'Non-hypotensive' hypovolaemia reduces ascending aortic dimensions in humans

Autonomic ganglionic blockade does not prevent reduction in cerebral blood flow velocity during orthostasis in humans

BACKGROUND AND PURPOSE

The underlying mechanisms for reductions in cerebral blood flow (CBF) during orthostasis are not completely understood. This study tested the hypothesis that sympathetic activation causes cerebral vasoconstriction leading to reductions in CBF during lower body negative pressure (LBNP).

METHODS

CBF velocity, arterial pressure, and end-tidal CO(2) were measured during LBNP (-30 to -50 mm Hg) in 11 healthy subjects before and after autonomic ganglionic blockade with trimethaphan. Arterial partial pressure of CO(2) also was measured in a subgroup of 5 subjects. Mean arterial pressure during LBNP after blockade was maintained by infusion of phenylephrine.

RESULTS

Before blockade, mean arterial pressure did not change during LBNP. However, CBF velocity was reduced in all subjects by 14% (P<0.05). Systolic and pulsatile (systolic-diastolic) CBF velocity were reduced by 18% and 28%, respectively, associated with significant reductions in pulse arterial pressure and end-tidal CO(2) (all P<0.05). After blockade, mean arterial pressure during LBNP was well-maintained and even increased slightly with infusion of phenylephrine. However, reductions in mean, systolic, and pulsatile CBF velocity, pulse arterial pressure, and ETCO(2) were similar to those before blockade. In contrast to reductions in end-tidal CO(2), arterial partial pressure of CO(2) did not change during LBNP.

CONCLUSIONS

These data, contrary to our hypothesis, demonstrate that sympathetic vasoconstriction is not the primary mechanism underlying reductions in CBF during moderate LBNP. We speculate that diminished pulse arterial pressure or pulsatile blood flow may reduce cerebral vessel wall shear stress and contribute to reductions in CBF during orthostasis through flow mediated regulatory mechanisms.

Transfer function analysis between arterial pressure and renal sympathetic nerve activity at cardiac pacing frequencies in the rat

This study examined the possible influence of changes in heart rate (HR) on the gain of the transfer function relating renal sympathetic nerve activity (RSNA) to arterial pressure (AP) at HR frequency in rats. In seven urethane-anesthetized rats, AP and RSNA were recorded under baseline conditions (spontaneous HR = 338 ± 6 beats/min, i.e., 5.6 ± 0.1 Hz) and during 70-s periods of cardiac pacing at 6–9 Hz applied in random order. Cardiac pacing slightly increased mean AP (0.8 ± 0.2 mmHg/Hz) and decreased pulse pressure (−3.6 ± 0.3 mmHg/Hz) while leaving the mean level of RSNA essentially unaltered ( P = 0.680, repeated-measures ANOVA). The gain of the transfer function from AP to RSNA measured at HR frequency was always associated with a strong, significant coherence and was stable between 6 and 9 Hz ( P = 0.185). The transfer function gain measured under baseline conditions [2.44 ± 0.28 normalized units (NU)/mmHg] did not differ from that measured during cardiac pacing (2.46 ± 0.27 NU/mmHg). On the contrary, phase decreased linearly as a function of HR, which indicated the presence of a fixed time delay (97 ± 6 ms) between AP and RSNA. In conclusion, the dynamic properties of arterial baroreflex pathways do not affect the gain of the transfer function between AP and RSNA measured at HR frequency in the upper part of the physiological range of HR variations in the rat.

Increases in central blood volume modulate carotid baroreflex resetting during dynamic exercise in humans

We sought to determine if resetting of the carotid-vasomotor baroreflex function curve during exercise is modulated by changes in central blood volume (CBV). CBV was increased during exercise by altering: (1) subject posture (supine versus upright) and (2) pedal frequency (80 versus 60 revolutions min(-1) (r.p.m.)); while oxygen uptake ( ) was kept constant. Eight male subjects performed three exercise trials: upright cycling at 60 r.p.m. (control); supine cycling at 60 r.p.m. (SupEX) and upright cycling at 80 r.p.m. to enhance the muscle pump (80EX). During each condition, carotid baroreflex (CBR) function was determined using the rapid neck pressure (NP) and neck suction (NS) protocol. Although mean arterial pressure (MAP) was significantly elevated from rest (88 +/- 2 mmHg) during all exercise conditions (P < 0.001), the increase in MAP was lower during SupEX (94 +/- 2 mmHg) and 80EX (95 +/- 2 mmHg) compared with control (105 +/- 2 mmHg, P < 0.05). Importantly, the blood pressure responses to NP and NS were maintained around these changed operating points of MAP. However, in comparison to control, the carotid-vasomotor baroreflex function curve was relocated downward and leftward when CBV was increased during SupEX and 80EX. These alterations in CBR resetting occurred without any differences in or heart rate between the exercise conditions. Thus, increasing CBV and loading the cardiopulmonary baroreflex reduces the magnitude of exercise-induced increases in MAP and CBR resetting. These findings suggest that changes in cardiopulmonary baroreceptor load influence carotid baroreflex resetting during dynamic exercise.

Sympathetic neural control of integrated cardiovascular function: Insights from measurement of human sympathetic nerve activity

Sympathetic neural control of cardiovascular function is essential for normal regulation of blood pressure and tissue perfusion. In the present review we discuss sympathetic neural mechanisms in human cardiovascular physiology and pathophysiology, with a focus on evidence from direct recordings of sympathetic nerve activity using microneurography. Measurements of sympathetic nerve activity to skeletal muscle have provided extensive information regarding reflex control of blood pressure and blood flow in conditions ranging from rest to postural changes, exercise, and mental stress in populations ranging from healthy controls to patients with hypertension and heart failure. Measurements of skin sympathetic nerve activity have also provided important insights into neural control, but are often more difficult to interpret since the activity contains several types of nerve impulses with different functions. Although most studies have focused on group mean differences, we provide evidence that individual variability in sympathetic nerve activity is important to the ultimate understanding of these integrated physiological mechanisms.

Mathematical modeling of gravitational effects on the circulation: importance of the time course of venous pooling and blood volume changes in the lungs

A dip in blood pressure (BP) in response to head-up tilt (HUT) or active standing might be due to rapid pooling in the veins below the heart (preload) or muscle activation-induced drop in systemic vascular resistance (afterload). We hypothesized that, in the cardiovascular response to passive HUT, where, in contrast to active standing, little BP dip is observed, features affecting the preload play a key role. We developed a baroreflex model combined with a lumped-parameter model of the circulation, including viscoelastic stress-relaxation of the systemic veins. Cardiac contraction is modeled using the varying-elastance concept. Gravity affects not only the systemic, but also the pulmonary, circulation. In accordance with the experimental results, model simulations do not show a BP dip on HUT; the tilt-back response is also realistic. If it is assumed that venous capacities are steady-state values, the introduction of stress-relaxation initially reduces venous pooling. The resulting time course of venous pooling is comparable to measured impedance changes. When venous pressure-volume dynamics are neglected, rapid (completed within 30 s) venous pooling leads to a drop in BP. The direct effect of gravity on the pulmonary circulation influences the BP response in the first ∼5 s after HUT and tilt back. In conclusion, the initial BP response to HUT is mainly determined by the response of the venous system. The time course of lower body pooling is essential in understanding the response to passive HUT.

Modulation of the control of muscle sympathetic nerve activity during severe orthostatic stress

We tested the hypothesis that arterial baroreflex (ABR)-mediated beat-to-beat control over muscle sympathetic nerve activity (MSNA) is progressively modulated as orthostatic stress increases in humans, but that this control becomes impaired just before the onset of orthostatic syncope. In 17 healthy subjects, the ABR control over MSNA (burst incidence, burst strength and total MSNA) was evaluated by analysing the relationship between beat-to-beat spontaneous variations in diastolic blood pressure (DAP) and MSNA during supine rest (control) and during progressive, stepwise increases in lower body negative pressure (LBNP) that were incremented by -10 mmHg every 5 min until presyncope (nine subjects) or -60 mmHg was reached. (1) The linear relationships between DAP and burst strength and between DAP and total MSNA were shifted progressively upward as LBNP increased until the level at which syncope occurred. The relationship between DAP and burst incidence, however, gradually shifted upward from control only to LBNP = -30 mmHg; there was no further upward shift at higher LBNPs. (2) Although the slope of the relationship between DAP and burst strength and between DAP and total MSNA remained constant at all LBNPs tested, except at the level where syncope occurred, the slope of the relationship between DAP and burst incidence was reduced at LBNPs of -40 mmHg and higher (versus control). (3) In syncopal subjects, the slopes of the relationships between DAP and burst incidence, burst strength, and total MSNA were all substantially reduced during the 1-2 min period prior to the onset of syncope. Taken together, these results suggest baroreflex control over MSNA is progressively modulated as orthostatic stress increases, so that its sensitivity is substantially reduced during the period immediately preceding the severe hypotension associated with orthostatic syncope.

Time course analysis of baroreflex sensitivity during postural stress

Postural stress requires immediate autonomic nervous action to maintain blood pressure. We determined time-domain cardiac baroreflex sensitivity (BRS) and time delay (tau) between systolic blood pressure and interbeat interval variations during stepwise changes in the angle of vertical body axis (alpha). The assumption was that with increasing postural stress, BRS becomes attenuated, accompanied by a shift in tau toward higher values. In 10 healthy young volunteers, alpha included 20 degrees head-down tilt (-20 degrees), supine (0 degree), 30 and 70 degrees head-up tilt (30 degrees, 70 degrees), and free standing (90 degrees). Noninvasive blood pressures were analyzed over 6-min periods before and after each change in alpha. The BRS was determined by frequency-domain analysis and with xBRS, a cross-correlation time-domain method. On average, between 28 (-20 degrees) to 45 (90 degrees) xBRS estimates per minute became available. Following a change in alpha, xBRS reached a different mean level in the first minute in 78% of the cases and in 93% after 6 min. With increasing alpha, BRS decreased: BRS = -10.1.sin(alpha) + 18.7 (r(2) = 0.99) with tight correlation between xBRS and cross-spectral gain (r(2) approximately 0.97). Delay tau shifted toward higher values. In conclusion, in healthy subjects the sensitivity of the cardiac baroreflex obtained from time domain decreases linearly with sin(alpha), and the start of baroreflex adaptation to a physiological perturbation like postural stress occurs rapidly. The decreases of BRS and reduction of short tau may be the result of reduced vagal activity with increasing alpha.

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.

Probing the arterial baroreflex: is there a ‘spontaneous’ baroreflex?

The arterial baroreflex is important for beat-to-beat arterial pressure control and its sensitivity has predictive value for clinical outcomes in a myriad of cardiovascular conditions. Given this, researchers have sought approaches for baroreflex assessment that are not invasive and easily obtained. These techniques have exploited the beat-by-beat parallel changes in arterial pressure and heart period to produce estimates that have been termed 'spontaneous' baroreflex indices. The two most commonly used analyses--frequency domain or spectral analyses and sequence analysis have been evaluated in both animals and humans. The animal data suggests an important baroreflex role in linking spontaneous heart period and pressure variabilities, but do not resolve the extent to which these fluctuations reflect baroreflex gain. The human data suggest a high correlation between spontaneous indices and pharmacologically derived baroreflex gain, but also indicate a poor correspondence between them. This may be due to the fact that short-term fluctuations in RR interval are not intimately and always linked to those in pressure via the baroreflex and thus simple observation of arterial pressure and heart period alone may not reveal the extent of arterial baroreflex involvement. If baroreflex function is to be assessed with the fewest and safest assumptions, the input to the system should be driven externally to create large and apparent responses. Nonetheless, spontaneous baroreflex indices may have predictive power; although it remains unknown whether spontaneous indices provide predictive power beyond that provided by heart rate variability indices alone.

Cardiopulmonary baroreflex is reset during dynamic exercise

The purpose of this study was to examine the hypothesis that the operating point of the cardiopulmonary baroreflex resets to the higher cardiac filling pressure of exercise associated with the increased cardiac filling volumes. Eight men (age 26 ± 1 yr; height 180 ± 3 cm; weight 86 ± 6 kg; means ± SE) participated in the present study. Lower body negative pressure (LBNP) was applied at 8 and 16 Torr to decrease central venous pressure (CVP) at rest and during steady-state leg cycling at 50% peak oxygen uptake (104 ± 20 W). Subsequently, two discrete infusions of 25% human serum albumin solution were administered until CVP was increased by 1.8 ± 0.6 and 2.4 ± 0.4 mmHg at rest and 2.9 ± 0.9 and 4.6 ± 0.9 mmHg during exercise. During all protocols, heart rate, arterial blood pressure, and CVP were recorded continuously. At each stage of LBNP or albumin infusion, forearm blood flow and cardiac output were measured. During exercise, forearm vascular conductance increased from 7.5 ± 0.5 to 8.7 ± 0.6 U ( P = 0.024) and total systemic vascular conductance from 7.2 ± 0.2 to 13.5 ± 0.9 l·min−1·mmHg−1 ( P < 0.001). However, there was no significant difference in the responses of both forearm vascular conductance and total systemic vascular conductance to LBNP and the infusion of albumin between rest and exercise. These data indicate that the cardiopulmonary baroreflex had been reset during exercise to the new operating point associated with the exercise-induced change in cardiac filling volume.

Cerebral blood flow during orthostasis: role of arterial CO2

Reductions in end-tidal Pco(2) (Pet(CO(2))) during upright posture have been suggested to be the result of hyperventilation and the cause of decreases in cerebral blood flow (CBF). The goal of this study was to determine whether decreases in Pet(CO(2)) reflected decreases in arterial Pco(2) (Pa(CO(2))) and their relation to increases in alveolar ventilation (Va) and decreases in CBF. Fifteen healthy subjects (10 women and 5 men) were subjected to a 10-min head-up tilt (HUT) protocol. Pa(CO(2)), Va, and cerebral flow velocity (CFV) in the middle and anterior cerebral arteries were examined. In 12 subjects who completed the protocol, reductions in Pet(CO(2)) and Pa(CO(2)) (-1.7 +/- 0.5 and -1.1 +/- 0.4 mmHg, P < 0.05) during minute 1 of HUT were associated with a significant increase in Va (+0.7 +/- 0.3 l/min, P < 0.05). However, further decreases in Pa(CO(2)) (-0.5 +/- 0.5 mmHg, P < 0.05), from minute 1 to the last minute of HUT, occurred even though Va did not change significantly (-0.2 +/- 0.3 l/min, P = not significant). Similarly, CFV in the middle and anterior cerebral arteries decreased (-7 +/- 2 and -8 +/- 2%, P < 0.05) from minute 1 to the last minute of HUT, despite minimal changes in Pa(CO(2)). These data suggest that decreases in Pet(CO(2)) and Pa(CO(2)) during upright posture are not solely due to increased Va but could be due to ventilation-perfusion mismatch or a redistribution of CO(2) stores. Furthermore, the reduction in Pa(CO(2)) did not fully explain the decrease in CFV throughout HUT. These data suggest that factors in addition to a reduction in Pa(CO(2)) play a role in the CBF response to orthostatic stress.

Effect of pimobendan on cardiopulmonary baroreflex control of sympathetic nerve activity in healthy young men

In order to determine the effect of pimobendan on sympathetic nerve activity and cardiopulmonary baroreflex (CPB), electrocardiogram, direct arterial pressure, central venous pressure (CVP) and cardiac output were recorded along with muscle sympathetic nerve activity (MSNA) in 8 healthy young men. CPB function was evaluated before and 60 min after oral administration of 5 mg pimobendan using the response of MSNA to lower body negative pressure (LBNP) of -5 and -10 mm Hg. The same protocol also was performed during handgrip exercise. Cardiac index, MSNA increased and CVP decreased significantly (p<0.01, respectively), but arterial pressure and heart rate unchanged after pimobendan administration. During LBNP, CVP decreased and MSNA increased significantly. CPB sensitivity was augmented from 5.53+/-0.75 to 8.59+/-0.78 burst incidence/mm Hg after pimobendan administration (p<0.01). Pimobendan did not alter the percentage increase of MSNA during handgrip exercise. In conclusion, pimobendan induces an increase in basal sympathetic nerve activity by decreasing CVP and augmenting CPB sensitivity without changing arterial pressure in healthy young men.

Cortical regions associated with autonomic cardiovascular regulation during lower body negative pressure in humans

The purpose of the present study was to determine the cortical structures involved with integrated baroreceptor-mediated modulation of autonomic cardiovascular function in conscious humans independent of changes in arterial blood pressure. We assessed the brain regions associated with lower body negative pressure (LBNP)-induced baroreflex control using functional magnetic resonance imaging with blood oxygen level-dependent (BOLD) contrast in eight healthy male volunteer subjects. The levels of LBNP administered were 5, 15 and 35 mmHg. Heart rate (HR; representing the cardiovascular response) and LBNP (representing the baroreceptor activation level) were simultaneously monitored during the scanning period. In addition, estimated central venous pressure (CVP), arterial blood pressure (ABP) and muscle sympathetic nerve activity were recorded on a separate session. Random effects analyses (SPM2) were used to evaluate significant (P < 0.05) BOLD signal changes that correlated separately with both LBNP and HR (15- and 35-mmHg versus 5-mmHg LBNP). Compared to baseline, steady-state LBNP at 15 and 35 mmHg decreased CVP (from 7 +/- 1 to 5 +/- 1 and 4 +/- 1 mmHg, respectively) and increased MSNA (from 12 +/- 1 to 23 +/- 3 and 36 +/- 4 bursts min(-1), respectively, both P < 0.05 versus baseline). Furthermore, steady-state LBNP elevated HR from 54 +/- 2 beats min(-1) at baseline to 64 +/- 2 beats min(-1) at 35-mmHg suction. Both mean arterial and pulse pressure were not different between rest and any level of LBNP. Cortical regions demonstrating increased activity that correlated with higher HR and greater LBNP included the right superior posterior insula, frontoparietal cortex and the left cerebellum. Conversely, using the identical statistical paradigm, bilateral anterior insular cortices, the right anterior cingulate, orbitofrontal cortex, amygdala, midbrain and mediodorsal nucleus of the thalamus showed decreased neural activation. These data corroborate previous investigations highlighting the involved roles of the insula, anterior cingulate cortex and amygdala in central autonomic cardiovascular control. In addition, we have provided the first evidence for the identification of the cortical network involved specifically with baroreflex-mediated autonomic cardiovascular function in conscious humans.

Balance between cardiac output and sympathetic nerve activity in resting humans: role in arterial pressure regulation

Large, reproducible interindividual differences exist in resting sympathetic nerve activity among normotensive humans with similar arterial pressures, resulting in a lack of correlation between muscle sympathetic nerve activity (MSNA) and arterial pressure among individuals. Although it is known that the arterial pressure is the main short-term determinant of MSNA in humans via the arterial baroreflex, the lack of correlation among individuals suggests that the level of arterial pressure is not the only important input in regulation of MSNA in humans. We studied the relationship between cardiac output (CO) and baroreflex control of sympathetic activity by measuring MSNA (peroneal microneurography), arterial pressure (arterial catheter), CO (acetylene uptake technique) and heart rate (HR; electrocardiogram) in 17 healthy young men during 20 min of supine rest. Across individuals, MSNA did not correlate with mean or diastolic blood pressure (r<0.01 for both), but displayed a significant negative correlation with CO (r=-0.71, P=0.001). To assess whether CO is related to arterial baroreflex control of MSNA, we constructed a baroreflex threshold diagram for each individual by plotting the percentage occurrence of a sympathetic burst against diastolic pressure. The mid-point of the diagram (T50) at which 50% of cardiac cycles are associated with bursts, was inversely related to CO (r=-0.75, P<0.001) and stroke volume (SV) (r=-0.57, P=0.015). We conclude that dynamic inputs from CO and SV are important in regulation of baroreflex control of MSNA in healthy, normotensive humans. This results in a balance between CO and sympathetically mediated vasoconstriction that may contribute importantly to normal regulation of arterial pressure in humans.

Carotid distensibility, baroreflex sensitivity, and orthostatic stress

In this study, we tested the hypothesis that carotid arteries undergo rapid changes in distensibility on moving from the supine to head-up tilt (HUT) postures and, subsequently, that this change in carotid distensibility (cDa) might be associated with concurrent reductions in cardiovagal baroreflex sensitivity (BRS). Thus the effect of posture on carotid vascular mechanics and cardiovagal BRS with consideration for altered central hemodynamics (i.e., stroke volume; Doppler ultrasound) was examined. Carotid pulse pressure (cPP; Millar transducer) and contralateral B-mode ultrasound images were assessed at the carotid artery during supine and 60° HUT postures. From these measures, cDa was calculated at 5-mmHg pressure increments experienced during the cardiac cycle ( n = 6). cPP ( n = 9) was not different in the two postures. A smaller stroke volume being ejected into a smaller carotid artery in HUT explained the maintenance of cPP in HUT. Also, compared with supine, cDa was reset to a lower level in HUT (main effect of posture; P < 0.05). Cardiovagal BRS (sequence method) was diminished in HUT vs. supine ( P < 0.05). A positive correlation was observed between the tilt-induced changes in maximal cDa (in early systole) and cardiovagal BRS ( r2 = 0.75; P < 0.05), but there was little predictive relationship between changes in cPP, systolic vessel dimensions, or average cDa and the corresponding change in BRS. The present results indicate that HUT elicits rapid changes in carotid artery mechanics and further suggest that reductions in the maximal cDa measured in early systole contribute to reduced cardiovagal BRS with HUT.

Resetting of the arterial baroreflex increases orthostatic sympathetic activation and prevents postural hypotension in rabbits

Since humans are under ceaseless orthostatic stress, the mechanism to maintain arterial pressure (AP) under orthostatic stress against gravitational fluid shift is of great importance. We hypothesized that (1) orthostatic stress resets the arterial baroreflex control of sympathetic nerve activity (SNA) to a higher SNA, and (2) resetting of the arterial baroreflex contributes to preventing postural hypotension. Renal SNA and AP were recorded in eight anaesthetized, vagotomized and aortic-denervated rabbits. Isolated intracarotid sinus pressure (CSP) was increased stepwise from 40 to 160 mmHg with increments of 20 mmHg (60 s for each CSP level) while the animal was placed supine and at 60 deg upright tilt. Upright tilt shifted the CSP-SNA relationship (the baroreflex neural arc) to a higher SNA, shifted the SNA-AP relationship (the baroreflex peripheral arc) to a lower AP, and consequently moved the operating point to marked high SNA while maintaining AP. A simulation study suggests that resetting in the neural arc would double the orthostatic activation of SNA and increase the operating AP in upright tilt by 10 mmHg, compared with the absence of resetting. In addition, upright tilt did not change the CSP-AP relationship (the baroreflex total arc). A simulation study suggests that although a downward shift of the peripheral arc could shift the total arc downward, resetting in the neural arc would compensate this fall and prevent the total arc from shifting downward to a lower AP. In conclusion, upright tilt increases SNA by resetting the baroreflex neural arc. This resetting may compensate for the reduced pressor responses to SNA in the peripheral cardiovascular system and contribute to preventing postural hypotension.

Modulation of control of muscle sympathetic nerve activity during orthostatic stress in humans

We tested the hypothesis that orthostatic stress would modulate the arterial baroreflex (ABR)-mediated beat-by-beat control of muscle sympathetic nerve activity (MSNA) in humans. In 12 healthy subjects, ABR control of MSNA (burst incidence, burst strength, and total activity) was evaluated by analysis of the relation between beat-by-beat spontaneous variations in diastolic blood pressure (DAP) and MSNA during supine rest (CON) and at two levels of lower body negative pressure (LBNP: -15 and -35 mmHg). At -15 mmHg LBNP, the relation between burst incidence (bursts per 100 heartbeats) and DAP showed an upward shift from that observed during CON, but the further shift seen at -35 mmHg LBNP was only marginal. The relation between burst strength and DAP was shifted upward at -15 mmHg LBNP (vs. CON) and further shifted upward at -35 mmHg LBNP. At -15 mmHg LBNP, the relation between total activity and DAP was shifted upward from that obtained during CON and further shifted upward at -35 mmHg LBNP. These results suggest that ABR control of MSNA is modulated during orthostatic stress and that the modulation is different between a mild (nonhypotensive) and a moderate (hypotensive) level of orthostatic stress.

Test of dynamic closed-loop baroreflex and autoregulatory control of total peripheral resistance in intact and conscious sheep

This is the first study able to examine and delineate the actual actions of the physiological mechanisms responsible for the dynamic couplings between cardiac output (CO), arterial pressure (Pa), right atrial pressure (PRA), and total peripheral resistance (TPR) in an individual subject without altering the underlying regulatory mechanisms. Eight conscious male sheep were used, where both types of baroreceptors were independently exposed to simultaneous beat-to-beat pressure perturbations under intact closed-loop conditions while CO, Pa, PRA, and TPR were measured. We applied the cardiovascular system identification method proposed in a companion paper ( 4 ) to quantitatively characterize the dynamic closed-loop transfer relations CO→Pa, PRA→Pa, Pa→TPR, and PRA→TPR from the measured signals. To validate the dynamic properties of the estimated transfer relations, the essential parts of the linear dynamics of the model were independently and comprehensively evaluated via error model cross-validation, and the overall model's steady-state behavior was compared with a separate random effects regression approach. In addition to numerous physiological findings, we found that the cardiovascular system identification results were exceptionally consistent with the analytically derived solutions previously discussed in Ref. 4 . In conclusion, this study presents the first time validation of a cardiovascular system identification method by means of experimentally acquired animal data in the intact and conscious animal and offers a set of powerful quantitative tools essential to advancing our knowledge of cardiovascular regulatory physiology.

Lower body negative pressure as a model to study progression to acute hemorrhagic shock in humans

Hemorrhage is a leading cause of death in both civilian and battlefield trauma. Survival rates increase when victims requiring immediate intervention are correctly identified in a mass-casualty situation, but methods of prioritizing casualties based on current triage algorithms are severely limited. Development of effective procedures to predict the magnitude of hemorrhage and the likelihood for progression to hemorrhagic shock must necessarily be based on carefully controlled human experimentation, but controlled study of severe hemorrhage in humans is not possible. It may be possible to simulate hemorrhage, as many of the physiological compensations to acute hemorrhage can be mimicked in the laboratory by applying negative pressure to the lower extremities. Lower body negative pressure (LBNP) sequesters blood from the thorax into dependent regions of the pelvis and legs, effectively decreasing central blood volume in a similar fashion as acute hemorrhage. In this review, we compare physiological responses to hemorrhage and LBNP with particular emphasis on cardiovascular compensations that both share in common. Through evaluation of animal and human data, we present evidence that supports the hypothesis that LBNP, and resulting volume sequestration, is an effective technique to study physiological responses and mechanisms associated with acute hemorrhage in humans. Such experiments could lead to clinical algorithms that identify bleeding victims who will likely progress to hemorrhagic shock and require lifesaving intervention(s).

Altered profile of baroreflex and autonomic responses to lower body negative pressure in chronic orthostatic intolerance

BACKGROUND

Chronic orthostatic intolerance (COI) is a common and disabling autonomic syndrome of unclear pathophysiology. We tested the hypothesis that baroreflex and autonomic responses to graded lower body suction (LBNP, up to -40 mmHg) could be altered in COI patients.

METHODS

Electrocardiogram (ECG), non-invasive arterial blood pressure and respiratory activity were measured during progressive LBNP (seven patients and seven volunteers). Lumped arterial baroreflex sensitivity (alpha index), and its arterial and cardiopulmonary components, were assessed by multivariate closed-loop analysis of RR interval and systolic arterial pressure (SAP) spontaneous variabilities and respiration. Monovariate spectral analysis of RR interval and SAP variability provided markers of autonomic regulation of the sinoatrial (SA) node and of vascular sympathetic modulation.

RESULTS

Similar reductions in overall and cardiopulmonary baroreflex gain were observed in both groups in response to graded LBNP. In contrast, only controls demonstrated a selective increase in arterial baroreflex sensitivity, at low-grade LBNP. Clear increases in the low-frequency component of RR interval variability (LFRR) [and decreases in the high-frequency component of RR interval variability (HFRR), both in normalized units] were observed in controls with graded LBNP, while insignificant changes occurred in COI patients, who showed, conversely, exaggerated sympathetic vasomotor responses [as assessed by the low frequency component of SAP variability (LFSAP)].

CONCLUSIONS

Patients with chronic orthostatic intolerance show distinct signs of altered baroreflex and autonomic regulation of the SA node and of the vasculature in response to graded LBNP.

Sublingual Nitroglycerin Used in Routine Tilt Testing Provokes a Cardiac Output-Mediated Vasovagal Response

OBJECTIVES

We set out to determine the effect of sublingual nitroglycerin (NTG), as used during routine tilt testing in patients with unexplained syncope, on hemodynamic characteristics and baroreflex control of heart rate (HR) and systemic vascular resistance (SVR).

BACKGROUND

Nitroglycerin is used in tilt testing to elicit a vasovagal response. It is known to induce venous dilation and enhance pooling. Also, NTG is lipophilic and readily passes cell membranes, and animal studies suggest a sympatho-inhibitory effect of NTG on circulatory control.

METHODS

Routine tilt testing was conducted in 39 patients with suspected vasovagal syncope (age 36 +/- 16 years, 18 females). Patients were otherwise healthy and free of medication. Before a loss of consciousness set in, oncoming syncope was cut short by tilt-back or counter-maneuvers. Finger arterial pressure was monitored continuously (Finapres). Left ventricular stroke volume (SV) was computed from the pressure pulsations (Modelflow). Spontaneous baroreflex control of HR was estimated in the time and frequency domains.

RESULTS

During tilt testing, 22 patients developed presyncope. After NTG administration but before presyncope, SV and cardiac output (CO) decreased (p < 0.001), whereas SVR and HR increased (p < 0.001) in all patients. Arterial pressure was initially maintained. Baroreflex sensitivity decreased after NTG. On Cox regression analysis, the occurrence of a vasovagal response was related to a drop in SV after NTG (hazard ratio 0.86, p = 0.005).

CONCLUSIONS

The cardiovascular response to NTG is similar in vasovagal and non-vasovagal patients, but more pronounced in those with tilt-positive results. The NTG-facilitated presyncope appears to be CO-mediated, and there is no evidence of NTG-induced sympathetic inhibition.

Heat stress modifies human baroreflex function independently of heat-induced hypovolemia

Since human thermoregulatory heat loss responses, cutaneous vasodilation and sweating, cause hypovolemia, they should resultantly stimulate human baroreflexes. However, it is possible that the thermoregulatory system directly interacts with the baroreflex system through central neural connections independently of the heat-induced hypovolemia. We hypothesized that heat stress modifies the baroreflex control of sympathetic nerve activity independently of heat-induced hypovolemia in humans. We made whole-body heating with tube-lined suits perfused with warm water (46-47 degrees C) on 10 healthy male subjects. The heating increased skin and tympanic temperatures by 10.0 and 0.4 degrees C, respectively. It increased resting total muscle sympathetic nerve activity (MSNA, microneurography) by 94 +/- 9% and decreased central venous pressure (CVP, dependent arm technique) by 2.6 +/- 0.9 mmHg. The heating increased arterial baroreflex gain by 193%, assessed as a response of MSNA to a decrease in diastolic arterial pressure during Valsalva's maneuver, but it did not change threshold arterial pressure for MSNA activation. Although the heating did not change the cardiopulmonary baroreflex gain assessed as a response of MSNA to a change in estimated central venous pressure (CVP) during a 10 degrees head-down and -up tilt test, it upwardly shifted the stimulus-response baroreflex relationship. These changes in baroreflex functions during heating were not restored by an intravenous infusion of warmed isotonic saline (37 degrees C, 15 ml/kg) that restored the heat-induced reduction of CVP. Our results support our hypothesis that heat stress modifies the baroreflex control of MSNA independently of heat-induced hypovolemia in humans. Our results also suggest that the hyperthermal modification of baroreflex results from central neural interaction between thermoregulatory and baroreflex systems.

Influences of hydration on post-exercise cardiovascular control in humans

Dehydration is known to decrease orthostatic tolerance and cause tachycardia, but little is known about the cardiovascular control mechanisms involved. To test the hypothesis that arterial baroreflex sensitivity increases during exercise-induced dehydration, we assessed arterial baroreflex responsiveness in 13 healthy subjects (protocol 1) at baseline (PRE-EX) and 1 h after (EX-DEH) 90 min of exercise to cause dehydration, and after subsequent intravenous rehydration with saline (EX-REH). Six of these subjects were studied a second time (protocol 2) with intravenous saline during exercise to prevent dehydration. We measured heart rate, central venous pressure and arterial pressure during all trials, and muscle sympathetic nerve activity (MSNA) during the post-exercise trials. Baroreflex responses were assessed using sequential boluses of nitroprusside and phenylephrine (modified Oxford technique). After exercise in protocol 1 (EX-DEH), resting blood pressure was decreased and resting heart rate was increased. Cardiac baroreflex gain, assessed as the responsiveness of heart rate or R-R interval to changes in systolic pressure, was diminished in the EX-DEH condition (9.17 +/- 1.06 ms mmHg-1 vs. PRE-EX: 18.68 +/- 2.22 ms mmHg-1, P < 0.05). Saline infusion after exercise did not alter the increase in HR post-exercise or the decrease in baroreflex gain (EX-REH: 10.20 +/- 1.43 ms mmHg-1; P > 0.10 vs. EX-DEH). Saline infusion during exercise (protocol 2) resulted in less of a post-exercise decrease in blood pressure and a smaller change in cardiac baroreflex sensitivity. Saline infusion caused a decrease in MSNA in protocol 1. We conclude that exercise-induced dehydration causes post-exercise changes in the baroreflex control of blood pressure that may contribute to, rather than offset, orthostatic intolerance.

Influence of increased central venous pressure on baroreflex control of sympathetic activity in humans

Volume expansion often ameliorates symptoms of orthostatic intolerance; however, the influence of this increased volume on integrated baroreflex control of vascular sympathetic activity is unknown. We tested whether acute increases in central venous pressure (CVP) diminished subsequent responsiveness of muscle sympathetic nerve activity (MSNA) to rapid changes in arterial pressure. We studied healthy humans under three separate conditions: control, acute 10 degrees head-down tilt (HDT), and saline infusion (SAL). In each condition, heart rate, arterial pressure, CVP, and peroneal MSNA were measured during 5 min of rest and then during rapid changes in arterial pressure induced by sequential boluses of nitroprusside and phenylephrine (modified Oxford technique). Sensitivities of integrated baroreflex control of MSNA and heart rate were assessed as the slopes of the linear portions of the MSNA-diastolic blood pressure and R-R interval-systolic pressure relations, respectively. CVP increased approximately 2 mmHg in both SAL and HDT conditions. Resting heart rate and mean arterial pressure were not different among trials. Sensitivity of baroreflex control of MSNA was decreased in both SAL and HDT condition, respectively: -3.1 +/- 0.6 and -3.3 +/- 1.0 versus -5.0 +/- 0.6 units.beat(-1).mmHg(-1) (P < 0.05 for SAL and HDT vs. control). Sensitivity of baroreflex control of the heart was not different among conditions. Our results indicate that small increases in CVP decrease the sensitivity of integrated baroreflex control of sympathetic nerve activity in healthy humans.

Modulation of arterial baroreflex dynamic response during mild orthostatic stress in humans

We tested the hypothesis that in humans, carotid-baroreflex dynamic responses (evaluated by examining the time course of the carotid-baroreflex-induced alterations in muscle sympathetic nerve activity (MSNA), mean arterial blood pressure (MAP) and heart rate (HR)) would be altered during mild orthostatic stress in ways that serve to limit orthostatic hypotension. In 12 healthy subjects (10 male, 2 female), 5-s periods of neck pressure (NP) (50 mmHg) and neck suction (NS) (-60 mmHg) were used to evaluate carotid baroreflex function at rest (CON) and during lower body negative pressure (LBNP) (-15 mmHg). During LBNP (as compared with CON) (a) the augmentations in MSNA and MAP elicited by NP were greater, (b) the NS-induced period of MSNA suppression was, if anything, shorter, (c) the peak decrement in MAP elicited by NS, although not different in amplitude, occurred earlier and recovered to its initial level more quickly after NS, and (d) the HR responses to NP and NS were greater. These results suggest that during mild orthostatic stress, carotid-baroreflex dynamic responses are modulated in ways that should help maintain blood pressure and limit orthostatic hypotension.

Muscle sympathetic nerve activity during lower body negative pressure is accentuated in heat-stressed humans

The purpose of this project was to test the hypothesis that increases in muscle sympathetic nerve activity (MSNA) during an orthostatic challenge is attenuated in heat-stressed individuals. To accomplish this objective, MSNA was measured during graded lower body negative pressure (LBNP) in nine subjects under normothermic and heat-stressed conditions. Progressive LBNP was applied at -3, -6, -9, -12, -15, -18, -21, and -40 mmHg for 2 min per stage. Whole body heating caused significant increases in sublingual temperature, skin blood flow, sweat rate, heart rate, and MSNA (all P < 0.05) but not in mean arterial blood pressure (P > 0.05). Progressive LBNP induced significant increases in MSNA in both thermal conditions. However, during the heat stress trial, increases in MSNA at LBNP levels higher than -9 mmHg were greater compared with during the same LBNP levels in normothermia (all P < 0.05). These data suggest that the increase in MSNA to orthostatic stress is not attenuated but rather accentuated in heat-stressed humans.

Human autonomic responses to blood donation

In order to characterize autonomic responses to acute volume loss, supine ECG, blood pressure (BP) and uncalibrated breathing signal (UBS) recordings were taken before and after blood donation in 48 healthy volunteers. Time and frequency domain parameters of RR interval (RRI), BP and UBS variability were determined. Baroreflex gain was calculated by the technique of the spontaneous sequences and cross-spectral analysis. The systolic (SAP), diastolic (DAP) and mean BP (MAP) increased after the blood withdrawal. The central frequency of breathing and mean heart rate did not change. RRI variability increased in low frequency band (LF), tended to decrease in high frequency band (HF). Systolic BP variability increased in both frequency bands, but was statistically significant only in the high frequency band. Diastolic BP power increased in both frequencies. From the different baroreflex gain estimates, up sequence BRS and HF alpha index decreased significantly. The phase angle between RRI and systolic blood pressure powers in LF band did not change (-58 +/- 24 degrees and -54 +/- 26 degrees ). In the high frequency range, the phase became more negative (-1 +/- 29 degrees and -17 +/- 32 degrees, p = 0.001). The withdrawal of 350-400 ml blood in 5 min resulted in sympathetic activation, which was reflected in increased systolic, diastolic and mean BP. The increased BP oscillation was a sensitive marker of the minor volume depletion. This was coupled by increased RRI oscillation via baroreflex mechanisms in the LF band. Changes in the RRI and BP oscillations in the HF band showed no similar coupling. That points to the fact that RRI oscillations in this band should not be explained entirely by baroreflex mechanisms. Vagal withdrawal was reflected in decreased root mean square of successive differences (RMSSD), decreased HF RRI power and decreased up sequence BRS.

Beat-to-beat noninvasive stroke volume from arterial pressure and Doppler ultrasound

The proper understanding of the cardiovascular mechanisms involved in complaints of short-lasting dizziness and the evaluation of unexplained recurrent syncope requires continuous monitoring of cardiac stroke volume (SV) in addition to blood pressure and heart rate. The primary aim of the present study was to evaluate a pulse wave analysis method that calculates beat-to-beat flow from non-invasive arterial pressure by simulating a non-linear, time-varying model of human aortic input impedance (Modelflow; MF), by comparing MF stroke volume (SV(MF)) to Doppler ultrasound (US) flow velocity SV (SV(US)). A second purpose was to compare the two methods under two different conditions: the supine and head-up tilt (30 degrees ) position. SV(US) and SV(MF) with non-invasive arterial pressure (Finapres) as input to the aortic model were measured beat-to-beat during spontaneous supine breathing and in the passive 30 degrees head-up tilt (HUT30) position in six normotensive healthy humans [three females, mean age 24 (21-26) years]. There were variations in supine SV track between the two methods with zero difference and a SD of the beat-to-beat difference (MF-US) of 4.2%. HUT30 induced a systematic difference of 10.5% and an increase in SD to 6.9%, which was reproducible. Beat-to-beat changes in SV in the supine resting condition were equally well assessed by both methods. Systematic differences appear during HUT30 and show opposite signs. The difference between the two methods upon a change in body position may be attributed to limitations in each method.

Carotid baroreflex responsiveness to head-up tilt-induced central hypovolaemia: effect of aerobic fitness

This investigation examined the interaction between carotid baroreflex (CBR) responsiveness during head-up tilt (HUT)-induced central hypovolaemia and aerobic fitness. Seven average fit (AF) individuals, with a mean maximal oxygen uptake (VO2max) of 49 +/- 1 (ml O2) kg-1 min-1, and seven high fit (HF) individuals, with a VO2max of 61 +/- 1 (ml O2) kg-1 min-1, voluntarily participated in the investigation. After 10-15 min supine, each subject was exposed to nine levels of progressively increasing HUT by 10 deg increments from -20 deg to +60 deg. During the final 3 min of each stage of HUT, the CBR responsiveness was measured using a rapid pulse (500 ms) train of neck pressure (NP) and neck suction (NS) ranging from +40 to -80 Torr. The maximal gain of the carotid-HR (Gmax-HR) and carotid-MAP (Gmax-MAP) baroreflex function curves was identified as measures of CBR responsiveness. During HUT-induced decreases in thoracic admittance, an index of central blood volume (CBV), the Gmax-HR and Gmax-MAP of the AF subjects increased more than the Gmax-HR and Gmax-MAP of the HF subjects (P < 0.05). The data demonstrate that the increase in the CBR responsiveness during a tilt-induced progressive unloading of the cardiopulmonary baroreceptors was attenuated in endurance-trained subjects. These findings provide an explanation for the predisposition to orthostatic hypotension and intolerance in well-trained athletes.

Heart rate control and mechanical cardiopulmonary coupling to assess central volume: a systems analysis

Small negative changes of central volume reduce cardiac output without significant alterations of arterial blood pressure (ABP), suggesting an adequate regulatory response. Furthermore, evidence has arisen supporting a Bainbridge reflex (tachycardia with hypervolemia) in humans. To investigate these phenomena, multivariate autoregressive techniques were used to evaluate the beat-to-beat interactions between respiration, R-R interval, and ABP at six levels of decreased and increased central volume. With reductions of central volume below control, baroreflex and respiratory sinus arrhythmia gains were reduced, while with increases of volume above control, gains increased for the first two levels but decreased again at the highest volume level, suggesting the presence of a Bainbridge reflex in healthy human subjects. The mechanical influence of respiration on central venous pressure (CVP) had an unexpected shift in phase at the point of mild central hypervolemia, with the expected negative relation at lower volumes (inspiration lowers CVP) but a positive relation at higher volumes (inspiration raises CVP). We conclude that multivariate techniques can quantify the relations between a variety of respiratory and hemodynamic parameters, allowing for the in vivo assessment of complex cardiorespiratory interactions during manipulations of central volume. The results identify the presence of a Bainbridge reflex in humans and suggest that short-term cardiovascular control is optimized at mild hypervolemia.

Influence of microgravity on astronauts' sympathetic and vagal responses to Valsalva's manoeuvre

When astronauts return to Earth and stand, their heart rates may speed inordinately, their blood pressures may fall, and some may experience frank syncope. We studied brief autonomic and haemodynamic transients provoked by graded Valsalva manoeuvres in astronauts on Earth and in space, and tested the hypothesis that exposure to microgravity impairs sympathetic as well as vagal baroreflex responses. We recorded the electrocardiogram, finger photoplethysmographic arterial pressure, respiration and peroneal nerve muscle sympathetic activity in four healthy male astronauts (aged 38-44 years) before, during and after the 16 day Neurolab space shuttle mission. Astronauts performed two 15 s Valsalva manoeuvres at each pressure, 15 and 30 mmHg, in random order. Although no astronaut experienced presyncope after the mission, microgravity provoked major changes. For example, the average systolic pressure reduction during 30 mmHg straining was 27 mmHg pre-flight and 49 mmHg in flight. Increases in muscle sympathetic nerve activity during straining were also much greater in space than on Earth. For example, mean normalized sympathetic activity increased 445% during 30 mmHg straining on earth and 792% in space. However, sympathetic baroreflex gain, taken as the integrated sympathetic response divided by the maximum diastolic pressure reduction during straining, was the same in space and on Earth. In contrast, vagal baroreflex gain, particularly during arterial pressure reductions, was diminished in space. This and earlier research suggest that exposure of healthy humans to microgravity augments arterial pressure and sympathetic responses to Valsalva straining and differentially reduces vagal, but not sympathetic baroreflex gain.

Sequential modulation of cardiac autonomic control induced by cardiopulmonary and arterial baroreflex mechanisms

BACKGROUND

Nonhypotensive lower body negative pressure (LBNP) induces a reflex increase in forearm vascular resistance and muscle sympathetic neural discharge without affecting mean heart rate. We tested the hypothesis that a reflex change of the autonomic modulation of heartbeat might arise during low intensity LBNP without changes of mean heart rate.

METHODS AND RESULTS

Ten healthy volunteers underwent plasma catecholamine evaluation and a continuous recording of ECG, finger blood pressure, respiratory activity, and central venous pressure (CVP) during increasing levels of LBNP up to -40 mm Hg. Spectrum and cross-spectrum analyses assessed the changes in the spontaneous variability of R-R interval, respiration, systolic arterial pressure (SAP), and CVP and in the gain (alpha(LF)) of arterial baroreflex control of heart rate. Baroreceptor sensitivity was also evaluated by the SAP/R-R spontaneous sequences technique. LBNP began decreasing significantly: CVP at -10, R-R interval at -20, SAP at -40, and the indexes alpha(LF) and baroreceptor sensitivity at -30 and -20 mm Hg, compared with baseline conditions. Plasma norepinephrine increased significantly at -20 mm Hg. The normalized low-frequency component of R-R variability (LF(R-R)) progressively increased and was significantly higher than in the control condition at -15 mm Hg.

CONCLUSIONS

Nonhypotensive LBNP elicits a reflex increase of cardiac sympathetic modulation, as evaluated by LF(R-R), which precedes the changes in the hemodynamics and in the indexes of arterial baroreflex control.

Renal and neurohormonal responses to increasing levels of lower body negative pressure in men

BACKGROUND

The stimulation of efferent renal sympathetic nerve activity induces sequential changes in renin secretion, sodium excretion, and renal hemodynamics that are proportional to the magnitude of the stimulation of sympathetic nerves. This study in men investigated the sequence of the changes in proximal and distal renal sodium handling, renal and systemic hemodynamics, as well as the hormonal profile occurring during a sustained activation of the sympathetic nervous system induced by various levels of lower body negative pressure (LBNP).

METHODS

Ten healthy subjects were submitted to three levels of LBNP ranging between 0 and -22.5 mm Hg for one hour according to a triple crossover design, with a minimum of five days between each level of LBNP. Systemic and renal hemodynamics, renal water and sodium handling (using the endogenous lithium clearance technique), and the neurohormonal profile were measured before, during, and after LBNP.

RESULTS

LBNP (0 to -22.5 mm Hg) induced an important hormonal response characterized by a significant stimulation of the sympathetic nervous system and gradual activations of the vasopressin and the renin-angiotensin systems. LBNP also gradually reduced water excretion and increased urinary osmolality. A significant decrease in sodium excretion was apparent only at -22.5 mm Hg. It was independent of any change in the glomerular filtration rate and was mediated essentially by an increased sodium reabsorption in the proximal tubule (a significant decrease in lithium clearance, P < 0.05). No significant change in renal hemodynamics was found at the tested levels of LBNP. As observed experimentally, there appeared to be a clear sequence of responses to LBNP, the neurohormonal response occurring before the changes in water and sodium excretion, these latter preceding any change in renal hemodynamics.

CONCLUSIONS

These data show that the renal sodium retention developing during LBNP, and thus sympathetic nervous stimulation, is due mainly to an increase in sodium reabsorption by the proximal segments of the nephron. Our results in humans also confirm that, depending on its magnitude, LBNP leads to a step-by-step activation of neurohormonal, renal tubular, and renal hemodynamic responses.

Differential sympathetic nerve and heart rate spectral effects of nonhypotensive lower body negative pressure

Lower body negative pressure (LBNP; -5 and -15 mmHg) was applied to 14 men (mean age 44 yr) to test the hypothesis that reductions in preload without effect on stroke volume or blood pressure increase selectively muscle sympathetic nerve activity (MSNA), but not the ratio of low- to high-frequency harmonic component of spectral power (P(L)/P(H)), a coarse-graining power spectral estimate of sympathetic heart rate (HR) modulation. LBNP at -5 mmHg lowered central venous pressure and had no effect on stroke volume (Doppler) or systolic blood pressure but reduced vagal HR modulation. This latter finding, a manifestation of arterial baroreceptor unloading, refutes the concept that low levels of LBNP interrogate, selectively, cardiopulmonary reflexes. MSNA increased, whereas P(L)/P(H) and HR were unchanged. This discordance is consistent with selectivity of efferent sympathetic responses to nonhypotensive LBNP and with unloading of tonically active sympathoexcitatory atrial reflexes in some subjects. Hypotensive LBNP (-15 mmHg) increased MSNA and P(L)/P(H), but there was no correlation between these changes within subjects. Therefore, HR variability has limited utility as an estimate of the magnitude of orthostatic changes in sympathetic discharge to muscle.

Beat-to-beat cardiovascular responses to rapid, low-level LBNP in humans

The hypothesis tested was that there are significant transient changes in the cardiovascular variables after rapid onset and release of mild lower body negative pressure (LBNP, -20 mmHg), even in experimental situations where there is no detectable change in steady-state values. Twelve subjects participated in the study. Heart rate, stroke volume (SV), cardiac output, mean arterial pressure (MAP), total peripheral resistance (TPR), acral and nonacral skin blood flow, and blood flow velocity in the brachial artery were continuously recorded during the pre-LBNP period (0-120 s), during LBNP (120-420 s), and during the post-LBNP period (420-600 s). The main finding was that MAP is transiently but strongly affected by rapid changes in LBNP as small as -20 mmHg. There was also a characteristic asymmetry in cardiovascular responses to the onset and release of LBNP, particularly in the responses in SV. The transient changes in MAP indicate that the neural responses that affect TPR are not fast enough to compensate for the rapid changes in LBNP. In this case, the arterial baroreceptors will be activated as well as the low-pressure baroreceptors that sense central venous pressure. This must be taken into consideration in future discussions of the results of LBNP protocols.

Regulation of muscle sympathetic nerve activity after bed rest deconditioning

Cardiovascular deconditioning reduces orthostatic tolerance. To determine whether changes in autonomic function might produce this effect, we developed stimulus-response curves relating limb vascular resistance, muscle sympathetic nerve activity (MSNA), and pulmonary capillary wedge pressure (PCWP) with seven subjects before and after 18 days of -6 degrees head-down bed rest. Both lower body negative pressure (LBNP; -15 and -30 mmHg) and rapid saline infusion (15 and 30 ml/kg body wt) were used to produce a wide variation in PCWP. Orthostatic tolerance was assessed with graded LBNP to presyncope. Bed rest reduced LBNP tolerance from 23.9 +/- 2.1 to 21.2 +/- 1.5 min, respectively (means +/- SE, P = 0.02). The MSNA-PCWP relationship was unchanged after bed rest, though at any stage of the LBNP protocol PCWP was lower, and MSNA was greater. Thus bed rest deconditioning produced hypovolemia, causing a shift in operating point on the stimulus-response curve. The relationship between limb vascular resistance and MSNA was not significantly altered after bed rest. We conclude that bed rest deconditioning does not alter reflex control of MSNA, but may produce orthostatic intolerance through a combination of hypovolemia and cardiac atrophy.

Inconsistent link between low-frequency oscillations: R-R interval responses to augmented Mayer waves

Low-frequency oscillations in arterial blood pressure (Mayer waves) and R-R interval are thought to be linked through the arterial baroreflex. To delve into this relationship, we applied low (10 mmHg) and moderate (30 mmHg) lower body negative pressure (LBNP) in 10-s cycles to 18 healthy young male subjects. They showed no change in average blood pressure with this oscillatory stimulus but did show a significant decrease in R-R interval (P < 0.05) during both levels of LBNP. In addition, we succeeded in augmenting low-frequency blood pressure oscillations in a graded response to oscillatory LBNP level (P < 0.05) while significantly increasing low-frequency R-R interval oscillations (P < 0.05). However, cross-spectral coherence between these increased oscillations was highly variable across individuals and stimulus level. Although nearly all subjects showed significant coherence during basal conditions (n = 17), only seven subjects maintained significant coherence during both levels of LBNP. These results suggest that a complex interaction of regulatory mechanisms determines the link between low-frequency oscillations and the responses to even low levels of LBNP.

Mechanisms of hypotensive effects of a posture change from seated to supine in humans

The hypothesis tested was that the hydrostatic stimulation of carotid baroreceptors is pivotal to decrease mean arterial pressure at heart level during a posture change from seated to supine. In eight males, the cardiovascular responses to a 15-min posture change from seated to supine were compared with those of water immersion to the xiphoid process and to the neck, respectively. Left atrial diameter and cardiac output (rebreathing) increased similarly during the posture change and water immersion to the xiphoid process and further so during neck immersion. Mean arterial pressure decreased by 12 +/- 2 mmHg during the posture change, by 5 +/- 1 mmHg during xiphoid immersion, and was unchanged during neck immersion. Arterial pulse pressure increased by 12 +/- 3 mmHg during the posture change (P < 0.05) and less during xiphoid and neck immersion by 7 +/- 3 mmHg (P < 0.05). Total peripheral vascular resistance decreased similarly during the posture change and neck immersion and slightly less during xiphoid immersion (P < 0.05). In conclusion, the hydrostatic stimulation of carotid baroreceptors combined with some additional increase in arterial pulse pressure, which also stimulates aortic baroreceptors, accounts for more than half of the hypotensive response at heart level to a posture change from seated to supine.

Dynamic regulation of heart rate during acute hypotension: new insight into baroreflex function

To examine the dynamic properties of baroreflex function, we measured beat-to-beat changes in arterial blood pressure (ABP) and heart rate (HR) during acute hypotension induced by thigh cuff deflation in 10 healthy subjects under supine resting conditions and during progressive lower body negative pressure (LBNP). The quantitative, temporal relationship between ABP and HR was fitted by a second-order autoregressive (AR) model. The frequency response was evaluated by transfer function analysis.

RESULTS

HR changes during acute hypotension appear to be controlled by an ABP error signal between baseline and induced hypotension. The quantitative relationship between changes in ABP and HR is characterized by a second-order AR model with a pure time delay of 0.75 s containing low-pass filter properties. During LBNP, the change in HR/change in ABP during induced hypotension significantly decreased, as did the numerator coefficients of the AR model and transfer function gain.

CONCLUSIONS

1) Beat-to-beat HR responses to dynamic changes in ABP may be controlled by an error signal rather than directional changes in pressure, suggesting a "set point" mechanism in short-term ABP control. 2) The quantitative relationship between dynamic changes in ABP and HR can be described by a second-order AR model with a pure time delay. 3) The ability of the baroreflex to evoke a HR response to transient changes in pressure was reduced during LBNP, which was due primarily to a reduction of the static gain of the baroreflex.

Abnormal baroreflex responses in patients with idiopathic orthostatic intolerance

BACKGROUND

Patients diagnosed with idiopathic orthostatic intolerance report symptoms of lightheadedness, fatigue, and nausea accompanied by an exaggerated tachycardia when assuming the upright posture. Often, these symptoms are present in the absence of any decrease in arterial pressure. We hypothesized that patients with idiopathic orthostatic intolerance would have impaired cardiac vagal and integrated baroreflex function, lower blood volume, and increased venous compliance.

METHODS AND RESULTS

Sixteen patients and 14 healthy control subjects underwent the modified Oxford technique to assess cardiac vagal baroreflex sensitivity. Progressive lower-body negative pressure (to -50 mm Hg; LBNP) was used to examine the integrated baroreflex response to progressive hypovolemic stimuli. Blood volume and venous compliance were also assessed. Patients with idiopathic orthostatic intolerance had lower cardiac vagal baroreflex sensitivity (12+/-1 versus 25+/-4 ms/mm Hg; P</=0.01). The integrated baroreflex response to low levels of LBNP was characterized by shorter R-R intervals and more symptoms such as lightheadedness, despite similar levels of blood pressure. There was a trend toward lower blood volume in the patient group (56+/-2 versus 63+/-3 mL/kg; P=0.054).

CONCLUSIONS

Patients with idiopathic orthostatic intolerance have lower cardiac vagal baroreflex sensitivity and marginally lower blood volume and respond with faster heart rates despite similar levels of arterial pressure during LBNP. These findings may contribute to the exaggerated postural tachycardia and symptoms observed in patients with this disorder.

Decreased cardiopulmonary baroreflex sensitivity in Chagas' heart disease

No study has been performed on reflexes originating from receptors in the heart that might be involved in the pathological lesions of Chagas' heart disease. Our study was undertaken to analyze the role of cardiopulmonary reflex on cardiovascular control in Chagas' disease. We studied 14 patients with Chagas' disease without heart failure and 12 healthy matched volunteers. Central venous pressure, arterial blood pressure, heart rate, forearm blood flow, and forearm vascular resistance were recorded during deactivation of cardiopulmonary receptors. By reducing central venous pressure by applying -10 and -15 mm Hg of negative pressure to the lower body, we observed (a) a similar decrease of central venous pressure in both groups; (b) a marked increase in forearm vascular resistance in the control group but a blunted increase in the Chagas' group; and (c) no significant changes in blood pressure and heart rate. To analyze cardiopulmonary and arterial receptors, we applied -40 mm Hg of lower-body negative pressure. As a consequence, (a) central venous pressure decreased similarly in both groups; (b) blood pressure was maintained in the control group, whereas in patients with Chagas' disease, a decrease in systolic and mean arterial pressure occurred; (c) heart rate increased in both groups; and (d) forearm vascular resistance increased significantly and similarly in both groups. Unloading of receptors with low levels of lower-body negative pressure did not increase forearm vascular resistance in patients with Chagas' disease, which suggests that the reflex mediated by cardiopulmonary receptors is impaired in patients with Chagas' disease without heart failure. Overall control of circulation appears to be compromised because patients did not maintain blood pressure under high levels of lower-body negative pressure.

No effect of venoconstrictive thigh cuffs on orthostatic hypotension induced by head-down bed rest

Orthostatic intolerance (OI) is the most serious symptom of cardiovascular deconditioning induced by head-down bed rest or weightlessness. Wearing venoconstrictive thigh cuffs is an empirical countermeasure used by Russian cosmonauts to limit the shift of fluid from the lower part of the body to the cardio-cephalic region. Our aim was to determine whether or not thigh cuffs help to prevent orthostatic hypotension induced by head-down bed rest. We studied the effect of thigh cuffs on eight healthy men. The cuffs were worn during the day for 7 days of head-down bed rest. We measured: orthostatic tolerance (stand tests and lower body negative pressure tests), plasma volume (Evans blue dilution), autonomic influences (plasma noradrenaline) and baroreflex sensitivity (spontaneous baroreflex slope). Thigh cuffs limited the loss of plasma volume (thigh cuffs: -201 +/- 37 mL vs. control: -345 +/- 42 mL, P < 0.05), the degree of tachycardia and reduction in the spontaneous baroreflex sensitivity induced by head-down bed rest. However, the impact of thigh cuffs was not sufficient to prevent OI (thigh cuffs: 7.0 min of standing time vs. control: 7.1 min). Decrease in absolute plasma volume and in baroreflex sensitivity are known to be important factors in the aetiology of OI induced by head-down bed rest. However, dealing with these factors, using thigh cuffs for example, is not sufficient to prevent OI. Other factors such as venous compliance, microcirculatory changes, peripheral arterial vasoconstriction and vestibular afferents must also be considered.

Baroreflex dysfunction induced by microgravity: potential relevance to postflight orthostatic intolerance

Microgravity imposes adaptive changes in the human body. This review focuses on the changes in baroreflex function produced by actual spaceflight, or by experimental models that simulate microgravity, e.g., bed rest. We will analyze separately studies involving baroreflexes arising from carotid sinus and aortic arch afferents ("high-pressure baroreceptors"), and cardiopulmonary afferents ("low-pressure receptors"). Studies from unrelated laboratories using different techniques have concluded that actual or simulated exposure to microgravity reduces baroreflex function arising from carotid sinus afferents ("carotic-cardiac baroreflex"). The techniques used to study the carotid-cardiac baroreflex, using neck suction and compression to simulate changes in blood pressure, have been extensively validated. In contrast, it is more difficult to selectively study aortic arch or cardiopulmonary baroreceptors. Nonetheless, studies that have examined these baroreceptors suggest that microgravity produces the opposite effect, ie, an increase in the gain of aortic arch and cardiopulmonary baroreflexes. Furthermore, most studies have focus on instantaneous changes in heart rate, which almost exclusively examines the vagal limb of the baroreflex. In comparison, there is limited information about the effect of microgravity on sympathetic function. A substantial proportion of subjects exposed to microgravity develop transient orthostatic intolerance. It has been proposed that alterations in baroreflex function play a role in the orthostatic intolerance induced by microgravity. The evidence in favor and against this hypothesis is reviewed.

Baroreflex modulation of peripheral vasoconstriction during progressive hypothermia in anesthetized humans

Mild hypothermia is a major concomitant of surgery under general anesthesia. We examined the hypothesis that baroreceptor loading/unloading modifies thermoregulatory peripheral vasoconstriction and, consequently, body core temperature in subjects undergoing lower abdominal surgery with general anesthesia. Thirty-six patients were divided into four groups: control group (C), applied positive end-expiratory pressure (PEEP; 10 cmH(2)O) group (P), applied leg-up position group (L), and a group of leg-up position patients with PEEP starting 90 min after induction of anesthesia (L + P). The esophageal temperature (T(es)) and the forearm-fingertip temperature gradient, as an index of peripheral vasoconstriction, were monitored for 3 h after induction of anesthesia. Mean arterial pressure and pulse pressure did not change during the study in any group. The change in right atrial transmural pressure from the baseline value was 0.3 +/- 0.1 mmHg in C, -3.0 +/- 0.5 mmHg in P, and 2.3 +/- 0.4 mmHg in L (P < 0.01). The change in T(es) at the end of the study was -1.7 +/- 0.1 (35.1 +/- 0.1) degrees C in C, -1.1 +/- 0.1 (35.7 +/- 0.1) degrees C in P, and -2.7 +/- 0.1 (34.1 +/- 0.1) degrees C in L, showing significant differences (P < 0.01). The T(es) threshold for thermal peripheral vasoconstriction was 35.6 +/- 0.1 degrees C in C, 36.2 +/- 0.2 degrees C in P, and 34.8 +/- 0.2 degrees C in L (P < 0.01). Excessive T(es) decrease in the leg-up-position operation was attenuated by applying PEEP (L + P group; P < 0.05). Our data indicate that baroreceptor loading augments and unloading prevents perioperative hypothermia in anesthetized and paralyzed subjects by reducing and increasing the body temperature threshold for peripheral vasoconstriction, respectively.

Systolic pressure predicts plasma vasopressin responses to hemorrhage and vena caval constriction in dogs

We have proposed that the reflex increase in arginine vasopressin (AVP) secretion in response to hypovolemia is due to arterial baroreceptor unloading. If arterial pressure is the key to the mechanism, the slope relating plasma AVP to arterial pressure should be the same in response to hemorrhage, a model of true hypovolemia, and in response to thoracic inferior vena caval constriction (IVCC), a model of central hypovolemia. We tested this hypothesis in conscious, chronically instrumented dogs (n = 8). The mean coefficient of determination (r(2)) values obtained from the individual regressions of log AVP onto systolic pressure (SP) and mean arterial pressure (MAP) in response to hemorrhage were 0.953 +/- 0.009 and 0.845 +/- 0.047, respectively. Paired comparisons indicated a significant difference between the means (P < 0.05), hence, SP was used in subsequent analyses. The mean slopes relating the log of plasma AVP to SP in response to hemorrhage and IVCC were -0.034 +/- 0.003 and -0.032 +/- 0.002, respectively, and the means were not significantly different (P = 0.7). The slopes were not altered when the experiments were repeated during acute blockade of cardiac receptors by intrapericardial procaine. Finally, sinoaortic denervation (n = 4) markedly reduced the slope in both the hemorrhage and IVCC treatments. We conclude that baroreceptors monitoring arterial pressure provide the principal reflex control of AVP secretion in response to hypovolemia.

Cardiopulmonary baroreceptor control of muscle sympathetic nerve activity in heat-stressed humans

Whole body heating decreases central venous pressure (CVP) while increasing muscle sympathetic nerve activity (MSNA). In normothermia, similar decreases in CVP elevate MSNA, presumably via cardiopulmonary baroreceptor unloading. The purpose of this project was to identify whether increases in MSNA during whole body heating could be attributed to cardiopulmonary baroreceptor unloading coincident with the thermal challenge. Seven subjects were exposed to whole body heating while sublingual temperature, skin blood flow, heart rate, arterial blood pressure, and MSNA were monitored. During the heat stress, 15 ml/kg warmed saline was infused intravenously over 7-10 min to increase CVP and load the cardiopulmonary baroreceptors. We reported previously that this amount of saline was sufficient to return CVP to pre-heat stress levels. Whole body heating increased MSNA from 25 +/- 3 to 39 +/- 3 bursts/min (P < 0. 05). Central blood volume expansion via rapid saline infusion did not significantly decrease MSNA (44 +/- 4 bursts/min, P > 0.05 relative to heat stress period) and did not alter mean arterial blood pressure (MAP) or pulse pressure. To identify whether arterial baroreceptor loading decreases MSNA during heat stress, in a separate protocol MAP was elevated via steady-state infusion of phenylephrine during whole body heating. Increasing MAP from 82 +/- 3 to 93 +/- 4 mmHg (P < 0.05) caused MSNA to decrease from 36 +/- 3 to 15 +/- 4 bursts/min (P < 0.05). These data suggest that cardiopulmonary baroreceptor unloading during passive heating is not the primary mechanism resulting in elevations in MSNA. Moreover, arterial baroreceptors remain capable of modulating MSNA during heat stress.

Cardiac autonomic responses to volume overload in normal subjects and in patients with dilated cardiomyopathy

This study evaluated the effects of acute isotonic volume expansion on heart rate variability (HRV) in 10 patients with dilated cardiomyopathy (DCM) and in 10 age- and sex-matched normal volunteers. Echocardiographic left ventricular volumes and HRV measurements by continuous Holter recording were assessed at baseline, at 60 and 120 min during intravenous saline load (0.9% NaCl, 0.25 ml. kg(-1). min(-1)), and 60 min after infusion was terminated. Data analysis was performed by repeated-measures ANOVA. After volume expansion, left ventricular ejection fraction increased (F = 9.8; P < 0.001) in normal subjects and decreased (F = 8.7; P < 0.001) in DCM patients. During volume expansion a significant difference was also detectable between the two groups in root-mean-square successive difference (F = 25.2; P < 0.001), percentage of differences between successive normal R-R intervals >50 ms (F = 97.6; P < 0.001), high-frequency power (F = 50.1; P < 0.001), and low-frequency power (F = 41.6; P < 0.001), all of which reflect parasympathetic modulation of heart rate; in fact, these measurements increased in normal subjects and decreased in DCM patients. In normal subjects, the increase in HRV measurements during volume expansion suggests a parasympathetic activation, mediated by stimulation of cardiopulmonary and arterial mechanoreceptors. On the contrary, in DCM patients the parasympathetic withdrawal, already detectable at baseline, increases during volume expansion.

Cerebrovascular mechanisms in neurocardiogenic syncope with and without postural tachycardia syndrome

BACKGROUND AND PURPOSE

Recent transcranial Doppler studies in patients with neurocardiogenic syncopes (NCS) have demonstrated that the cerebrovascular response to sudden systemic hypotension is vasoconstriction instead of compensatory vasodilation (autoregulation). We tried to characterize the conditions leading to this unexpected response in NCS patients further by continuously monitoring autoregulation and autonomic parameters during a standardized tilt-table test (TTT).

METHODS

Sixteen patients below the age of 50 years with a history of at least three syncopes of undetermined cause and tilt-table verified NCS and 20 normal controls were studied. Arterial blood pressure (ABP) and heart rate (HR) were monitored by Finapres and cerebral blood flow velocity (CBFV) of the left middle cerebral artery by transcranial Doppler. Baroreflex sensitivity and autoregulation parameters were measured continuously, using cross-spectral analysis of Mayer waves (3-9 cycles per minute oscillations) in ABP, HR and CBFV, respectively. Pulsatility indices (PI) of CBFV and ABP were determined continuously. Measurements were taken during 5 min in supine and during 5 min in tilted position. In patients, tilting was continued for a maximum of 45 min until the onset of syncope or presyncope.

RESULTS

According to the maximum increase in heart rate (deltaHR) during the first 5 min of standing, heart rate responses were classified as postural tachycardia syndrome (POTS) (deltaHR > 35/min) or as normal. Only one out of 20 control subjects showed a POTS (5%) in contrast to seven patients (44%). Patients with a POTS had significantly lower PI values in ABP and higher ratios between the PI of CBFV and the PI of ABP both in supine and in tilted positions. Baroreflex sensitivity during standing decreased significantly in POTS patients when compared to controls. Although autoregulation remained intact during standing, mean CBFV decreased significantly and continuously. The nine patients without a POTS showed almost the same cardiovascular and cerebrovascular responses as the control subjects. All 16 patients showed similar circulatory responses during syncope (sudden hypotension, relative or absolute bradycardia, reduced CBFV and increased PI in CBFV).

CONCLUSIONS

The development of a POTS during tilting indicates a high risk for fainting. The characteristic hemodynamic features in the initial phase of standing in these patients can be interpreted in terms of central hypovolemia (low PI of ABP) with sufficient ABP regulation and increased cerebrovascular resistance (defined as the ratio between PI of CBFV and ABP). Cerebral autoregulation seems not to be affected in patients suffering from NCS.

Cardiopulmonary baroreflex inhibition of sympathetic nerve activity is preserved with age in healthy humans

1. We tested the hypothesis that the ability of the cardiopulmonary baroreflex to produce sympathoinhibition is reduced with age in humans. Eleven young (23 +/- 1 years, mean +/- s.e.m.) and ten older (64 +/- 1) healthy adult males were studied under supine conditions (control) and in response to cardiopulmonary baroreflex stimulation evoked by acute central circulatory hypervolaemia (10 deg head-down tilt). The two groups were normotensive and free of overt cardiovascular disease. 2. Supine baseline (control) levels of efferent muscle sympathetic nerve activity (MSNA) burst frequency were twice as high in the older men (41 +/- 2 vs. 21 +/- 2 bursts min-1, P < 0.05). In both groups in response to head-down tilt arterial blood pressure and heart rate were unchanged, peripheral venous pressure (PVP) increased (P < 0.05), MSNA total activity decreased (P < 0.05), antecubital venous plasma noradrenaline concentrations did not change significantly, and forearm blood flow and vascular conductance increased (vascular resistance decreased) (all P < 0.05). The mean absolute DeltaMSNA/DeltaPVP was similar in the young and older men, although the higher control levels of MSNA in the older men resulted in a smaller percentage DeltaMSNA/DeltaPVP (P < 0.05). Per DeltaPVP, the reduction in forearm vascular resistance was smaller in the older men, but there were no age group differences when expressed as increases in forearm vascular conductance. 3. These results indicate that the ability of the cardiopulmonary baroreflex to inhibit MSNA is well preserved with age in healthy adult humans. As such, these findings are not consistent with the concept that this mechanism plays a role in the age-associated elevation in basal MSNA.