpm2; P = 0.036) and high (0.895 ± 0.301 versus 0.156 ± 0.045 bpm2; P = 0.044) frequency domains. CBR-HR gains (bpm/mmHg) were greater (fitness factor P < 0.001) in active versus sedentary women at rest (-0.146 ± 0.014 versus -0.088 ± 0.011) and during LBNP (-0.105 ± 0.014 versus -0.065 ± 0.008). CBR-MAP gains (mmHg/mmHg) tended to be greater (fitness factor P = 0.077) in active versus sedentary women at rest (-0.132 ± 0.013 versus -0.110 ± 0.011) and during LBNP (-0.129 ± 0.015 versus -0.113 ± 0.013). However, LBNP did not potentiate CBR-MAP gains in either sedentary or active women (LBNP factor P = 0.94), and it depressed CBR-HR gains in both groups (LBNP factor P = 0.003). CBR-HR gains in the sedentary women did not differ (sex factor P = 0.65) from gains reported in age-matched sedentary men, although CBR-MAP gains tended to be greater (sex factor P = 0.109) in the men. Thus, tonic vagal modulation indicated by HR variability and dynamic vagal responses assessed by CBR-HR gain were augmented in physically active women. Enhanced vagal-cardiac function may protect against senescence-associated cardiac electrical and hemodynamic instability in elderly women.

Effector mechanisms in the baroreceptor control of blood pressure

While the effects of changing heart rate and systemic vascular resistance have been generally understood and appreciated, the effects of changes in left ventricular contractility on end-systolic volume may have been less understood and appreciated and the effects of changes in venous capacitance on end-diastolic volume may have been unknown to many readers. Herein, we have provided a brief review for the medical student and beginning graduate student highlighting these sometimes-complex relationships.

Effects of resistance training on baroreflex sensitivity function in healthy males

[Purpose] This experimental study examined the effects of resistance exercises with three intensities on baroreflex sensitivity (BRS) in healthy males. [Participants and Methods] This study enrolled 27 sedentary healthy males with the following demographic characteristics: mean age, 19.9 years; body mass, 63.4 kg; height, 171.7 cm; and body mass index, 21.5 kg/m². The participants performed 20 alternating knee extensions while sitting, and each excursion comprised 5-seconds contraction and rest periods with 20%, 50% and 80% loads of one repetition maximum. The main outcome measures examined in this study were autonomic nerve activities and BRS during the low-intensity resistance exercise utilizing cardiographic and hemodynamic impedance parameters for cardiac function. While measuring the spectral powers of high-frequency (HF) and low frequency (LF) regions, a continuous RR series of the heart rate was calculated. Moreover, the LF/HF ratio of the RR interval variability power and the HF normalized unit (HFnu) were calculated as parameters for sympathetic and parasympathetic nerve activities. [Results] After the low-intensity resistance training, a significant decrease in the LF/HF ratio associated with significant increases in HFnu and BRS were observed. [Conclusion] Low-intensity resistance training may enhance BRS function in healthy males.

Neck Chamber Technique Revisited: Low-Noise Device Delivering Negative and Positive Pressure and Enabling Concomitant Carotid Artery Imaging With Ultrasonography

Background and Objectives: Recently, novel noiseless device for the assessment of baroreceptor function with the neck suction (NS) has been presented. In this study, we present another in-house approach to the variable-pressure neck chamber method. Our device offers further critical improvements. First, it enables delivery of negative (NS) as well as positive pressure (neck pressurizing, NP) in a noiseless manner. Second, we used small, 3D-printed cups positioned over the carotid sinuses instead of cumbersome neck collar to improve subject comfort and to test feasibility of tracking the pressure-induced changes in carotid artery with ultrasonography. Methods: Five healthy, non-smoking, normal-weight subjects aged 29 ± 3 years (mean ± SD) volunteered for the study. Heart rate (HR, bpm) and mean arterial pressure (MAP, mmHg) responses to short, 7-s long episodes of NS and NP were recorded. Each trial consisted of 12 episodes of variable-pressure: six episodes of NS (suction ranging between -10 and -80 mmHg) and six episodes of NP (pressure ranging between + 10 and + 80 mmHg). Carotid artery sonography was performed during the NS and NP in four subjects, on another occasion. Results: The variable-pressure episodes resulted consistently in the expected pattern of hemodynamic alterations: HR and MAP increases or decreases following the NP and NS, respectively, as evidenced by the coefficient of determination (R²) of ≥0.78 for the carotid-HR response curve (for all five participants) and the carotid-MAP response curve (for four out of five participants; the curve cannot be calculated for one subject). We found a linear, dose-dependent relation between the applied pressure and the systolic-diastolic difference in carotid artery diameter. Conclusion: The novel device enables noiseless stimulation and unloading of the carotid baroreceptors with the negative and positive pressure, respectively, applied on the subject's neck via small, asymmetric and one-side flattened, 3D-printed cups. The unique design of the cups enables concomitant visualizing of the carotid artery during the NS or NP administration, and thereby direct monitoring of the intensity of mechanical stimulus targeting the carotid baroreceptors.

Auxiliary Tests of Autonomic Functions

SUMMARY

The autonomic nervous system is a complex neural network that controls several organ systems. Its assessment includes a detailed history of autonomic functions, clinical examination, and autonomic tests. Most widely used is a battery of tests that assess cardiovascular reflex autonomic and sudomotor tests, which include deep breathing (assesses parasympathetic function), Valsalva maneuver, tilt test (both assess parasympathetic and adrenergic functions), and sudomotor testing for the evaluation of postganglionic sudomotor fibers. These basic tests represent a foundation of autonomic testing. Nevertheless, the autonomic nervous system also controls organ systems not directly assessed by basic tests. This review describes a number of auxiliary autonomic tests that can be used in addition to basic autonomic tests or can be used independently to explore particular autonomic functions or to answer a specific clinical question. The auxiliary tests described in this review evaluate cardiovascular, thermoregulatory, gastrointestinal, genitourinary, eye, and exocrine functions. These tests are cold pressor test, sustained handgrip maneuver, reverse tilt test, venoarteriolar reflex, laser Doppler flare imaging, microneurography, neck suction, lower body negative pressure, venous occlusion plethysmography, pharmacologic assessment of postganglionic sympathetic outflow, plasma norepinephrine, sympathetic skin response, video cinefluoroscopic swallowing test, esophageal manometry test, small bowel manometry test, wireless motility capsule test, urodynamic studies, penile plethysmography, intracavernosal papaverine injection, infrared video pupillography, corneal confocal microscopy, pupillary response to dilute pilocarpine and hydroxyamphetamine, Schirmer test, tear osmolarity test, and salivary secretion test. The protocol of each test is described in detail. This review can be used as a quick reference for the auxiliary autonomic tests.

Cuff-Method Thigh Arterial Occlusion Counteracts Cerebral Hypoperfusion Against the Push-Pull Effect in Humans

Exposure to acute transition from negative (−Gz) to positive (+ Gz) gravity significantly impairs cerebral perfusion in pilots of high-performance aircraft during push—pull maneuver. This push—pull effect may raise the risk for loss of vision or consciousness. The aim of the present study was to explore effective countermeasures against cerebral hypoperfusion induced by the push—pull effect. Twenty healthy young volunteers (male, 21 ± 1 year old) were tested during the simulated push–pull maneuver by tilting. A thigh cuff (TC) pressure of 200 mmHg was applied before and during simulated push—pull maneuver (−0.87 to + 1.00 Gz). Beat-to-beat cerebral and systemic hemodynamics were measured continuously. During rapid −Gz to + Gz transition, mean cerebral blood flow velocity (CBFV) was decreased, but to a lesser extent, in the TC bout compared with the control bout (−3.1 ± 4.9 vs. −7.8 ± 4.4 cm/s, P < 0.001). Similarly, brain-level mean blood pressure showed smaller reduction in the TC bout than in the control bout (−46 ± 12 vs. −61 ± 13 mmHg, P < 0.001). The systolic CBFV was lower but diastolic CBFV was higher in the TC bout. The systemic blood pressure response was blunted in the TC bout, along with similar heart rate increase, smaller decrease, and earlier recovery of total peripheral resistance index than control during the gravitational transition. These data demonstrated that restricting thigh blood flow can effectively mitigate the transient cerebral hypoperfusion induced by rapid shift from −Gz to + Gz, characterized by remarkable improvement of cerebral diastolic flow.

Sitting-induced hemodynamic changes and association with sitting intolerance in children and adolescents: a cross-sectional study

Hemodynamic alteration with postural change from supine to sitting has been unclear in the young. In the cross-sectional study, 686 participants (371 boys and 315 girls, aged 6-18 years) were recruited from 4 schools in Kaifeng city, the central area of China. The active sitting test was performed to obtain heart rate (HR) and blood pressure (BP) changes from supine to sitting in children and adolescents. Hemodynamic change-associated sitting intolerance was analyzed. In the study participants, the 95th percentile (P95) values of changes in HR and BP within 3 min from supine to sitting were 25 beats/min and 18/19 mm Hg, respectively. Sixty-six participants had sitting intolerance symptoms. Compared with participants without sitting intolerance symptoms, those with symptoms more frequently had HR increase ≥ P95 or BP increase ≥ P95 within 3 min from supine to sitting (P < 0.001). Risk factors for sitting intolerance were age (odds ratio 1.218, 95% confidence interval 1.072-1.384, P = 0.002) and changes in HR or BP ≥ P95 within 3 min after sitting (odds ratio 2.902, 95% confidence interval 1.572-5.357, P = 0.001). We firstly showed hemodynamic changing profiles from supine to sitting and their association with sitting intolerance in children and adolescents. Sitting tachycardia is likely suggested with a change in HR ≥ 25 beats/min and sitting hypertension with a change in BP ≥ 20/20 mm Hg when changing from supine to sitting within 3 min. The age and changes in HR or BP were independent risk factors for sitting intolerance.

Lower body negative pressure protects brain perfusion in aviation gravitational stress induced by push-pull manoeuvre

KEY POINTS

Rapid alterations of gravitational stress during high-performance aircraft push-pull manoeuvres induce dramatic shifts in volume and pressure within the circulation system, which may result in loss of consciousness due to the rapid and significant reduction in cerebral perfusion. There are still no specific and effective countermeasures so far. We found that lower body negative pressure (LBNP), applied prior to and during -Gz and released at the subsequent transition to +Gz, could effectively counteract gravitational haemodynamic stress induced by a simulated push-pull manoeuvre and improve cerebral diastolic perfusion in human subjects. We developed a LBNP strategy that effectively protects cerebral perfusion at rapid -Gz to +Gz transitions via improving cerebral blood flow and blood pressure during push-pull manoeuvres and highlight the importance of the timing of the intervention. Our findings provide a systemic link of integrated responses between the peripheral and cerebral haemodynamic changes during push-pull manoeuvres.

ABSTRACT

The acute negative (-Gz) to positive (+Gz) gravity stress during high-performance aircraft push-pull manoeuvres dramatically reduces transient cerebral perfusion, which may lead to loss of vision or even consciousness. The aim of this study was to explore a specific and effective counteractive strategy. Twenty-three healthy young male volunteers (age 21 ± 1 year) were subjected to tilting-simulated push-pull manoeuvres. Lower body negative pressure (LBNP) of -40 mmHg was applied prior to and during -Gz stress (-0.50 or -0.87 Gz) and released at the subsequent transition to +1.00 Gz stress. Beat-to-beat cerebral and systemic haemodynamics were continuously recorded during the simulated push-pull manoeuvre in LBNP bouts and corresponding control bouts. During the rapid gravitational transition from -Gz to +Gz, the mean cerebral blood flow velocity decreased significantly in control bouts, while it increased in LBNP bouts (control vs. LBNP bouts, -6.6 ± 4.6 vs. 5.1 ± 6.8 cm s^-1 for -0.50 Gz, and -7.4 ± 4.8 vs. 3.4 ± 4.6 cm s^-1 for -0.87 Gz, P < 0.01), which was attributed mainly to the elevation of diastolic flow. The LBNP bouts showed much smaller reduction of mean arterial blood pressure at the brain level than control bouts (control bouts vs. LBNP bouts, -38 ± 12 vs. -23 ± 10 mmHg for -0.50 to +1.00 Gz, and -62 ± 16 vs. -43 ± 11 mmHg for -0.87 to +1.00 Gz, P < 0.01). LBNP applied at -Gz and released at subsequent +Gz had biphasic counteractive effects against the gravitational responses to the push-pull manoeuvre. These data demonstrate that this LBNP strategy could effectively protect cerebral perfusion with dominant improvement of diastolic flow during push-pull manoeuvres.

Augmented resting beat-to-beat blood pressure variability in young, healthy, non-Hispanic black men

NEW FINDINGS

What is the central question of this study? The prevalence of hypertension in black individuals exceeds that in other racial groups. Despite this well-known heightened risk, the underlying contributory factors remain incompletely understood. We hypothesized that young black men would exhibit augmented beat-to-beat blood pressure variability compared with white men and that black men would exhibit augmented total peripheral resistance variability. What is the main finding and its importance? We demonstrate that young, healthy black men exhibit greater resting beat-to-beat blood pressure variability compared with their white counterparts, which is accompanied by greater variability in total peripheral resistance. These swings in blood pressure over time might contribute to the enhanced cardiovascular risk profile in black individuals.

ABSTRACT

The prevalence of hypertension in black (BL) individuals exceeds that in other racial groups. Recently, resting beat-to-beat blood pressure (BP) variability has been shown to predict cardiovascular risk and detect target organ damage better than ambulatory BP monitoring. Given the heightened risk in BL individuals, we hypothesized young BL men would exhibit augmented beat-to-beat BP variability compared with white (WH) men. Furthermore, given studies reporting reduced vasodilatation and augmented vasoconstriction in BL individuals, we hypothesized that BL men would exhibit augmented variability in total peripheral resistance (TPR). In 45 normotensive men (24 BL), beat-to-beat BP (Finometer) was measured during 10-20 min of quiet rest. Cardiac output and TPR were estimated (Modelflow method). Despite similar resting BP, BL men exhibited greater BP standard deviation (e.g. systolic BP SD; BL, 7.1 ± 2.2 mmHg; WH, 5.4 ± 1.5 mmHg; P = 0.006) compared with WH men, which was accompanied by a greater TPR SD (P = 0.003), but not cardiac output SD (P = 0.390). Other traditional measures of variability provided similar results. Histogram analysis indicated that BL men exhibited a greater percentage of cardiac cycles with BPs higher (> +10 mmHg higher) and lower (< -8 mmHg lower) than mean systolic BP compared with WH men (interaction, P < 0.001), which was accompanied by a greater percentage of cardiac cycles with high/low TPR (P < 0.001). In a subset of subjects (n = 30), reduced sympathetic baroreflex sensitivity was associated with augmented BP variability (r = -0.638, P < 0.001), whereas cardiac baroreflex sensitivity had no relationship (P = 0.447). Herein, we document an augmented beat-to-beat BP variability in young BL men, which coincided with fluctuations in vascular resistance and reduced sympathetic BRS.

Carotid baroreflex control of central and peripheral hemodynamics during recovery after moderate leg cycling exercise

This study aimed to examine the carotid baroreflex (CBR) control of the central and peripheral hemodynamics after exercise using the neck pressure (NP) and neck suction (NS) technique. Sixteen healthy young male participants (age: 27 ± 1.5 yr) were in a supine position for 30 min preexercise, followed by 60 min of cycling exercise, and then returned to a supine position for an additional 60 min postexercise. Both pre- and postexercise, the CBR-mediated responses of the central and peripheral hemodynamics were evaluated using 5-s periods of NP and NS (-60, -40, or +40 mmHg). As the central hemodynamics measurements, heart rate (HR), mean arterial pressure (MAP), cardiac output, and total vascular conductance were assessed. To determine peripheral circulation, vascular conductance in active and inactive limbs was measured. Eight participants [responder (RE) group] showed substantial postexercise hypotension (PEH) during recovery from exercise (Δ MAP: approximately -5 ± 0.9 mmHg, P < 0.05). The other eight participants did not display a reduction in MAP after exercise (non-RE group). In the non-RE group, the responsiveness of CBR-mediated changes in HR, MAP, and vascular conductance increased, particularly in response to -40 mmHg NS during postexercise compared with preexercise. However, in the RE group, any alterations in responsiveness to NP and NS were unchanged during PEH compared with preexercise. In conclusion, some normotensive individuals do not show PEH because the responsiveness of the CBR in central and peripheral hemodynamics following exercise is augmented, particularly to high blood pressure.NEW & NOTEWORTHY The carotid baroreflex (CBR) control of central and peripheral hemodynamics was investigated after exercise in both the presence and absence of postexercise hypotension (PEH). In individuals with no PEH, the responsiveness of CBR-mediated changes in all hemodynamics was augmented after exercise, particularly to high blood pressure; conversely, the CBR responsiveness remained unchanged in individuals with PEH. These findings provide insight into the mechanism of CBR control after exercise.

Effect of a neck compression collar on cardiorespiratory and cerebrovascular function in postural orthostatic tachycardia syndrome (POTS)

Postural orthostatic tachycardia syndrome (POTS) is accompanied by reduced brain blood flow, autonomic dysfunction, and orthostatic intolerance. We hypothesized that wearing a neck compression collar would attenuate orthostatic symptoms, increase brain blood flow, and influence autonomic reflexes. Ten participants with POTS (9 women, age: 36 ± 10) underwent two trials of supine rest, paced deep breathing (6 breaths/min), Valsalva maneuver (40 mmHg for 15 s), and 70° upright tilt. For one trial, participants wore a neck compression device (Q30 Innovations). Blood pressure, heart rate (HR), brain blood flow velocity, stroke volume, respiratory rate, and end-tidal gases were continuously measured. The Vanderbilt Orthostatic Symptom Score was compiled at the end of tilt. The use of the collar reduced the orthostatic symptom score of participants with POTS during upright tilt (26.9 ± 12.5 to 18.7 ± 13.1, P = 0.04). Collar compression in the supine condition reduced the low-frequency domain of HR variability (60 ± 18 to 51 ± 23 normalized units, P = 0.04) and increased the change in HR (15 ± 5 to 17 ± 6 bpm, P = 0.02) and E:I ratio (1.2 ± 0.1 to 1.3 ± 0.1, P = 0.01) during paced deep breathing. Throughout tilt, wearing the collar reduced respiratory rate (baseline: 13 ± 3 to 12 ± 4 breath/min; tilt: 18 ± 5 to 15 ± 5 breath/min; main effect of collar P = 0.048), end-tidal oxygen (baseline: 115 ± 5 to 112 ± 5 mmHg; tilt: 122 ± 10 to 118 ± 11 mmHg; main effect of collar P = 0.026). In participants with POTS, wearing the Q-collar reduced orthostatic symptoms, increased the HR response to deep breathing, and decreased resting ventilation.NEW & NOTEWORTHY We found that using a neck compression collar alleviated orthostatic symptoms in upright posture in participants with postural orthostatic tachycardia syndrome (POTS). This could be due to compression of the baroreceptors and subsequent changes in autonomic function. Indeed, we observed increased heart rate responsiveness to paced deep breathing and reductions of respiratory rate and end-tidal O₂ (suggesting reduced ventilation). Further, wearing the collar reduced mean blood velocity in the brain during Valsalva perhaps due to higher brain blood volume.

Cerebral blood flow regulation and cognitive function: a role of arterial baroreflex function

A strict adequate perfusion pressure via arterial baroreflex for the delivery of oxygen to the tissues of the body is well established; however, the importance of baroreflex for cerebral blood flow (CBF) is unclear. On the other hand, there is convincing evidence for arterial baroreflex function playing an important role in maintaining brain homeostasis, e.g., cerebral metabolism, cerebral hemodynamics, and cognitive function. For example, mild cognitive impairment attenuates the sensitivity of baroreflex, and Alzheimer's disease further decreases it. These clinical findings suggest that CBF and cerebral function are affected by systemic blood pressure regulation via the arterial baroreflex. However, dysfunction of arterial baroreflex is likely to affect CBF regulation as well as the underlying neuronal function, but identifying how this is achieved is arduous since neurological diseases affect systemic as well as cerebral circulation independently. Recent insights into the influence of blood pressure regulation via the arterial baroreflex on cerebral function and blood flow regulation may help elucidate this important question. This review summarizes some update findings regarding direct (autonomic regulation) and indirect (systemic blood pressure regulation) contributions of the arterial baroreflex to the maintenance of cerebral vasculature regulation.

The influence of the carotid baroreflex on dynamic regulation of cerebral blood flow and cerebral tissue oxygenation in humans at rest and during exercise

PURPOSE

This preliminary study tested the hypothesis that the carotid baroreflex (CBR) mediated sympathoexcitation regulates cerebral blood flow (CBF) at rest and during dynamic exercise.

METHODS

In seven healthy subjects (26 ± 1 years), oscillatory neck pressure (NP) stimuli of + 40 mmHg were applied to the carotid baroreceptors at a pre-determined frequency of 0.1 Hz at rest, low (10 ± 1W), and heavy (30 ± 3W) exercise workloads (WLs) without (control) and with α - 1 adrenoreceptor blockade (prazosin). Spectral power analysis of the mean arterial blood pressure (MAP), mean middle cerebral artery blood velocity (MCAV), and cerebral tissue oxygenation index (ScO₂) in the low-frequency range (0.07-0.20 Hz) was estimated to examine NP stimuli responses.

RESULTS

From rest to heavy exercise, WLs resulted in a greater than three-fold increase in MCAV power (42 ± 23.8-145.2 ± 78, p < 0.01) and an almost three-fold increase in ScO₂ power (0.51 ± 0.3-1.53 ± 0.8, p = 0.01), even though there were no changes in MAP power (from 24.5 ± 21 to 22.9 ± 11.9) with NP stimuli. With prazosin, the overall MAP (p = 0.0017), MCAV (p = 0.019), and ScO₂ (p = 0.049) power was blunted regardless of the exercise conditions. Prazosin blockade resulted in increases in the Tf gain index between MAP and MCAV compared to the control (p = 0.03).

CONCLUSION

CBR-mediated changes in sympathetic activity contribute to dynamic regulation of the cerebral vasculature and CBF at rest and during dynamic exercise in humans.

Baroreflex autonomic control in human spinal cord injury: Physiology, measurement, and potential alterations

The arterial baroreflex is a primary regulator of autonomic outflow to effectively regulate acute changes in blood pressure. After a spinal cord injury (SCI), regulation of autonomic function is disrupted, although the damage of the autonomic pathways may not necessarily be related to the severity of injury (i.e. level and completeness). Nonetheless, it can be assumed that there would be greater loss of sympathetic innervation with higher level of injury and that cardiac parasympathetic control would remain intact regardless of injury level. In those with SCI, impaired baroreflex regulation has implications not only for adequate pressure regulation, but also for long term cardiovascular health. In this review, we discuss the expected impact ofan SCI on baroreflex control and the studies that have investigated baroreflex sensitivity in this population. The data generally indicates that baroreflex sensitivity is lesser in those with chronic injuries. However, these findings are counter to the expected effect of an SCI and hence may indicate that the effect of an SCI on baroreflex control might be secondary to long term deconditioning and/or vascular stiffening of baroreceptive arteries. Furthermore, the alterations in the ability to regulate pressure do not impact the relationship between spontaneous heart rate and blood pressure variabilities. In addition, those with SCI are not adequately able to control blood pressure changes in response to orthostasis, resulting in frank hypotension in a significant proportion of those with high level injuries.

The carotid baroreflex modifies the pressor threshold of the muscle metaboreflex in humans

The purpose of the present study was to test our hypothesis that unloading the carotid baroreceptors alters the threshold and gain of the muscle metaboreflex in humans. Ten healthy subjects performed a static handgrip exercise at 50% of maximum voluntary contraction. Contraction was sustained for 15, 30, 45, and 60 s and was followed by 3 min of forearm circulatory arrest, during which forearm muscular pH is known to decrease linearly with increasing contraction time. The carotid baroreceptors were unloaded by applying 0.1-Hz sinusoidal neck pressure (oscillating from +15 to +50 mmHg) during ischemia. We estimated the threshold and gain of the muscle metaboreflex by analyzing the relationship between the cardiovascular responses during ischemia and the amount of work done during the exercise. In the condition with unloading of the carotid baroreceptors, the muscle metaboreflex thresholds for mean arterial blood pressure (MAP) and total vascular resistance (TVR) corresponded to significantly lower work levels than the control condition (threshold for MAP: 795 ± 102 vs. 662 ± 208 mmHg and threshold for TVR: 818 ± 213 vs. 572 ± 292 kg·s, P < 0.05), but the gains did not differ between the two conditions (gain for MAP: 4.9 ± 1.7 vs. 4.4 ± 1.6 mmHg·kg·s^-1·100 and gain for TVR: 1.3 ± 0.8 vs. 1.3 ± 0.7 mmHg·l^-1·min^-1·kg·s^-1·100). We conclude that the carotid baroreflex modifies the muscle metaboreflex threshold in humans. Our results suggest the carotid baroreflex brakes the muscle metaboreflex, thereby inhibiting muscle metaboreflex-mediated pressor and vasoconstriction responses.NEW & NOTEWORTHY We found that unloading the carotid baroreceptors shifts the pressor threshold of the muscle metaboreflex toward lower metabolic stimulation levels in humans. This finding indicates that, in the normal loading state, the carotid baroreflex inhibits the muscle metaboreflex pressor response by shifting the reflex threshold to higher metabolic stimulation levels.

Carotid baroreflex function at the onset of cycling in men

Arterial baroreflex function is important for blood pressure control during exercise, but its contribution to cardiovascular adjustments at the onset of cycling exercise remains unclear. Fifteen healthy male subjects (24 ± 1 yr) performed 45-s trials of low- and moderate-intensity cycling, with carotid baroreceptor stimulation by neck suction at −60 Torr applied 0–5, 10–15, and 30–35 s after the onset of exercise. Cardiovascular responses to neck suction during cycling were compared with those obtained at rest. An attenuated reflex decrease in heart rate following neck suction was detected during moderate-intensity exercise, compared with the response at rest ( P < 0.05). Furthermore, compared with the reflex decrease in blood pressure elicited at rest, neck suction elicited an augmented decrease in blood pressure at 0–5 and 10–15 s during low-intensity exercise and in all periods during moderate-intensity exercise ( P < 0.05). The reflex depressor response at the onset of cycling was primarily mediated by an increase in the total vascular conductance. These findings evidence altered carotid baroreflex function during the first 35 s of cycling compared with rest, with attenuated bradycardic response, and augmented depressor response to carotid baroreceptor stimulation.

Carotid baroreflex control of heart rate is enhanced, while control of mean arterial pressure is preserved during whole body heat stress in young healthy men

Whole body heat stress (WBH) results in numerous cardiovascular alterations that ultimately reduce orthostatic tolerance. While impaired carotid baroreflex (CBR) function during WBH has been reported as a potential reason for this decrement, study design considerations may limit interpretation of previous findings. We sought to test the hypothesis that CBR function is unaltered during WBH. CBR function was assessed in 10 healthy male subjects (age: 26 ± 3; height: 185 ± 7 cm; weight: 82 ± 10 kg; BMI: 24 ± 3 kg/m2; means ± SD) using 5-s trials of neck pressure (+45, +30, and +15 Torr) and neck suction (−20, −40, −60, and −80 Torr) during normothermia (NT) and passive WBH (Δ core temp ∼1°C). Analyses of stimulus response curves (four-parameter logistic model) for CBR control of heart rate (CBR-HR) and mean arterial pressure (CBR-MAP), as well as separate two-way ANOVA of the hypotensive and hypertensive stimuli (factor 1: thermal condition, factor 2: chamber pressure), were performed. For CBR-HR, maximal gain was increased during WBH (−0.73 ± 0.11) compared with NT (−0.39 ± 0.04, mean ± SE, P = 0.03). In addition, the CBR-HR responding range was increased during WBH (33 ± 5) compared with NT (19 ± 2 bpm, P = 0.03). Separate analysis of hypertensive stimulation revealed enhanced HR responses during WBH at −40, −60, and −80 Torr (condition × chamber pressure interaction, P = 0.049) compared with NT. For CBR-MAP, both logistic analysis and separate two-way ANOVA revealed no differences during WBH. Therefore, in response to passive WBH, CBR control of heart rate (enhanced) and arterial pressure (no change) is well preserved.

Reflex control of the circulation during exercise

Appropriate cardiovascular and hemodynamic adjustments are necessary to meet the metabolic demands of working skeletal muscle during exercise. Alterations in the sympathetic and parasympathetic branches of the autonomic nervous system are fundamental in ensuring these adjustments are adequately made. Several neural mechanisms are responsible for the changes in autonomic activity with exercise and through complex interactions, contribute to the cardiovascular and hemodynamic changes in an intensity-dependent manner. This short review is from a presentation made at the Saltin Symposium June 2-4, 2015 in Copenhagen, Denmark. As such, the focus will be on reflex control of the circulation with an emphasis on the work of the late Dr. Bengt Saltin. Moreover, a concerted effort is made to highlight the novel and insightful concepts put forth by Dr. Saltin in his last published review article on the regulation of skeletal muscle blood flow in humans. Thus, the multiple roles played by adenosine triphosphate (ATP) including its ability to induce vasodilatation, override sympathetic vasoconstriction and stimulate skeletal muscle afferents (exercise pressor reflex) are discussed and a conceptual framework is set suggesting a major role of ATP in blood flow regulation during exercise.

Impaired carotid baroreflex control of arterial blood pressure in multiple sclerosis

Multiple sclerosis (MS), a progressive neurological disease, can lead to impairments in the autonomic control of cardiovascular function. We tested the hypothesis that individuals with relapsing-remitting MS (n = 10; 7 females, 3 males; 13 ± 4 yr from diagnosis) exhibit impaired carotid baroreflex control of blood pressure and heart rate compared with sex, age, and body weight-matched healthy individuals (CON: n = 10; 7 females, 3 males). At rest, 5-s trials of neck pressure (NP; +40 Torr) and neck suction (NS; -60 Torr) were applied to simulate carotid hypotension and hypertension, respectively, while mean arterial pressure (MAP; finger photoplethysmography), heart rate (HR), cardiac output (CO; Modelflow), and total vascular conductance (TVC) were continuously measured. In response to NP, there was a blunted increase in peak MAP responses (MS: 5 ± 2 mmHg) in individuals with MS compared with healthy controls (CON: 9 ± 3 mmHg; P = 0.005), whereas peak HR responses were not different between groups. At the peak MAP response to NP, individuals with MS demonstrated an attenuated decrease in TVC (MS, -10 ± 4% baseline vs. CON, -15 ± 4% baseline, P = 0.012), whereas changes in CO were similar between groups. Following NS, all cardiovascular responses (i.e., nadir MAP and HR and percent changes in CO and TVC) were not different between MS and CON groups. These data suggest that individuals with MS have impaired carotid baroreflex control of blood pressure via a blunted vascular conductance response resulting in a diminished ability to increase MAP in response to a hypotensive challenge.

The interplay between sympathetic overactivity, hypertension and heart rate variability (review, invited)

The control of arterial pressure is a complex interaction of the long- and short-term influences of hormones, local vascular factors, and neural mechanisms. The autonomic nervous system and its sympathetic arm play important roles in the regulation of blood pressure, and overactivity of sympathetic nerves may have an important role in the development of hypertension and related cardiovascular disorders. The baroreceptor system opposes either increases or decreases in arterial pressure, and the primary purpose of the arterial baroreflex is to keep blood pressure close to a particular set point over a relatively short period of time. The ability of the baroreflex to powerfully buffer acute changes in arterial pressure is well established, but the role of the arterial baroreceptor reflex in long-term control of arterial pressure has been a topic of many debate and controversy for decades. The sympathetic nervous system and arterial baroreceptor reflex control of renal sympathetic nerve activity has been proposed to play a role in long-term control of arterial pressure. The aim of this paper has been to review the postulated role of sympathetic activation.

Effect of aging on carotid baroreflex control of blood pressure and leg vascular conductance in women

Recent work suggests that β-adrenergic vasodilation offsets α-adrenergic vasoconstriction in young women, but this effect is lost after menopause. Given these age-related vascular changes, we tested the hypothesis that older women would exhibit a greater change in vascular conductance following baroreflex perturbation compared with young women. In 10 young (21 ± 1 yr) and 10 older (62 ± 2 yr) women, mean arterial pressure (MAP; Finometer), heart rate (HR), cardiac output (CO; Modelflow), total vascular conductance (TVC), and leg vascular conductance (LVC, duplex-Doppler ultrasound) were continuously measured in response to 5-s pulses of neck suction (NS; -60 Torr) and neck pressure (NP; +40 Torr) to simulate carotid hypertension and hypotension, respectively. Following NS, decreases in MAP were similar between groups; however, MAP peak response latency was slower in older women (P < 0.05). Moreover, at the time of peak MAP, increases in LVC (young, -11.5 ± 3.9%LVC vs. older, +19.1 ± 7.0%LVC; P < 0.05) and TVC were greater in older women, whereas young women exhibited larger decreases in HR and CO (young, -10 ± 3% CO vs. older, +0.8 ± 2% CO; P < 0.05). Following NP, increases in MAP were blunted (young, +14 ± 1 mmHg vs. older, +8 ± 1 mmHg; P < 0.05) in older women, whereas MAP response latencies were similar. Interestingly, decreases in LVC and TVC were similar between groups, but HR and CO (young, +7.0 ± 2% CO vs. older, -4.0 ± 2% CO; P < 0.05) responses were attenuated in older women. These findings suggest that older women have greater reliance on vascular conductance to modulate MAP via carotid baroreflex, whereas young women rely more on cardiac responsiveness. Furthermore, older women demonstrate a blunted ability to increase MAP to hypotensive stimuli.

Blood pressure regulation II: what happens when one system must serve two masters--oxygen delivery and pressure regulation?

During high-intensity dynamic exercise, O2 delivery to active skeletal muscles is enhanced through marked increases in both cardiac output and skeletal muscle blood flow. When the musculature is vigorously engaged in exercise, the human heart lacks the pumping capacity to meet the blood flow demands of both the skeletal muscles and other organs such as the brain. Vasoconstriction must therefore be induced through activation of sympathetic nervous activity to maintain blood flow to the brain and to produce the added driving pressure needed to increase flow to the skeletal muscles. In this review, we first briefly summarize the local vascular and neural control mechanisms operating during high-intensity exercise. This is followed by a review of the major neural mechanisms regulating blood pressure during high-intensity exercise, focusing mainly on the integrated activities of the arterial baroreflex and muscle metaboreflex. In high cardiac output situations, such as during high-intensity dynamic exercise, small changes in total peripheral resistance can induce large changes in blood pressure, which means that rapid and fine regulation is necessary to avoid unacceptable drops in blood pressure. To accomplish this rapid regulation, arterial baroreflex function may be modulated in various ways through activation of the muscle metaboreflex and/or other neural mechanisms. Moreover, this modulation of the arterial baroreflex may change over the time course of an exercise bout, or to accommodate changes in exercise intensity. Within this model, integration of arterial baroreflex modulation with other neural mechanisms plays an important role in cardiovascular control during high-intensity exercise.

Muscle metaboreflex activation speeds the recovery of arterial blood pressure following acute hypotension in humans

It has been suggested that the arterial baroreflex and muscle metaboreflex are both activated during heavy exercise and that they interact to modulate primary cardiovascular reflex responses. This proposed interaction and its consequences are not fully understood, however. The purpose of present study was to test our hypothesis that dynamic arterial baroreflex-mediated cardiovascular responses to acute systemic hypotension in humans are augmented when the muscle metaboreflex is active and that this results in a faster recovery of arterial blood pressure. Acute hypotension was induced nonpharmacologically in 12 healthy subjects by releasing bilateral thigh cuffs after 9 min of suprasystolic resting ischemia, with and without muscle metaboreflex activation via postexercise muscle ischemia (PEMI) after 1 min of isometric handgrip exercise at 50% maximum voluntary contraction. The thigh-cuff release evoked rapid reductions in mean arterial pressure (MAP) and increases in heart rate, cardiac output (Doppler), and total vascular conductance (TVC) under control conditions and during PEMI. The reductions in MAP from baseline were greater and the increases in TVC were smaller during PEMI than control. In addition, arterial baroreflex-mediated peripheral vasoconstriction was augmented during PEMI, as evidenced by a near doubling of the rate of recovery of MAP and TVC. These results show that when the muscle metaboreflex is activated in humans, arterial baroreflex-mediated peripheral vasoconstriction elicited in response to acute hypotension is augmented, which halves the time needed for MAP recovery. Such modulation of baroreflex function would be advantageous for maintaining an elevated arterial blood pressure during activation of the muscle metaboreflex.

New insights into the effects of age and sex on arterial baroreflex function at rest and during dynamic exercise in humans

The arterial baroreflex (ABR) performs an important role in regulating blood pressure (BP) both at rest and during exercise, by carefully orchestrating autonomic neural activity to the heart and blood vessels. Reduced ABR sensitivity (i.e., gain) has been associated with increased cardiovascular risk, cardiac electrical instability and orthostatic intolerance, while 'normal' ABR function during exercise is important for ensuring an appropriate cardiovascular response is elicited. Previous studies examining the influence of age and sex on resting ABR function in humans have primarily used pharmacological methods (e.g., modified Oxford technique) to change BP and alter baroreceptor input. With this approach only reflex control of heart rate and sympathetic nerve activity may be evaluated, and as such the influence of age and sex on ABR control of BP per se remains incompletely understood. Furthermore, the majority of previous studies examining ABR function during exercise have principally assessed young men. Whether these findings can be extrapolated to young women or older men and women remains unclear. Recently the potential for age and sex to modulate the integrative neural control of the cardiovascular system is becoming appreciated. This review article will provide a detailed update of such recent advances into our understanding of the effects of age and sex on ABR control of BP both at rest and during dynamic exercise in humans.

Arterial baroreflex control of muscle sympathetic nerve activity under orthostatic stress in humans

The mechanisms by which blood pressure is maintained against the orthostatic stress caused by gravity's effect on the fluid distribution within the body are important issues in physiology, especially in humans who usually adopt an upright posture. Peripheral vasoconstriction and increased heart rate (HR) are major cardiovascular adjustments to orthostatic stress and comprise part of the reflex response elicited via the carotid sinus and aortic baroreceptors (arterial baroreflex: ABR) and cardiopulmonary stretch receptors (cardiopulmonary baroreflex). In a series of studies, we have been characterizing the ABR-mediated regulation of cardiovascular hemodynamics and muscle sympathetic nerve activity (MSNA) while applying orthostatic stress in humans. We have found that under orthostatic stress, dynamic carotid baroreflex responses are modulated as exemplified by the increases in the MSNA, blood pressure, and HR responses elicited by carotid baroreflex unloading and the shorter period of MSNA suppression, comparable reduction and faster recovery of mean arterial blood pressure (MAP) and greater HR response to carotid baroreflex stimulation. Our results also show that ABR-mediated beat-to-beat control over burst incidence, burst strength and total MSNA is progressively modulated as orthostatic stress is increased until induction of syncope, and that the sensitivity of ABR control over the aforementioned MSNA variables is substantially reduced during the development of syncope. We suggest that in humans, the modulation of ABR function under orthostatic stress may be one of the mechanisms by which blood pressure is maintained and orthostatic hypotension limited, and impairment of ABR control over sympathetic vasomotor activity leads to the severe hypotension associated with orthostatic syncope.

Sex differences in carotid baroreflex control of arterial blood pressure in humans: relative contribution of cardiac output and total vascular conductance

It is presently unknown whether there are sex differences in the magnitude of blood pressure (BP) responses to baroreceptor perturbation or if the relative contribution of cardiac output (CO) and total vascular conductance (TVC) to baroreflex-mediated changes in BP differs in young women and men. Since sympathetic vasoconstrictor tone is attenuated in women, we hypothesized that carotid baroreflex-mediated BP responses would be attenuated in women by virtue of a blunted vascular response (i.e., an attenuated TVC response). BP, heart rate (HR), and stroke volume were continuously recorded during the application of 5-s pulses of neck pressure (NP; carotid hypotension) and neck suction (NS; carotid hypertension) ranging from +40 to -80 Torr in women (n = 20, 21 ± 0.5 yr) and men (n = 20, 21 ± 0.4 yr). CO and TVC were calculated on a beat-to-beat basis. Women demonstrated greater depressor responses to NS (e.g., -60 Torr, -17 ± 1%baseline in women vs. -11 ± 1%baseline in men, P < 0.05), which were driven by augmented decreases in HR that, in turn, contributed to larger reductions in CO (-60 Torr, -15 ± 2%baseline in women vs. -6 ± 2%baseline in men, P < 0.05). In contrast, pressor responses to NP were similar in women and men (e.g., +40 Torr, +14 ± 2%baseline in women vs. +10 ± 1%baseline in men, P > 0.05), with TVC being the primary mediating factor in both groups. Our findings indicate that sex differences in the baroreflex control of BP are evident during carotid hypertension but not carotid hypotension. Furthermore, in contrast to our hypothesis, young women exhibited greater BP responses to carotid hypertension by virtue of a greater cardiac responsiveness.

Carotid baroreflex responsiveness is impaired in normotensive African American men

There are important differences in autonomic function and cardiovascular responsiveness between African Americans (AA) and Caucasian Americans (CA). This study tested the hypothesis that carotid baroreflex (CBR) responsiveness is impaired in normotensive AA compared with normotensive CA at rest. CBR control of heart rate (HR) and mean arterial blood pressure (MAP) was assessed in 30 nonhypertensive male subjects (15 AA; 15 CA; age 18-33 yr) with 5-s periods of neck pressure (NP; simulated hypotension) and neck suction (NS; simulated hypertension) ranging from +45 to -80 Torr during rest. Carotid-cardiac stimulus-response curves revealed a significantly lower minimum HR response in the CA compared with AA (40.8 ± 2.4 vs. 49.8 ± 2.9 beats/min, respectively; P < 0.05). In addition, the magnitude of the mean HR response to all trials of NS (-20, -40, -60, and -80 Torr) was attenuated in the AA group (AA, -10.1 ± 1.7 vs. CA, -14.9 ± 2.2 beats/min; P < 0.05), while no significant differences were found in the magnitude of the mean HR response to NP (+15, +30, and +45 Torr) between racial groups. There were no significant differences in the carotid-vasomotor stimulus-response curves between racial groups. Also, while no racial differences were found in the magnitude of the mean MAP response to all trials of NS, the magnitude of the mean MAP response to all trials of NP was attenuated in the AA group (AA, 7.2 ± 1.3 vs. CA, 9.3 ± 1.1 mmHg; P < 0.05). Together, these findings support inherent differences in short-term blood pressure regulation between racial groups that exhibit different relative risk for the development of hypertension.

Vagal modulation of heart rate variability during atrial fibrillation in pigs

Atrial fibrillation (AF) is the most common sustained cardiac dysrhythmia and is associated with an increased risk for sudden cardiac death. The ventricular rhythm is irregular and displays both non-linear and linear patterns; however, it has not been determined whether vagally derived patterns are manifest within the irregular rhythm. Moreover, indices of increased vagal control are associated with reduced risk of sudden cardiac death. In this study, we sought to determine whether the ventricular rhythm pattern during AF is, in part, modulated by vagal activity. Vagal oscillations were forced at 0.15 Hz by neck suction in 12 pigs with sustained AF with and without glycopyrrolate (0.15 microg/kg, intravenously) vagal blockade. Vagal activity was evaluated using time- and frequency-domain heart rate variability measures. The standard deviation of RR intervals (SDRRI) was significantly increased during vagal activation compared with baseline (P = 0.006). Moreover, SDRRI correlated significantly with spectral power at 0.15 Hz during baseline (r = 0.90, P < .001) and vagal activation (r = 0.86, P < 0.05). Glycopyrrolate blocked the increase in SDRRI (P < 0.001) and blunted spectral power at 0.15 Hz (P < 0.05). These results indicate that: (1) power spectral analysis may be used to assess parasympathetic regulation during AF, and (2) vagal oscillations produce an entrainment of the ventricular rhythm during AF in pigs.

Influence of ageing on carotid baroreflex peak response latency in humans

The stability of a physiological control system, such as the arterial baroreflex, depends critically upon both the magnitude (i.e. gain or sensitivity) and timing (i.e. latency) of the effector response. Although studies have examined resting arterial baroreflex sensitivity in older subjects, little attention has been given to the influence of ageing on the latency of peak baroreflex responses. First, we compared the temporal pattern of heart rate (HR) and mean arterial blood pressure (BP) responses to selective carotid baroreceptor (CBR) unloading and loading in 14 young (22 +/- 1 years) and older (61 +/- 1 years) subjects, using 5 s pulses of neck pressure (NP, +35 Torr) and neck suction (NS, -80 Torr). Second, CBR latency was assessed following pharmacological blockade of cardiac parasympathetic nerve activity in eight young subjects, to better understand how known age-related reductions in parasympathetic nerve activity influence CBR response latency. In response to NP, the time to the peak increase in HR and mean BP were similar in young and older groups. In contrast, in response to NS the time to peak decrease in HR (2.1 +/- 0.2 vs. 3.8 +/- 0.2 s) and mean BP (6.7 +/- 0.4 vs. 8.3 +/- 0.2 s) were delayed in older individuals (young vs. older, P < 0.05). The time to peak HR and mean BP were delayed in young subjects following cardiac parasympathetic blockade (glycopyrrolate). Collectively, these data suggest that ageing is associated with delayed peak cardiovascular responses to acute carotid baroreceptor loading that may be, in part, due to age-related reductions in cardiac parasympathetic tone.

Transfer function characteristics of the neural and peripheral arterial baroreflex arcs at rest and during postexercise muscle ischemia in humans

Previous studies have demonstrated an increase in the arterial baroreflex (ABR) control of muscle sympathetic nerve activity (MSNA) during isolated activation of the muscle metaboreflex with postexercise muscle ischemia (PEMI). However, the increased ABR-MSNA control does not appear to manifest in an enhancement in the ABR control of arterial blood pressure (BP), suggesting alterations in the transduction of MSNA into a peripheral vascular response and a subsequent ABR-mediated change in BP. Thus we examined the operating gains of the neural and peripheral arcs of the ABR and their interactive relationship at rest and during muscle metaboreflex activation. In nine healthy subjects, graded isolation of the muscle metaboreflex was achieved by PEMI following isometric handgrip performed at 15% and 30% maximal voluntary contraction (MVC). To obtain the sensitivities of the ABR neural and peripheral arcs, the transfer function gain from BP to MSNA and MSNA to femoral vascular conductance, respectively, was analyzed. No changes from rest were observed in the ABR neural or peripheral arcs during PEMI after 15% MVC handgrip. However, PEMI following 30% MVC handgrip increased the low frequency (LF) transfer function gain between BP and MSNA (ABR neural arc; +58 +/- 28%, P = 0.036), whereas the LF gain between MSNA and femoral vascular conductance (ABR peripheral arc) was decreased from rest (-36 +/- 8%, P = 0.017). These findings suggest that during high-intensity muscle metaboreflex activation an increased ABR gain of the neural arc appears to offset an attenuation of the peripheral arc gain to help maintain the overall ABR control of systemic BP.

Carotid baroreflex testing using the neck collar device

A neck chamber device for stimulation of carotid sinus baroreceptors by changing carotid transmural pressure was first described in 1957 by Ernsting and Parry and, with several modifications, has been extensively used in a number of physiological and clinical studies. This article outlines the evolution of neck chamber devices and describes some of the advantages and limitations of the technique. We also describe the responses in healthy subjects and the changes observed in patients with some disorders affecting the autonomic nervous system.

Stroke volume decreases during mild dynamic and static exercise in supine humans

AIM

The contributions of cardiac output (CO) and total peripheral resistance to changes in arterial blood pressure are debated and differ between dynamic and static exercise. We studied the role stroke volume (SV) has in mild supine exercise.

METHODS

We investigated 10 healthy, supine volunteers by continuous measurement of heart rate (HR), mean arterial blood pressure, SV (ultrasound Doppler) and femoral beat volume (ultrasound Doppler) during both dynamic mild leg exercise and static forearm exercise. This made it possible to study CO, femoral flow (FF) and both total and femoral peripheral resistance beat-by-beat.

RESULTS

During a countdown period immediately prior to exercise, HR and mean arterial pressure increased, while SV decreased. During mild supine exercise, SV decreased by 5-8%, and most of this was explained by increased mean arterial pressure. Dynamic leg exercise doubled femoral beat volume, while static hand grip decreased femoral beat volume by 18%. FF is tightly regulated according to metabolic demand during both dynamic leg exercise and static forearm exercise.

CONCLUSION

Our three major findings are, firstly, that SV decreases during both dynamic and static mild supine exercise due to an increase in mean arterial pressure. Secondly, femoral beat volume decreases during static hand grip, but FF is unchanged due to the increase in HR. Finally, anticipatory responses to exercise are apparent prior to both dynamic and static exercise. SV changes contribute to CO changes and should be included in studies of central haemodynamics during exercise.

Spontaneous baroreflex measures are unable to detect age-related impairments in cardiac baroreflex function during dynamic exercise in humans

The dynamic relationship between 'spontaneous' fluctuations in arterial blood pressure (BP) and heart rate (HR) is increasingly being used to provide an estimate of resting cardiac baroreflex sensitivity. Given the ease of use and clinical utility, spontaneous methods are now also being used to examine cardiac baroreflex sensitivity in distinct subject groups during various laboratory stressors and tasks encountered during daily life, such as physical activity. However, the utility of such spontaneous measures to estimate cardiac baroreflex function during exercise remains unclear, particularly when comparing groups. Therefore, we tested the ability of spontaneous indices to detect age-related differences in cardiac baroreflex function during dynamic exercise. Beat-to-beat HR and BP were measured in eighteen healthy young subjects (24 +/- 1 years) and sixteen healthy middle-aged subjects (59 +/- 1 years) at rest and during steady-state leg cycling. Estimates of spontaneous cardiac baroreflex sensitivity using the sequence technique (G(SEQ)) and low-frequency transfer function gain (G(TF)) were compared with the operating point (G(OP)) and maximal gain (G(MAX)) of the full carotid-cardiac baroreflex function curve. At rest G(SEQ), G(TF), G(OP) and G(MAX) were all significantly lower in older subjects. During moderate-intensity steady-state exercise no differences were observed in G(SEQ) and G(TF) (older 0.26 +/- 0.03 beats min(-1) mmHg(-1) versus younger 0.32 +/- 0.04 beats min(-1) mmHg(-1); P > 0.05), whereas G(OP) and G(MAX) (older -0.21 +/- 0.02 beats min(-1) mmHg(-1) versus younger -0.39 +/- 0.03 beats min(-1) mmHg(-1); P < 0.05) remained lower in older subjects. These data indicate that spontaneous measures of cardiac baroreflex sensitivity alone provide limited information when comparing age-groups during exercise, which makes genuine differences in baroreflex function difficult to identify.

Physically Active Lifestyle Enhances Vagal-Cardiac Function but Not Central Autonomic Neural Interaction in Elderly Humans

The cause of the age-related impairment of arterial baroreflex function remains ill-defined; moreover, it is unknown whether this impairment results from aging per se or from an inactive lifestyle associated with aging. In this study, we sought to: 1) determine whether elderly individuals who maintained an active lifestyle had an enhanced carotid baroreflex function as compared with their sedentary counterparts; and 2) determine whether this difference was due in part to altered function of the arterial baroreceptor and/or altered central modulation. Eight healthy, sedentary (SED, 68 ± 2 yr) and eight physically active (ACT, 68 ± 1 yr) elderly men with peak O2 consumption 25.5 ± 1.2 vs 35.7 ± 2.4 ml/min/kg (P < 0.01), respectively, were assessed with carotid baroreceptor (CBR) function using 5s pulses of neck pressure or suction (ranging from +40 to −80 Torr) delivered to the carotid sinus region at rest and during lower body negative pressure (LBNP) of −15 and −40 Torr. Changes in heart rate (HR) and mean arterial pressure (MAP) were assessed for CBR-HR and CBR-MAP gains, respectively. Overall CBR-HR gains in a range of ∼ 120 mmHg of carotid sinus pressure were greater (P < 0.01) in ACT than SED at rest and during LBNP. The derived peak CBR-HR slopes between ACT and SED at rest were −0.32 ± 0.07 vs −0.11 ± 0.02 bpm/mmHg (P = 0.007), respectively. However, there was no statistical difference (P = 0.37) in CBR-MAP gains between the groups. Neither CBR-MAP (P = 0.08) nor CBR-HR (P = 0.41) gain was augmented by LBNP in the elderly. Conclusion: Active lifestyle enhances the CBR-HR reflex sensitivity as a result of the improved vagal-cardiac function in elderly people. Aging is associated with an absence of central autonomic interaction in the control of blood pressure regardless of physical fitness.

Altered hormonal regulation and blood flow distribution with cardiovascular deconditioning after short-duration head down bed rest

This study tested the hypothesis that cardiovascular and hormonal responses to lower body negative pressure (LBNP) would be altered by 4-h head down bed rest (HDBR) in 11 healthy young men. In post-HDBR testing, three subjects failed to finish the protocol due to presyncopal symptoms, heart rate was increased during LBNP compared with pre-HDBR, mean arterial blood pressure was elevated at 0, −10, and −20 mmHg and reduced at −40 mmHg, central venous pressure (CVP) and cardiac stroke volume were reduced at all levels of LBNP. Plasma concentrations of renin, angiotensin II, and aldosterone were significantly lower after HDBR. Renin and angiotensin II increased in response to LBNP only post-HDBR. There was no effect of HDBR or LBNP on norepinephrine while epinephrine tended to increase at −40 mmHg post-HDBR ( P = 0.07). Total blood volume was not significantly reduced. Splanchnic blood flow taken from ultrasound measurement of the portal vein was higher at each level of LBNP post-compared with pre-HDBR. The gain of the cardiopulmonary baroreflex relating changes in total peripheral resistance to CVP was increased after HDBR, but splanchnic vascular resistance was actually reduced. These results are consistent with our hypothesis and suggest that cardiovascular instability following only 4-h HDBR might be related to altered hormonal and/or neural control of regional vascular resistance. Impaired ability to distribute blood away from the splanchnic region was associated with reduced stroke volume, elevated heart rate, and the inability to protect mean arterial pressure.

Effect of propranolol on sympathetically mediated leg vasoconstriction in humans

Sympatho-excitatory manoeuvres are used to study vascular responsiveness in humans, but it is unclear if circulating adrenaline attenuates peripheral vasoconstriction during these manoeuvres. We hypothesized that vasoconstrictor responses to three manoeuvres (neck pressure, unilateral thigh-cuff release and isometric handgrip) would be greater after the administration of the beta-adrenergic blocker propranolol. Seven men and six women underwent these manoeuvres while beat-by-beat arterial pressure (finger photoplethysmography), femoral mean blood velocity (Doppler ultrasound) and femoral artery diameter (edge-detection software) were measured. Femoral vascular conductance was calculated as flow/pressure. Propranolol had no effect on baseline femoral vascular conductance (P > 0.05). As a result of neck pressure, femoral vascular conductance was reduced 23.9 +/- 3.5% before vs. 33.2 +/- 3.2% after infusion of propranolol (P = 0.033). After thigh-cuff release, femoral vascular conductance declined 50.2 +/- 5.8% before vs. 57.4 +/- 9.6% after propranolol infusion (P = 0.496). During handgrip, femoral vascular conductance was reduced 47.2 +/- 9.6% before vs. 55.2 +/- 9.2% after propranolol administration (P = 0.447). After handgrip, women had a greater rise in conductance than men (women: 153 +/- 16.2%; men: 36.4 +/- 10.6%; P < 0.001), which was blunted by 54.8% by propranolol (P < 0.001 vs. control), but unaffected by propranolol in men (P = 0.355 vs. control). The finding that beta-adrenergic receptor-mediated vasodilatation minimally affects vascular responses to these sympatho-excitatory manoeuvres reinforces their utility in the investigation of sympathetic vascular regulation in humans. Interestingly, post-handgrip hyperaemia is greater in women than men and is, in part, beta-adrenergic receptor mediated.

Modulation of the spontaneous beat-to-beat fluctuations in peripheral vascular resistance during activation of muscle metaboreflex

Continuous measurement of leg blood flow (LBF) using Doppler ultrasound with simultaneous noninvasive mean arterial blood pressure (MAP) measurement permits beat-to-beat estimates of leg vascular resistance (LVR) in humans. We tested the hypothesis that the beat-to-beat fluctuations in LVR and the dynamic relationship between MAP and LVR are modulated by the activation of muscle metaboreflex. Twelve healthy subjects performed a 1-min isometric handgrip exercise at 50% maximal voluntary contraction, which was followed by a period of imposed postexercise muscle ischemia (PEMI). We then employed transfer function analysis to examine the dynamic relationships between MAP and LBF and between MAP and LVR, both at rest (control) and during PEMI. We found the following. 1) The spectral power for LBF and LVR in low-frequency (∼0.03–0.15 Hz) range significantly increased from control during PEMI without a significant change in the high-frequency (∼0.15–0.35 Hz) power. 2) During PEMI, the transfer function gains for MAP-LBF and MAP-LVR relationships in the low-frequency (∼0.05–0.15 Hz) range were significantly increased during PEMI (vs. control) but were unchanged in the high-frequency (∼0.2–0.3 Hz) range. 3) The phases for MAP-LBF and MAP-LVR relationships were not different during control and PEMI. The phase for MAP-LVR relationship revealed that changes in MAP were followed by directionally similar changes in LVR, which is consistent with the characteristics of intrinsic vascular regulatory mechanisms such as the myogenic response of the resistance arteries. We suggest that, in humans, modulation of the dynamic MAP-LVR relationship during activation of the muscle metaboreflex reflects complex interactions between intrinsic vascular regulatory mechanisms and sympathetic vascular regulation.

Cardiac and vasomotor components of the carotid baroreflex control of arterial blood pressure during isometric exercise in humans

We sought to examine the importance of the cardiac component of the carotid baroreflex (CBR) in control of blood pressure during isometric exercise. Nine subjects performed 4 min of ischaemic isometric calf exercise at 20% of maximum voluntary contraction. Trials were repeated with beta1-adrenergic blockade (metoprolol, 0.15 +/- 0.003 mg kg(-1)) or parasympathetic blockade (glycopyrrolate, 13.6 +/- 1.5 microg kg(-1)). CBR function was determined using rapid pulses of neck pressure and neck suction from +40 to -80 mmHg, while heart rate (HR), mean arterial pressure (MAP) and changes in stroke volume (SV, Modelflow method) were measured. Metoprolol decreased and glycopyrrolate increased HR and cardiac output both at rest and during exercise (P < 0.05), while resting and exercising blood pressure were unchanged. Glycopyrrolate reduced the maximal gain (G(max)) ofthe CBR-HR function curve (-0.58 +/- 0.10 to -0.06 +/- 0.01 beats min(-1) mmHg(-1), P < 0.05), but had no effect on the G(max) of the CBR-MAP function curve. During isometric exercise the CBR-HR curve was shifted upward and rightward in the metoprolol and no drug conditions, while the control of HR was significantly attenuated with glycopyrrolate (P < 0.05). Regardless of drug administration isometric exercise produced an upward and rightward resetting of the CBR control of MAP with no change in G(max). Thus, despite marked reductions in CBR control of HR following parasympathetic blockade, CBR control of blood pressure was well maintained. These data suggest that alterations in vasomotor tone are the primary mechanism by which the CBR modulates blood pressure during low intensity isometric exercise.

Carotid baroreflex regulation of vascular resistance in high-altitude Andean natives with and without chronic mountain sickness

We investigated carotid baroreflex control of vascular resistance in two groups of high-altitude natives: healthy subjects (HA) and a group with chronic mountain sickness (CMS), a maladaptation condition characterized by high haematocrit values and symptoms attributable to chronic hypoxia. Eleven HA controls and 11 CMS patients underwent baroreflex testing, using the neck collar method in which the pressure distending the carotid baroreceptors was changed by applying pressures of -40 to +60 mmHg to the chamber. Responses of forearm vascular resistance were assessed from changes in the quotient of blood pressure divided by brachial artery blood velocity. Stimulus-response curves were defined at high altitude (4338 m) and within 1 day of descent to sea level. We applied a sigmoid function or third-order polynomial to the curves and determined the maximal slope (equivalent to peak gain) and the corresponding carotid pressure (equivalent to 'set point'). The results showed that the peak gains of the reflex were similar in both groups and at both locations. The 'set point' of the reflex, however, was significantly higher in the CMS patients compared to HA controls, indicating that the reflex operates over higher pressures in the patients (94.4 +/- 3.0 versus 79.6 +/- 4.1 mmHg; P < 0.01). This, however, was seen only when subjects were studied at altitude; after descent to sea level the curve reset to a lower pressure with no significant difference between HA and CMS subjects. These results indicate that carotid baroreceptor control of vascular resistance may be abnormal in CMS patients but that descent to sea level rapidly normalizes it. We speculate that this may be explained by CMS patients having greater vasoconstrictor activity at altitude owing to greater hypoxic stimulation of chemoreceptors.

Effects of changes in central blood volume on carotid-vasomotor baroreflex sensitivity at rest and during exercise

The purpose of this investigation was to examine whether the effect of changes in central blood volume on carotid-vasomotor baroreflex sensitivity at rest was the same during exercise. Eight men (means ± SE: age 26 ± 1 yr; height 180 ± 3 cm; weight 86 ± 6 kg) participated in the present study. Sixteen Torr of lower body negative pressure (LBNP) were applied to decrease central venous pressure (CVP) at rest and during steady-state leg cycling at 50% peak O2 uptake (104 ± 20 W). Subsequently, infusions of 25% human serum albumin solution were administered to increase CVP at rest and during exercise. During all protocols, heart rate, arterial blood pressure, and CVP were recorded continuously. At each stage of LBNP or albumin infusion, the maximal gain (Gmax) of the carotid-vasomotor baroreflex function curve was measured using the neck pressure and neck suction technique. LBNP reduced CVP and increased the Gmax of the carotid-vasomotor baroreflex function curve at rest (+63 ± 25%, P = 0.006) and during exercise (+69 ± 19%, P = 0.002). In contrast to the LBNP, increases in CVP resulted in the Gmax of the carotid-vasomotor baroreflex function curve being decreased at rest −8 ± 4% and during exercise −18 ± 5% ( P > 0.05). These findings indicate that the relationship between CVP and carotid-vasomotor baroreflex sensitivity was nonlinear at rest and during exercise and suggests a saturation load of the cardiopulmonary baroreceptors at which carotid-vasomotor baroreflex sensitivity remains unchanged.