n-3 Polyunsaturated fatty acids supplementation in peripheral artery disease: the OMEGA-PAD trial

Despite current consensus guidelines recommending intensive cardiovascular risk factor management for peripheral artery disease (PAD), patients suffering from PAD continue to experience significant morbidity and mortality. This excess morbid burden is at least partially related to impaired vascular function and systemic inflammation. Interventions bridging this gap are critical. Dietary supplementation of n-3 polyunsaturated fatty acids (n-3 PUFA) has been shown to improve endothelial function and reduce inflammation in different cohorts, as well as to decrease cardiovascular events in secondary prevention trials in patients with coronary artery disease. Their effects in the PAD population are, however, less well understood. The OMEGA-PAD trial is a double-blinded, randomized, placebo-controlled trial that examines the impact of a high-dose, short-duration dietary oral supplementation of n-3 PUFA on vascular function and inflammation in patients with established PAD. The purpose of this article is to provide a detailed description of the design and methods of the OMEGA-PAD trial, and a summary of baseline characteristics of the cohort.

Peripheral artery disease and risk of cardiovascular events in patients with coronary artery disease: insights from the Heart and Soul Study

Among patients with coronary artery disease (CAD), those with peripheral artery disease (PAD) have a greater vulnerability to cardiovascular (CV) events than those with CAD alone. In a prospective cohort study of patients with CAD, we evaluated potential mechanisms that might explain the adverse CV outcomes associated with PAD. We performed a prospective cohort study of 1018 patients with stable CAD who were recruited from 2000 to 2002. Incident symptomatic PAD events were adjudicated during a follow-up period of 7.2 ± 2.6 years. We used Cox proportional hazards models to evaluate the association between incident symptomatic PAD events and subsequent risk of CV events or death. Models were adjusted for demographics, traditional risk factors, inflammation, insulin resistance and health behaviors. Among the 1018 patients, 50 patients who did not report a history of PAD at baseline suffered incident symptomatic PAD events during the follow-up period. Those patients had a higher risk of subsequent CV events and death compared to those who did not develop PAD. After adjustment for traditional risk factors, symptomatic PAD events remained associated with a 70% increased risk of subsequent CV events (adjusted HR 1.7; 95% CI 1.0, 2.9; p = 0.04) and an 80% increased risk of death (adjusted HR 1.8; 95% CI 1.2, 2.7; p = 0.006). Inflammatory biomarkers were the strongest risk factor contributing to the excess risk. In a contemporary cohort of patients with CAD, incident symptomatic PAD events were associated with an increased risk for subsequent CV events. The increased vulnerability to CV events was partially explained by shared CV risk factors and inflammation.

Can I take a space flight? Considerations for doctors

Commercial investment is bringing space tourism closer to reality. Marlene Grenon and colleagues: outline what doctors will need to know.

Association between depression and peripheral artery disease: insights from the heart and soul study

Background

Depression is known to increase the risk of coronary artery disease, but few studies have evaluated the association between depression and peripheral artery disease (PAD). We examined the association of depression with PAD and evaluated potential mediators of this association.

Methods and Results

We used data from the Heart and Soul Study, a prospective cohort of 1024 men and women with coronary artery disease recruited in 2000–2002 and followed for a mean of 7.2±2.6 years. Depressive symptoms were assessed with the validated 9-item Patient Health Questionnaire. Prevalent PAD at baseline was determined by self-report. Prospective PAD events were adjudicated on the basis of review of medical records. We used logistic regression and Cox proportional-hazards models to estimate the independent associations of depressive symptoms with prevalent PAD and subsequent PAD events. At baseline, 199 patients (19%) had depressive symptoms (Patient Health Questionnaire ≥10). Prevalent PAD was reported by 12% of patients with depression and 7% of those without depression (base model adjusted for age and sex: odds ratio 1.79, 95% confidence interval 1.06–3.04, P=0.03; full model adjusted for comorbidities, medications, PAD risk factors, inflammation, and health behaviors: odds ratio 1.59, 95% confidence interval 0.90–2.83, P=0.11). During follow-up, PAD events occurred in 7% of patients with depression and 5% of those without depression (base model adjusted for age and sex: hazard ratio 2.09, 95% confidence interval 1.09–4.00, P=0.03; full model adjusted for comorbidities, medications, PAD risk factors, inflammation, and health behaviors: hazard ratio 1.33, 95% confidence interval 0.65–2.71, P=0.44). Factors explaining >5% of the association between depression and incident PAD events included race/ethnicity, diabetes, congestive heart failure, high-density lipoprotein, triglyceride levels, serum creatinine, inflammation, smoking, and levels of physical activity.

Conclusions

Depressive symptoms were associated with a greater risk of PAD. Because the association was explained partly by modifiable risk factors, our findings suggest that more aggressive treatment of these risk factors could reduce the excess risk of PAD associated with depression. (J Am Heart Assoc. 2012;1:e002667 doi: 10.1161/JAHA.112.002667.)

Polyunsaturated fatty acids and peripheral artery disease

There is substantial evidence that polyunsaturated fatty acids (PUFAs) such as n-3 and n-6 fatty acids (FAs) play an important role in prevention of atherosclerosis. In vitro and in vivo studies focusing on the interactions between monocytes and endothelial cells have explored the molecular effects of FAs on these interactions. Epidemiological surveys, followed by large, randomized, control trials have demonstrated a reduction in major cardiovascular events with supplementation of n-3 FAs in secondary prevention settings. The evidence of beneficial effects specific to patients with peripheral artery disease (PAD) remains elusive, and is the focus of this review.

Abstract 418: Is Depression a Risk Factor for Peripheral Arterial Disease? Insights from the Heart and Soul Study

Objectives: Risk factors for peripheral artery disease (PAD) are tightly linked to those for coronary artery disease (CAD). Although depression is known to increase risk of CAD, fewer studies have evaluated depression and PAD, and the mechanisms linking depression and PAD remain unknown. We examined the association of depression with prevalent PAD and with prospective PAD events and evaluated potential mediators of these associations.

Methods: We used data from the Heart and Soul Study, a prospective cohort of 1,024 men and women with CAD recruited in 2000-2002 and followed for a mean of 7.2 years with <5% loss to follow-up. Depression was assessed with the validated nine-item Patient Health Questionnaire. Prevalent PAD at baseline was determined by self-report. Prospective PAD events were adjudicated based on physician diagnosis, radiological imaging and/or need for PAD surgery. We used logistic and Cox models to estimate the independent associations of depression with prevalent PAD at baseline and subsequent PAD events, adjusting for potential mediators of these associations.

Results: At baseline, depression affected 19% of patients. Prevalent PAD was reported by 12.1% of patients with depression and 7.3% of those without (age-adjusted OR 1.79, 95% CI 1.06-3.03, p=0.03). In age-adjusted models, physical inactivity, diabetes, and CRP level each explained more than 5% of the association between depression and prevalent PAD. During follow-up, PAD events occurred in 9% of patients with depression and 6% of those without (age-adjusted HR 1.77, 95% CI 1.02-3.07, p=0.04). Factors explaining more than 5% of the association between depression and incident PAD events included inactivity, diabetes, prevalent PAD, race/ethnicity, smoking, HDL, and triglyceride levels.

Conclusions: Depression was associated with a greater risk of PAD at baseline and with prospective PAD events, providing evidence that depression is an important risk factor for the development of PAD. Since elevations in traditional, modifiable cardiovascular risk factors partially explain these associations, such risk factors should be assessed and aggressively treated. Furthermore, this study emphasizes the importance of depression screening and treatment in patients with PAD.

Abstract 353: Mechanical (or “Gravitational”) Unloading Reduces Inflammatory and Cell Adhesion Molecule Gene Expression in Human Endothelial Cells

Objectives: Mechanical forces including gravity affect mechanotransduction and subsequent cell function. The goal of this study was to investigate the impact of mechanical unloading (MU) and loading (ML) of endothelial cells (ECs) with microgravity and hypergravity respectively, with the hypothesis that MU alters expression of inflammatory and adhesion molecule gene expression and these changes are reversed by ML.

Methods: Human umbilical vascular endothelial cells (HUVECs) grown to confluency were studied. A desktop random positioning machine and a gravitational cell-loading apparatus provided MU and ML conditions, respectively. The experimental conditions included: 1) controls exposed to 1-gravity environment for 24 h (CL), 2) MU for 24 hours, 3) MU for 24 hours with three 30-minutes periods of ML of 12-gravity (MU/ML). Gene expression was studied with reverse transcription followed by real-time quantitative polymerase chain reaction (qRTPCR).

Results: MU led to a significant decrease in gene expression of the adhesion molecules ICAM-1, VCAM-1, E-Selectin, as well as TNF-α, IL-6 and VEGF. In contrast, NOS-3, Caveolin-1 and -2 were significantly increased with MU. The changes observed in gene expression with MU were reversed by gravitational mechanical loading (MU/ML).

Conclusions: Gravitational MU decreases inflammatory and adhesion molecule gene expression and these changes are reversed by short periods of ML. This points towards the importance of gravitational loading in ECs function and cellular interactions.

Abstract 289: Peripheral Arterial Disease and Vulnerability to Cardiovascular Events: Data from the Heart and Soul Study

Objectives: Among patients with coronary artery disease (CAD), those with comorbid peripheral arterial disease (PAD) have a greater vulnerability to cardiovascular (CV) events than those with CAD alone. It is unclear why the presence of PAD predisposes patients with CAD to adverse CV outcomes. In a prospective cohort study of patients with CAD, we evaluated potential mechanisms that might explain the adverse CV outcomes associated with PAD.

Methods: We evaluated 1020 patients with stable CAD who were recruited from 2000-2002 and followed for an average of 7.2 ± 2.6 years. Incident PAD events were adjudicated based on physician diagnosis, radiological imaging and/or need for PAD surgery. We used Cox proportional hazards models to evaluate the association between development of PAD events (entered as a time-dependent covariate) and subsequent risk of cardiovascular (CV) events (myocardial infarction, congestive heart failure, cerebrovascular event, revascularization) or death based on detailed review of medical records. Models were adjusted for traditional cardiovascular risk factors (age, race, sex, diabetes, hypertension, hypercholesterolemia, kidney function, inflammation) and self-reported history of PAD at baseline.

Results: Among the 1020 patients, 67 patients developed incident PAD events during the follow-up period. Patients who developed PAD events had a higher risk of subsequent CV events and death compared to those who did not develop PAD (Figure). After adjustment for traditional cardiovascular risk factors and self-reported history of PAD, development of PAD events remained associated with an 84% increased risk of subsequent cardiovascular events [adjusted HR 1.84, 95% CI 1.15, 2.96, p=0.01] and an 89% increased risk of death [adjusted HR 1.89, 95% CI 1.30-2.74, p=0.0009]. Only a small portion of this association was explained by baseline differences in levels of IL-6, TNF-alpha and fibrinogen.

Conclusions: In a contemporary cohort of patients with CAD, development of incident PAD was associated with adverse CV outcomes. The increased risk of CV events was not explained by shared risk factors or inflammation. Further research is necessary to understand how the presence of PAD increases risk of subsequent CV events.

Surgery in Space: Where are we at now?

In the coming decades, as we continue our path of space exploration beyond Earth's orbit, we will be required to provide sound medical and surgical care for the safety of space travellers and space flight participants. A few investigations have taken place in the field of surgery in space. In this paper, the authors review the present literature in order to identify possible limitations that currently exist and that could impair our ability to provide surgical care during spaceflight, from the pre-operative to the post-operative period.

Challenges in aerospace medicine education

Aerospace medicine training and research represents a dream for many and a challenge for most. In Canada, although some opportunities exist for the pursuit of education and research in the aerospace medicine field, they are limited despite the importance of this field for enabling safe human space exploration. In this commentary, we aim to identify some of the challenges facing individuals wishing to get involved in the field as well as the causal factors for these challenges. We also explore strategies to mitigate against these.

Occlusion of the common and internal iliac arteries for aortoiliac aneurysm repair: experience with the Amplatzer vascular plug

BACKGROUND

We sought to evaluate and describe our centre's experience with the Amplatzer vascular plug (AVP) for the occlusion of common and internal iliac arteries (CIA; IIA) during endovascular aortic aneurysm repair (EVAR).

METHODS

We performed a retrospective analysis of 20 consecutive patients between October 2006 and December 2007, who underwent occlusion of the CIA or IIA before or during EVAR to prevent endoleak.

RESULTS

Among these 20 patients, 21 occlusion procedures occurred and 20 were successful. In the only unsuccessful case, the patient had EVAR, but occlusion with an AVP was not possible because of severe narrowing at the origin of the vessel. Of the successfully treated patients, 2 presented with ruptured aneurysms, whereas the others had elective procedures. Eleven patients received aortouni-iliac grafts and femoral-femoral bypass, and 9 patients received a bifurcated stent graft. In 5 patients, the AVP occlusion and EVAR procedures were staged; in these cases occlusion occurred first, followed by EVAR on average 29 (standard deviation [SD] 23) days later. We deployed 7 AVPs in the CIA, whereas 13 were deployed in the IIA. The average diameter of the vessels occluded was 10 (SD 1) mm and the average size of the device used was 13 (SD 1) mm, representing a device diameter 28% (SD 2%) greater than the vessel diameter. We used a single device in 18 patients, whereas 2 devices were deployed in the same artery in 2 patients. Four patients underwent concomitant coil embolization. On follow-up computed tomography (CT) scans, all occlusion procedures were clinically successful. At the 14-month (SD 1 mo) follow-up, 4 patients had a small type-II endoleak unrelated to the occlusion procedure and 1 had a type-I endoleak that required graft limb extension. Four patients had buttock claudication but none had changes in sexual function, ischemic complications or device dislodgement on CT scans.

CONCLUSION

The AVP is a safe and effective method to occlude the CIA and IIA in patients undergoing EVAR.

Canadian experience with percutaneous endovascular aneurysm repair: short-term outcomes

BACKGROUND

To decrease the morbidity associated with cut-downs during endovascular aneurysm repair, some authors have suggested the totally percutaneous endovascular repair (PEVAR). The goal of this report is to evaluate and describe our centre's experience with the total percutaneous endovascular aneurysm repair (PEVAR) for aortic abdominal aneurysm (AAA).

METHODS

We performed a retrospective analysis of 15 consecutive patients with AAA, including 1 with right common iliac artery aneurysm.

RESULTS

There were 12 men and 3 women with a mean age of 74 (standard deviation [SD] 2) years who underwent PEVAR with a Perclose ProGlide suture-mediated closure system between July 2007 and July 2008. All surgeries were elective. Forty percent of patients had a history of smoking, 73% were hypertensive, 33% were diabetic, 20% had chronic obstructive pulmonary disease and 40% had coronary artery disease. Fourteen patients had bilateral deployment for bifurcated devices (7 bifurcated Gore Excluder, 7 bifurcated Cook Zenith grafts), and 1 patient had unilateral deployment for a Cook Zenith device. The outer diameter of the sheaths used for puncture sites was on average 18.1-Fr (SD 0.6), with main bodies being 21.1-Fr (SD 0.3) and contralateral sides 15-Fr (SD 0.3). Procedural success was 93%, with 1 patient requiring a femoral artery cut-down because of failure of the Perclose device to deploy in the groin. Another patient had persistent venous bleeding in 1 puncture site that stopped with skin suturing. Endovascular aneurysm repair was 100% with no conversion to open surgery and no type-I endoleaks. The mean length of stay in hospital was 2.2 (SD 0.4) days. There were no long-term groin complications at 6 (SD 1) months' follow-up.

CONCLUSION

To our knowledge, this is the first Canadian report of experience with PEVAR using the Perclose device. The technique is safe, reliable and allows discharge of patients soon after surgery.

Use of short-radius centrifugation to augment ankle-brachial indices

BACKGROUND

Peripheral arterial disease is mainly caused by atherosclerosis and is characterized by decreased circulation, lower blood pressure, and insufficient tissue perfusion in the lower extremities. The hemodynamics of standing and altered gravity environments have been well studied relative to arm blood pressures but are less well understood for ankle pressures.

METHODS

Because regional blood pressure depends, in part, on the gravitational pressure gradient, we hypothesized that artificial gravity exposure on a short-arm centrifuge with the center of rotation above the head would increase blood pressure in the lower extremities. Cardiovascular parameters for 12 healthy subjects were measured during exposure to supine short-arm centrifugation at 20, 25, and 30 revolutions per minute (rpm), corresponding to centripetal accelerations of 0.94, 1.47, and 2.11 Gz at the foot level, respectively.

RESULTS

Systolic ankle blood pressure significantly increased at all levels of centrifugation. Ankle-brachial indices (the ratio of systolic ankle to arm blood pressures) increased significantly from 1.17 +/- 0.03 to 1.58 +/- 0.03 at 0.94 Gz (P < 0.005), 1.74 +/- 0.02 at 1.47 Gz (P < 0.005), and 1.89 +/- 0.06 at 2.11 Gz (P < 0.005). Systolic arm blood pressure significantly increased at 2.11 Gz, but heart rate did not change significantly. All parameters returned to normal after cessation of centrifugation.

CONCLUSIONS

We demonstrated that short-radius centrifugation leads to an increase in ankle-brachial indices. This could have potential implications for the treatment of peripheral arterial disease.

A type IV paraesophageal hernia containing a volvulized sigmoid colon

A type IV paraesophageal hernia is a rare complication in esophageal hiatus and is an uncommon presentation of hiatal hernia. We report herein a 71-year-old man who presented with abdominal pain, nausea and constipation that was attributed to a sigmoid volvulus. At laparotomy, the sigmoid volvulus was identified as strangulated and involved in a type IV paraesophageal hernia in which the esophageal junction was located in its normal anatomic position. The esophageal hiatus was impressively dilated and there was no evidence suggesting previous mechanical disruption of the esophageal hiatus.

Surgical strategies for severe calcification of the aorta (porcelain aorta) in two patients with homozygous familial hypercholesterolemia

Homozygous familial hypercholesterolemia (HzFH) is a rare genetic defect caused predominantly by mutations at the low-density lipoprotein receptor. Until recent advances in the management of this complex disorder, patients affected by HzFH rarely survived beyond 30 years of age. Two patients with HzFH who survived to adulthood and developed cardiovascular complications requiring surgery are reported. In these patients, a porcelain aorta complicated surgical management. Lipid profile, mutational analysis and pathological assessment of the aorta were performed in two patients referred for cardiac surgery. The first patient was a 46-year-old man with a history of coronary artery bypass grafting (CABG) and recurrent severe angina who, because of a heavily calcified ascending aorta, required a complex repeat CABG. The second patient was a 42-year-old woman who underwent CAGB at 28 years of age and presented 13 years later with aortic stenosis. The extensive calcifications of the whole aortic root required performance of a modified Cabrol procedure. A porcelain aorta appears to be a feature of HzFH. This has an important impact on surgical planning and management and on possible pathophysiological processes related to the cardiovascular complications of HzFH.

Why is orthostatic tolerance lower in women than in men? Renal and cardiovascular responses to simulated microgravity and the role of midodrine

BACKGROUND

Exposure to microgravity induces cardiovascular deconditioning, manifested by orthostatic intolerance (OI). We assessed the renal, cardioendocrine, and cardiovascular responses of women and men to simulated microgravity to examine the impact of gender on OI.

METHODS

Fifteen healthy female and 14 healthy male subjects were given a constant diet for 3 to 5 days, after which they underwent a tilt-stand test (pre-TST) and began 14 to 16 days of head-down tilt bed rest (HDTB), followed by a repeat tilt-stand test (post-TST). Female subjects began HDTB so that the post-TST was at the same time in their menstrual cycle as their pre-TST. Twenty-four-hour urine collections (daily), hormonal measurements, plethysmography, and cardiovascular system identification were performed.

RESULTS

The times to presyncope were significantly different for men and women before (p= .005) and after HDTB (p= .001), with all of the women but only 50% of the men experiencing presyncope during the pre-TST (p= .002) and all of the women but only 64% of the men experiencing presyncope during the post-TST. At baseline, the following differences between women and men were observed: women had higher serum aldosterone levels (p = .02), higher parasympathetic responsiveness (p = .01), lower sympathetic responsiveness (p = .05), and lower venous compliance (p = .05). Several parameters changed with HDTB in both men and women. In a double-blinded randomized trial, midodrine (5 mg orally) or placebo given to female subjects 1 hour before post-TST was ineffective in preventing 01.

CONCLUSION

In conclusion, the frequency of OI is higher in women than in men and is not modified by midodrine at the dose used. This increased susceptibility is likely secondary to intrinsic basal differences in the activity of volume-mediated parasympathetic and adrenergic systems and in venous tone. Thus, approaches to reduce OI in women are likely to differ from those effective in men.

Simulated microgravity induces microvolt T wave alternans

BACKGROUND

There are numerous anecdotal reports of ventricular arrhythmias during spaceflight; however, it is not known whether spaceflight or microgravity systematically increases the risk of cardiac dysrhythmias. Microvolt T wave alternans (MTWA) testing compares favorably with other noninvasive risk stratifiers and invasive electrophysiological testing in patients as a predictor of sudden cardiac death, ventricular tachycardia, and ventricular fibrillation. We hypothesized that simulated microgravity leads to an increase in MTWA.

METHODS

Twenty-four healthy male subjects underwent 9 to 16 days of head-down tilt bed rest (HDTB). MTWA was measured before and after the bed rest period during bicycle exercise stress. For the purposes of this study, we defined MTWA outcome to be positive if sustained MTWA was present with an onset heart rate<or=125 bpm. During various phases of HDTB, the following were also performed: daily 24-hour urine collections, serum electrolytes and catecholamines, and cardiovascular system identification (measure of autonomic function).

RESULTS

Before HDTB, 17% of the subjects were MTWA positive [95%CI: (0.6%, 37%)]; after HDTB, 42% of the subjects were MTWA positive [95%CI: (23%, 63%)] (P=0.03). The subjects who were MTWA positive after HDTB compared with MTWA negative subjects had an increased versus decreased sympathetic responsiveness (P=0.03) and serum norepinephrine levels (P=0.05), and a trend toward higher potassium excretion (P=0.06) after bed rest compared to baseline.

CONCLUSIONS

HDTB leads to an increase in MTWA, providing the first evidence that simulated microgravity has a measurable effect on electrical repolarization processes. Possible contributing factors include loss in potassium and changes in sympathetic function.

Bed rest effects on human calf hemodynamics and orthostatic intolerance: a model-based analysis

INTRODUCTION

Microgravity-induced orthostatic intolerance continues to be a primary problem after space missions. Its etiology remains uncertain despite significant research efforts over the past years. We hypothesized that calf hemodynamic parameters (compliance and resistance) are significantly affected by 14 to 16-d head-down bed rest (simulated microgravity), and their alterations play a role in the pathogenesis of orthostatic intolerance (OI) following bed rest.

METHODS

To estimate these parameters, we developed a model-based approach to quantitatively simulate calf vascular response to venous occlusion, which only necessitates measurement of plethysmography data. In this study, plethysmography data were obtained from 29 subjects before and after 14-16 d of head-down bed rest. The subjects also underwent a tilt/stand test before and after bed rest.

RESULTS

Statistical analyses demonstrated an increase in calf compliance (1.87 +/- 0.08, mean +/- SE, pre-bed rest; 2.16 +/- 0.10, end-bed rest) but no significant change in vascular resistance following bed rest. Compared with the tilt-intolerant subjects, those who were tilt-tolerant before bed rest had significantly higher calf compliance [2.00 +/- 0.09 (tolerant); 1.58 +/- 0.09 (intolerant)] and higher vascular resistance [7.79 +/- 0.18 (tolerant); 6.91 +/- 0.40 (intolerant)]. After bed rest, no such difference was detected.

DISCUSSION

Based on these results, we validated the hypothesis that, instead of causing orthostatic intolerance, higher calf compliance before bed rest leads to recruitment of compensatory mechanisms (validated by the enhanced vascular resistance during venous occlusion) for a better toleration of orthostatic stress. With the absence of orthostatic challenge during bed rest, the difference in calf hemodynamic parameters is attenuated between the tilt-tolerant and tilt-intolerant groups.

Readaptation from simulated microgravity as a stimulus for improved orthostatic tolerance: role of the renal, cardioendocrine, and cardiovascular systems

BACKGROUND

Microgravity and simulated microgravity (SM) lead to important changes in orthostatic tolerance (OT), the autonomic nervous system (ANS), and the volume-regulating systems. After one is exposed to microgravity or SM, a period of readaptation to gravity is known to take place, but it is not certain if orthostatic function returns to baseline within the initial recovery and what mechanisms are involved. We hypothesized that after a period of recovery, OT, ANS, and volume-regulating systems would return to pre-SM levels.

METHODS

To test this hypothesis, 24 healthy men were placed on a constant diet for 3 to 5 days, after which a tilt-stand test (pre-TST) was performed. The TST was repeated after 14 to 16 days of head-down tilt bed rest (HDTB) (post-TST) and a 3-day period of recovery (rec-TST), at which times measurements of renal, cardioendocrine, and cardiovascular systems were conducted.

RESULTS

Presyncope occurred in 46% of subjects pre-TST, in 72% post-TST, and in 23% during rec-TST. OT was significantly better during the recovery period than at baseline (p = .03). There was a significant decrease in urinary sodium and potassium excretion, along with a decrease in plasma renin activity and serum and urine aldosterone compared with baseline. Serum norepinephrine and sympathetic responsiveness remained below baseline values.

CONCLUSION

In summary, OT improved compared with baseline after a period of readaptation. Retention of electrolytes (sodium, potassium) could be involved. These findings indicate that recovery after SM is not simply a gradual return to baseline values but is instead a dynamic process reflecting interaction of multiple regulatory systems.

Role of individual predisposition in orthostatic intolerance before and after simulated microgravity

Orthostatic intolerance (OI) is a major problem after spaceflight. Its etiology remains uncertain, but reports have pointed toward an individual susceptibility to OI. We hypothesized that individual predisposition plays an important role in post-bed rest OI. Twenty-four healthy male subjects were equilibrated on a constant diet, after which they underwent tilt-stand test (pre-TST). They then completed 14-16 days of head-down-tilt bed rest, and 14 of the subjects underwent repeat tilt-stand test (post-TST). During various phases, the following were performed: 24-h urine collections and hormonal measurements, plethysmography, and cardiovascular system identification (a noninvasive method to assess autonomic function and separately quantify parasympathetic and sympathetic responsiveness). Development of presyncope or syncope defined OI. During pre-TST, 11 subjects were intolerant and 13 were tolerant. At baseline, intolerant subjects had lower serum aldosterone (P < 0.01), higher excretion of potassium (P = 0.01), lower leg venous compliance (P = 0.03), higher supine parasympathetic responsiveness (P = 0.02), and lower standing sympathetic responsiveness (P = 0.048). Of the 14 subjects who completed post-TST, 9 were intolerant and 5 were tolerant. Intolerant subjects had lower baseline serum cortisol (P = 0.03) and a higher sodium level (P = 0.02) compared with tolerant subjects. Thus several physiological characteristics were associated with increased susceptibility to OI. We propose a new model for OI, whereby individuals with greater leg venous compliance recruit compensatory mechanisms (activation of the renin-angiotensin-aldosterone system and sympathetic nervous system, and withdrawal of the parasympathetic nervous system) in the face of daily postural challenges, which places them at an advantage to face orthostatic stress. With head-down-tilt bed rest, the stimulus to recruit compensatory mechanisms disappears, and differences between the two subgroups attenuate.

Sleep restriction does not affect orthostatic tolerance in the simulated microgravity environment

Orthostatic intolerance (OI) is a major problem following spaceflight, and, during flight, astronauts also experience sleep restriction. We hypothesized that sleep restriction will compound the risk and severity of OI following simulated microgravity and exaggerate the renal, cardioendocrine, and cardiovascular adaptive responses to it. Nineteen healthy men were equilibrated on a constant diet, after which they underwent a tilt-stand test. They then completed 14–16 days of simulated microgravity [head-down tilt bed rest (HDTB)], followed by repeat tilt-stand test. During HDTB, 11 subjects were assigned to an 8-h sleep protocol (non-sleep restricted), and 8 were assigned to a sleep-restricted protocol with 6 h of sleep per night. During various phases, the following were performed: 24-h urine collections, hormonal measurements, and cardiovascular system identification. Development of presyncope or syncope defined OI. There was a significant decrease in time free of OI ( P = 0.02) and an increase in OI occurrence ( P = 0.06) after HDTB among all subjects. However, the increase in OI occurrence did not differ significantly between the two groups ( P = 0.60). The two groups also experienced similar physiological changes with HDTB (initial increase in sodium excretion; increased excretion of potassium at the end of HDTB; increase in plasma renin activity secretion without a change in serum or urine aldosterone). No significant change in autonomic function or catecholamines was noted. Simulated microgravity leads to increased OI, and sleep restriction does not additively worsen OI in simulated microgravity. Furthermore, conditions of sleep restriction and nonsleep restriction are similar with respect to renal, cardioendocrine, and cardiovascular responses to simulated microgravity.

Renal, endocrine, and cardiovascular responses to bed rest in male subjects on a constant diet

BACKGROUND

Exposure to actual and simulated microgravity induces cardiovascular deconditioning through a variety of factors. Although the mechanisms involved remain uncertain, one involves alterations in volume-regulating systems--the hypothesis being tested in this study. To maximize our ability to detect subtle changes in the volume-regulating systems, subjects were studied on a high-average salt intake to maximally suppress these systems basally.

METHODS

Fourteen healthy male subjects underwent 14-day head-down tilt bed rest (HDTB) during which a constant 200 mEq sodium, 100 mEq potassium diet was maintained. Daily 24-hour urine collection was performed; plasma renin activity, serum aldosterone, plethysmography, and cardiovascular system identification were performed during a control period (pre-HDTB) and at the end of HDTB (end HDTB).

RESULTS

Sodium excretion increased initially (pre-HDTB = 182.8 +/- 10.4 mEq/total volume; early HDTB = 236.4 +/- 13.0; p = .002) and then returned to baseline values. Potassium excretion increased 4 days after the initiation of HDTB and remained elevated thereafter (pre-HDTB = 82.2 +/- 2.4/total volume; mid- to late HDTB = 89.4 +/- 2.1; p = .02). Plasma renin activity increased significantly with HDTB (pre-HDTB = 1.28 +/- 0.21 ng/mL/h; end HDTB = 1.69 +/- 0.18; p = .01), but serum aldosterone did not change. A significant decrease in autonomic responsiveness and an increase in leg compliance were observed.

CONCLUSIONS

We conclude that even in the presence of a high-average salt intake diet, simulated microgravity leads to renal, cardioendocrine, and cardiovascular system alterations that likely contribute to cardiovascular deconditioning.

Renal, Endocrine, and Cardiovascular Responses to Bed Rest in Male Subjects on a Constant Diet

Background

Exposure to actual and simulated microgravity induces cardiovascular deconditioning through a variety of factors. Although the mechanisms involved remain uncertain, one involves alterations in volume-regulating systems—the hypothesis being tested in this study. To maximize our ability to detect subtle changes in the volume-regulating systems, subjects were studied on a high-average salt intake to maximally suppress these systems basally.

Methods

Fourteen healthy male subjects underwent 14-day head-down tilt bed rest (HDTB) during which a constant 200 mEq sodium, 100 mEq potassium diet was maintained. Daily 24-hour urine collection was performed; plasma renin activity, serum aldosterone, plethysmography, and cardiovascular system identification were performed during a control period (pre-HDTB) and at the end of HDTB (end HDTB).

Results

Sodium excretion increased initially (pre-HDTB = 182.8 ± 10.4 mEq/total volume; early HDTB = 236.4 ± 13.0; p = .002) and then returned to baseline values. Potassium excretion increased 4 days after the initiation of HDTB and remained elevated thereafter (pre-HDTB = 82.2 ± 2.4/total volume; mid- to late HDTB = 89.4 ± 2.1; p = .02). Plasma renin activity increased significantly with HDTB (pre-HDTB = 1.28 ± 0.21 ng/mL/h; end HDTB = 1.69 ± 0.18; p = .01), but serum aldosterone did not change. A significant decrease in autonomic responsiveness and an increase in leg compliance were observed.

Conclusions

We conclude that even in the presence of a high-average salt intake diet, simulated microgravity leads to renal, cardioendocrine, and cardiovascular system alterations that likely contribute to cardiovascular deconditioning.

Effects of simulated microgravity on closed-loop cardiovascular regulation and orthostatic intolerance: analysis by means of system identification

Microgravity-induced orthostatic intolerance (OI) continues to be a primary concern for the human space program. To test the hypothesis that exposure to simulated microgravity significantly alters autonomic nervous control and, thus, contributes to increased incidence of OI, we employed the cardiovascular system identification (CSI) technique to evaluate quantitatively parasympathetic and sympathetic regulation of heart rate (HR). The CSI method analyzes second-to-second fluctuations in noninvasively measured HR, arterial blood pressure, and instantaneous lung volume. The coupling mechanisms between these signals are characterized by using a closed-loop model. Parameters reflecting parasympathetic and sympathetic responsiveness with regard to HR regulation can be extracted from the identified coupling mechanisms. We analyzed data collected from 29 human subjects before and after 16 days of head-down-tilt bed rest (simulated microgravity). Statistical analyses showed that parasympathetic and sympathetic responsiveness was impaired by bed rest. A lower sympathetic responsiveness and a higher parasympathetic responsiveness measured before bed rest identified individuals at greater risk of OI before and after bed rest. We propose an algorithm to predict OI after bed rest from measures obtained before bed rest.