Cardiac output by pulse contour analysis does not match the increase measured by rebreathing during human spaceflight

Simulation of murine retinal hemodynamics in response to tail suspension

The etiology of spaceflight-associated neuro-ocular syndrome (SANS) remains unclear. Recent murine studies indicate there may be a link between the space environment and retinal endothelial dysfunction. Post-fixed control (N = 4) and 14-day tail-suspended (TS) (N = 4) mice eye samples were stained and imaged for the vessel plexus and co-located regions of endothelial cell death. A custom workflow combined whole-mounted and tear reconstructed three-dimensional (3D) spherical retinal plexus models with computational fluid dynamics (CFD) simulation that accounted for the Fåhræus-Lindqvist effect and boundary conditions that accommodated TS fluid pressure measurements and deeper capillary layer blood flow distribution. TS samples exhibited reduced surface area (4.6 ± 0.5 mm2 vs. 3.5 ± 0.3 mm2, P = 0.010) and shorter lengths between branches in small vessels (<10 μm, 69.5 ± 0.6 μm vs. 60.4 ± 1.1 μm, P < 0.001). Wall shear stress (WSS) and pressure were higher in TS mice compared to controls, particularly in smaller vessels (<10 μm, WSS: 6.57 ± 1.08 Pa vs. 4.72 ± 0.67 Pa, P = 0.034, Pressure: 72.04 ± 3.14 mmHg vs. 50.64 ± 6.74 mmHg, P = 0.004). Rates of retinal endothelial cell death were variable in TS mice compared to controls. WSS and pressure were generally higher in cell death regions, both within and between cohorts, but significance was variable and limited to small to medium-sized vessels (<20 μm). These findings suggest a link may exist between emulated microgravity and retinal endothelial dysfunction that may have implications for SANS development. Future work with increased sample sizes of larger species or spaceflight cohorts should be considered.

A perspective on the evidence for glymphatic obstruction in spaceflight associated neuro-ocular syndrome and fatigue

Spaceflight associated neuro-ocular syndrome (SANS) alters the vision of astronauts during long-duration spaceflights. Previously, the current authors have discussed the similarities and differences between SANS and idiopathic intracranial hypertension to try to elucidate a possible pathophysiology. Recently, a theory has been advanced that SANS may occur secondary to failure of the glymphatic system caused by venous dilatation within the brain and optic nerves. There is recent evidence to suggest glymphatic obstruction occurs in childhood hydrocephalus, multiple sclerosis and syringomyelia due to venous outflow dilatation similar to that proposed in SANS. The purpose of the current paper is to discuss the similarities and differences between the known CSF and venous pathophysiology in SANS with these other terrestrial diseases, to see if they can shed any further light on the underlying cause of this microgravity-induced disease.

Modelling large scale artery haemodynamics from the heart to the eye in response to simulated microgravity

We investigated variations in haemodynamics in response to simulated microgravity across a semi-subject-specific three-dimensional (3D) continuous arterial network connecting the heart to the eye using computational fluid dynamics (CFD) simulations. Using this model we simulated pulsatile blood flow in an upright Earth gravity case and a simulated microgravity case. Under simulated microgravity, regional time-averaged wall shear stress (TAWSS) increased and oscillatory shear index (OSI) decreased in upper body arteries, whilst the opposite was observed in the lower body. Between cases, uniform changes in TAWSS and OSI were found in the retina across diameters. This work demonstrates that 3D CFD simulations can be performed across continuously connected networks of small and large arteries. Simulated results exhibited similarities to low dimensional spaceflight simulations and measured data-specifically that blood flow and shear stress decrease towards the lower limbs and increase towards the cerebrovasculature and eyes in response to simulated microgravity, relative to an upright position in Earth gravity.

Selective elevation in external carotid artery flow during acute gravitational transition to microgravity during parabolic flight

This study sought to determine to what extent acute exposure to microgravity (0 G) and related increases in central blood volume (CBV) during parabolic flight influence the regional redistribution of intra and extra cranial cerebral blood flow (CBF). Eleven healthy participants performed during two parabolic flights campaigns aboard the Airbus A310-ZERO G aircraft. The response of select variables for each of the 15 parabolas involving exposure to both 0 G and hypergravity (1.8 G) were assessed in the seated position. Mean arterial blood pressure (MAP) and heart rate (HR) were continuously monitored and used to calculate stroke volume (SV), cardiac output ([Formula: see text]), and systemic vascular resistance (SVR). Changes in CBV were measured using an impedance monitor. Extracranial flow through the internal carotid, external carotid, and vertebral artery ([Formula: see text]ICA, [Formula: see text]ECA, and [Formula: see text]VA), and intracranial blood velocity was measured by duplex ultrasound. When compared with 1-G baseline condition, 0 G increased CBV (+375 ± 98 mL, P = 0.004) and [Formula: see text] (+16 ± 14%, P = 0.024) and decreased SVR (-7.3 ± 5 mmHg·min·L-1, P = 0.002) and MAP (-13 ± 4 mmHg, P = 0.001). [Formula: see text]ECA increased by 43 ± 46% in 0 G (P = 0.030), whereas no change was observed for CBF, [Formula: see text]ICA, or [Formula: see text]VA (P = 0.102, P = 0.637, and P = 0.095, respectively).NEW & NOTEWORTHY Our findings demonstrate that in microgravity there is a selective increase in external carotid artery blood flow whereas global and regional cerebral blood flow remained preserved. To what extent this reflects an adaptive, neuroprotective response to counter overperfusion remains to be established.

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.

Implementation of exercise countermeasures during spaceflight and microgravity analogue studies: Developing countermeasure protocols for bedrest in older adults (BROA)

Significant progress has been made in the development of countermeasures to attenuate the negative consequences of prolonged exposure to microgravity on astronauts’ bodies. Deconditioning of several organ systems during flight includes losses to cardiorespiratory fitness, muscle mass, bone density and strength. Similar deconditioning also occurs during prolonged bedrest; any protracted time immobile or inactive, especially for unwell older adults (e.g., confined to hospital beds), can lead to similar detrimental health consequences. Due to limitations in physiological research in space, the six-degree head-down tilt bedrest protocol was developed as ground-based analogue to spaceflight. A variety of exercise countermeasures have been tested as interventions to limit detrimental changes and physiological deconditioning of the musculoskeletal and cardiovascular systems. The Canadian Institutes of Health Research and the Canadian Space Agency recently provided funding for research focused on Understanding the Health Impact of Inactivity to study the efficacy of exercise countermeasures in a 14-day randomized clinical trial of six-degree head-down tilt bedrest study in older adults aged 55–65 years old (BROA). Here we will describe the development of a multi-modality countermeasure protocol for the BROA campaign that includes upper- and lower-body resistance exercise and head-down tilt cycle ergometry (high-intensity interval and continuous aerobic exercise training). We provide reasoning for the choice of these modalities following review of the latest available information on exercise as a countermeasure for inactivity and spaceflight-related deconditioning. In summary, this paper sets out to review up-to-date exercise countermeasure research from spaceflight and head-down bedrest studies, whilst providing support for the proposed research countermeasure protocols developed for the bedrest study in older adults.

A perspective on spaceflight associated neuro-ocular syndrome causation secondary to elevated venous sinus pressure

Spaceflight associated neuro-ocular syndrome (SANS) alters the vision of astronauts during long-duration spaceflights. There is controversy regarding SANS being similar to patients with idiopathic intracranial hypertension (IIH). IIH has been shown to be due to an elevation in venous sinus pressure. The literature suggests an increase in jugular vein pressure secondary to a headward shift of fluid occurs in SANS but this may not be enough to significantly alter the intracranial pressure (ICP). The literature regarding cardiac output and cerebral blood flow (CBF) in long-duration spaceflight is contradictory, however, more recent data suggests increased flow. Recent modelling has shown that an increase in CBF can significantly increase sinus pressure. The purpose of the present paper is to review the SANS vascular dynamics literature and through mathematical modelling suggest the possible underlying cause of SANS as an elevation in venous sinus pressure, secondary to the redistribution of fluids towards the head, together with a significant increase in pressure drop across the venous system related to the CBF.

Indirect measurement of absolute cardiac output during exercise in simulated altered gravity is highly dependent on the method

PURPOSE

Altered gravity environments introduce cardiovascular changes that may require continuous hemodynamic monitoring in both spaceflight and terrestrial analogs. Conditions in such environments are often prohibitive to direct/invasive methods and therefore, indirect measurement techniques must be used. This study compares two common cardiac measurement techniques used in the human spaceflight domain, pulse contour analysis (PCA-Nexfin) and inert gas rebreathing (IGR-Innocor), in subjects completing ergometer exercise under altered gravity conditions simulated using a tilt paradigm.

METHODS

Seven subjects were tilted to three different angles representing Martian, Lunar, and microgravity conditions in the rostrocaudal direction. They completed a 36-min submaximal cardiovascular exercise protocol in each condition. Hemodynamics were continuously monitored using Nexfin and Innocor.

RESULTS

Linear mixed-effects models revealed a significant bias of [Formula: see text] ml ([Formula: see text]) in stroke volume and [Formula: see text] l/min ([Formula: see text]) in cardiac output, with Nexfin measuring greater than Innocor in both variables. These values are in agreement with a Bland-Altman analysis. The correlation of stroke volume and cardiac output measurements between Nexfin and Innocor were [Formula: see text] ([Formula: see text]) and [Formula: see text] ([Formula: see text]) respectively.

CONCLUSION

There is a poor agreement in absolute stroke volume and cardiac output values between measurement via PCA (Nexfin) and IGR (Innocor) in subjects who are exercising in simulated altered gravity environments. These results suggest that the chosen measurement method and device greatly impacts absolute measurements of cardiac output. However, there is a good level of agreement between the two devices when measuring relative changes. Either of these devices seem adequate to capture cardiac changes, but should not be solely relied upon for accurate measurement of absolute cardiac output.

Gravitational Transitions Increase Posterior Cerebral Perfusion and Systemic Oxidative-nitrosative Stress: Implications for Neurovascular Unit Integrity

The present study examined if repeated bouts of micro- and hypergravity during parabolic flight (PF) alter structural integrity of the neurovascular unit (NVU) subsequent to free radical-mediated changes in regional cerebral perfusion. Six participants (5♂, 1♀) aged 29 ± 11 years were examined before, during and after a 3 h PF and compared to six sex and age-matched (27 ± 6 years) normogravity controls. Blood flow was measured in the anterior (middle cerebral artery, MCA; internal carotid artery, ICA) and posterior (vertebral artery, VA) circulation (duplex ultrasound) in-flight over the course of 15 parabolas. Venous blood was assayed for free radicals (electron paramagnetic resonance spectroscopy), nitric oxide (NO, ozone-based chemiluminescence) and NVU integrity (chemiluminescence/ELISA) in normogravity before and after exposure to 31 parabolas. While MCA velocity did not change (P > 0.05), a selective increase in VA flow was observed during the most marked gravitational transition from micro- to hypergravity (P < 0.05). Increased oxidative-nitrosative stress defined by a free radical-mediated reduction in NO and elevations in glio-vascular GFAP and S100ß were observed after PF (P < 0.05), the latter proportional to the increase in VA flow (r = 0.908, P < 0.05). In contrast, biomarkers of neuronal-axonal damage (neuron-specific enolase, neurofilament light-chain, ubiquitin carboxy-terminal hydrolase L1 and tau) did not change (P > 0.05). Collectively, these findings suggest that the cumulative effects of repeated gravitational transitions may promote minor blood-brain barrier disruption, potentially related to the combined effects of haemodynamic (posterior cerebral hyperperfusion) and molecular (systemic oxidative-nitrosative) stress.

Adaptation of the cardiovascular system to weightlessness: Surprises, paradoxes and implications for deep space missions

Weightlessness in space induces a fluid shift from the dependent to the cephalad parts of the body leading to distension of the cardiac chambers and an accumulation of blood in the veins of the head and neck. Surprisingly, central venous pressure (CVP) during the initial hours of spaceflight decreases compared to being horizontal supine on the ground. The explanation is that the thorax is expanded by weightlessness leading to a decrease in inter-pleural pressure (IPP), which exceeds the measured decrease in CVP. Thus, transmural CVP (TCVP = CVP - IPP) is increased indicating an augmented cardiac preload. Simultaneously, stroke volume and cardiac output (CO) are increased by 18%-26% within the initial weeks and more so by 35%-56% during the subsequent months of flight relative to in the upright posture on the ground. Mean arterial pressure (MAP) is decreased indicating a lower systemic vascular resistance (MAP/CO). It is therefore a surprise that sympathetic nerve activity is not suppressed in space and thus cannot be a mechanism for the systemic vasodilation, which still needs to be explored. Recent observations indicate that the fluid shift during long duration (months) flights is associated with increased retinal thickness that sometimes leads to optical disc oedema. Ocular and cerebral structural changes, increases in left atrial size and decreased flows with thrombi formation in the left internal jugular vein have also been observed. This is of concern for future long duration deep space missions because the health implications are unknown.

Inflight leg cuff test does not identify the risk for orthostatic hypotension after long-duration spaceflight

Landing day symptoms from orthostatic hypotension after prolonged spaceflight can be debilitating, but severity of these symptoms can be unpredictable and highly individual. We tested the hypothesis that an impaired baroreflex response to an inflight leg cuff test could predict orthostatic intolerance on return to Earth. Eight male astronauts (44 ± 7 years of age (mean ± SD); mean mission length: 167 ± 12 days) participated in a standardized supine-to-sit-to-stand test (5 min-30s-3 min) pre- and postflight, and a 3 min thigh cuff occlusion test pre- and inflight with continuous monitoring of heart rate and arterial blood pressure. The arterial baroreflex was not changed inflight as shown by similar reductions in mean arterial pressure (MAP) response to leg cuff deflation (preflight -19 ± 2 mmHg vs. inflight -18 ± 5 mmHg). With the sit/stand test, the nadir of MAP was lower postflight (-17 ± 9 mmHg) than preflight (-11 ± 6 mmHg, p < 0.05). A greater increase in heart rate (25 ± 7; 16 ± 3 bpm) and decrease in stroke volume (-24 ± 11; -6 ± 4 mL) occurred with sit/stand postflight than leg cuffs inflight (p < 0.001). Inflight testing was influenced by elevated cardiac output resulting in a smaller drop in total peripheral resistance. Two of eight subjects exhibited orthostatic hypotension during the postflight stand test; their responses were not predicted by the inflight leg cuff deflation test. These results suggest that the baroreflex response examined by inflight leg cuff deflation was not a reliable indicator of postflight stand responses.

Comparison of pulse contour, aortic Doppler ultrasound and bioelectrical impedance estimates of stroke volume during rapid changes in blood pressure

NEW FINDINGS

What is the central question of this study? Pulse contour analysis of the finger arterial pressure by Windkessel modelling is commonly used to estimate stroke volume continuously. But is it valid during dynamic changes in blood pressure? What is the main finding and its importance? Second-by-second analysis revealed that pulse contour analysis underestimated stroke volume by up to 25% after standing from a squat, and 16% after standing thigh-cuff release, when compared with aortic Doppler ultrasound estimates. These results reveal that pulse contour analysis of stroke volume should be interpreted with caution during rapid changes in physiological state.

ABSTRACT

Dynamic measurements of stroke volume (SV) and cardiac output provide an index of central haemodynamics during transitional states, such as postural changes and onset of exercise. The most widely used method to assess dynamic fluctuations in SV is the Modelflow method, which uses the arterial blood pressure waveform along with age- and sex-specific aortic properties to compute beat-to-beat estimates of aortic flow. Modelflow has been validated against more direct methods in steady-state conditions, but not during dynamic changes in physiological state, such as active orthostatic stress testing. In the present study, we compared the dynamic SV responses from Modelflow (SV_MF ), aortic Doppler ultrasound (SV_U/S ) and bioelectrical impedance analysis (SV_BIA ) during two different orthostatic stress tests, a squat-to-stand (S-S) transition and a standing bilateral thigh-cuff release (TCR), in 15 adults (six females). Second-by-second analysis revealed that when compared with estimates of SV by aortic Doppler ultrasound, Modelflow underestimated SV by up to 25% from 3 to 11 s after standing from the squat position and by up to 16% from 3 to 7 s after TCR (P < 0.05). The SV_MF and SV_BIA were similar during the first minute of the S-S transition, but were different 3 s after TCR and at intermittent time points between 34 and 44 s (P < 0.05). These findings indicate that the physiological conditions elicited by orthostatic stress testing violate some of the inherent assumptions of Modelflow and challenge models used to interpret bioelectrical impedance responses, resulting in an underestimation in SV during rapid changes in physiological state.

Efficacy of fluid loading as a countermeasure to the hemodynamic and hormonal changes of 28-h head-down bed rest

After exposure to microgravity, or head-down bed rest (HDBR), fluid loading is often used with the intent of increasing plasma volume and maintaining mean arterial pressure during orthostatic stress. Nine men (aged 18-32 years) underwent three randomized trials with lower body negative pressure (LBNP) before and after: (1) 4-h of sitting with fluid loading (1 g sodium chloride/125 mL of water starting 2.5-h before LBNP), (2) 28-h of 6-degree HDBR without fluid loading, and (3) 28-h of 6-degree HDBR with fluid loading. LBNP was progressive from 0 to -40 mmHg. After 28-h HDBR, fluid loading did not protect against the loss of plasma volume (-280 ± 64 mL without fluid loading, -207 ± 86 with fluid loading, P = 0.472) nor did it protect against a drop of mean arterial pressure (P = 0.017) during LBNP (Post-28 h HDBR response from 0 to -40 mmHg LBNP: 88 ± 4 to 85 ± 4 mmHg without fluid loading and 93 ± 4 to 88 ± 5 mmHg with fluid loading, P = 0.557 between trials). However, fluid loading did protect against the loss of stroke volume index and central venous pressure observed after 28-h HDBR. Fluid loading also attenuated the increase of angiotensin II seen after 28-h HDBR and throughout the LBNP protocol (Post-28 h HDBR response from 0 to -40 mmHg LBNP: 16.6 ± 3.4 to 23.7 ± 5.0 pg/mL without fluid loading and 6.1 ± 0.8 to 12.2 ± 2.3 pg/mL with fluid loading, P < 0.001 between trials). Our results indicate that fluid loading did not protect against plasma volume loss due to HDBR or change blood pressure responses to LBNP. However, changes in central venous pressure, stroke volume and fluid regulatory hormones could potentially influence longer duration studies and those with more severe orthostatic stress.