Changes in arterial cerebral blood volume during lower body negative pressure measured with MRI

Blood pressure variability at rest and during pressor challenges in patients with acute ischemic stroke

INTRODUCTION

Patients with acute ischemic stroke (AIS) have elevated blood pressure (BP) variability (BPV) and reduced baroreflex sensitivity (BRS) at rest for several days after initial stroke symptoms. We aimed to assess BPV and BRS in AIS patients during pressor challenge maneuvers in the acute and subacute phases of stroke. Pressor challenge maneuvers simulate day-to-day activities and can predict the quality of life.

METHODS

Continuous beat-to-beat BP and ECG in 15 AIS patients (mean age 69 ± 7.5 years) and 15 healthy controls (57 ± 16 years) were recorded at rest and during a 5-min rapid head positioning (RHP) paradigm. Patients were assessed within 24 h (acute phase) and 7 days (subacute phase) of stroke onset. Low frequency (LF) SBP power (measure of BPV), LF-α, and combined α-index (measure of BRS) were calculated from the recordings.

RESULTS

In the acute phase, at rest, LF-SBP power was higher (P = 0.024) and α-index was lower (P = 0.006) in AIS patients than in healthy controls. There was no change in LF-SBP during RHP in the patients but in healthy controls, it increased significantly (P = 0.018). In the subacute phase, at rest, the alpha-index increased (P = 0.037) and LF-SBP decreased (P = 0.029) significantly in the AIS patients, however, there was still no rise in the LF-SBP power during RHP (P = 0.240).

CONCLUSION

AIS patients have a high resting BPV. High resting BPV may be responsible for blunted BPV responses during pressor challenge maneuvers such as RHP, suggesting ongoing autonomic dysfunction and compromised quality of life.

The Spatiotemporal Dynamics of Cerebral Autoregulation in Functional Magnetic Resonance Imaging

The thigh-cuff release (TCR) maneuver is a physiological challenge that is widely used to assess dynamic cerebral autoregulation (dCA). It is often applied in conjunction with Transcranial Doppler ultrasound (TCD), which provides temporal information of the global flow response in the brain. This established method can only yield very limited insights into the regional variability of dCA, whereas functional MRI (fMRI) has the ability to reveal the spatial distribution of flow responses in the brain with high spatial resolution. The aim of this study was to use whole-brain blood-oxygenation-level-dependent (BOLD) fMRI to characterize the spatiotemporal dynamics of the flow response to the TCR challenge, and thus pave the way toward mapping dCA in the brain. We used a data driven approach to derive a novel basis set that was then used to provide a voxel-wise estimate of the TCR associated haemodynamic response function (HRF TCR ). We found that the HRF TCR evolves with a specific spatiotemporal pattern, with gray and white matter showing an asynchronous response, which likely reflects the anatomical structure of cerebral blood supply. Thus, we propose that TCR challenge fMRI is a promising method for mapping spatial variability in dCA, which will likely prove to be clinically advantageous.

Cerebral Hemodynamics During Exposure to Hypergravity (+Gz) or Microgravity (0 G)

BACKGROUND: Optimal human performance and health is dependent on steady blood supply to the brain. Hypergravity (+Gz) may impair cerebral blood flow (CBF), and several investigators have also reported that microgravity (0 G) may influence cerebral hemodynamics. This has led to concerns for safe performance during acceleration maneuvers in aviation or the impact long-duration spaceflights may have on astronaut health.METHODS: A systematic PEO (Population, Exposure, Outcome) search was done in PubMed and Web of Science, addressing studies on how elevated +Gz forces or absence of such may impact cerebral hemodynamics. All primary research containing anatomical or physiological data on relevant intracranial parameters were included. Quality of the evidence was analyzed using the GRADE tool.RESULTS: The search revealed 92 eligible articles. It is evident that impaired CBF during +Gz acceleration remains an important challenge in aviation, but there are significant variations in individual tolerance. The reports on cerebral hemodynamics during weightlessness are inconsistent, but published data indicate that adaptation to sustained microgravity is also characterized by significant variations among individuals.DISCUSSION: Despite a high number of publications, the quality of evidence is limited due to observational study design, too few included subjects, and methodological challenges. Clinical consequences of high +Gz exposure are well described, but there are significant gaps in knowledge regarding the intracranial pathophysiology and individual hemodynamic tolerance to both hypergravity and microgravity environments.Saehle T. Cerebral hemodynamics during exposure to hypergravity (+Gz) or microgravity (0 G). Aerosp Med Hum Perform. 2022; 93(7):581–592.

Cerebrovascular Effects of Lower Body Negative Pressure at 3T MRI: Implications for Long-Duration Space Travel

BACKGROUND

Optic disc edema develops in most astronauts during long-duration spaceflight. It is hypothesized to result from weightlessness-induced venous congestion of the head and neck and is an unresolved health risk of space travel.

PURPOSE

Determine if short-term application of lower body negative pressure (LBNP) could reduce internal jugular vein (IJV) expansion associated with the supine posture without negatively impacting cerebral perfusion or causing IJV flow stasis.

STUDY TYPE

Prospective.

SUBJECTS

Nine healthy volunteers (six women).

FIELD STRENGTH/SEQUENCE

3T/cine two-dimensional phase-contrast gradient echo; pseudo-continuous arterial spin labeling single-shot gradient echo echo-planar.

ASSESSMENT

The study was performed with two sequential conditions in randomized order: supine posture and supine posture with 25 mmHg LBNP (LBNP₂₅ ). LBNP was achieved by enclosing the lower extremities in a semi-airtight acrylic chamber connected to a vacuum. Heart rate, bulk cerebrovasculature flow, IJV cross-sectional area, fractional IJV outflow relative to arterial inflow, and cerebral perfusion were assessed in each condition.

STATISTICAL TESTS

Paired t-tests were used to compare measurement means across conditions. Significance was defined as P < 0.05.

RESULTS

LBNP₂₅ significantly increased heart rate from 64 ± 9 to 71 ± 8 beats per minute and significantly decreased IJV cross-sectional area, IJV outflow fraction, cerebral arterial flow rate, and cerebral arterial stroke volume from 1.28 ± 0.64 to 0.56 ± 0.31 cm² , 0.75 ± 0.20 to 0.66 ± 0.28, 780 ± 154 to 708 ± 137 mL/min and 12.2 ± 2.8 to 9.7 ± 1.7 mL/cycle, respectively. During LBNP₂₅ , there was no significant change in gray or white matter cerebral perfusion (P = 0.26 and P = 0.24 respectively) and IJV absolute mean peak flow velocity remained ≥4 cm/sec in all subjects.

DATA CONCLUSION

Short-term application of LBNP₂₅ reduced IJV expansion without decreasing cerebral perfusion or inducing IJV flow stasis.

LEVEL OF EVIDENCE

1 TECHNICAL EFFICACY STAGE: 1.

Elevated cerebral blood flow in patients with pure autonomic failure

PURPOSE

Pure autonomic failure (PAF) results from an impaired peripheral autonomic nervous system, and clinical symptoms present with orthostatic hypotension. While the impact on cardiovascular indices of orthostatic intolerance are well-characterized, more limited information is available regarding cerebral hemodynamic dysfunction in PAF. The objective of this study was to test the hypothesis that cerebral blood flow (CBF) is reduced in PAF, and to quantify the relationship between CBF and clinical indicators of disease severity, including peripheral supine arterial blood pressure.

METHODS

Participants with PAF (n = 17) and age- and sex-matched normotensive healthy controls (n = 17) were examined using established clinical rating scales, cardiovascular autonomic function tests, and 3T MRI measurements of CBF. CBF-weighted images were also used to determine the prevalence of venous hyperintensities from the major dural sinuses as evidence of abnormal capillary flow. Nonparametric tests and general linear models were used to evaluate differences and correlations between study variables.

RESULTS

Gray matter CBF was higher in PAF (51.1 ± 13.4 mL/100 g/min) compared to controls (42.9 ± 6.5 mL/100 g/min, p = 0.007). Venous hyperintensities were more prevalent in PAF relative to controls, and the presence and degree of venous hyperintensities was associated with higher mean CBF (p = 0.027). In PAF participants, CBF and supine systolic blood pressure were inversely related (Spearman's rho = -0.545, p = 0.024).

CONCLUSIONS

Findings suggest that PAF patients may exhibit elevated CBF and provide evidence that this condition exerts a hemodynamic impact in the central nervous system.

Rude mechanicals in brain haemodynamics: non-neural actors that influence blood flow

Fluctuations in blood oxygenation and flow are widely used to infer brain activity during resting-state functional magnetic resonance imaging (fMRI). However, there are strong systemic and vascular contributions to resting-state signals that are unrelated to ongoing neural activity. Importantly, these non-neural contributions to haemodynamic signals (or 'rude mechanicals') can be as large as or larger than the neurally evoked components. Here, we review the two broad classes of drivers of these signals. One is systemic and is tied to fluctuations in external drivers such as heart rate and breathing, and the robust autoregulatory mechanisms that try to maintain a constant milieu in the brain. The other class comprises local, active fluctuations that appear to be intrinsic to vascular tissue and are likely similar to active local fluctuations seen in vasculature all over the body. In this review, we describe these non-neural fluctuations and some of the tools developed to correct for them when interpreting fMRI recordings. However, we also emphasize the links between these vascular fluctuations and brain physiology and point to ways in which fMRI measurements can be used to exploit such links to gain valuable information about neurovascular health and about internal brain states. This article is part of the theme issue 'Key relationships between non-invasive functional neuroimaging and the underlying neuronal activity'.

The effect of hypercapnia on regional cerebral blood flow regulation during progressive lower-body negative pressure

PURPOSE

Previous work indicates that dynamic cerebral blood flow (CBF) regulation is impaired during hypercapnia; however, less is known about the impact of resting hypercapnia on regional CBF regulation during hypovolemia. Furthermore, there is disparity within the literature on whether differences between anterior and posterior CBF regulation exist during physiological stressors. We hypothesized: (a) lower-body negative pressure (LBNP)-induced reductions in cerebral blood velocity (surrogate for CBF) would be more pronounced during hypercapnia, indicating impaired CBF regulation; and (b) the anterior and posterior cerebral circulations will exhibit similar responses to LBNP.

METHODS

In 12 healthy participants (6 females), heart rate (electrocardiogram), mean arterial pressure (MAP; finger photoplethosmography), partial pressure of end-tidal carbon dioxide (P_ETCO₂), middle cerebral artery blood velocity (MCAv) and posterior cerebral artery blood velocity (PCAv; transcranial Doppler ultrasound) were measured. Cerebrovascular conductance (CVC) was calculated as MCAv or PCAv indexed to MAP. Two randomized incremental LBNP protocols were conducted (- 20, - 40, - 60 and - 80 mmHg; three-minute stages), during coached normocapnia (i.e., room air), and inspired 5% hypercapnia (~ + 7 mmHg P_ETCO₂ in normoxia).

RESULTS

The main findings were: (a) static CBF regulation in the MCA and PCA was similar during normocapnic and hypercapnic LBNP trials, (b) MCA and PCA CBV and CVC responded similarly to LBNP during normocapnia, but (c) PCAv and PCA CVC were reduced to a greater extent at - 60 mmHg LBNP (P = 0.029; P < 0.001) during hypercapnia.

CONCLUSION

CBF regulation during hypovolemia was preserved in hypercapnia, and regional differences in cerebrovascular control may exist during superimposed hypovolemia and hypercapnia.

Cerebral Autoregulation Evidenced by Synchronized Low Frequency Oscillations in Blood Pressure and Resting-State fMRI

Resting-state functional magnetic resonance imaging (rs-fMRI) is a widely used technique for mapping the brain’s functional architecture, so delineating the main sources of variance comprising the signal is crucial. Low frequency oscillations (LFO) that are not of neural origin, but which are driven by mechanisms related to cerebral autoregulation (CA), are present in the blood-oxygenation-level-dependent (BOLD) signal within the rs-fMRI frequency band. In this study we use a MR compatible device (Caretaker, Biopac) to obtain a non-invasive estimate of beat-to-beat mean arterial pressure (MAP) fluctuations concurrently with rs-fMRI at 3T. Healthy adult subjects (n = 9; 5 male) completed two 20-min rs-fMRI scans. MAP fluctuations were decomposed into different frequency scales using a discrete wavelet transform, and oscillations at approximately 0.1 Hz show a high degree of spatially structured correlations with matched frequency fMRI fluctuations. On average across subjects, MAP fluctuations at this scale of the wavelet decomposition explain ∼2.2% of matched frequency fMRI signal variance. Additionally, a simultaneous multi-slice multi-echo acquisition was used to collect 10-min rs-fMRI at three echo times at 7T in a separate group of healthy adults (n = 5; 5 male). Multiple echo times were used to estimate the R₂^∗ decay at every time point, and MAP was shown to strongly correlate with this signal, which suggests a purely BOLD (i.e., blood flow related) origin. This study demonstrates that there is a significant component of the BOLD signal that has a systemic physiological origin, and highlights the fact that not all localized BOLD signal changes necessarily reflect blood flow supporting local neural activity. Instead, these data show that a proportion of BOLD signal fluctuations in rs-fMRI are due to localized control of blood flow that is independent of local neural activity, most likely reflecting more general systemic autoregulatory processes. Thus, fMRI is a promising tool for studying flow changes associated with cerebral autoregulation with high spatial resolution.

MRI techniques to measure arterial and venous cerebral blood volume

The measurement of cerebral blood volume (CBV) has been the topic of numerous neuroimaging studies. To date, however, most in vivo imaging approaches can only measure CBV summed over all types of blood vessels, including arterial, capillary and venous vessels in the microvasculature (i.e. total CBV or CBVtot). As different types of blood vessels have intrinsically different anatomy, function and physiology, the ability to quantify CBV in different segments of the microvascular tree may furnish information that is not obtainable from CBVtot, and may provide a more sensitive and specific measure for the underlying physiology. This review attempts to summarize major efforts in the development of MRI techniques to measure arterial (CBVa) and venous CBV (CBVv) separately. Advantages and disadvantages of each type of method are discussed. Applications of some of the methods in the investigation of flow-volume coupling in healthy brains, and in the detection of pathophysiological abnormalities in brain diseases such as arterial steno-occlusive disease, brain tumors, schizophrenia, Huntington's disease, Alzheimer's disease, and hypertension are demonstrated. We believe that the continual development of MRI approaches for the measurement of compartment-specific CBV will likely provide essential imaging tools for the advancement and refinement of our knowledge on the exquisite details of the microvasculature in healthy and diseased brains.