Arterial pulse pressure and vasopressin release in humans during lower body negative pressure

Assessment of changes in blood volume during lower body negative pressure-induced hypovolemia using bioelectrical impedance analysis

BACKGROUND

Lower body negative Pressure (LBNP)-induced hypovolemia is simulating acute hemorrhage by sequestrating blood into lower extremities. Bioelectrical Impedance Analysis (BIA) is based on the electrical properties of biological tissues, as electrical current flows along highly conductive body tissues (such as blood). Changes in blood volume will lead to changes in bioimpedance. This study aims to study changes in upper (UL) and lower (LL) extremities bioimpedance during LBNP-induced hypovolemia.

METHODS

This was a prospective observational study of healthy volunteers who underwent gradual LBNP protocol which consisted of 3-minute intervals: at baseline, -15, -30, -45, -60 mmHg, then recovery phases at -30 mmHg and baseline. The UL&LL extremities bioimpedance were measured and recorded at each phase of LBNP and the percentage changes of bioimpedance from baseline were calculated and compared using student's t-test. A P-value of < 0.05 was considered significant. Correlation between relative changes in UL&LL bioimpedance and estimated blood loss (EBL) from LBNP was calculated using Pearson correlation.

RESULTS

26 healthy volunteers were enrolled. As LBNP-induced hypovolemia progressed, there were a significant increase in UL bioimpedance and a significant decrease in LL bioimpedance. During recovery phases (where blood was shifted from the legs to the body), there were a significant increase in LL bioimpedance and a reduction in UL bioimpedance. There were significant correlations between estimated blood loss from LBNP model with UL (R = 0.97) and LL bioimpedance (R = - 0.97).

CONCLUSION

During LBNP-induced hypovolemia, there were reciprocal changes in UL&LL bioimpedance. These changes reflected hemodynamic compensatory mechanisms to hypovolemia.

Acute fasting reduces tolerance to progressive central hypovolemia in humans

Potential health benefits of an acute fast include reductions in blood pressure and increases in vagal cardiac control. These purported health benefits could put fasted humans at risk for cardiovascular collapse when exposed to central hypovolemia. The purpose of this study was to test the hypothesis that an acute 24-h fast (vs. 3-h postprandial) would reduce tolerance to central hypovolemia induced via lower body negative pressure (LBNP). We measured blood ketones (β-OHB) to confirm a successful fast (n = 18). We recorded the electrocardiogram (ECG), beat-to-beat arterial pressure, muscle sympathetic nerve activity (MSNA; n = 7), middle cerebral artery blood velocity (MCAv), and forearm blood flow. Following a 5-min baseline, LBNP was increased by 15 mmHg until -60 mmHg and then increased by 10 mmHg in a stepwise manner until onset of presyncope. Each LBNP stage lasted 5-min. Data are expressed as means ± SE β-OHB increased (β-OHB; 0.12 ± 0.04 fed vs. 0.47 ± 0.11, P < 0.01 mmol/L fast). Tolerance to central hypovolemia was decreased by ∼10% in the fasted condition measured via total duration of negative pressure (1,370 [Formula: see text] 89 fed vs. 1,229 ± 94 s fast, P = 0.04), and was negatively associated with fasting blood ketones (R = -0.542, P = 0.02). During LBNP, heart rate and MSNA increased similarly, but in the fasted condition forearm vascular resistance was significantly reduced. Our results suggest that acute fasting reduces tolerance to central hypovolemia by blunting increases in peripheral resistance, indicating that prolonged fasting may hinder an individual's ability to compensate to a loss of blood volume.NEW & NOTEWORTHY An acute 24 h fasting reduces tolerance to central hypovolemia, and tolerance is negatively associated with blood ketone levels. Compared with a fed condition (3-h postprandial), fasted participants exhibited blunted peripheral vasoconstriction and greater reductions in stroke volume during stepwise lower body negative pressure. These findings suggest that a prolonged fast may lead to quicker decompensation during central hypovolemia.

Optical Hemodynamic Imaging of Jugular Venous Dynamics During Altered Central Venous Pressure

OBJECTIVE

An optical imaging system is proposed for quantitatively assessing jugular venous response to altered central venous pressure.

METHODS

The proposed system assesses sub-surface optical absorption changes from jugular venous waveforms with a spatial calibration procedure to normalize incident tissue illumination. Widefield frames of the right lateral neck were captured and calibrated using a novel flexible surface calibration method. A hemodynamic optical model was derived to quantify jugular venous optical attenuation (JVA) signals, and generate a spatial jugular venous pulsatility map. JVA was assessed in three cardiovascular protocols that altered central venous pressure: acute central hypovolemia (lower body negative pressure), venous congestion (head-down tilt), and impaired cardiac filling (Valsalva maneuver).

RESULTS

JVA waveforms exhibited biphasic wave properties consistent with jugular venous pulse dynamics when time-aligned with an electrocardiogram. JVA correlated strongly (median, interquartile range) with invasive central venous pressure during graded central hypovolemia (r = 0.85, [0.72, 0.95]), graded venous congestion (r = 0.94, [0.84, 0.99]), and impaired cardiac filling (r = 0.94, [0.85, 0.99]). Reduced JVA during graded acute hypovolemia was strongly correlated with reductions in stroke volume (SV) (r = 0.85, [0.76, 0.92]) from baseline (SV: 79 ± 15 mL, JVA: 0.56 ± 0.10 a.u.) to -40 mmHg suction (SV: 59 ± 18 mL, JVA: 0.47 ± 0.05 a.u.; p 0.01).

CONCLUSION

The proposed non-contact optical imaging system demonstrated jugular venous dynamics consistent with invasive central venous monitoring during three protocols that altered central venous pressure.

SIGNIFICANCE

This system provides non-invasive monitoring of pressure-induced jugular venous dynamics in clinically relevant conditions where catheterization is traditionally required, enabling monitoring in non-surgical environments.

Elevated Muscle Sympathetic Nerve Activity Contributes to Central Artery Stiffness in Young and Middle-Age/Older Adults

Muscle sympathetic nerve activity (MSNA) influences the mechanical properties (ie, vascular smooth muscle tone and stiffness) of peripheral arteries, but it remains controversial whether MSNA contributes to stiffness of central arteries, such as the aorta and carotids. We examined whether elevated MSNA (age-related) would be independently associated with greater stiffness of central (carotid-femoral pulse wave velocity [PWV]) and peripheral (carotid-brachial PWV) arteries, in addition to lower carotid compliance coefficient, in healthy men and women (n=88, age: 19-73 years, 52% men). We also examined whether acute elevations in MSNA without increases in mean arterial pressure using graded levels of lower body negative pressure would augment central and peripheral artery stiffness in young (n=15, 60% men) and middle-age/older (MA/O, n=14, 43% men) adults. Resting MSNA burst frequency (bursts·min^-1) was significantly correlated with carotid-femoral PWV ( R=0.44, P<0.001), carotid-brachial PWV ( R=0.32, P=0.004), and carotid compliance coefficient ( R=0.28, P=0.01) after controlling for sex, mean arterial pressure, heart rate, and waist-to-hip ratio (central obesity), but these correlations were abolished after further controlling for age (all P>0.05). In young and MA/O adults, MSNA was elevated during lower body negative pressure ( P<0.001) and produced significant increases in carotid-femoral PWV (young: Δ+1.3±0.3 versus MA/O: Δ+1.0±0.3 m·s^-1, P=0.53) and carotid-brachial PWV (young: Δ+0.7±0.3 versus MA/O: Δ+0.7±0.5 m·s^-1, P=0.92), whereas carotid compliance coefficient during lower body negative pressure was significantly reduced in young but not MA/O (young: Δ-0.04±0.01 versus MA/O: Δ0.001±0.008 mm²·mm Hg^-1, P<0.01). Collectively, these data demonstrate the influence of MSNA on central artery stiffness and its potential contribution to age-related increases in stiffness of both peripheral and central arteries.

Influence of perioperative stress on central and peripheral oxytocin and arginine-vasopressin concentrations

Perioperative stress provides not only physical, but also psychic and emotional aspects, which may influence the hypothalamic neuropeptide system. Studies investigating the perioperative course of central neuropeptide activity are missing. Therefore, the present study aimed to determine perioperative fluctuations in central and concomitant peripheral concentrations of the hypothalamic neuropeptides oxytocin (OXT) and arginine-vasopressin (AVP), as well as their impact on perioperative anxiety and depression. Cerebrospinal fluid (CSF), blood and saliva were collected from 12 patients who underwent elective endovascular aortic repair with a routinely inserted spinal catheter. AVP and OXT concentrations were analysed at four timepoints: (i) the evening before the operation; (ii) the operation day immediately before anaesthesia induction; (iii) intraoperatively after the stent was placed; and (iv) on day 1 after the operation. Patients completed the Hospital Anxiety and Depression Scale (HADS) at timepoints 1 and 4. For CSF OXT, the present study showed a significant intraoperative decline, accompanied by a decrease in saliva. OXT blood concentrations before anaesthesia induction were higher than at the evening before the operation. OXT concentrations in CSF and saliva correlated well at timepoints 2-4. AVP concentrations in CSF, blood and saliva did not show any significant changes perioperatively. However, postoperative AVP blood concentrations showed a significant negative correlation with anxiety and depression scores according to the HADS. This pilot study demonstrates perioperative fluctuations in central OXT concentrations, which are better reflected by saliva than by blood. Further studies are required to determine whether OXT and AVP can predict postoperative post-traumatic stress disorder.

Age-dependent impairment of the erythropoietin response to reduced central venous pressure in HFpEF patients

Despite growing research interest in the pathophysiology of heart failure with preserved ejection fraction (HFpEF), it remains unknown whether central hemodynamic alterations inherently present in this condition do affect blood pressure and blood volume (BV) regulation. The present study sought to determine hemodynamic and endocrine responses to prolonged orthostatic stress in HFpEF patients. Central venous pressure (CVP) assessed via the internal jugular vein (IJV) aspect ratio with ultrasonography, arterial pressure and heart rate were determined at supine rest and during 2 hours of moderate (25-30°) head-up tilt (HUT) in 18 stable HFpEF patients (71.2 ± 7.3 years), 14 elderly (EC), and 10 young (YC) healthy controls. Parallel endocrine measurements comprised main BV-regulating hormones: pro-atrial natriuretic peptide, copeptin, aldosterone, and erythropoietin (EPO). At supine rest, the IJV aspect ratio was higher (>30%) in HFpEF patients compared with EC and YC, while mean arterial pressure was elevated in HFpEF patients (98.0 ± 13.1 mm Hg) and EC (95.6 ± 8.3 mm Hg) versus YC (87.3 ± 5.0 mm Hg) (P < 0.05). HUT increased heart rate (+10%) and reduced the IJV aspect ratio (-52%), with similar hemodynamic effects in all groups (P for interaction ≥ 0.322). The analysis of endocrine responses to HUT revealed a group×time interaction for circulating EPO, which was increased in YC (+10%) but remained unaltered in HFpEF patients and EC. The EPO response to a given reduction in CVP is similarly impaired in HFpEF patients and elderly controls, suggesting an age-dependent dissociation of EPO production from hemodynamic regulation in the HFpEF condition.

Lower Body Negative Pressure: Physiological Effects, Applications, and Implementation

This review presents lower body negative pressure (LBNP) as a unique tool to investigate the physiology of integrated systemic compensatory responses to altered hemodynamic patterns during conditions of central hypovolemia in humans. An early review published in Physiological Reviews over 40 yr ago (Wolthuis et al. Physiol Rev 54: 566-595, 1974) focused on the use of LBNP as a tool to study effects of central hypovolemia, while more than a decade ago a review appeared that focused on LBNP as a model of hemorrhagic shock (Cooke et al. J Appl Physiol (1985) 96: 1249-1261, 2004). Since then there has been a great deal of new research that has applied LBNP to investigate complex physiological responses to a variety of challenges including orthostasis, hemorrhage, and other important stressors seen in humans such as microgravity encountered during spaceflight. The LBNP stimulus has provided novel insights into the physiology underlying areas such as intolerance to reduced central blood volume, sex differences concerning blood pressure regulation, autonomic dysfunctions, adaptations to exercise training, and effects of space flight. Furthermore, approaching cardiovascular assessment using prediction models for orthostatic capacity in healthy populations, derived from LBNP tolerance protocols, has provided important insights into the mechanisms of orthostatic hypotension and central hypovolemia, especially in some patient populations as well as in healthy subjects. This review also presents a concise discussion of mathematical modeling regarding compensatory responses induced by LBNP. Given the diverse applications of LBNP, it is to be expected that new and innovative applications of LBNP will be developed to explore the complex physiological mechanisms that underline health and disease.

Circulatory responses to lower body negative pressure in young Afghans and Danes: implications for understanding ethnic effects on blood pressure regulation

PURPOSE

We have previously shown that Afghans residing in Denmark for at least 12 years exhibit a lower 24-h ambulatory blood pressure compared to Danes. The purpose of this study was to test the hypothesis that the lower blood pressure reflects attenuated compensatory baroreflex responses in the Afghans.

METHODS

On a controlled diet (2,822 cal/day, 55-75 mmol + 2 mmol Na+/kg/day), 12 young males of Afghan (Afghans) and 12 young males of Danish (Danes) origin were exposed to a two-step lower body negative pressure (LBNP) protocol of -20 and -50 mmHg, respectively, each of 10-min duration.

RESULTS

Afghans had lower 24-h systolic blood pressure compared to Danes (115 ± 2 vs. 123 ± 1 mmHg, p < 0.05). Cardiac output and stroke volume were significantly lower in Afghans compared to Danes prior to and during each level of LBNP. However, it decreased to the same extent in both groups during LBNP. During LBNP of -20 mmHg, plasma noradrenaline concentration and plasma renin activity (PRA) increased significantly only in the Afghans. At LBNP of -50 mmHg plasma noradrenaline concentration and PRA both increased significantly and similarly in the two groups.

CONCLUSION

The lower 24-h ambulatory blood pressure in the Afghans is probably caused by a lower stroke volume, which augmented the circulatory and vasoactive hormonal responses to LBNP in the Afghans. The lower stroke volume in Afghans residing in Denmark compared to that of matched native Danes remains to be explained.

Reductions in central venous pressure by lower body negative pressure or blood loss elicit similar hemodynamic responses

The purpose of this study was to compare hemodynamic and blood analyte responses to reduced central venous pressure (CVP) and pulse pressure (PP) elicited during graded lower body negative pressure (LBNP) to those observed during graded blood loss (BL) in conscious humans. We hypothesized that the stimulus-response relationships of CVP and PP to hemodynamic responses during LBNP would mimic those observed during BL. We assessed CVP, PP, heart rate, mean arterial pressure (MAP), and other hemodynamic markers in 12 men during LBNP and BL. Blood samples were obtained for analysis of catecholamines, hematocrit, hemoglobin, arginine vasopressin, and blood gases. LBNP consisted of 5-min stages at 0, 15, 30, and 45 mmHg of suction. BL consisted of 5 min at baseline and following three stages of 333 ml of hemorrhage (1,000 ml total). Individual r(2) values and linear regression slopes were calculated to determine whether the stimulus (CVP and PP)-hemodynamic response trajectories were similar between protocols. The CVP-MAP trajectory was the only CVP-response slope that was statistically different during LBNP compared with BL (0.93 ± 0.27 vs. 0.13 ± 0.26; P = 0.037). The PP-heart rate trajectory was the only PP-response slope that was statistically different during LBNP compared with BL (-1.85 ± 0.45 vs. -0.46 ± 0.27; P = 0.024). Norepinephrine, hematocrit, and hemoglobin were all lower at termination in the BL protocol compared with LBNP (P < 0.05). Consistent with our hypothesis, LBNP mimics the hemodynamic stimulus-response trajectories observed during BL across a significant range of CVP in humans.

Blood pressure regulation IV: adaptive responses to weightlessness

During weightlessness, blood and fluids are immediately shifted from the lower to the upper body segments, and within the initial 2 weeks of spaceflight, brachial diastolic arterial pressure is reduced by 5 mmHg and even more so by some 10 mmHg from the first to the sixth month of flight. Blood pressure thus adapts in space to a level very similar to that of being supine on the ground. At the same time, stroke volume and cardiac output are increased and systemic vascular resistance decreased, whereas sympathetic nerve activity is kept surprisingly high and similar to when ground-based upright seated. This was not predicted from simulation models and indicates that dilatation of the arteriolar resistance vessels is caused by mechanisms other than a baroreflex-induced decrease in sympathetic nervous activity. Results of baroreflex studies in space indicate that compared to being ground-based supine, the carotid (vagal)-cardiac interaction is reduced and sympathetic nerve activity, heart rate and systemic vascular resistance response more pronounced during baroreflex inhibition by lower body negative pressure. The future challenge is to identify which spaceflight mechanism induces peripheral arteriolar dilatation, which could explain the decrease in blood pressure, the high sympathetic nerve activity and associated cardiovascular changes. It is also a challenge to determine the cardiovascular risk profile of astronauts during future long-duration deep space missions.

Renal renin secretion as regulator of body fluid homeostasis

The renin-angiotensin system is essential for body fluid homeostasis and blood pressure regulation. This review focuses on the homeostatic regulation of the secretion of active renin in the kidney, primarily in humans. Under physiological conditions, renin secretion is determined mainly by sodium intake, but the specific pathways involved and the relations between them are not well defined. In animals, renin secretion is a log-linear function of sodium intake. Close associations exist between sodium intake, total body sodium, extracellular fluid volume, and blood volume. Plasma volume increases by about 1.5 mL/mmol increase in daily sodium intake. Several lines of evidence indicate that central blood volume may vary substantially without measurable changes in arterial blood pressure. At least five intertwining feedback loops of renin regulation are identifiable based on controlled variables (blood volume, arterial blood pressure), efferent pathways to the kidney (nervous, humoral), and pathways operating via the macula densa. Taken together, the available evidence favors the notion that under physiological conditions (1) volume-mediated regulation of renin secretion is the primary regulator, (2) macula densa mediated mechanisms play a substantial role as co-mediator although the controlled variables are not well defined so far, and (3) regulation via arterial blood pressure is the exception rather than the rule. Improved quantitative analyses based on in vivo and in silico models are warranted.

Treatment of severe sepsis: where next? Current and future treatment approaches after the introduction of drotrecogin alfa

Severely septic patients continue to experience excessive morbidity and mortality despite recent advances in critical care. Although significant resources have been invested in new treatments, almost all have failed to improve outcomes. An improved understanding of sepsis pathophysiology, including the complex interactions between inflammatory, coagulation, and fibrinolytic systems, has accelerated the development of novel treatments. Recombinant human activated protein C (rhAPC), or drotrecogin alfa (activated) (DAA), is currently the only US Food and Drug Administration (FDA)-approved medicine for the treatment of severe sepsis, and only in patients with a high risk of death. This review will discuss the treatment of severe sepsis, focusing on recent discoveries and unresolved questions about DAA's optimal use. Increasing pharmacological experience has generated enthusiasm for investigating medicines already approved for other indications as treatments for severe sepsis. Replacement doses of hydrocortisone and vasopressin may reduce mortality and improve hypotension, respectively, in a subgroup of patients with catecholamine-refractory septic shock. In addition to discussing these new indications, this review will detail the provocative preliminary data from four promising treatments, including two novel modalities: antagonizing high mobility group box protein and inhibiting tissue factor (TF). Observational data from the uncontrolled administration of heparin or statins in septic patients will also be reviewed.

Exercise in Heart Failure: Should Aqua Therapy and Swimming Be Allowed?

PURPOSE

Although exercise training is established as an integrated part of treatment regimes in both patients with transmural myocardial infarction (MI) and chronic congestive heart failure (CHF), there is no consensus yet on the appropriateness of water exercises and swimming. One reason is the lack of information concerning both central hemodynamic volume and pressure responses during immersion in these patients.

METHODS

This paper presents explorative studies on changes in cardiac dimensions and central hemodynamics during graded immersion and swimming in patients with moderate and/or severe MI and in patients with moderate and/or compensated severe CHF. For comparison purposes, healthy subjects were assessed. Measurements were performed by using Swan-Ganz right heart catheterization, subxiphoidal echocardiography, and Doppler-echocardiography.

RESULTS

The major findings were: 1) Indicators of an increase in preload were seen in patients with moderate and severe MI. In both patient groups, upright immersion to the neck and supine body position at rest in the water resulted in abnormal mean pulmonary artery pressure (PAm) and mean pulmonary capillary pressures (PCPm), respectively. During low-speed swimming (20-25 m.min(-1)), the PAm and/or PCPm were higher than during supine cycle ergometry at a load of 100 W. 2) Left ventricular overload and decrease and/or no change in stroke volume occurred in patients with severe CHF who were immersed up to the neck. 3) Patient's well-being was maintained despite hemodynamic deterioration.

CONCLUSION

The acute responses during immersion and swimming suggest the need for additional studies on long-term changes in cardiac dimensions and central hemodynamic in both patients with severe MI and severe CHF who undergo a swimming program, compared with nonswimming patients with MI and CHF of similar etiology and severity of disease.

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).

Clinical review: Vasopressin and terlipressin in septic shock patients

Vasopressin (antidiuretic hormone) is emerging as a potentially major advance in the treatment of septic shock. Terlipressin (tricyl-lysine-vasopressin) is the synthetic, long-acting analogue of vasopressin, and has comparable pharmacodynamic but different pharmacokinetic properties. Vasopressin mediates vasoconstriction via V₁ receptor activation on vascular smooth muscle. Septic shock first causes a transient early increase in blood vasopressin concentrations; these concentrations subsequently decrease to very low levels as compared with those observed with other causes of hypotension. Infusions of 0.01–0.04 U/min vasopressin in septic shock patients increase plasma vasopressin concentrations. This increase is associated with reduced need for other vasopressors. Vasopressin has been shown to result in greater blood flow diversion from nonvital to vital organ beds compared with adrenaline (epinephrine). Of concern is a constant decrease in cardiac output and oxygen delivery, the consequences of which in terms of development of multiple organ failure are not yet known. Terlipressin (one or two boluses of 1 mg) has similar effects, but this drug has been used in far fewer patients. Large randomized clinical trials should be conducted to establish the utility of these drugs as therapeutic agents in patients with septic shock.

Hypovolemia and neurovascular control during orthostatic stress

Humans exposed to real or simulated microgravity experience decrements in blood pressure regulation during orthostatic stress that may be related to autonomic dysregulation and/or hypovolemia. We examined the hypothesis that hypovolemia, without the deconditioning effects of bed rest or spaceflight, would augment the sympathoneural and vasomotor response to graded orthostatic stress. Radial artery blood pressure (tonometry), stroke volume (SV), brachial blood flow (Doppler ultrasound), heart rate (electrocardiogram), peroneal muscle sympathetic nerve activity (MSNA; microneurography), and estimated central venous pressure (CVP) were recorded during five levels (-5, -10, -15, -20 and -40 mmHg) of randomly assigned lower body negative pressure (LBNP) (n = 8). Forearm (FVR) and total peripheral vascular resistance (TPR) were calculated. The test was repeated under randomly assigned placebo (normovolemia) or diuretic (spironolactone: 100 mg/day, 3 days) (hypovolemia) conditions. The diuretic produced an approximately 16% reduction in plasma volume. Compared with normovolemia, SV and cardiac output were reduced by approximately 12% and approximately 10% at baseline and during LBNP after the diuretic. During hypovolemia, there was an upward shift in the %DeltaMSNA/DeltaCVP, DeltaFVR/DeltaCVP, and DeltaTPR/DeltaCVP relationships during 0 to -20 mmHg LBNP. In contrast to normovolemia, blood pressure increased at -40 mmHg LBNP during hypovolemia due to larger gains in the %DeltaMSNA/DeltaCVP and DeltaTPR/DeltaCVP relationships. It was concluded that acute hypovolemia augmented the neurovascular component of blood pressure control during moderate orthostasis, effectively compensating for decrements in SV and cardiac output.

Changes in cerebral oxygenation and blood flow during LBNP in spinal cord-injured individuals

Spinal cord-injured (SCI) individuals, having a sympathetic nervous system lesion, experience hypotension during sitting and standing. Surprisingly, they experience few syncopal events. This suggests adaptations in cerebrovascular regulation. Therefore, changes in systemic circulation, cerebral blood flow, and oxygenation in eight SCI individuals were compared with eight able-bodied (AB) individuals. Systemic circulation was manipulated by lower body negative pressure at several levels down to -60 mmHg. At each level, we measured steady-state blood pressure, changes in cerebral blood velocity with transcranial Doppler, and cerebral oxygenation using near-infrared spectroscopy. We found that mean arterial pressure decreased significantly in SCI but not in AB individuals, in accordance with the sympathetic impairment in the SCI group. Cerebral blood flow velocity decreased during orthostatic stress in both groups, but this decrease was significantly greater in SCI individuals. Cerebral oxygenation decreased in both groups, with a tendency to a greater decrease in SCI individuals. Thus present data do not support an advantageous mechanism during orthostatic stress in the cerebrovascular regulation of SCI individuals.

Central volume expansion is pivotal for sustained decrease in heart rate during seated to supine posture change

During prolonged, static carotid baroreceptor stimulation by neck suction (NS) in seated humans, heart rate (HR) decreases acutely and thereafter gradually increases. This increase has been explained by carotid baroreceptor adaptation and/or buffering by aortic reflexes. During a posture change from seated to supine (Sup) with similar carotid stimulation, however, the decrease in HR is sustained. To investigate whether this discrepancy is caused by changes in central blood volume, we compared (n = 10 subjects) the effects of 10 min of seated NS (adjusted to simulate carotid stimulation of a posture change), a posture change from seated to Sup, and the same posture change with left atrial (LA) diameter maintained unchanged by lower body negative pressure (Sup + LBNP). During Sup, the prompt decreases in HR and mean arterial pressure (MAP) were sustained. HR decreased similarly within 30 s of NS (65 +/- 2 to 59 +/- 2 beats/min) and Sup + LBNP (65 +/- 2 to 58 +/- 2 beats/min) and thereafter gradually increased to values of seated. MAP decreased similarly within 5 min during Sup + LBNP and NS (by 7 +/- 1 to 9 +/- 1 mmHg) and thereafter tended to increase toward values of seated subjects. Arterial pulse pressure was increased the most by Sup, less so by Sup + LBNP, and was unchanged by NS. LA diameter was only increased by Sup. In conclusion, static carotid baroreceptor stimulation per se causes the acute (<30 s) decrease in HR during a posture change from seated to Sup, whereas the central volume expansion (increased LA diameter and/or arterial pulse pressure) is pivotal to sustain this decrease. Thus the effects of central volume expansion override adaptation of the carotid baroreceptors and/or buffering of aortic reflexes.

Fluid volume and osmoregulation in humans after a week of head-down bed rest

Body fluid homeostasis was investigated during chronic bed rest (BR) and compared with that of acute supine conditions. The hypothesis was tested that 6 degrees head-down BR leads to hypovolemia, which activates antinatriuretic mechanisms so that the renal responses to standardized saline loading are attenuated. Isotonic (20 ml/kg body wt) and hypertonic (2.5%, 7.2 ml/kg body wt) infusions were performed in eight subjects over 20 min following 7 and 10 days, respectively, of BR during constant sodium intake (200 meq/day). BR decreased body weight (83.0 +/- 4.8 to 81.8 +/- 4.4 kg) and increased plasma osmolality (285.9 +/- 0.6 to 288.5 +/- 0.9 mosmol/kgH(2)O, P < 0.05). Plasma ANG II doubled (4.2 +/- 1.2 to 8.8 +/- 1.8 pg/ml), whereas other endocrine variables decreased: plasma atrial natriuretic peptide (42 +/- 3 to 24 +/- 3 pg/ml), urinary urodilatin excretion rate (4.5 +/- 0.3 to 3.2 +/- 0.1 pg/min), and plasma vasopressin (1.7 +/- 0.3 to 0.8 +/- 0.2 pg/ml, P < 0.05). During BR, the natriuretic response to the isotonic saline infusion was augmented (39 +/- 8 vs. 18 +/- 6 meq sodium/350 min), whereas the response to hypertonic saline was unaltered (32 +/- 8 vs. 29 +/- 5 meq/350 min, P < 0.05). In conclusion, BR elicits antinatriuretic endocrine signals, but it does not attenuate the renal natriuretic response to saline stimuli in men; on the contrary, the response to isotonic saline is augmented.

Cardiovascular effects of static carotid baroreceptor stimulation during water immersion in humans

We hypothesized that the more-pronounced hypotensive and bradycardic effects of an antiorthostatic posture change from seated to supine than water immersion are caused by hydrostatic carotid baroreceptor stimulation. Ten seated healthy males underwent five interventions of 15-min each of 1) posture change to supine, 2) seated water immersion to the Xiphoid process (WI), 3) seated neck suction (NS), 4) WI with simultaneous neck suction (−22 mmHg) adjusted to simulate the carotid hydrostatic pressure increase during supine (WI + NS), and 5) seated control. Left atrial diameter increased similarly during supine, WI + NS, and WI and was unchanged during control and NS. Mean arterial pressure (MAP) decreased the most during supine (7 ± 1 mmHg, P < 0.05) and less during WI + NS (4 ± 1 mmHg) and NS (3 ± 1 mmHg). The decrease in heart rate (HR) by 13 ± 1 beats/min ( P < 0.05) and the increase in arterial pulse pressure (PP) by 17 ± 4 mmHg ( P< 0.05) during supine was more pronounced ( P < 0.05) than during WI + NS (10 ± 2 beats/min and 7 ± 2 mmHg, respectively) and WI (8 ± 2 beats/min and 6 ± 1 mmHg, respectively, P < 0.05). Plasma vasopressin decreased only during supine and WI, and plasma norepinephrine, in addition, decreased during WI + NS ( P < 0.05). In conclusion, WI + NS is not sufficient to decrease MAP and HR to a similar extent as a 15-min seated to supine posture change. We suggest that not only static carotid baroreceptor stimulation but also the increase in PP combined with low-pressure receptor stimulation is a possible mechanism for the more-pronounced decrease in MAP and HR during the posture change.

Cardiovascular and neuroendocrine responses to water immersion in compensated heart failure

The hypothesis was tested that cardiovascular and neuroendocrine (norepinephrine, renin, and vasopressin) responses to central blood volume expansion are blunted in compensated heart failure (HF). Nine HF patients [New York Heart Association class II-III, ejection fraction = 0.28 +/- 0.02 (SE)] and 10 age-matched controls (ejection fraction = 0.68 +/- 0.03) underwent 30 min of thermoneutral (34.7 +/- 0.02 degrees C) water immersion (WI) to the xiphoid process. WI increased (P < 0.05) central venous pressure by 3.7 +/- 0.6 and 3.2 +/- 0.4 mmHg and stroke volume index by 12.2 +/- 2.1 and 7.2 +/- 2.1 ml. beat(-1). m(-2) in controls and HF patients, respectively. During WI, systemic vascular resistance decreased (P < 0.05) similarly by 365 +/- 66 and 582 +/- 227 dyn. s. cm(-5) in controls and HF patients, respectively. Forearm subcutaneous vascular resistance decreased by 19 +/- 7% (P < 0.05) in controls but did not change in HF patients. Heart rate decreased less during WI in HF patients, whereas release of norepinephrine, renin, and vasopressin was suppressed similarly in the two groups. We suggest that reflex control of forearm vascular beds and heart rate is blunted in compensated HF but that baroreflex-mediated systemic vasodilatation and neuroendocrine responses to central blood volume expansion are preserved.

Low LBNP tolerance in men is associated with attenuated activation of the renin-angiotensin system

Plasma vasoactive hormone concentrations [epinephrine (p(Epi)), norepinephrine (p(NE)), ANG II (p(ANG II)), vasopressin (p(VP)), endothelin-1 (p(ET-1))] and plasma renin activity (p(RA)) were measured periodically and compared during lower body negative pressure (LBNP) to test the hypothesis that responsiveness of the renin-angiotensin system, the latter being one of the most powerful vasoconstrictors in the body, is of major importance for LBNP tolerance. Healthy men on a controlled diet (2,822 cal/day, 2 mmol. kg(-1). day(-1) Na(+)) were exposed to 30 min of LBNP from -15 to -50 mmHg. LBNP was uneventful for seven men [25 +/- 2 yr, high-tolerance (HiTol) group], but eight men (26 +/- 3 yr) reached presyncope after 11 +/- 1 min [P < 0.001, low-tolerance (LoTol) group]. Mean arterial pressure (MAP) did not change measurably, but central venous pressure and left atrial diameter decreased similarly in both groups (5-6 mmHg, by approximately 30%, P < 0.05). Control (0 mmHg LBNP) hormone concentrations were similar between groups, however, p(RA) differed between them (LoTol 0.6 +/- 0.1, HiTol 1.2 +/- 0.1 ng ANG I. ml(-1). h(-1), P < 0.05). LBNP increased (P < 0. 05) p(RA) and p(ANG II), respectively, more in the HiTol group (9.9 +/- 2.2 ng ANG I. ml(-1). h(-1) and 58 +/- 12 pg/ml) than in LoTol subjects (4.3 +/- 0.9 ng ANG I. ml(-1). h(-1) and 28 +/- 6 pg/ml). In contrast, the increase in p(VP) was higher (P < 0.05) in the LoTol than in the HiTol group. The increases (P < 0.05) for p(NE) were nonsignificant between groups, and p(ET-1) remained unchanged. Thus there may be a causal relationship between attenuated activation of p(RA) and p(ANG II) and presyncope, with p(VP) being a possible cofactor. Measurement of resting p(RA) may be of predictive value for those with lower hypotensive tolerance.

Arterial pulse pressure and vasopressin release during graded water immersion in humans

Previous results indicate that arterial pulse pressure modulates release of arginine vasopressin (AVP) in humans. The hypothesis was therefore tested that an increase in arterial pulse pressure is the stimulus for suppression of AVP release during central blood volume expansion by water immersion. A two-step immersion model (n = 8) to the xiphoid process and neck, respectively, was used to attain two different levels of augmented cardiac distension. Left atrial diameter (echocardiography) increased from 28 +/- 1 to 34 +/- 1 mm (P < 0.05) during immersion to the xiphoid process and more so (P < 0.05), to 36 +/- 1 mm, during immersion to the neck. During immersion to the xiphoid process, arterial pulse pressure (invasively measured in a brachial artery) increased (P < 0.05) from 44 +/- 1 to 51 +/- 2 mmHg and to the same extent from 42 +/- 1 to 52 +/- 2 mmHg during immersion to the neck. Mean arterial pressure was unchanged during immersion to the xiphoid process and increased during immersion to the neck by 7 +/- 1 mmHg (P < 0.05). Arterial plasma AVP decreased from 2.5 +/- 0.7 to 1.8 +/- 0.5 pg/ml (P < 0. 05) during immersion to the xiphoid process and significantly more so (P < 0.05), to 1.4 +/- 0.5 pg/ml, during immersion to the neck. In conclusion, other factors besides the increase in arterial pulse pressure must have participated in the graded suppression of AVP release, comparing immersion to the xiphoid process with immersion to the neck. We suggest that when arterial pulse pressure is increased, graded distension of cardiopulmonary receptors modulate AVP release.