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Syncope: Physiology, Pathophysiology and Aeromedical Implications
Published in Anthony N. Nicholson, The Neurosciences and the Practice of Aviation Medicine, 2017
David A. Low, Christopher J. Mathias
Blood flow to the brain is controlled via adjustments in systemic haemodynamics – the perfusion pressure (Ogoh et al., 2005; Ogoh et al., 2008; van Lieshout et al., 2003) – and also through local vascular regulation (Panerai, 2008; Paulson et al., 1990). The brain is able to maintain its blood flow in the face of a wide range of arterial blood pressures – a process termed cerebral autoregulation (CA) that has both fast- and slow-acting regulatory components. Fast (or dynamic CA) refers to the ability to restore cerebral blood flow in the face of blood pressure changes within seconds and reflects the latency of the cerebral vasoregulatory system, whereas static CA reflects the overall ‘steady-state’ efficiency of the system (Panerai, 2008; Tiecks et al., 1995; van Beek et al., 2008). Cerebral blood flow is also locally adjusted to changes in the metabolic activity of the brain (alterations in carbon dioxide, oxygen and glucose content), myogenic and humoral factors and autonomic neural activity (Panerai, 2008; Paulson et al., 1990). Although there is some variation, syncope typically ensues when cerebral perfusion has decreased by ~ 50 per cent, when mental confusion becomes prominent and cerebral oxygenation becomes affected (Madsen et al., 1998; van Lieshout et al., 2003) (Figure 13.4).
Trauma Outcome Prediction in the Era of Big Data: From Data Collection to Analytics
Published in Ervin Sejdić, Tiago H. Falk, Signal Processing and Machine Learning for Biomedical Big Data, 2018
Shiming Yang, Peter F. Hu, Colin F. Mackenzie
Maintaining normal cerebral perfusion and oxygenation is important in managing severe TBI patients. Monitoring of the cerebral autoregulation could inform clinicians if a patient has lost the ability to maintain a constant perfusion when blood pressure changes [47]. Cerebrovascular pressure-reactivity index (PRx) was proposed as an indicator of loss of autoregulatory reserve [48,49]. PRx is calculated as a moving correlation coefficient between the mean arterial pressure (MAP) and ICP. Given a short time window, about 40 consecutive data points of MAP and ICP in 4–5 minutes are used for calculation [50]. When cerebral autoregulation is intact, cerebral blood flow (CBF) remains a normal constant speed and does not change significantly with mean blood pressure. In such a situation, PRx should be close to zero, indicating no or weak linear correlation between MAP and ICP. When cerebral autoregulation is damaged after severe head injury, CBF increases or decreases with blood pressure. The absolute value of PRx moves away from zero, indicating strong linear correlation between MAP and ICP. In this way, PRx can serve to continuously monitor the existence of cerebral autoregulation. When indicators of autoregulation are plotted against cerebral perfusion pressure (CPP), a U-shaped curve is generated, consistent with a loss of autoregulation in conditions of hypoperfusion or hyperemia. A study in 2002 by Steiner et al. took advantage of this relationship to construct curves of CPP against PRx, and hypothesized that the minima of these would indicate an ideal CPP at which pressure reactivity is maximized [51]. This group and others since have validated this model by finding a correlation between patients’ deviation away from this optimal CPP and eventual neurologic outcome.
Cardiac cycle timing and contractility following acute sport-related concussion
Published in Research in Sports Medicine, 2022
Jyotpal Singh, Chase J. Ellingson, Cody A. Ellingson, Parker Scott, J. Patrick Neary
Research in acute concussion has emphasized changes in cerebrovascular function, with altered cerebral blood flow (CBF) and cerebral autoregulation known to be common impairments (Giza & Hovda, 2014; Len & Neary, 2011; Wright et al., 2018). CBF decreases during the acute stage, suggesting altered ionic and metabolic activity (Champagne et al., 2021; Giza & Hovda, 2014). This can be reflective of the vasoreactivity which can occur due to acute concussed (Aaron et al., 2021), which along with the CBF decrease can lead to a mismatch between metabolic requirement in comparison to the perfusion during the CBF perturbations (Giza & Hovda, 2014). The perfusion impairments may be due to a reduction in systole in the presence of sustained HR, thereby limiting systemic circulation and altering cerebral haemodynamics during the acute concussion period. The reduction in systolic time can also explain why some patients with isolated TBI exhibit a reduced left ventricular ejection fraction, as there may be limitations in ensuring adequate stroke volume under resting conditions. This reduction in stroke volume (likely due to reduced systolic time) can further be suggestive of a potential cardiovascular dysfunction sequelae which occurs following concussion.
Brain oxygenation during multiple sets of isometric and dynamic resistance exercise of equivalent workloads: Association with systemic haemodynamics
Published in Journal of Sports Sciences, 2022
Andreas Zafeiridis, Anastasios Kounoupis, Stavros Papadopoulos, Aggelos Koutlas, Afroditi K Boutou, Ilias Smilios, Konstantina Dipla
We simultaneously assessed cerebral (using NIRS) and systemic haemodynamics during isometric and dynamic-RE. This allowed us to examine, for the first time, the role of systemic haemodynamics to changes in cerebral oxygenation. Although the pattern of changes in O2Hb and tHb within set and from set to set followed the same trend to those in CO/MAP/HR in both RE protocols (Figures 1 and 3), we did not find significant correlations between the changes in NIRS parameters and those in systemic haemodynamics, irrespective of protocol. These findings were confirmed by the time-variant ANCOVA tests. Thus, other factors, such as cerebral vasodilation, might have contributed more to changes in cerebral oxygenation and blood volume. In a previous study, changes in cerebral oxygenation during a 2-min intermittent isometric elbow flexions were also independent of changes in MAP (Bhambhani et al., 2014). Whether differences in cerebral autoregulation exist between the two types of RE protocols warrant investigation.
Numerical investigation of carotid stenosis in three-dimensional aortic-cerebral vasculature: pulsatility index, resistive index, time to peak velocity, and flow characteristics
Published in Engineering Applications of Computational Fluid Mechanics, 2021
Taehak Kang, Debanjan Mukherjee, Jaiyoung Ryu
The boundary conditions at the distal ends of the outlets were assumed to remain unchanged despite CS progression. This assumption raised a concern: the closed-loop nature of vasculatures may be neglected and could affect the resistances in the distal regions of the outlets. The fixed parameters of the Windkessel outlet model (, , and ), however, do not fix the haemodynamic outcomes such as pressure and flow rate; the parameters alter the pressure when the flow rate changes. (Alastruey et al., 2007; Cassot et al., 1995; Ryu et al., 2015) Although we did not adopt any cerebral autoregulation models, the total cerebral blood flow (tCBF) and the collateral flow through the communicating arteries were similar to those with the autoregulation models (Cassot et al., 1995; Ryu et al., 2015); we noted this phenomenon as a bulk-autoregulatory effect of Windkessel model in a large arterial domain (T. Kang et al., 2021).