Reduced baroreflex control of heart period after bed rest is normalized by acute plasma volume restoration

Regional changes in cerebral blood flow between the upright and supine posture and over 3 days of bed rest

A reduction in cerebral blood flow (CBF) has been observed during spaceflight and bed rest. We aimed to examine the magnitude and regional heterogeneity of the decrease in CBF during bed rest compared to posture changes on Earth. Seventeen participants (age, 29 ± 9 years, 7 females) were studied in the upright and supine posture and over 3 days of bed rest. We assessed blood flow via duplex ultrasonography in the internal carotid (ICA) and vertebral arteries (VA), and via transcranial Doppler of the middle cerebral artery (MCAv). Mean arterial pressure (MAP) and end-tidal CO2 (E T C O 2 ${\mathrm{E}}{{{\mathrm{T}}}_{{\mathrm{C}}{{{\mathrm{O}}}_{\mathrm{2}}}}}$ ) were assessed at all time points. By day 3, total CBF (1078 ± 302 to 853 ± 245 mL min-1, P < 0.0001) and MCAv (61 ± 15 to 49 ± 12 mL min-1, P < 0.0001) were decreased compared to the supine posture. CBF values did not fall below the upright posture (all P > 0.05) but were lower than a calculated 24-h mean baseline (P = 0.0132). MAP remained stable (P = 0.971), as didE T C O 2 ${\mathrm{E}}{{{\mathrm{T}}}_{{\mathrm{C}}{{{\mathrm{O}}}_{\mathrm{2}}}}}$ (P = 0.0803), while VA blood flow decreased after 24 h and again after 72 h (P = 0.0024). These findings indicate that CBF decreases during short-term bed rest, but not below values observed in the upright posture.

Impact of Daily Lower-Body Negative Pressure or Cycling Followed by Venous Constrictive Thigh Cuffs on Bedrest-Induced Orthostatic Intolerance

BACKGROUND

Orthostatic intolerance occurs following immobilization in patients on Earth and in astronauts after spaceflight. Head-down tilt bedrest is a terrestrial model for weightlessness and induces orthostatic intolerance. We hypothesized that lower-body negative pressure (LBNP) or cycling followed by wearing venous constrictive thigh cuffs mitigates orthostatic intolerance after head-down tilt bedrest.

METHODS AND RESULTS

We enrolled 47 healthy individuals (20 women, 35±9 years) to a 30-day strict head-down tilt bedrest study. During bedrest, they were assigned to 6 hours of 25 mm Hg LBNP (n=12) per day and 1 hour of supine cycling followed by 6 hours of venous constriction through thigh cuffs 6 days per week (n=12), 6 hours of daily upright sitting (positive control, n=11), or no countermeasure (negative control, n=12). We measured orthostatic tolerance as the time to presyncope during 80° head-up tilt testing with incremental LBNP before and immediately after bedrest. We determined plasma volume with carbon monoxide rebreathing before and at the end of bedrest. After bedrest, orthostatic tolerance decreased 540±457 seconds in the control group, 539±68 seconds in the cycling group, 217±379 seconds in the LBNP group, and 289±89 seconds in the seated group (P<0.0001 time point, P=0.009 for group differences). Supine and upright heart rate increased in all groups following bedrest. Plasma volume was only maintained in the cycling group but decreased in all others (interaction countermeasure×time point P<0.0001).

CONCLUSIONS

Six hours of moderate LBNP training was as effective as sitting in attenuating orthostatic intolerance after 30 days of head-down tilt bedrest. Daily cycling exercise followed by 6 hours of wearing venous constrictive thigh cuffs, while maintaining plasma volume, did not improve orthostatic tolerance.

REGISTRATION

URL: https://www.bfarm.de/EN; Identifiers: DRKS00027643 and DRKS00030848.

Effects of weightlessness on the cardiovascular system: a systematic review and meta-analysis

Background: The microgravity environment has a direct impact on the cardiovascular system due to the fluid shift and weightlessness that results in cardiac dysfunction, vascular remodeling, and altered Cardiovascular autonomic modulation (CAM), deconditioning and poor performance on space activities, ultimately endangering the health of astronauts. Objective: This study aimed to identify the acute and chronic effects of microgravity and Earth analogues on cardiovascular anatomy and function and CAM. Methods: CINAHL, Cochrane Library, Scopus, Science Direct, PubMed, and Web of Science databases were searched. Outcomes were grouped into cardiovascular anatomic, functional, and autonomic alterations, and vascular remodeling. Studies were categorized as Spaceflight (SF), Chronic Simulation (CS), or Acute Simulation (AS) based on the weightlessness conditions. Meta-analysis was performed for the most frequent outcomes. Weightlessness and control groups were compared. Results: 62 articles were included with a total of 963 participants involved. The meta-analysis showed that heart rate increased in SF [Mean difference (MD) = 3.44; p = 0.01] and in CS (MD = 4.98; p < 0.0001), whereas cardiac output and stroke volume decreased in CS (MD = -0.49; p = 0.03; and MD = -12.95; p < 0.0001, respectively), and systolic arterial pressure decreased in AS (MD = -5.20; p = 0.03). According to the qualitative synthesis, jugular vein cross-sectional area (CSA) and volume were greater in all conditions, and SF had increased carotid artery CSA. Heart rate variability and baroreflex sensitivity, in general, decreased in SF and CS, whereas both increased in AS. Conclusion: This review indicates that weightlessness impairs the health of astronauts during and after spaceflight, similarly to the effects of aging and immobility, potentially increasing the risk of cardiovascular diseases. Systematic Review Registration: https://www.crd.york.ac.uk/prospero/, identifier CRD42020215515.

Retinal venular vessel diameters are smaller during ten days of bed rest

Older individuals experience cardiovascular dysfunction during extended bedridden hospital or care home stays. Bed rest is also used as a model to simulate accelerated vascular deconditioning occurring during spaceflight. This study investigates changes in retinal microcirculation during a ten-day bed rest protocol. Ten healthy young males (22.9 ± 4.7 years; body mass index: 23.6 ± 2.5 kg·m-2) participated in a strictly controlled repeated-measures bed rest study lasting ten days. High-resolution images were obtained using a hand-held fundus camera at baseline, daily during the 10 days of bed rest, and 1 day after re-ambulation. Retinal vessel analysis was performed using a semi-automated software system to obtain metrics for retinal arteriolar and venular diameters, central retinal artery equivalent and central retinal vein equivalent, respectively. Data analysis employed a mixed linear model. At the end of the bed rest period, a significant decrease in retinal venular diameter was observed, indicated by a significantly lower central retinal vein equivalent (from 226.1 µm, CI 8.90, to 211.4 µm, CI 8.28, p = .026), while no significant changes in central retinal artery equivalent were noted. Prolonged bed rest confinement resulted in a significant (up to 6.5%) reduction in retinal venular diameter. These findings suggest that the changes in retinal venular diameter during bedrest may be attributed to plasma volume losses and reflect overall (cardio)-vascular deconditioning.

Impact of daily artificial gravity on autonomic cardiovascular control following 60-day head-down tilt bed rest

Impaired cardiovascular autonomic control following space flight or immobilization may limit the ability to cope with additional hemodynamic stimuli. Head-down tilt bedrest is an established terrestrial analog for space flight and offers the opportunity to test potential countermeasures for autonomic cardiovascular deconditioning. Previous studies revealed a possible benefit of daily artificial gravity on cardiovascular autonomic control following head-down tilt bedrest, but there is a need for efficiency in a long-term study before an artificial gravity facility would be brought to space. We hypothesized that artificial gravity through short-arm centrifugation attenuates functional adaptions of autonomic function during head-down tilt bed rest. 24 healthy persons (8 women, 33.4 ± 9.3 years, 24.3 ± 2.1 kg/m2) participated in the 60-day head-down tilt bed rest (AGBRESA) study. They were assigned to three groups, 30 min/day continuous, or 6(5 min intermittent short-arm centrifugation, or a control group. We assessed autonomic cardiovascular control in the supine position and in 5 minutes 80° head-up tilt position before and immediately after bed rest. We computed heart rate variability (HRV) in the time (rmssd) and frequency domain, blood pressure variability, and baroreflex sensitivity (BRS). RR interval corrected rmssd was reduced supine (p = 0.0358) and during HUT (p = 0.0161). Heart rate variability in the high-frequency band (hf-RRI; p = 0.0004) and BRS (p < 0.0001) decreased, whereas blood pressure variability in the low-frequency band (lf-SBP, p = 0.0008) increased following bedrest in all groups. We did not detect significant interactions between bedrest and interventions. We conclude that up to daily 30 min of artificial gravity on a short-arm centrifuge with 1Gz at the center of mass do not suffice to prevent changes in autonomic cardiovascular control following 60-day of 6° head-down tilt bed rest. Clinical Trial Registration: https://drks.de/search/en/trial/DRKS00015677, identifier, DRKS00015677.

Cardiac function, structural, and electrical remodeling by microgravity exposure

Space medicine is key to the human exploration of outer space and pushes the boundaries of science, technology, and medicine. Because of harsh environmental conditions related to microgravity and other factors and hazards in outer space, astronauts and spaceflight participants face unique health and medical challenges, including those related to the heart. In this review, we summarize the literature regarding the effects of spaceflight on cardiac structure and function. We also provide an in-depth review of the literature regarding the effects of microgravity on cardiac calcium handling. Our review can inform future mechanistic and therapeutic studies and is applicable to other physiological states similar to microgravity such as prolonged horizontal bed rest and immobilization.

Regulation of cerebral blood flow in humans: physiology and clinical implications of autoregulation

Brain function critically depends on a close matching between metabolic demands, appropriate delivery of oxygen and nutrients, and removal of cellular waste. This matching requires continuous regulation of cerebral blood flow (CBF), which can be categorized into four broad topics: 1) autoregulation, which describes the response of the cerebrovasculature to changes in perfusion pressure; 2) vascular reactivity to vasoactive stimuli [including carbon dioxide (CO2)]; 3) neurovascular coupling (NVC), i.e., the CBF response to local changes in neural activity (often standardized cognitive stimuli in humans); and 4) endothelium-dependent responses. This review focuses primarily on autoregulation and its clinical implications. To place autoregulation in a more precise context, and to better understand integrated approaches in the cerebral circulation, we also briefly address reactivity to CO2 and NVC. In addition to our focus on effects of perfusion pressure (or blood pressure), we describe the impact of select stimuli on regulation of CBF (i.e., arterial blood gases, cerebral metabolism, neural mechanisms, and specific vascular cells), the interrelationships between these stimuli, and implications for regulation of CBF at the level of large arteries and the microcirculation. We review clinical implications of autoregulation in aging, hypertension, stroke, mild cognitive impairment, anesthesia, and dementias. Finally, we discuss autoregulation in the context of common daily physiological challenges, including changes in posture (e.g., orthostatic hypotension, syncope) and physical activity.

Risk Factors of Patient-Related Safety Events during Active Mobilization for Intubated Patients in Intensive Care Units-A Multi-Center Retrospective Observational Study

The aim of this study is to clarify the incidence and risk factors of patient-related safety events (PSE) in situations limited to intubated patients in which active mobilization, such as sitting on the edge of the bed/standing/walking, was carried out. A multi-center retrospective observational study was conducted at nine hospitals between January 2017 and March 2018. The safety profiles and PSE of 87 patients were analyzed. PSE occurred in 10 out of 87 patients (11.5%) and 13 out of 198 sessions (6.6%). The types of PSE that occurred were hypotension (8, 62%), heart rate instability (3, 23%), and desaturation (2, 15%). Circulation-related events occurred in 85% of overall cases. No accidents, such as line/tube removal or falls, were observed. The highest incidence of PSE was observed during the mobilization level of standing (8 out of 39 sessions, 20.5%). The occurrence of PSE correlated with the highest activity level under logistic regression analysis. Close vigilance is required for intubated patients during active mobilization in the standing position with regard to circulatory dynamics.

Evaluation of Cardiac Circadian Rhythm Deconditioning Induced by 5-to-60 Days of Head-Down Bed Rest

Head-down tilt (HDT) bed rest elicits changes in cardiac circadian rhythms, generating possible adverse health outcomes such as increased arrhythmic risk. Our aim was to study the impact of HDT duration on the circadian rhythms of heart beat (RR) and ventricular repolarization (QTend) duration intervals from 24-h Holter ECG recordings acquired in 63 subjects during six different HDT bed rest campaigns of different duration (two 5-day, two 21-day, and two 60-day). Circadian rhythms of RR and QTend intervals series were evaluated by Cosinor analysis, resulting in a value of midline (MESOR), oscillation amplitude (OA) and acrophase (φ). In addition, the QTc (with Bazett correction) was computed, and day-time, night-time, maximum and minimum RR, QTend and QTc intervals were calculated. Statistical analysis was conducted, comparing: (1) the effects at 5 (HDT5), 21 (HDT21) and 58 (HDT58) days of HDT with baseline (PRE); (2) trends in recovery period at post-HDT epochs (R) in 5-day, 21-day, and 60-day HDT separately vs. PRE; (3) differences at R + 0 due to bed rest duration; (4) changes between the last HDT acquisition and the respective R + 0 in 5-day, 21-day, and 60-day HDT. During HDT, major changes were observed at HDT5, with increased RR and QTend intervals' MESOR, mostly related to day-time lengthening and increased minima, while the QTc shortened. Afterward, a progressive trend toward baseline values was observed with HDT progression. Additionally, the φ anticipated, and the OA was reduced during HDT, decreasing system's ability to react to incoming stimuli. Consequently, the restoration of the orthostatic position elicited the shortening of RR and QTend intervals together with QTc prolongation, notwithstanding the period spent in HDT. However, the magnitude of post-HDT changes, as well as the difference between the last HDT day and R + 0, showed a trend to increase with increasing HDT duration, and 5/7 days were not sufficient for recovering after 60-day HDT. Additionally, the φ postponed and the OA significantly increased at R + 0 compared to PRE after 5-day and 60-day HDT, possibly increasing the arrhythmic risk. These results provide evidence that continuous monitoring of astronauts' circadian rhythms, and further investigations on possible measures for counteracting the observed modifications, will be key for future missions including long periods of weightlessness and gravity transitions, for preserving astronauts' health and mission success.

Effects of Prolonged Head-Down Bed Rest on Cardiac and Vascular Baroreceptor Modulation and Orthostatic Tolerance in Healthy Individuals

Orthostatic intolerance commonly occurs after prolonged bed rest, thus increasing the risk of syncope and falls. Baroreflex-mediated adjustments of heart rate and sympathetic vasomotor activity (muscle sympathetic nerve activity – MSNA) are crucial for orthostatic tolerance. We hypothesized that prolonged bed rest deconditioning alters overall baroreceptor functioning, thereby reducing orthostatic tolerance in healthy volunteers. As part of the European Space Agency Medium-term Bed Rest protocol, 10 volunteers were studied before and after 21 days of −6° head down bed rest (HDBR). In both conditions, subjects underwent ECG, beat-by-beat blood pressure, respiratory activity, and MSNA recordings while supine (REST) and during a 15-min 80° head-up tilt (TILT) followed by a 3-min −10 mmHg stepwise increase of lower body negative pressure to pre-syncope. Cardiac baroreflex sensitivity (cBRS) was obtained in the time (sequence method) and frequency domain (spectrum and cross-spectrum analyses of RR interval and systolic arterial pressure – SAP, variability). Baroreceptor modulation of sympathetic discharge activity to the vessels (sBRS) was estimated by the slope of the regression line between the percentage of MSNA burst occurrence and diastolic arterial pressure. Orthostatic tolerance significantly decreased after HDBR (12 ± 0.6 min) compared to before (21 ± 0.6 min). While supine, heart rate, SAP, and cBRS were unchanged before and after HDBR, sBRS gain was slightly depressed after than before HDBR (sBRS: −6.0 ± 1.1 versus −2.9 ± 1.5 burst% × mmHg⁻¹, respectively). During TILT, HR was higher after than before HDBR (116 ± 4 b/min versus 100 ± 4 b/min, respectively), SAP was unmodified in both conditions, and cBRS indexes were lower after HDBR (α index: 3.4 ± 0.7 ms/mmHg; BRS_SEQ 4.0 ± 1.0) than before (α index: 6.4 ± 1.0 ms/mmHg; BRS_SEQ 6.8 ± 1.2). sBRS gain was significantly more depressed after HDBR than before (sBRS: −2.3 ± 0.7 versus −4.4 ± 0.4 burst% × mmHg⁻¹, respectively). Our findings suggest that baroreflex-mediated adjustments in heart rate and MSNA are impaired after prolonged bed rest. The mechanism likely contributes to the decrease in orthostatic tolerance.

Endothelial Cell-Targeted Deletion of PPARγ Blocks Rosiglitazone-Induced Plasma Volume Expansion and Vascular Remodeling in Adipose Tissue

Thiazolidinediones (TZDs) are peroxisome proliferator-activated receptor γ (PPARγ) agonists that represent an effective class of insulin-sensitizing agents; however, clinical use is associated with weight gain and peripheral edema. To elucidate the role of PPARγ expression in endothelial cells (ECs) in these side effects, EC-targeted PPARγ knockout (Pparg ΔEC) mice were placed on a high-fat diet to promote PPARγ agonist-induced plasma volume expansion, and then treated with the TZD rosiglitazone. Compared with Pparg-floxed wild-type control (Pparg f/f) mice, Pparg ΔEC treated with rosiglitazone are resistant to an increase in extracellular fluid, water content in epididymal and inguinal white adipose tissue, and plasma volume expansion. Interestingly, histologic assessment confirmed significant rosiglitazone-mediated capillary dilation within white adipose tissue of Pparg f/f mice, but not Pparg ΔEC mice. Analysis of ECs isolated from untreated mice in both strains suggested the involvement of changes in endothelial junction formation. Specifically, compared with cells from Pparg f/f mice, Pparg ΔEC cells had a 15-fold increase in focal adhesion kinase, critically important in EC focal adhesions, and >3-fold significant increase in vascular endothelial cadherin, the main component of focal adhesions. Together, these results indicate that rosiglitazone has direct effects on the endothelium via PPARγ activation and point toward a critical role for PPARγ in ECs during rosiglitazone-mediated plasma volume expansion.

Clinical neurophysiology of postural tachycardia syndrome

Postural tachycardia syndrome (POTS) is one of several disorders of orthostatic intolerance (OI). It is defined by the development of symptoms of cerebral hypoperfusion or sympathetic activation and a sustained heart rate increment of 30 beats/min or more (40 beats/min for teenagers) within 10min of standing or head-up tilt in the absence of orthostatic hypotension; the standing heart rate is often 120 beats/min or higher. POTS is approximately five times more common in women than men. This heterogeneous syndrome is caused by several pathophysiologic mechanisms (limited autonomic neuropathy, hyperadrenergic state, hypovolemia, venous pooling, deconditioning), which are not mutually exclusive. Anxiety and somatic hypervigilance play significant roles in POTS. Common comorbidities include visceral pain and dysmotility, chronic fatigue and fibromyalgia, migraine, joint hypermobility, mitral valve prolapse, and inappropriate sinus tachycardia. Patients with suspected POTS should undergo comprehensive cardiac and neurologic examinations and autonomic and laboratory tests to determine the most likely pathophysiologic basis of OI. The objectives of POTS management are to (1) increase the time that patients can stand, perform daily activities, and exercise and (2) avoid syncope. Management involves nonpharmacologic (fluid and salt loading, physical countermaneuvers, compression garments, exercise training) and pharmacologic (β-blockers, pyridostigmine, fludrocortisone, midodrine) approaches.

Analogs of microgravity: head-down tilt and water immersion

This article briefly reviews the fidelity of ground-based methods used to simulate human existence in weightlessness (spaceflight). These methods include horizontal bed rest (BR), head-down tilt bed rest (HDT), head-out water immersion (WI), and head-out dry immersion (DI; immersion with an impermeable elastic cloth barrier between subject and water). Among these, HDT has become by far the most commonly used method, especially for longer studies. DI is less common but well accepted for long-duration studies. Very few studies exist that attempt to validate a specific simulation mode against actual microgravity. Many fundamental physical, and thus physiological, differences exist between microgravity and our methods to simulate it, and between the different methods. Also, although weightlessness is the salient feature of spaceflight, several ancillary factors of space travel complicate Earth-based simulation. In spite of these discrepancies and complications, the analogs duplicate many responses to 0 G reasonably well. As we learn more about responses to microgravity and spaceflight, investigators will continue to fine-tune simulation methods to optimize accuracy and applicability.

Sustained mild hypergravity reduces spontaneous cardiac baroreflex sensitivity

Head-to-foot gravitational force >1G (+Gz hypergravity) augments venous pooling in the lower body and reduces central blood volume during exposure, compared with 1Gz. Central hypovolemia has been reported to reduce spontaneous cardiac baroreflex sensitivity. However, no investigations have examined spontaneous cardiac baroreflex sensitivity during exposure to sustained mild +Gz hypergravity. We therefore hypothesized that mild +Gz hypergravity would reduce spontaneous cardiac baroreflex sensitivity, compared with 1Gz. To test this hypothesis, we examined spontaneous cardiac baroreflex sensitivity in 16 healthy men during exposure to mild +Gz hypergravity using a short-arm centrifuge. Beat-to-beat arterial blood pressure (tonometry) and R-R interval (electrocardiography) were obtained during 1Gz and 1.5Gz exposures. Spontaneous cardiac baroreflex sensitivity was assessed by sequence slope and transfer function gain. Stroke volume was calculated from the arterial pressure waveform using a three-element model. All indices of spontaneous cardiac baroreflex sensitivity decreased significantly (up slope: 18.6±2.3→12.7±1.6ms/mmHg, P<0.001; down slope: 19.0±2.5→13.2±1.3ms/mmHg, P=0.002; transfer function gain in low frequency: 14.4±2.2→10.1±1.1ms/mmHg, P=0.004; transfer function gain in high frequency: 22.2±7.5→12.4±3.5ms/mmHg, P<0.001). Stroke volume decreased significantly (88±5→80±6ml, P=0.025). Moreover, although systolic arterial pressure variability increased, R-R interval variability did not increase. These results suggest that even mild +Gz hypergravity reduces spontaneous cardiac baroreflex sensitivity, increasing the risk of cardiovascular disturbance during the exposure.

Cardiac parasympathetic reactivation following exercise: implications for training prescription

The objective of exercise training is to initiate desirable physiological adaptations that ultimately enhance physical work capacity. Optimal training prescription requires an individualized approach, with an appropriate balance of training stimulus and recovery and optimal periodization. Recovery from exercise involves integrated physiological responses. The cardiovascular system plays a fundamental role in facilitating many of these responses, including thermoregulation and delivery/removal of nutrients and waste products. As a marker of cardiovascular recovery, cardiac parasympathetic reactivation following a training session is highly individualized. It appears to parallel the acute/intermediate recovery of the thermoregulatory and vascular systems, as described by the supercompensation theory. The physiological mechanisms underlying cardiac parasympathetic reactivation are not completely understood. However, changes in cardiac autonomic activity may provide a proxy measure of the changes in autonomic input into organs and (by default) the blood flow requirements to restore homeostasis. Metaboreflex stimulation (e.g. muscle and blood acidosis) is likely a key determinant of parasympathetic reactivation in the short term (0-90 min post-exercise), whereas baroreflex stimulation (e.g. exercise-induced changes in plasma volume) probably mediates parasympathetic reactivation in the intermediate term (1-48 h post-exercise). Cardiac parasympathetic reactivation does not appear to coincide with the recovery of all physiological systems (e.g. energy stores or the neuromuscular system). However, this may reflect the limited data currently available on parasympathetic reactivation following strength/resistance-based exercise of variable intensity. In this review, we quantitatively analyse post-exercise cardiac parasympathetic reactivation in athletes and healthy individuals following aerobic exercise, with respect to exercise intensity and duration, and fitness/training status. Our results demonstrate that the time required for complete cardiac autonomic recovery after a single aerobic-based training session is up to 24 h following low-intensity exercise, 24-48 h following threshold-intensity exercise and at least 48 h following high-intensity exercise. Based on limited data, exercise duration is unlikely to be the greatest determinant of cardiac parasympathetic reactivation. Cardiac autonomic recovery occurs more rapidly in individuals with greater aerobic fitness. Our data lend support to the concept that in conjunction with daily training logs, data on cardiac parasympathetic activity are useful for individualizing training programmes. In the final sections of this review, we provide recommendations for structuring training microcycles with reference to cardiac parasympathetic recovery kinetics. Ultimately, coaches should structure training programmes tailored to the unique recovery kinetics of each individual.

Understanding critically ill patients hemodynamic response to mobilization: using the evidence to make it safe and feasible

In today's critical care environment, we face a difficult but essential task. We must provide comprehensive, compassionate, complex, technological care without causing harm to our patients. To foster a patient-safe environment, we must examine care practices and processes to reduce the chance of error. Successful early mobilization of critically ill patients can reduce several complications including atelectasis and ventilator-associated pneumonia and shorten ventilator time along with cognitive and functional limitations that linger 1 to 5 years after discharge from the intensive care unit. A long-standing challenge to successful mobilization of critically ill patients is the safety concern of hemodynamic instability. An in-depth exploration of what happens to a critically ill patient physiology during mobilization was done to foster a better understanding of strategies that promote adaptation. The article examines the evidence supporting the need to assess readiness for mobilization to reduce the risk of adverse events. Evidence-based tools and techniques to help clinicians prevent hemodynamic instability before, during, and after in-bed or out-of-bed mobilization are discussed. With safety serving as the overriding goal, we can overcome the barriers and succeed in creating and sustaining a culture of early progressive mobility programs within the intensive care unit.

Mobilization Patterns of Patients After an Acute Myocardial Infarction

This study was aimed to identify the mobilization patterns of acute myocardial infarction (AMI) patients during their first three days in the coronary care unit (CCU) by performing a prospective observational pilot study design. The study included 31 diagnosed AMI patients admitted to three CCUs. Mobilization patterns classified as bed rest, semi-fowler, transfer to chair, and standing/walking were documented by CCU nurses for 72 consecutive hours after patient admission to the CCU. Of 2,232 possible mobilization periods (72 hr × 31 patients), 1,385 recorded observations of mobilization (62%) were obtained. Bed rest and semi-fowler positions were the most common mobilization patterns; together they accounted for 70% of the documented positions over the first 72 hr in the CCU. Patients who experience an uncomplicated AMI spend the majority of their first 72 hr in CCU in bed.

Sex differences in blood pressure control during 6° head-down tilt bed rest

Spaceflight-induced orthostatic intolerance has been studied for decades. Although ∼22% of the astronaut corps are women, most mechanistic studies use mostly male subjects, despite known sex differences in autonomic control and postflight orthostatic intolerance. We studied adrenergic, baroreflex, and autonomic indexes during continuous infusions of vasoactive drugs in men and women during a 60-day head-down bed rest. Volunteers were tested before bed rest (20 men and 10 women) and around day 30 (20 men and 10 women) and day 60 (16 men and 8 women) of bed rest. Three increasing doses of phenylephrine (PE) and sodium nitroprusside were infused for 10 min after an infusion of normal saline. A 20-min rest period separated the phenylephrine and sodium nitroprusside infusions. Autonomic activity was approximated by spectral indexes of heart rate and blood pressure variability, and baroreflex sensitivity was measured by the spontaneous baroreflex slope. Parasympathetic modulation and baroreflex sensitivity decreased with bed rest, with women experiencing a larger decrease in baroreflex sensitivity by day 30 than men. The sympathetic activation of men and parasympathetic responsiveness of women in blood pressure control during physiological stress were preserved throughout bed rest. During PE infusions, women experienced saturation of the R-R interval at high frequency, whereas men did not, revealing a sex difference in the parabolic relationship between high-frequency R-R interval, a measurement of respiratory sinus arrhythmia, and R-R interval. These sex differences in blood pressure control during simulated microgravity reveal the need to study sex differences in long-duration spaceflight to ensure the health and safety of the entire astronaut corps.

Measuring postural-related changes of spontaneous baroreflex sensitivity after repeated long-duration diving: frequency domain approaches

Sustained water immersion is thought to modulate orthostatic tolerance to an extent dependent on the duration and repetition over consecutive days of the diving sessions. We tested this hypothesis investigating in ten healthy subjects the potential changes in the cardiovascular response to head-up tilt induced by single and multiple resting air dives. Parametric cross-spectral analysis of spontaneous RR interval and systolic arterial pressure variability was performed in three experimental sessions: before diving (BD), after single 6-hour dive (ASD), and after multiple 6-hour dives (AMD, 5 consecutive days with 18-hour surface interval). From this analysis, baroreflex sensitivity (BRS) was computed as spectral power ratio (αBRS), non-causal transfer function gain (tfBRS) and causal transfer function gain (γBRS) evaluated at low frequency (0.04-0.14Hz) in the supine position (su) as well as in the standing upright position in the early tilt (et) and late tilt (lt) epochs. We found that, while αBRS decreased significantly in et and lt compared to su during all sessions, tfBRS and γBRS decreased during ASD and AMD but not during BD; moreover γBRS evidenced a progressive decrease from BD to ASD and to AMD in both et and lt epochs. These results indicate the necessity of following a causal approach for the estimation of BRS in the frequency domain, and suggest a progressive impairment of the baroreflex response to postural stress after single and multiple dives, which may reflect symptoms of increasing orthostatic intolerance.

Antagonistic effect of flumazenil after midazolam sedation on arterial-cardiac baroreflex

BACKGROUND

Flumazenil is generally administered to antagonise the sedative effect of midazolam. However, although flumazenil completely antagonises the sedative effect of midazolam, a few effects remain unantagonised. Hence, it is unclear whether flumazenil restores the attenuation of the arterial-cardiac baroreflex (i.e. arterial-heart rate reflex) induced by midazolam. We investigated the antagonistic effect of flumazenil administered after midazolam on cardiac baroreflex, to reveal whether complete recovery from midazolam-induced sedation by flumazenil administration is accompanied by restoration of midazolam's attenuating effects on the cardiac baroreflex.

METHOD

Twelve healthy male subjects received midazolam followed by flumazenil until complete recovery from midazolam sedation. Before and during midazolam sedation, and after flumazenil administration, cardiac baroreflex function was assessed by sequence analysis and transfer function analysis between spontaneous oscillations in systolic arterial pressure and R-R interval.

RESULTS

During midazolam sedation, defined by an Observer's Assessment of Alertness/Sedation scale score of 3, BIS value decreased significantly. Simultaneously, the baroreflex indices of the two analyses decreased significantly compared with baseline, suggesting attenuated cardiac baroreflex function. With complete recovery from midazolam sedation by flumazenil, indicated by an Observer's Assessment of Alertness/Sedation scale score of 5, BIS values returned to the baseline level. Simultaneously, cardiac baroreflex indices also returned to baseline levels.

CONCLUSION

The present results suggest that complete recovery from midazolam sedation by flumazenil is accompanied by restoration of the attenuated cardiac baroreflex function induced by midazolam.

Left ventricular diastolic dysfunction is associated with impaired baroreflex at rest and during orthostatic stress in hypertensive patients with left ventricular hypertrophy

The study aimed to determine the relationship between left ventricular (LV) diastolic function and the heart's spontaneous baroreflex at rest and in response to orthostatic stress during a prospective follow-up of hypertensive patients with LV hypertrophy (LVH+). LV structure and function and baroreflex sensitivity (BRS) during tilt testing were evaluated in 24 LVH+ patients and compared with 25 age-matched healthy controls and 25 hypertensive patients without LVH (LVH-). Clinical status, diastolic function and BRS were then assessed in LVH+ patients during treatment with telmisartan (monotherapy or combined with hydrochlorothiazide and/or amlodipine) at 6- and 18-month follow-ups. LVH+ patients had significantly altered diastolic function indices and decreased BRS as compared with healthy controls and LVH- patients. During the 18-month follow-up, favorable changes in diastolic function were associated with improvement in BRS at rest and during tilting. In multivariate regression models, an index reflecting rate of LV myocardial relaxation (E'sept) where E'sept denotes peak early diastolic velocity at the septal mitral annulus and a surrogate for LV filling pressure (E/E'sept), independently from other clinical and echocardiographic variables related to the low-frequency component of BRS during tilting. In conclusion, the LV diastolic function indices have independent associations with BRS parameters obtained at rest and during orthostatic stress in LVH+ patients receiving long-term pharmacological intervention.

Postural tachycardia syndrome: a heterogeneous and multifactorial disorder

Postural tachycardia syndrome (POTS) is defined by a heart rate increment of 30 beats/min or more within 10 minutes of standing or head-up tilt in the absence of orthostatic hypotension; the standing heart rate is often 120 beats/min or higher. POTS manifests with symptoms of cerebral hypoperfusion and excessive sympathoexcitation. The pathophysiology of POTS is heterogeneous and includes impaired sympathetically mediated vasoconstriction, excessive sympathetic drive, volume dysregulation, and deconditioning. POTS is frequently included in the differential diagnosis of chronic unexplained symptoms, such as inappropriate sinus tachycardia, chronic fatigue, chronic dizziness, or unexplained spells in otherwise healthy young individuals. Many patients with POTS also report symptoms not attributable to orthostatic intolerance, including those of functional gastrointestinal or bladder disorders, chronic headache, fibromyalgia, and sleep disturbances. In many of these cases, cognitive and behavioral factors, somatic hypervigilance associated with anxiety, depression, and behavioral amplification contribute to symptom chronicity. The aims of evaluation in patients with POTS are to exclude cardiac causes of inappropriate tachycardia; elucidate, if possible, the most likely pathophysiologic basis of postural intolerance; assess for the presence of treatable autonomic neuropathies; exclude endocrine causes of a hyperadrenergic state; evaluate for cardiovascular deconditioning; and determine the contribution of emotional and behavioral factors to the patient's symptoms. Management of POTS includes avoidance of precipitating factors, volume expansion, physical countermaneuvers, exercise training, pharmacotherapy (fludrocortisone, midodrine, β-blockers, and/or pyridostigmine), and behavioral-cognitive therapy. A literature search of PubMed for articles published from January 1, 1990, to June 15, 2012, was performed using the following terms (or combination of terms): POTS; postural tachycardia syndrome, orthostatic; orthostatic; syncope; sympathetic; baroreceptors; vestibulosympathetic; hypovolemia; visceral pain; chronic fatigue; deconditioning; headache; Chiari malformation; Ehlers-Danlos; emotion; amygdala; insula; anterior cingulate; periaqueductal gray; fludrocortisone; midodrine; propranolol; β-adrenergic; and pyridostigmine. Studies were limited to those published in English. Other articles were identified from bibliographies of the retrieved articles.

[Relationship between baroreflex function and training effects on altitude training]

OBJECTIVE

Altitude training is frequently used for athletes requiring competitive endurance in an attempt to improve their sea-level performance. However, there has been no study in which the mechanisms by which spontaneous arterial-cardiac baroreflex function changes was examined in responders or nonresponders of altitude training. The purpose of this study was to clarify the different effects of altitude training on baroreflex function between responders and nonresponders.

METHODS

Twelve university student cross-country skiers (6 men, 6 women; age, 19±1 years) participated in the altitude training in a camp for 3 weeks, which was carried out in accordance with the method of Living High-Training Low. Baroreflex function was estimated by transfer function analysis before and after the training.

RESULTS

The responders of the training were 3 men and 2 women, and the nonresponders were 3 men and 4 women. In the responders, the transfer function gain in the high-frequency range significantly increased after the training (28.9→46.5 ms/mmHg p=0.021). On the other hand, no significant change in this index was observed in the nonresponders (25.9→21.2 ms/mmHg p=0.405).

CONCLUSION

As indicated by the results of transfer function gain in the high-frequency range, the baroreflex function in the responders increased significantly after the altitude training, whereas no significant change was observed in the nonresponders.

Low-magnitude whole body vibration with resistive exercise as a countermeasure against cardiovascular deconditioning after 60 days of head-down bed rest

Whole body vibration with resistive exercise is a promising countermeasure against some weightlessness-induced dysfunctions. Our objective was to study whether the combination of low-magnitude whole body vibration with a resistive exercise can prevent the cardiovascular deconditioning induced by a nonstrict 60-day head-down bed rest (Earth Star International Bed Rest Experiment Project). Fourteen healthy men participated in this study. We recorded electrocardiograms and blood pressure waves by means of a noninvasive beat-by-beat measurement system (Cardiospace, integrated by Centre National d'Etudes Spatiales and Astronaut Center of China) during an orthostatic test (20 min of 75-degree head-up tilt test) before and immediately after bed rest. We estimated heart rate, blood pressure, cardiac output, stroke volume, total peripheral resistance, baroreflex sensitivity, and heart rate variability. Low-magnitude whole body vibration with resistive exercise prevented an increase of the sympathetic index (reflecting the sympathovagal balance of cardiac autonomic control) and limited the decrease of the spontaneous baroreflex sensitivity induced by 60 days of head-down bed rest. However, this countermeasure had very little effect on cardiac hemodynamics and did not improve the orthostatic tolerance. This combined countermeasure did not efficiently prevent orthostatic intolerance but prevents changes in the autonomic nervous system associated with cardiovascular deconditioning. The underlying mechanisms remain hypothetical but might involve cutaneous and muscular mechanoreceptors.

Exercise plus volume loading prevents orthostatic intolerance but not reduction in cerebral blood flow velocity after bed rest

This study tested the hypothesis that reduction in cerebral blood flow (CBF) during orthostatic stress after bed rest can be ameliorated with volume loading, exercise, or both. Transcranial Doppler was used to measure changes in CBF velocity during lower body negative pressure (LBNP) before and after an 18-day bed rest in 33 healthy subjects. Subjects were assigned into four groups with similar age and sex: 1) supine cycling during bed rest (Exercise group; n = 7), 2) volume loading with Dextran infusion after bed rest to restore reduced left ventricular filling pressure (Dextran group; n = 7), 3) exercise combined with volume loading to prevent orthostatic intolerance (Ex-Dex group; n = 7), and 4) a control group (n = 12). LBNP tolerance was measured using a cumulative stress index (CSI). After bed rest, CBF velocity was reduced at a lower level of LBNP in the Control group, and the magnitude of reduction was greater in the Ex-Dex group. However, reduction in orthostatic tolerance was prevented in the Ex-Dex group. Notably, volume loading alone prevented greater reductions in CBF velocity after bed rest, but CSI was reduced still by 25%. Finally, decreases in CBF velocity during LBNP were correlated with reduction in cardiac output under all conditions (r(2) = 0.86; P = < 0.001). Taken together, these findings demonstrate that volume loading alone can ameliorate reductions in CBF during LBNP. However, the lack of associations between changes in CBF velocity and orthostatic tolerance suggests that reductions in CBF during LBNP under steady-state conditions by itself are unlikely to be a primary factor leading to orthostatic intolerance.

Enhanced open-loop but not closed-loop cardiac baroreflex sensitivity during orthostatic stress in humans

The neural interaction between the cardiopulmonary and arterial baroreflex may be critical for the regulation of blood pressure during orthostatic stress. However, studies have reported conflicting results: some indicate increases and others decreases in cardiac baroreflex sensitivity (i.e., gain) with cardiopulmonary unloading. Thus the effect of orthostatic stress-induced central hypovolemia on regulation of heart rate via the arterial baroreflex remains unclear. We sought to comprehensively assess baroreflex function during orthostatic stress by identifying and comparing open- and closed-loop dynamic cardiac baroreflex gains at supine rest and during 60° head-up tilt (HUT) in 10 healthy men. Closed-loop dynamic "spontaneous" cardiac baroreflex sensitivities were calculated by the sequence technique and transfer function and compared with two open-loop carotid-cardiac baroreflex measures using the neck chamber system: 1) a binary white-noise method and 2) a rapid-pulse neck pressure-neck suction technique. The gain from the sequence technique was decreased from -1.19 ± 0.14 beats·min(-1)·mmHg(-1) at rest to -0.78 ± 0.10 beats·min(-1)·mmHg(-1) during HUT (P = 0.005). Similarly, closed-loop low-frequency baroreflex transfer function gain was reduced during HUT (P = 0.033). In contrast, open-loop low-frequency transfer function gain between estimated carotid sinus pressure and heart rate during white-noise stimulation was augmented during HUT (P = 0.01). This result was consistent with the maximal gain of the carotid-cardiac baroreflex stimulus-response curve (from 0.47 ± 0.15 beats·min(-1)·mmHg(-1) at rest to 0.60 ± 0.20 beats·min(-1)·mmHg(-1) at HUT, P = 0.037). These findings suggest that open-loop cardiac baroreflex gain was enhanced during HUT. Moreover, under closed-loop conditions, spontaneous baroreflex analyses without external stimulation may not represent open-loop cardiac baroreflex characteristics during orthostatic stress.

Blood vessel remodeling and physical inactivity in humans

Physical inactivity is associated with an increase in cardiovascular risk that cannot be fully explained by traditional or novel risk factors. Inactivity is also associated with changes in hemodynamic stimuli, which exert direct effects on the vasculature leading to remodeling and a proatherogenic phenotype. In this review, we synthesize and summarize in vivo evidence relating to the impact of local and systemic models of physical inactivity on conduit arteries, resistance vessels, and the microcirculation in humans. Taken together, the literature suggests that a rapid inward structural remodeling of vessels occurs in response to physical inactivity. The magnitude of this response is dependent on the "dose" of inactivity. Moreover, changes in vascular function are found at resistance and microvessel levels in humans. In conduit arteries, a strong interaction between vascular function and structure is present, which results in conflicting data regarding the impact of inactivity on conduit artery function. While much of the cardioprotective effect of exercise is related to the nitric oxide pathway, deconditioning may primarily be associated with activation of vasoconstrictor pathways. The effects of deconditioning on the vasculature are therefore not simply the opposite of those in response to exercise training. Given the importance of sedentary behavior, future studies should provide further insight into the impact of inactivity on the vasculature and other (novel) markers of vascular health. Moreover, studies should examine the role of (hemodynamic) stimuli that underlie the characteristic vascular adaptations during deconditioning. Our review concludes with some suggestions for future research directions.

PAR-1622 is a selective peroxisome proliferator-activated receptor gamma partial activator with preserved antidiabetic efficacy and broader safety profile for fluid retention

Peroxisome proliferator-activated receptor (PPAR) gamma is known to be a key regulator of insulin resistance. PAR-1622 is a novel small molecule compound synthesized in Dong-A research center. In this study, we characterized the pharmacological profiles of PAR-1622, a selective partial activator of PPARgamma. In transient transactivation assays, PAR-1622 [(S)-2-ethoxy-3(4-(5-(4-(5-(methoxymethyl)isoxazol-3-yl)phenyl)-3-methylthiophen-2-yl)methoxy)phenyl)propanoic acid] showed a partial activator against human PPARgamma with an EC(50) of 41 nM and a maximal response of 37% relative to the full agonist rosiglitazone without activating human PPARdelta. PAR-1622 was 56 folds more selective for human PPARgamma than for human PPARalpha (EC(50), 2304 nM), which means that it is a selective partial activator of PPARgamma. PAR-1622 also showed a partial activator against mouse PPARgamma with an EC(50) of 427 nM and a maximal response was 57% of that of rosiglitazone. INT-131, a selective PPARgamma partial agonist in clinical stage, also was a partial activator against human PPARgamma with an EC(50) of 83 nM and a maximal response achieved by INT-131 was 49% of that observed with full agonist rosiglitazone. In functional assays using human mesenchymal stem cells, PAR-1622 induced adipocyte differentiation, which was 3-fold more potent with a comparable maximum response compared to INT-131. Furthermore, PAR-1622 significantly improved hyperglycemia in db/db when orally administered at a dose of 1 mg/kg/day for 5 days. In hemodilution assays with Evans Blue, rosiglitazone significantly increased the plasma volume in ICR mice that were orally administered 30 mg/kg/day for 9 days; however, PAR-1622 showed no significant effects on plasma volume, similar to INT-131. These results suggest that PAR-1622 is a selective partial activator of PPARgamma and has excellent antihyperglycemic activities and a broad safety profile for fluid retention.

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

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

Dynamic adaptation of cardiac baroreflex sensitivity to prolonged exposure to microgravity: data from a 16-day spaceflight

This study explored the process of arterial baroreflex adaptation to microgravity, starting from the first day of flight, during the 16-day STS-107 Columbia Space Shuttle mission. Continuous blood pressure (BP), ECG, and respiratory frequency were collected in four astronauts on ground (baseline) and during flight at days 0–1, 6–7, and 12–13, both at rest and during moderate exercise (75 W) on a cycle ergometer. Sensitivity of the baroreflex heart rate control (BRS) was assessed by sequence and spectral alpha methods. Baroreflex effectiveness index (BEI); low-frequency (LF) power and high-frequency (HF) power of systolic BP (SBP), diastolic BP (DBP), and R-R interval (RRI); the RRI LF/HF ratio; and the RRI root mean square of successive differences (RMSSD) index were also estimated. We found that, at rest, BRS increased in early flight phase, compared with baseline (means ± SE: 18.3 ± 3.4 vs. 10.4 ± 1.2 ms/mmHg; P < 0.05), and it tended to return to baseline in subsequent days. During exercise, BRS was lower than at rest, without differences between preflight and in-flight values. At rest, in the early flight phase, RMSSD and RRI HF power increased ( P < 0.05) compared with baseline, whereas LF powers of SBP and DBP decreased. No statistical difference was found in these parameters during exercise before vs. during flight. These findings demonstrate that heart rate baroreflex sensitivity and markers of cardiac vagal modulation are enhanced during early exposure to microgravity, likely because of the blood centralization, and return to baseline values in subsequent flight phases, possibly because of the fluid loss. No deconditioning seems to occur in the baroreflex control of the heart.

Diastolic suction is impaired by bed rest: MRI tagging studies of diastolic untwisting

Bed rest deconditioning leads to physiological cardiac atrophy, which may compromise left ventricular (LV) filling during orthostatic stress by reducing diastolic untwisting and suction. To test this hypothesis, myocardial-tagged magnetic resonance imaging (MRI) was performed, and maximal untwisting rates of the endocardium, midwall, and epicardium were calculated by Harmonic Phase Analysis (HARP) before and after -6 degrees head-down tilt bed rest for 18 days with (n = 14) and without exercise training (n = 10). LV mass and LV end-diastolic volume were measured using cine MRI. Exercise subjects cycled on a supine ergometer for 30 min, three times per day at 75% maximal heart rate (HR). After sedentary bed rest, there was a significant reduction in maximal untwisting rates of the midwall (-46.8 +/- 14.3 to -35.4 +/- 12.4 degrees /s; P = 0.04) where untwisting is most reliably measured, and to a lesser degree of certainty in the endocardium (-50.3 +/- 13.8 to -40.1 +/- 18.5 degrees /s; P = 0.09); the epicardium was unchanged. In contrast, when exercise was performed in bed, untwisting rates were enhanced at the endocardium (-48.4 +/- 20.8 to -72.3 +/- 22.3 degrees /ms; P = 0.05) and midwall (-39.2 +/- 12.2 to -59.0 +/- 19.6 degrees /s; P = 0.03). The differential response was significant between groups at the endocardium (interaction P = 0.02) and the midwall (interaction P = 0.004). LV mass decreased in the sedentary group (156.4 +/- 30.3 to 149.5 +/- 27.9 g; P = 0.07), but it increased slightly in the exercise-trained subjects (156.4 +/- 34.3 to 162.3 +/- 40.5 g; P = 0.16); (interaction P = 0.03). We conclude that diastolic untwisting is impaired following sedentary bed rest. However, exercise training in bed can prevent the physiological cardiac remodeling associated with bed rest and preserve or even enhance diastolic suction.

Central hypervolemia with hemodilution impairs dynamic cerebral autoregulation

BACKGROUND

Frequent changes in the perioperative central blood volume could affect cerebral autoregulation through alterations in sympathetic nerve activity, cardiac output, blood viscosity, and cerebral vasomotor tone. However, the effect of dynamic cerebral autoregulation has not been studied during acute wide-ranging changes in central blood volume, especially with respect to central hypervolemia with hemodilution.

METHODS

We evaluated dynamic cerebral autoregulation during central hypovolemia and central hypervolemia with hemodilution using spectral and transfer function analysis between mean arterial blood pressure (MBP) and cerebral blood flow (CBF) velocity variability in 12 individuals. Rapid changes in central blood volume were achieved using two levels of lower body negative pressure (-15 and -30 mm Hg) and two discrete infusions of normal saline (15 mL/kg and total 30 mL/kg). We then estimated changes in central blood volume as central venous pressure (CVP) and/or cardiac output using impedance cardiography.

RESULTS

Steady-state CBF velocity and cardiac output decreased at -30 mm Hg lower body negative pressure (changes of CVP approximately -4 mm Hg) or were increased by each saline infusion (changes of CVP 4-6 mm Hg), without a significant change in MBP. However, transfer function gain (magnitude of transfer) between MBP and CBF velocity variability significantly increased only after saline infusion, suggesting an increased magnitude of transfer from MBP oscillations to CBF fluctuations during central hypervolemia with hemodilution.

CONCLUSION

Our results suggest that, although steady-state CBF velocity changes under both central hypervolemia and hypovolemia, only hypervolemic hemodilution impairs dynamic cerebral autoregulation.

Respiratory modulation of cardiovascular rhythms before and after short-duration human spaceflight

AIM

Astronauts commonly return from space with altered short-term cardiovascular dynamics and blunted baroreflex sensitivity. Although many studies have addressed this issue, post-flight effects on the dynamic circulatory control remain incompletely understood. It is not clear how long the cardiovascular system needs to recover from spaceflight as most post-flight investigations only extended between a few days and 2 weeks.

METHODS

In this study, we examined the effect of short-duration spaceflight (1-2 weeks) on respiratory-mediated cardiovascular rhythms in five cosmonauts. Two paced-breathing protocols at 6 and 12 breaths min(-1) were performed in the standing and supine positions before spaceflight, and after 1 and 25 days upon return. Dynamic baroreflex function was evaluated by transfer function analysis between systolic pressure and the RR intervals.

RESULTS

Post-flight orthostatic blood pressure control was preserved in all cosmonauts. In the standing position after spaceflight there was an increase in heart rate (HR) of approx. 20 beats min(-1) or more. Averaged for all five cosmonauts, respiratory sinus dysrhythmia and transfer gain reduced to 40% the day after landing, and had returned to pre-flight levels after 25 days. Low-frequency gain decreased from 6.6 (3.4) [mean (SD)] pre-flight to 3.9 (1.6) post-flight and returned to 5.7 (1.3) ms mmHg(-1) after 25 days upon return to Earth. Unlike alterations in the modulation of HR, blood pressure dynamics were not significantly different between pre- and post-flight sessions.

CONCLUSION

Our results indicate that short-duration spaceflight reduces respiratory modulation of HR and decreases cardiac baroreflex gain without affecting post-flight arterial blood pressure dynamics. Altered respiratory modulation of human autonomic rhythms does not persist until 25 days upon return to Earth.

From space to Earth: advances in human physiology from 20 years of bed rest studies (1986-2006)

Bed rest studies of the past 20 years are reviewed. Head-down bed rest (HDBR) has proved its usefulness as a reliable simulation model for the most physiological effects of spaceflight. As well as continuing to search for better understanding of the physiological changes induced, these studies focused mostly on identifying effective countermeasures with encouraging but limited success. HDBR is characterised by immobilization, inactivity, confinement and elimination of Gz gravitational stimuli, such as posture change and direction, which affect body sensors and responses. These induce upward fluid shift, unloading the body's upright weight, absence of work against gravity, reduced energy requirements and reduction in overall sensory stimulation. The upward fluid shift by acting on central volume receptors induces a 10-15% reduction in plasma volume which leads to a now well-documented set of cardiovascular changes including changes in cardiac performance and baroreflex sensitivity that are identical to those in space. Calcium excretion is increased from the beginning of bed rest leading to a sustained negative calcium balance. Calcium absorption is reduced. Body weight, muscle mass, muscle strength is reduced, as is the resistance of muscle to insulin. Bone density, stiffness of bones of the lower limbs and spinal cord and bone architecture are altered. Circadian rhythms may shift and are dampened. Ways to improve the process of evaluating countermeasures--exercise (aerobic, resistive, vibration), nutritional and pharmacological--are proposed. Artificial gravity requires systematic evaluation. This review points to clinical applications of BR research revealing the crucial role of gravity to health.

Human cerebral autoregulation before, during and after spaceflight

Exposure to microgravity alters the distribution of body fluids and the degree of distension of cranial blood vessels, and these changes in turn may provoke structural remodelling and altered cerebral autoregulation. Impaired cerebral autoregulation has been documented following weightlessness simulated by head-down bed rest in humans, and is proposed as a mechanism responsible for postspaceflight orthostatic intolerance. In this study, we tested the hypothesis that spaceflight impairs cerebral autoregulation. We studied six astronauts approximately 72 and 23 days before, after 1 and 2 weeks in space (n = 4), on landing day, and 1 day after the 16 day Neurolab space shuttle mission. Beat-by-beat changes of photoplethysmographic mean arterial pressure and transcranial Doppler middle cerebral artery blood flow velocity were measured during 5 min of spontaneous breathing, 30 mmHg lower body suction to simulate standing in space, and 10 min of 60 deg passive upright tilt on Earth. Dynamic cerebral autoregulation was quantified by analysis of the transfer function between spontaneous changes of mean arterial pressure and cerebral artery blood flow velocity, in the very low- (0.02-0.07 Hz), low- (0.07-0.20 Hz) and high-frequency (0.20-0.35 Hz) ranges. Resting middle cerebral artery blood flow velocity did not change significantly from preflight values during or after spaceflight. Reductions of cerebral blood flow velocity during lower body suction were significant before spaceflight (P < 0.05, repeated measures ANOVA), but not during or after spaceflight. Absolute and percentage reductions of mean (+/- s.e.m.) cerebral blood flow velocity after 10 min upright tilt were smaller after than before spaceflight (absolute, -4 +/- 3 cm s(-1) after versus -14 +/- 3 cm s(-1) before, P = 0.001; and percentage, -8.0 +/- 4.8% after versus -24.8 +/- 4.4% before, P < 0.05), consistent with improved rather than impaired cerebral blood flow regulation. Low-frequency gain decreased significantly (P < 0.05) by 26, 23 and 27% after 1 and 2 weeks in space and on landing day, respectively, compared with preflight values, which is also consistent with improved autoregulation. We conclude that human cerebral autoregulation is preserved, and possibly even improved, by short-duration spaceflight.

Sympathetic nervous activity decreases during head-down bed rest but not during microgravity

We tested the hypothesis that sympathoadrenal activity in humans is low during spaceflight and that this effect can be simulated by head-down bed rest (HDBR). Platelet norepinephrine and epinephrine were measured as indexes of long-term changes in sympathoadrenal activity. Ten normal healthy subjects were studied before and during HDBR of 2-wk duration, as well as during an ambulatory study period of a similar length. Platelet norepinephrine concentrations (half-life = 2 days) were studied in five cosmonauts, 2 wk before launch, within 12 h after landing after 11–12 days of flight, and at least 2 wk after return to Earth. Because of the long half-life of platelet norepinephrine, data obtained early after landing would still reflect the microgravity state. Platelet norepinephrine decreased markedly during HDBR ( P < 0.001), whereas there were no significant changes when subjects were ambulatory. Platelet epinephrine did not change during HDBR. During microgravity, platelet norepinephrine and epinephrine increased in four of the five cosmonauts. Platelet norepinephrine concentrations expressed in percentage of preflight and pre-HDBR values, respectively, were significantly different during microgravity compared with HDBR [153 ± 28% (mean ± SE) vs. 60 ± 6%, P < 0.004]. Corresponding values for platelet epinephrine were also significant (293 ± 85 vs. 90 ± 12%, P < 0.01). The mechanism of the platelet norepinephrine and epinephrine response during spaceflight flight is most likely related to the concomitant decrease in plasma volume. HDBR cannot be applied to simulate changes in sympathoadrenal activity during microgravity.

Collecting duct-specific deletion of peroxisome proliferator-activated receptor gamma blocks thiazolidinedione-induced fluid retention

The peroxisome proliferator-activated receptor subtype gamma (PPARgamma) ligands, namely the synthetic insulin-sensitizing thiazolidinedione (TZD) compounds, have demonstrated great potential in the treatment of type II diabetes. However, their clinical applicability is limited by a common and serious side effect of edema. To address the mechanism of TZD-induced edema, we generated mice with collecting duct (CD)-specific disruption of the PPARgamma gene. We found that mice with CD knockout of this receptor were resistant to the rosiglitazone- (RGZ) induced increases in body weight and plasma volume expansion found in control mice expressing PPARgamma in the CD. RGZ reduced urinary sodium excretion in control and not in conditional knockout mice. Furthermore, RGZ stimulated sodium transport in primary cultures of CD cells expressing PPARgamma and not in cells lacking this receptor. These findings demonstrate a PPARgamma-dependent pathway in regulation of sodium transport in the CD that underlies TZD-induced fluid retention.

Errors of measurement for blood volume parameters: a meta-analysis

The volume of red blood cells (V(RBC)) is used routinely in the diagnostic workup of polycythemia, in assessing the efficacy of erythropoietin administration, and to study factors affecting oxygen transport. However, errors of various methods of measurement of V(RBC) and related parameters are not well characterized. We meta-analyzed 346 estimates of error of measurement of V(RBC) for techniques based on Evans blue (V(RBC,Evans)), 51chromium-labeled red blood cells (V(RBC,51Cr)), and carbon monoxide (CO) rebreathing (V(RBC,CO)), as well as hemoglobin mass with the carbon-monoxide method (M(Hb,CO)), in athletes and active and inactive subjects undergoing various experimental and control treatments lasting minutes to months. Subject characteristics and experimental treatments had little effect on error of measurement, but measures with the smallest error showed some increase in error with increasing time between trials. Adjusted to 1 day between trials and expressed as coefficients of variation, mean errors for M(Hb,CO) (2.2%; 90% confidence interval 1.4-3.5%) and V(RBC,51Cr) (2.8%; 2.4-3.2%) were much less than those for V(RBC,Evans) (6.7%; 4.9-9.4%) and V(RBC,CO) (6.7%; 3.4-14%). Most of the error of V(RBC,Evans) was due to error in measurement of volume of plasma via Evans blue dye (6.0%; 4.5-7.8%), which is the basis of V(RBC,Evans). Most of the error in V(RBC,CO) was due to estimates from laboratories with a relatively large error in M(Hb,CO), the basis of V(RBC,CO). V(RBC,51Cr) and M(Hb,CO) are the best measures for research on blood-related changes in oxygen transport. With care, V(RBC,Evans) is suitable for clinical applications of blood-volume measurement.

Orthostatic blood pressure control before and after spaceflight, determined by time-domain baroreflex method

Reduction in plasma volume is a major contributor to orthostatic tachycardia and hypotension after spaceflight. We set out to determine time- and frequency-domain baroreflex (BRS) function during preflight baseline and venous occlusion and postflight orthostatic stress, testing the hypothesis that a reduction in central blood volume could mimic the postflight orthostatic response. In five cosmonauts, we measured finger arterial pressure noninvasively in supine and upright positions. Preflight measurements were repeated using venous occlusion thigh cuffs to impede venous return and “trap” an increased blood volume in the lower extremities; postflight sessions were between 1 and 3 days after return from 10- to 11-day spaceflight. BRS was determined by spectral analysis and by PRVXBRS, a time-domain BRS computation method. Although all completed the stand tests, two of five cosmonauts had drastically reduced pulse pressures and an increase in heart rate of ∼30 beats/min or more during standing after spaceflight. Averaged for all five subjects in standing position, high-frequency interbeat interval spectral power or transfer gain did not decrease postflight. Low-frequency gain decreased from 8.1 (SD 4.0) preflight baseline to 6.8 (SD 3.4) postflight ( P = 0.033); preflight with thigh cuffs inflated, low-frequency gain was 9.4 (SD 4.3) ms/mmHg. There was a shift in time-domain-determined pulse interval-to-pressure lag, Tau, toward higher values ( P < 0.001). None of the postflight results were mimicked during preflight venous occlusion. In conclusion, two of five cosmonauts showed abnormal orthostatic response 1 and 2 days after spaceflight. Overall, there were indications of increased sympathetic response to standing, even though we can expect (partial) restoration of plasma volume to have taken place. Preflight venous occlusion did not mimic the postflight orthostatic response.