Coupling between arterial and venous cerebral blood flow during postural change

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.

Arterial Diameter Trend Estimation Using Deep Learning on Ultrasound Spectral Doppler

This study presents an approach for estimating gaps in arterial diameter using flow velocity obtained from spectral Doppler data. We utilize short-time Fourier transform in conjunction with deep learning models designed for spectrograms to estimate arterial diameter trends. We train the Convolutional Recurrent Neural Network with Attention (CRNN-A) and Audio Spectrogram Transformer (AST) to provide scaled trend estimates in increments of 1, 2, or 4 seconds (s). We present an algorithm for Arterial ReScaling (AReS) that utilizes scaled trend predictions to fill in missing arterial diameter data at 1, 2 or 4 s increments. The CRNN-A model trained on 1 s lengths performed best at predicting scaled trends (R2 = 0.8083). However, when implementing AReS, the AST model trained on 2 s segments performed best at predicting the shortest gaps (1 s) in arterial diameter data (MAE: 0.0341 mm). Our study showcases that training to predict longer time segments can be beneficial in real-world performance, even if standardized metrics show otherwise.Clinical Relevance- In cases where an arterial ultrasound image becomes temporarily lost or inaccessible, the spectral Doppler signal can be used to estimate changes in the arterial diameter, offering continuous data. This allows for an accurate tracking of both blood flow and volume changes, assisting in the study of vascular reactivity and potentially aiding in clinical diagnoses such as carotid stenosis.

Pathophysiology, risk, diagnosis, and management of venous thrombosis in space: where are we now?

The recent incidental discovery of an asymptomatic venous thrombosis (VT) in the internal jugular vein of an astronaut on the International Space Station prompted a necessary, immediate response from the space medicine community. The European Space Agency formed a topical team to review the pathophysiology, risk and clinical presentation of venous thrombosis and the evaluation of its prevention, diagnosis, mitigation, and management strategies in spaceflight. In this article, we discuss the findings of the ESA VT Topical Team over its 2-year term, report the key gaps as we see them in the above areas which are hindering understanding VT in space. We provide research recommendations in a stepwise manner that build upon existing resources, and highlight the initial steps required to enable further evaluation of this newly identified pertinent medical risk.

Intracranial compliance in type 2 diabetes mellitus and its relationship with the cardiovascular autonomic nervous control

The intracranial compliance in type 2 diabetes mellitus (T2DM) patients and the association with cardiovascular autonomic control have not been fully elucidated. The aim of this study was to assess intracranial compliance using the noninvasive intracranial pressure (niICP) and the monitoring of waveform peaks (P1, P2, and P3) and the relationship with cardiovascular autonomic control in T2DM patients. Thirty-two men aged 40-60 years without cardiovascular autonomic neuropathy (CAN) were studied: T2DMG (n=16) and control group CG (n=16). The niICP was evaluated by a noninvasive extracranial sensor placed on the scalp. Cardiovascular autonomic control was evaluated by indices of the baroreflex sensitivity (BRS), from temporal series of R-R intervals of electrocardiogram and systolic arterial pressure, during supine and orthostatic positions. The participants remained in the supine position for 15 min and then 15 min more in orthostatism. T2DMG presented a decrease of the P2/P1 ratio during the orthostatic position (P<0.001). There was a negative moderate correlation between the P2 peak with cardiovascular coupling (K²_HP-SAPLF) in supine (r=-0.612, P=0.011) and orthostatic (r=-0.568, P=0.020) positions in T2DMG. We concluded that T2DM patients without CAN and cardiovascular complications presented intracranial compliance similar to healthy subjects. Despite preserved intracranial adjustments, T2DM patients had a response of greater magnitude in orthostatism. In addition, the decoupling between the heart period and blood pressure signal oscillations in low frequency appeared to be related to the worsening of intracranial compliance due to the increased P2 peak.

Influence of head-up tile and lower body negative pressure on the internal jugular vein

Head-up tilt (HUT)-induced gravitational stress causes collapse of the internal jugular vein (IJV) by decreasing central blood volume and through mass-effect from the surrounding tissues. Besides HUT, lower body negative pressure (LBNP) is used to stimulate orthostatic stress as an experimental model. Compared to HUT, LBNP has less of a gravitational effect because of the supine position; therefore, we hypothesized that LBNP causes less of a decrease in the cross-sectional area of the IJV compared to HUT. We tested the hypothesis by measuring the cross-sectional area of the IJV using B-mode ultrasonography while inducing orthostatic stress at levels of -40 mmHg LBNP and 60° HUT. The cross-sectional area of IJV decreased from the resting baseline during both LBNP and HUT trials, but the LBNP-induced decrease in the cross-sectional area of IJV was smaller than that of HUT (right, -45% ± 49% vs. -78% ± 27%, p = 0.008; left, -49% ± 27% vs. -78% ± 20%, p = 0.004). Since changes in venous outflow may affect cerebral arterial circulation, the findings of the present study suggest that orthostatic stress induced by different techniques modulates cerebral blood flow regulation through its effect on venous outflow.

Effect of jump exercise training on long-term head-down bed rest-induced cerebral blood flow responses in arteries and veins

NEW FINDINGS

What is the central question of this study? What is the effect of an exercise countermeasure on microgravity-induced change in cerebral blood flow? What is the main finding and its importance? Jump exercise training as a countermeasure did not modify the heterogeneous cerebral blood flow response to head-down bed rest, suggesting that this method is effective in preventing cardiovascular system deconditioning but is not good for cerebral haemodynamics.

ABSTRACT

This study aimed to examine the effect of an exercise countermeasure on cerebral blood flow (CBF) response to long-term -6° head-down bed rest (HDBR) in all cerebral arteries and veins. Twenty male volunteers were exposed to HDBR for 60 days with (training group, n = 10) or without (control group, n = 10) jump exercise training as a countermeasure to spaceflight. The blood flow in the neck conduit arteries (internal carotid and vertebral artery; ICA and VA) and veins (internal jugular and vertebral veins; IJV and VV) was measured, using ultrasonography before (baseline) HDBR, on the 30th and 57th day of HDBR. Long-term HDBR causes a heterogeneous CBF response between the anterior and the posterior brain or between arteries and veins. Long-term HDBR decreased anterior cerebral arterial and venous blood flow, while posterior cerebral arterial and venous blood flows were well maintained. However, exercise jump training did not change each arterial and venous CBF responses to HDBR (control vs. training; ICA, P = 0.643; VA, P = 0.542; external carotid artery, P = 0.644; IJV, P = 0.980; VV, P = 0.999). These findings suggest that jump exercise training did not modify the heterogeneous CBF response to long-term HDBR.

Reviving lower body negative pressure as a countermeasure to prevent pathological vascular and ocular changes in microgravity

Mitigation of spaceflight-related pathologies such as spaceflight-associated neuro-ocular syndrome (SANS) and the recently discovered risk of venous thrombosis must happen before deep space exploration can occur. Lower body negative pressure (LBNP) can simulate gravitational stress during spaceflight that is likely to counteract SANS and venous thrombosis, but the ideal dose and method of delivery have yet to be determined. We undertook a review of current LBNP literature and conducted a gap analysis to determine the steps needed to adapt LBNP for in-flight use. We found that to use LBNP in flight, it must be adapted to long time duration/low pressure use that should be compatible with crew activities. A lack of understanding of the etiology of the pathologies that LBNP can counteract hinders the application of LBNP as a countermeasure during spaceflight. Future research should aim at filling the knowledge gaps outlined in this review.

Gravitational effects on intracranial pressure and blood flow regulation in young men: a potential shunting role for the external carotid artery

We sought to determine whether gravity-induced changes in intracranial pressure influence cerebral blood flow regulation. Accordingly, nine young healthy men were studied while supine (0°) and during mild changes in hydrostatic pressure induced by head-up tilt at +20° and +10° (HUT+20 and HUT+10) and head-down tilt at -20° and -10° (HDT-20, HDT-10). Blood flows were measured in the internal and external carotid and vertebral arteries (ICA, ECA, and VA). Intraocular pressure (IOP) was measured as an indicator of hydrostatic changes in intracranial pressure. A posture change from HUT+20 to HDT-20 increased IOP by +5.1 ± 1.9 mmHg (P < 0.001) and ECA blood flow (from 61.7 ± 26.1 to 87.6 ± 46.4 mL/min, P = 0.004) but did not affect ICA (P = 0.528) or VA (P = 0.101) blood flow. The increase in ECA flow correlated with the tilt angle and resultant changes in intracranial pressures (by IOP), thus indicating a passive hydrostatic gravitational dependence (r = 0.371, P = 0.012). On the contrary, ICA flow remained constant and thus well protected against moderate orthostatic stress. When ICA flow was corrected for the gravitational changes in intracranial pressures (by IOP), it demonstrated the same magnitude of gravitational dependence as ECA. These findings suggest that passive hydrostatic increases in intracranial pressure outbalance the concurrent increase in arterial feeding pressure to the brain and thus prevent cerebral hyperperfusion during HDT. The mechanism for maintaining constant cerebral flow was by increased ECA flow, thus supporting the role of these vascular beds as a shunting pathway.NEW & NOTEWORTHY We investigated whether gravity-induced changes in intracranial pressure influence cerebral blood flow regulation in young men. We recorded extra- and intracerebral blood flow during changes in posture, and data indicate that the external carotid artery may serve as an overflow pathway to prevent cerebral hyperperfusion during increases in cerebral arterial blood pressure.

Relationship between cerebral arterial inflow and venous outflow during dynamic supine exercise

The regulation of cerebral venous outflow during exercise has not been studied systematically. To identify relations between cerebral arterial inflow and venous outflow, we assessed the blood flow (BF) of the cerebral arteries (internal carotid artery: ICA and vertebral artery: VA) and veins (internal jugular vein: IJV and vertebral vein: VV) during dynamic exercise using ultrasonography. Nine subjects performed a cycling exercise in supine position at a light and moderate workload. Similar to the ICA BF, the IJV BF increased from baseline during light exercise (P < 0.05). However, the IJV BF decreased below baseline levels during moderate exercise, whereas the ICA BF returned near resting levels. In contrast, BF of the VA and VV increased with the workload (P < 0.05). The change in the ICA or VA BF from baseline to exercise was significantly correlated with the change in the IJV (r = 0.73, P = 0.001) or VV BF (r = 0.52, P = 0.028), respectively. These findings suggest that dynamic supine exercise modifies the cerebral venous outflow, and there is coupling between regulations of arterial inflow and venous outflow in both anterior and posterior cerebral circulation. However, it remains unclear whether changes in cerebral venous outflow influence on the regulation of cerebral arterial inflow during exercise.

Transcatheter aortic valve implantation improves carotid and vertebral arterial blood flow in patients with severe aortic stenosis: practical role of orthostatic stress test

BACKGROUND

There are no data on the impact of transcatheter aortic valve implantation (TAVI) on carotid and vertebral arterial blood flow. Our aim was to assess the effects of the orthostatic stress test on carotid and vertebral artery blood flow in patients with severe aortic stenosis (AS) undergoing TAVI.

HYPOTHESIS

TAVI may have beneficial effect on carotid and vertebral artery flow in patients with severe aortic stenosis.

METHODS

Thirty carefully selected patients with severe AS undergoing TAVI were enrolled. Peak systolic blood-flow velocity and end-diastolic velocity in the common carotid artery, internal carotid artery, and vertebral artery, as well as spectral analysis of flow pattern with time-averaged maximum velocity (centimeters per second), time-averaged mean velocity (centimeters per second), and flow volume (milliliters per minute) on both sides were measured by duplex ultrasound. Measurements were performed in the supine position and at 1 to 2 minutes after the assumption of the standing position at baseline and 3 months after TAVI.

RESULTS

All duplex ultrasound parameters assessed in the supine position have significantly improved in patients after TAVI as compared to baseline (P < 0.001 for all). The orthostatic stress test induced decrease of carotid and vertebral arterial flow velocities in AS patients before and after TAVI. However, the drop in velocities and flow volume was numerically lower after TAVI.

CONCLUSIONS

TAVI may have some beneficial effect on extracranial artery blood flow by minimalization of its decrease as a response to orthostatic stress.