Imaging of cerebral blood flow and metabolism in brain injury in the ICU

Early Effects of Passive Leg-Raising Test, Fluid Challenge, and Norepinephrine on Cerebral Autoregulation and Oxygenation in COVID-19 Critically Ill Patients

Background: Coronavirus disease 2019 (COVID-19) patients are at high risk of neurological complications consequent to several factors including persistent hypotension. There is a paucity of data on the effects of therapeutic interventions designed to optimize systemic hemodynamics on cerebral autoregulation (CA) in this group of patients. Methods: Single-center, observational prospective study conducted at San Martino Policlinico Hospital, Genoa, Italy, from October 1 to December 15, 2020. Mechanically ventilated COVID-19 patients, who had at least one episode of hypotension and received a passive leg raising (PLR) test, were included. They were then treated with fluid challenge (FC) and/or norepinephrine (NE), according to patients' clinical conditions, at different moments. The primary outcome was to assess the early effects of PLR test and of FC and NE [when clinically indicated to maintain adequate mean arterial pressure (MAP)] on CA (CA index) measured by transcranial Doppler (TCD). Secondary outcomes were to evaluate the effects of PLR test, FC, and NE on systemic hemodynamic variables, cerebral oxygenation (rSo₂), and non-invasive intracranial pressure (nICP). Results: Twenty-three patients were included and underwent PLR test. Of these, 22 patients received FC and 14 were treated with NE. The median age was 62 years (interquartile range = 57-68.5 years), and 78% were male. PLR test led to a low CA index [58% (44-76.3%)]. FC and NE administration resulted in a CA index of 90.8% (74.2-100%) and 100% (100-100%), respectively. After PLR test, nICP based on pulsatility index and nICP based on flow velocity diastolic formula was increased [18.6 (17.7-19.6) vs. 19.3 (18.2-19.8) mm Hg, p = 0.009, and 12.9 (8.5-18) vs. 15 (10.5-19.7) mm Hg, p = 0.001, respectively]. PLR test, FC, and NE resulted in a significant increase in MAP and rSo₂. Conclusions: In mechanically ventilated severe COVID-19 patients, PLR test adversely affects CA. An individualized strategy aimed at assessing both the hemodynamic and cerebral needs is warranted in patients at high risk of neurological complications.

Imaging of cerebral blood flow in patients with severe traumatic brain injury in the neurointensive care

Ischemia is a common and deleterious secondary injury following traumatic brain injury (TBI). A great challenge for the treatment of TBI patients in the neurointensive care unit (NICU) is to detect early signs of ischemia in order to prevent further advancement and deterioration of the brain tissue. Today, several imaging techniques are available to monitor cerebral blood flow (CBF) in the injured brain such as positron emission tomography (PET), single-photon emission computed tomography, xenon computed tomography (Xenon-CT), perfusion-weighted magnetic resonance imaging (MRI), and CT perfusion scan. An ideal imaging technique would enable continuous non-invasive measurement of blood flow and metabolism across the whole brain. Unfortunately, no current imaging method meets all these criteria. These techniques offer snapshots of the CBF. MRI may also provide some information about the metabolic state of the brain. PET provides images with high resolution and quantitative measurements of CBF and metabolism; however, it is a complex and costly method limited to few TBI centers. All of these methods except mobile Xenon-CT require transfer of TBI patients to the radiological department. Mobile Xenon-CT emerges as a feasible technique to monitor CBF in the NICU, with lower risk of adverse effects. Promising results have been demonstrated with Xenon-CT in predicting outcome in TBI patients. This review covers available imaging methods used to monitor CBF in patients with severe TBI.

Brain endothelial HSP-70 stress response coincides with endothelial and pericyte death after brain trauma

OBJECTIVES

Our objective was to characterize the heat shock response (HSR) in a model of traumatic brain injury (TBI) and to determine the association of HSR to cell death.

METHODS

We used immunofluorescent detection of HSP-70 to characterize HSR and TUNEL labeling to determine the pattern of cell death.

RESULTS

HSP-70 immunofluorescence revealed a steady increase from 4 to 48 hours following TBI, culminating in a ubiquitous expression with the capillary bed 48 hours post-TBI. TUNEL labeling revealed a small subset of endothelial cell death and a most robust staining of putative pericyte cell death.

DISCUSSION

Our results show that while injury causes a detectable stress response, cell death is not a direct consequence of the HSR.

PET Imaging for Traumatic Brain Injury

Traumatic brain injury represents a substantial public health problem for which clinicians have limited treatment avenues. Traditional FDG-positron emission tomography (PET) brain imaging has provided unique insights into this disease including prognostic information. With the advent and implementation of novel tracers as well as improvement in instrumentation, molecular brain imaging using PET can further illustrate traumatic brain injury pathophysiology and point to novel treatment strategies.

Molecular mechanisms of traumatic brain injury in children. A review

Despite advances in molecular biology and genetics, the precise pathophysiology of traumatic brain injury (TBI) in children is unknown. In this paper the authors review what is currently known about intra- and extracellular responses to pediatric TBI and relate these factors to future investigations. Although hyperemia and vascular congestion have long been thought to be the hallmarks of pediatric TBI, on a cellular level, calcium influx as well as modulation of local neurotransmitters appears to play a major role in its onset. Recent genetic and proteomic research has identified specific neurotrophic factors as well as apoptotic and antiapoptotic genes that appear to control the progression of inflammation and neuronal damage. The search for a therapeutic target will ultimately require a thorough understanding of these factors and their interplay on a proteomic, genomic, and neuromic level.

Clinical review: Prognostic value of magnetic resonance imaging in acute brain injury and coma

Progress in management of critically ill neurological patients has led to improved survival rates. However, severe residual neurological impairment, such as persistent coma, occurs in some survivors. This raises concerns about whether it is ethically appropriate to apply aggressive care routinely, which is also associated with burdensome long-term management costs. Adapting the management approach based on long-term neurological prognosis represents a major challenge to intensive care. Magnetic resonance imaging (MRI) can show brain lesions that are not visible by computed tomography, including early cytotoxic oedema after ischaemic stroke, diffuse axonal injury after traumatic brain injury and cortical laminar necrosis after cardiac arrest. Thus, MRI increases the accuracy of neurological diagnosis in critically ill patients. In addition, there is some evidence that MRI may have potential in terms of predicting outcome. Following a brief description of the sequences used, this review focuses on the prognostic value of MRI in patients with traumatic brain injury, anoxic/hypoxic encephalopathy and stroke. Finally, the roles played by the main anatomical structures involved in arousal and awareness are discussed and avenues for future research suggested.

Neuroimaging in trauma

PURPOSE OF REVIEW

Developments in imaging following traumatic brain injury are outlined. Numerous techniques have evolved over the past several years giving us more information about the injury and prognosis for recovery. Some of these techniques are in clinical use while others are used primarily in research but have the potential to become clinically useful.

RECENT FINDINGS

Computed tomography (CT) scanning is the primary imaging technique for acute brain injury, giving rapid information and being part of a general trauma work up in the emergency situation. It has supplanted plain films in the immediate management of brain injury. Following stabilization, MRI is the method of choice for evaluating the full extent of brain injury. Information on diffuse axonal injury is obtained by several MRI sequences. Diffusion tensor imaging is able to show long tract damage and relates to prognosis. There are several techniques which are best suited to research in brain injury, including single photon emission CT, PET and xenon CT.

SUMMARY

CT and MRI are now the imaging techniques for acute and subacute brain injury, respectively. Diffusion tensor imaging is being developed to provide more information on structural damage in brain injury. There are several research techniques available for brain injury, particularly relating to cerebral blood flow and metabolism.

Neuroimaging in neuroophthalmology

Recent advancements in the speed and accuracy of data acquisition and resolution of neuroimaging and interventional techniques have revolutionized the early anatomical and functional diagnosis, prognosis, and treatment of many neuroophthalmological disorders. The relatively new techniques include magnetic resonance (MR) spectroscopy, computed tomography angiography, positron emission tomography, and functional MR imaging. In this paper the author describes the principles of the current techniques used by neuroophthalmologists and their value in the diagnosis, localization, and treatment of various afferent and efferent visual and ocular disorders.

Cerebral energy failure after subarachnoid hemorrhage: The role of relative hyperglycolysis

After subarachnoid hemorrhage (SAH) cerebral metabolism is significantly impaired. Hyperglycolysis describes the reduction of oxidative metabolism followed by a relative increase of anaerobic glycolysis to maintain energy supply. This phenomenon is known in head injury but has not as yet been shown after SAH. This study was conducted to test the hypothesis that hyperglycolysis is present in SAH patients and is associated with vasospasm. A total of 105 measurements were conducted on 21 SAH patients (age 49+/-15 years, median World Federation of Neurosurgical Societies Grade 4) over the first 5 days following admission. Arteriovenous differences were calculated for oxygen (avDO2) and glucose (avDGlc). Relative hyperglycolysis was defined as metabolic ratio (MR=avDO2[mmol/L]/avDGlc[mmol/L])<3.44. Jugular-venous saturation for oxygen (SjvO2), mean arterial blood pressure (MAP), intracranial pressure (ICP), cerebral perfusion pressure (CPP) were monitored. Relative hyperglycolyis was recorded in 34% of studies after SAH. In hyperglycolytic studies both jugular-venous lactate and SjvO2 were significantly elevated (jugular-venous lactate 14.9+/-9.9 vs. 11.8+/-5.5 mg/dL, p=0.04; SjvO2: 70.0+/-18% vs. 81.7+/-9%, p=0.002). Relative hyperglycolysis is associated with outcome after SAH. In patients who died after SAH almost 50% of studies showed hyperglycolysis, whereas patients who survived without neurological deficit had no hyperglycolytic events. Relative hyperglycolysis is a common event after SAH. It may be associated with relative hyperemia but most importantly with outcome.

Imaging of cerebral blood flow and metabolism

PURPOSE OF REVIEW

To review the techniques for imaging cerebral blood flow and metabolism following injury to the brain.

RECENT FINDINGS

Xenon enhanced computerized tomography (Xenon CT), CT perfusion and single photon emission CT provide measurements of cerebral perfusion, while positron emission tomography (PET), and magnetic resonance imaging and spectroscopy (MRI and MRS) are able to assess both perfusion and cerebral metabolism. Xenon CT and CT perfusion are readily available and have proved useful in a variety of causes of brain injury. PET is an extremely useful research tool for defining cerebral physiology, but is limited in its availability. Despite the continuing development of MRI and MRS imaging, the scanning environment remains hostile for critically ill patients, and further research is required before the techniques become generally available.

SUMMARY

Imaging of cerebral blood flow and metabolism has been shown to be useful following a variety of causes of brain injury, as it can help to define the cause and extent of injury, identify appropriate treatments and predict outcome. Imaging based on CT techniques (Xenon CT and CT perfusion) can be implemented easily in most hospital centres, and are able to provide quantitative perfusion data in addition to structural images.