Principles of cerebral hemodynamics when intracranial pressure is raised: lessons from the peripheral circulation

Hemodynamic Imaging in Cerebral Diffuse Glioma-Part A: Concept, Differential Diagnosis and Tumor Grading

Diffuse gliomas are the most common primary malignant intracranial neoplasms. Aside from the challenges pertaining to their treatment-glioblastomas, in particular, have a dismal prognosis and are currently incurable-their pre-operative assessment using standard neuroimaging has several drawbacks, including broad differentials diagnosis, imprecise characterization of tumor subtype and definition of its infiltration in the surrounding brain parenchyma for accurate resection planning. As the pathophysiological alterations of tumor tissue are tightly linked to an aberrant vascularization, advanced hemodynamic imaging, in addition to other innovative approaches, has attracted considerable interest as a means to improve diffuse glioma characterization. In the present part A of our two-review series, the fundamental concepts, techniques and parameters of hemodynamic imaging are discussed in conjunction with their potential role in the differential diagnosis and grading of diffuse gliomas. In particular, recent evidence on dynamic susceptibility contrast, dynamic contrast-enhanced and arterial spin labeling magnetic resonance imaging are reviewed together with perfusion-computed tomography. While these techniques have provided encouraging results in terms of their sensitivity and specificity, the limitations deriving from a lack of standardized acquisition and processing have prevented their widespread clinical adoption, with current efforts aimed at overcoming the existing barriers.

Change in Blood Flow Velocity Pulse Waveform during Plateau Waves of Intracranial Pressure

A reliable method for non-invasive detection of dangerous intracranial pressure (ICP) elevations is still unavailable. In this preliminary study, we investigate quantitatively our observation that superimposing waveforms of transcranial Doppler blood flow velocity (FV) and arterial blood pressure (ABP) may help in non-invasive identification of ICP plateau waves. Recordings of FV, ABP and ICP in 160 patients with severe head injury (treated in the Neurocritical Care Unit at Addenbrookes Hospital, Cambridge, UK) were reviewed retrospectively. From that cohort, we identified 18 plateau waves registered in eight patients. A “measure of dissimilarity” (Dissimilarity/Difference Index, DI) between ABP and FV waveforms was calculated in three following steps: 1. fragmentation of ABP and FV signal according to cardiac cycle; 2. obtaining the normalised representative ABP and FV cycles; and finally; 3. assessing their difference, represented by the area between both curves. DI appeared to discriminate ICP plateau waves from baseline episodes slightly better than conventional pulsatility index did: area under ROC curve 0.92 vs. 0.90, sensitivity 0.81 vs. 0.69, accuracy 0.88 vs. 0.84, respectively. The concept of DI, if further tested and improved, might be used for non-invasive detection of ICP plateau waves.

A multiscale computational model of angiogenesis after traumatic brain injury, investigating the role location plays in volumetric recovery

Vascular endothelial growth factor (VEGF) is a key protein involved in the process of angiogenesis. VEGF is of particular interest after a traumatic brain injury (TBI), as it re-establishes the cerebral vascular network in effort to allow for proper cerebral blood flow and thereby oxygenation of damaged brain tissue. For this reason, angiogenesis is critical in the progression and recovery of TBI patients in the days and weeks post injury. Although well established experimental work has led to advances in our understanding of TBI and the progression of angiogenisis, many constraints still exist with existing methods, especially when considering patient progression in the days following injury. To better understand the healing process on the proposed time scales, we develop a computational model that quickly simulates vessel growth and recovery by coupling VEGF and its interactions with its associated receptors to a physiologically inspired fractal model of the microvascular re-growth. We use this model to clarify the role that diffusivity, receptor kinetics and location of the TBI play in overall blood volume restoration in the weeks post injury and show that proper therapeutic angiogenesis, or vasculogenic therapies, could speed recovery of the patient as a function of the location of injury.

A Clinical Test for a Newly Developed Direct Brain Cooling System for the Injured Brain and Pattern of Cortical Brainwaves in Cooling, Noncooling, and Dead Brain

To ensure the direct delivery of therapeutic hypothermia at a selected constant temperature to the injured brain, a newly innovated direct brain cooling system was constructed. The practicality, effectiveness, and safety of this system were clinically tested in our initial series of 14 patients with severe head injuries. The patients were randomized into two groups: direct brain cooling at 32°C and the control group. All of them received intracranial pressure (ICP), focal brain oxygenation, brain temperature, and direct cortical brainwave monitoring. The direct brain cooling group did better in the Extended Glasgow Outcome Scale at the time of discharge and at 6 months after trauma. This could be owing to a trend in the monitored parameters; reduction in ICP, increment in cerebral perfusion pressure, optimal brain redox regulation, near-normal brain temperature, and lessening of epileptic-like brainwave activities are likely the reasons for better outcomes in the cooling group. Finally, this study depicts interesting cortical brainwaves during a transition time from being alive to dead. It is believed that the demonstrated cortical brainwaves follow the principles of quantum physics.

Effects of size and elasticity on the relation between flow velocity and wall shear stress in side-wall aneurysms: A lattice Boltzmann-based computer simulation study

Blood flow in an artery is a fluid-structure interaction problem. It is widely accepted that aneurysm formation, enlargement and failure are associated with wall shear stress (WSS) which is exerted by flowing blood on the aneurysmal wall. To date, the combined effect of aneurysm size and wall elasticity on intra-aneurysm (IA) flow characteristics, particularly in the case of side-wall aneurysms, is poorly understood. Here we propose a model of three-dimensional viscous flow in a compliant artery containing an aneurysm by employing the immersed boundary-lattice Boltzmann-finite element method. This model allows to adequately account for the elastic deformation of both the blood vessel and aneurysm walls. Using this model, we perform a detailed investigation of the flow through aneurysm under different conditions with a focus on the parameters which may influence the wall shear stress. Most importantly, it is shown in this work that the use of flow velocity as a proxy for wall shear stress is well justified only in those sections of the vessel which are close to the ideal cylindrical geometry. Within the aneurysm domain, however, the correlation between wall shear stress and flow velocity is largely lost due to the complexity of the geometry and the resulting flow pattern. Moreover, the correlations weaken further with the phase shift between flow velocity and transmural pressure. These findings have important implications for medical applications since wall shear stress is believed to play a crucial role in aneurysm rupture.

Evaluation of a New Catheter for Simultaneous Intracranial Pressure Monitoring and Cerebral Spinal Fluid Drainage: A Pilot Study

OBJECTIVES

Intracranial pressure (ICP) monitoring is a common practice when treating intracranial pathology with risk of elevated ICP. External ventricular drain (EVD) insertion is a standard approach for both monitoring ICP and draining cerebrospinal fluid (CSF). However, the conventional EVD cannot serve these two purposes simultaneously because it cannot accurately measure ICP and its pulsatile waveform while the EVD is open to CSF drainage. A new Integra^® Camino^® FLEX Ventricular Catheter (Integra Lifesciences, County Offaly, Ireland) with a double-lumen construction has been recently introduced into the market, and it can monitor ICP waveforms even during CSF drainage. The aim of this study was to evaluate and validate this new FLEX catheter for ICP monitoring in a neurological intensive care unit.

METHODS

Six patients with 34 EVD open/close episodes were retrospectively analyzed. Continuous ICP was detected in two ways: through the FLEX sensor at the tip (ICP_f) and through a fluid-coupled manometer within the FLEX catheter, functioning as a conventional EVD (ICP_e). The morphologies of ICP_f and ICP_e pulses were extracted using Morphological Clustering and Analysis of ICP algorithm, an algorithm that has been validated in previous publications. The mean ICP and waveform shapes of ICP pulses detected through the two systems were compared. Bland-Altman plots were used to assess the agreement of the two systems.

RESULTS

A significant linear relationship existed between mean ICP_f and mean ICP_e, which can be described as: mICP_f = 0.81 × mICP_e + 1.67 (r = 0.79). The Bland-Altman plot revealed that no significant difference existed between the two ICPs (average of [ICP_e-ICP_f] was - 1.69 mmHg, 95% limits of agreement: - 7.94 to 4.56 mmHg). The amplitudes of the landmarks of ICP pulse waveforms from the two systems showed strong, linear relationship (r ranging from 0.89 to 0.94).

CONCLUSIONS

This study compared a new FLEX ventricular catheter with conventional fluid-coupled manometer for ICP waveform monitoring. Strong concordance in ICP value and waveform morphology between the two systems indicates that this catheter can be used for reliability for both clinical and research applications.

Inconsistency in Reporting Variables Related to Intracranial Pressure Measurement in Scientific Literature

PURPOSE

To assess whether the collection and communication of intracranial pressure (ICP) values were standardized and reproducible.

METHODS

Integrative review of clinical trials (n = 357) reporting ICP as a variable.

RESULTS

Only 24.1% of studies reported adequate data required for replication. Of the 357 reports, 342 provided information about the design, 274 discussed sampling strategy, 294 identified the ICP device type, 312 provided a unit of measure, 121 provided anatomical localization for measuring ICP, and 83 provided information about patient positioning.

CONCLUSIONS

The majority of literature evaluated did not provide enough data for replication of results. Measuring and reporting ICP in the scientific literature is not standardized. A uniform standard would strengthen the quality of the evidence in neurocritical care and neurosurgical literature and better establish clinical guidelines for ICP management in neurologically injured patients.

Validity of self blood pressure measurement in the control of the hypertensive patient: factors involved

Background

Improving clinical practice aimed at controlling hypertension is a pending issue in health systems. One of the methods currently used for this purpose is self blood pressure measurement (SBPM) whose use increases every day. The aims of this study are to establish the optimal cut-off point for the 3-day SMBP protocol and to identify factors that could affect the precision of the 3-day SMBP protocol using 24-h ambulatory blood pressure monitoring (ABPM) as a reference.

Method

This is a cross-sectional descriptive study to validate a diagnostic test performed by a primary care team in Murcia, Spain. A total of 153 hypertensive patients under 80 years of age who met the inclusion criteria were evaluated. ABPM was performed for 24 h. The SBPM protocol consisted of recording 2 measurements in the morning and 2 at night for 3 days.

Results

The cut-off point for SBP was set at 135 mmHg (sensitivity: 80.39%, specificity: 74.19%), and for DBP, it was set at 83 mmHg (sensitivity: 76.48%, specificity: 84.89%), which yielded the highest combined sensitivity and specificity. After carrying out the validation study with the new figures, we proceeded to establish which socio-demographic factors prevented a correct classification of patients. These errors were more common in male patients for the assessments of both DBP (OR = 2.4) and SBP (OR = 2.5); hypertensive patients with age < 67,5 years (OR = 1,5); having no work activity (OR = 3,6) and with concomitant chronic kidney disease (CKD) (OR = 5.0).

Conclusion

Being male, older than 67.5 years, with CKD or with no work activity increases the probability of being misclassified for hypertension during follow-up as assessed by SBPM over 3 days.

Trial registration

This study was approved by the research ethics committee of the University of Murcia under registration number 1018/2015.

The Use of Computed Tomography Perfusion on Admission to Predict Outcomes in Surgical and Nonsurgical Traumatic Brain Injury Patients

Introduction: The objective of this study was to investigate if data obtained from a computed tomography (CT) perfusion study on admission could correlate to outcomes for the patient, including the patient’s length of stay in the hospital and their initial and final Glasgow Coma Scale (GCS), as well as the modified Rankin Scale (mRS) on discharge. We present an initial subset of patients fulfilling the inclusion criteria: over the age of 18 with mild, moderate, or severe traumatic brain injury (TBI). Patients admitted with a diagnosis of TBI had CT perfusion studies performed within 48 hours of admission. GCS, length of stay, mRS, and discharge location were tracked, along with the patient’s course of hospitalization. Initial results and discussion on the utility of CT perfusion for predicting outcomes are presented.

Methods: Patients exhibiting mild, moderate, or severe TBI were assessed using CT perfusion within 48 hours of admission from January to July 2019 at the Arrowhead Regional Medical Center (ARMC). The neurosurgery census and patient records were assessed for progression of outcomes. Data obtained from the perfusion scans were correlated to patient outcomes to evaluate the utility of CT perfusion in predicting outcomes in surgical and nonsurgical TBI patients.

Results: Preliminary data were obtained on six patients exhibiting TBI, ranging from mild to severe. The mean GCS of our patient cohort on admission was eight, with the most common mechanism of injury found to be falls (50%) and motor vehicle accidents (50%). Cerebral blood volume (CBV) seemed to increase with Rankin value (Pearson's correlations coefficient = 0.43 but was statistically insignificant (P = 0.21)). Cerebral blood flow (CBF) was found to be correlated with CBV, and both increased with Rankin score (Pearson's correlation coefficient = 0.56) but were statistically insignificant (P = 0.27). These results suggest that with a larger sample size, CBV and CBF may be correlated to patient outcome.

Conclusion: Although more data is needed, preliminary results suggest that with larger patient populations, CT perfusion may provide information that can be correlated clinically to patient outcomes. This study shows that CBF and CBV may serve as useful indicators for prognostication of TBI patients.

Structure and Function of Systemic Arteries: Reflections on the Arterial Pulse

"Structure and Function of Arteries"-is a topic of great importance to those who deal with arterial hypertension, since it links the source of flow, the left ventricle of the heart (whose output is pulsatile) to the peripheral tissues (whose flow is near continuous). The arterial tree acts passively as a conduit and cushion, and the interaction of heart, arterial tree, and organs is conventionally gauged on the basis of blood pressure measured by cuff in a conveniently located place (the brachial artery). For any precision and perspective to be gained, measurements of brachial systolic and diastolic pressure need be supplemented by other information. When such information is gained, one can understand how beautifully the arterial tree is tuned to the beat of the heart in animals of different size and shape and in humans at age 30 through the first third of a 3 billion beat lifetime. After age 30, the beats themselves progressively destroy the human arteries and their tuning to the heart, with emergence of clinical syndromes. In this review, the subject is tackled quantitatively on the basis of published numerical, physical, physiological, and pathophysiological basis, with principal focus on the beat of the heart, the pulse of the arteries, and their interaction.

Cerebral Haemodynamics: Effects of Systemic Arterial Pulsatile Function and Hypertension

PURPOSE OF REVIEW

Concepts of pulsatile arterial haemodynamics, including relationships between oscillatory blood pressure and flow in systemic arteries, arterial stiffness and wave propagation phenomena have provided basic understanding of underlying haemodynamic mechanisms associated with elevated arterial blood pressure as a major factor of cardiovascular risk, particularly the deleterious effects of isolated systolic hypertension in the elderly. This topical review assesses the effects of pulsatility of blood pressure and flow in the systemic arteries on the brain. The review builds on the emerging notion of the "pulsating brain", taking into account the high throughput of blood flow in the cerebral circulation in the presence of mechanisms involved in ensuring efficient and regulated cerebral perfusion.

RECENT FINDINGS

Recent studies have provided evidence of the relevance of pulsatility and hypertension in the following areas: (i) pressure and flow pulsatility and regulation of cerebral blood flow, (ii) cerebral and systemic haemodynamics, hypertension and brain pathologies (cognitive impairment, dementia, Alzheimer's disease), (iii) stroke and cerebral small vessel disease, (iv) cerebral haemodynamics and noninvasive estimation of cerebral vascular impedance, (v) cerebral and systemic pulsatile haemodynamics and intracranial pressure, (iv) response of brain endothelial cells to cyclic mechanical stretch and increase in amyloid burden. Studies to date, producing increasing epidemiological, clinical and experimental evidence, suggest a potentially significant role of systemic haemodynamic pulsatility on structure and function of the brain.

Assessing Cerebral Hemodynamic Stability After Brain Injury

OBJECTIVE

Following brain injury, unstable cerebral hemodynamics can be characterized by abnormal rises in intracranial pressure (ICP). This behavior has been quantified by the RAP index: the correlation (R) between ICP pulse amplitude (A) and mean (P). While RAP could be a valuable indicator of autoregulatory processes, its prognostic ability is not well established and its validity has been questioned due to potential errors in measurement. Here, we test (1) whether RAP is a consistent measure of intracranial hemodynamics and (2) whether RAP has prognostic value in predicting hemodynamic instability following brain injury.

MATERIALS AND METHODS

RAP was tested in seven brain injured patients treated in a surgical intensive care unit. A sample of ICP data was randomly chosen and segmented into 1 hour periods. Hours were then categorized as either stable, which contained no sharp rises in ICP, or unstable, which contained ≥1 sharp rise-where a sharp rise is defined as ICP exceeding a mean slope of 0.15 mmHg/s. Equal numbers of stable and unstable segments were then selected for each patient. RAP was calculated as the Pearson's correlation coefficient between ICP pulse amplitude (AMP) and mean (mICP), determined in 6 second windows, according to established methods.

RESULTS

Results showed that (1) average AMP and ICP levels were similar between stable and unstable periods and (2) unstable periods were identified by RAP values exceeding 0.6 with an average positive predictive value of 74%.

CONCLUSIONS

We conclude that RAP can provide a valid measure of ICP dynamics, is not affected by sensor drift, and can better distinguish periods of instability than ICP or AMP alone.

Aneurysm rebleeding after placement of external ventricular drainage: a systematic review and meta-analysis

BACKGROUND

The association between external ventricular drainage (EVD) and aneurysm rerupture is still a controversial issue. We performed a systematic review and meta-analysis of published series reporting data of EVD placement in aneurysmal subarachnoid hemorrhage (SAH) with the aim to evaluate the relationship between ventricular drainage and aneurysm rebleeding.

METHOD

PRISMA/MOOSE guidelines were followed. The Newcastle-Ottawa Scale was used to assess the quality of the studies. We conducted a comprehensive literature search of three databases (PubMed, Ovid MEDLINE, and Ovid EMBASE) on series reporting patients with EVD placement and aneurysmal SAH. The reported studies were analyzed with a primary aim to calculate the strength of the association (odds ratio) between rebleeding and EVD placement in patients with SAH.

RESULTS

Our systematic review included 16 articles with a total of 6804 patients with SAH of which 595 (8.7%) had aneurysmal rebleeding. Ventricular drainage was inserted in 19% of patients. Rebleeding occurred in 18.8% (95% CI = 16.4-20.6%) of patients requiring EVD and in 6.4% (95% CI = 5.8-7.1%) of patients who did not require EVD (OR = 3.92, p < 0.0001). The mean time between EVD placement and rebleeding was 1 h (range, 0-8 h). The maximal aneurysm diameter of patients with rerupture was larger when compared with patients without rerupture (9.9 mm vs. 7.5 mm; p < 0.04). High Fisher grades (III-IV) were present in 75% of patients with rerupture and in 60% of patients without aneurysmal rerupture (p < 0.0001).

CONCLUSIONS

Our study conclusively defined an association between EVD and rebleeding. However, the effective role of EVD in aneurysm rebleeding is insufficiently investigated in the literature. Other significant factors of rebleeding are usually observed in patients requiring ventricular drainage. According to our findings, a causal relationship between EVD and aneurysm rebleeding could be a plausible hypothesis, at least in certain cases.

Arterial Stiffening in Perspective: Advances in Physical and Physiological Science Over Centuries

Arterial stiffening is not a new issue in medicine or research but was the prime concern of Richard Bright in the early 19th century and of the prominent London physicians and pathologists who tried to unscramble the relationship between kidney, heart, and cerebrovascular disease and hardness of the pulse in the late 19th century. It was of major concern to medical educators including Osler and Mackenzie who were still active in practice 100 years ago. It is all too easy (when dependent on the Internet) to consider arterial stiffness to be a new issue. The terms arterial stiffness, aortic stiffness, or wave reflection do not appear as categories for articles such as this in respectable journals, nor in categories for meetings of specialized physicians. Yet as described in this article, the subject was of interest to clinicians, to investigators such as Harvey in the 17th century, and to physicists who developed laws and principles of elasticity from the study of biological materials including ligaments and arteries. This paper provides a perspective on arterial stiffness from the time of William Harvey and Isaac Newton to the present, with a glance into the future.

Vasculopathy of Aging and the Revised Cardiovascular Continuum

There have been attempts to explain the process of developments in overt cardiovascular disease, resulting in the presentation of the classic cardiovascular disease continuum and the aging cardiovascular continuum. Although the starting points of these two continua are different, they meet in the midstream of the cycle and reach a consensus at the end of the process. The announcement of the aging cardiovascular continuum made both continua complete, explaining the cardiovascular events in patients without atherosclerotic cardiovascular disease with aging. Impairment of the vascular structure by pulse wave and reflected wave is considered the cause of aortic damage, which influences the development of ischemic heart disease and the development of overt renal disease or cerebrovascular disease. The pathophysiology of vascular aging through pulse wave and its effect on other organs was discussed with Prof. Michael F. O'Rourke who devised the aging cardiovascular continuum.