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cerebral_blood_flow

Cerebral blood flow

Cerebral blood flow, or CBF, is the blood supply to the brain in a given time.

The arteries deliver oxygenated blood, glucose and other nutrients to the brain, and the veins carry deoxygenated blood back to the heart, removing carbon dioxide, lactic acid, and other metabolic products. Since the brain is very vulnerable to compromises in its blood supply, the cerebral circulatory system has many safeguards including autoregulation of the blood vessels and the failure of these safeguards can result in a stroke. The amount of blood that the cerebral circulation carries is known as cerebral blood flow. The presence of gravitational fields or accelerations also determine variations in the movement and distribution of blood in the brain, such as when suspended upside-down.

In an adult, CBF is typically 750 millilitres per minute or 15% of the cardiac output.

This equates to an average perfusion of 50 to 54 millilitres of blood per 100 grams of brain tissue per minute.

CBF is tightly regulated to meet the brain's metabolic demands.

Ischemia results if blood flow to the brain is below 18 to 20 ml per 100 g per minute, and tissue death occurs if flow dips below 8 to 10 ml per 100 g per minute.

CBF < 20 is generally associated with ischemia and if prolonged will produce cell death 1).

A study was undertaken to determine the minimum CBF and CMRO2 required by the human brain to maintain normal function and viability for more than a few hours. Positron emission tomography (PET) was used to perform regional measurements in 50 subjects with varying degrees of cerebral ischemia but no evidence of infarction. There were 24 normal subjects, 24 subjects with arteriographic evidence of vascular disease of the carotid system, and two subjects with reversible ischemic neurological deficits due to cerebral vasospasm. Minimum values found in the 48 subjects with normal neurological function were 19 ml/100 g-min for regional cerebral blood flow (rCBF) and 1.3 ml/100 g-min for regional cerebral metabolic rate of oxygen (rCMRO2). Minimum values for all 50 subjects with viable cerebral tissue were 15 ml/100 g-min for rCBF and 1.3 ml/100 g-min for rCMRO2. Comparison of these measurements with values from 20 areas of established cerebral infarction in 10 subjects demonstrated that 80% (16/20) of infarcted regions had rCMRO2 values below the lower normal limit of 1.3 ml/100 g-min. Measurements of rCBF, regional cerebral blood volume, and oxygen extraction fraction were less useful for distinguishing viable from infarcted tissue. These data indicate that quantitative regional measurements of rCMRO2 with PET accurately distinguish viable from nonviable cerebral tissue and may be useful in the prospective identification of patients with reversible ischemia 2).

Pathology

Hyperemia.

Ischemia.

Medical professionals must take steps to maintain proper CBF in patients who have conditions like shock, stroke, cerebral edema, and traumatic brain injury.

Insufficient cerebral perfusion pressure (CPP) after aneurysmal subarachnoid hemorrhage can impair cerebral blood flow.

Considerable studies showed that a reduction in cerebral blood flow (CBF) might affect learning and memory processes, resulting in the development, and progression of dementia, such as vascular dementia.

Assessment

Assessment of the cerebral blood flow (CBF) is crucial in the evaluation of patients with steno-occlusive diseases of the arteries supplying the brain for prediction of stroke risk.

Quantitative phase contrast magnetic resonance angiography (PC-MRA) can be utilised for noninvasive quantification of CBF.

Arterial spin labeling (ASL)-MRI is becoming a routinely used sequence for ischemic strokes, as it quantifies cerebral blood flow (CBF) without the need for contrast injection.


Perfusion computed tomography (CT) is a technique that allows rapid qualitative and quantitative evaluation of cerebral perfusion by generating maps of cerebral blood flow (CBF), cerebral blood volume (CBV), and mean transit time (MTT).


A Medline search was conducted to address essential pre-specified questions related to the utility of CBF monitoring. Peer-reviewed recommendations were constructed according to the GRADE criteria based upon the available supporting literature. Transcranial Doppler ultrasonography (TCD) and transcranial color-coded duplex sonography (TCCS) are predictive of angiographic vasospasm and delayed ischemic neurological deficits after aneurysmal subarachnoid hemorrhage. TCD and TCCS may be beneficial in identifying vasospasm after traumatic brain injury. TCD and TCCS have shortcomings in identifying some secondary ischemic risks. Implantable thermal diffusion flowmetry (TDF) probes may provide real-time continuous quantitative assessment of ischemic risks. Data are lacking regarding ischemic thresholds for TDF or their correlation with ischemic injury and clinical outcomes.TCD and TCCS can be used to monitor CBF in the neurocritical care unit. Better and more developed methods of continuous CBF monitoring are needed to limit secondary ischemic injury in the neurocritical care unit. 3).

Cerebral blood flow for outcome prediction after traumatic brain injury

The aim of a study was to examine cortical cerebral blood flow (CBF) in patients with traumatic brain injury (TBI) and determine whether lobar cortical CBF is a better predictor of long-term neurological outcome assessed by the Glasgow Outcome Scale (GOS) than global cortical CBF. Ninety-eight patients with TBI had a stable xenon computed tomography scan (Xe/CT-CBF study) performed at various time points after their initial injury. Spearman's correlation coefficients and Kruskall-Wallis' test were used to examine the relationship between patient age, emergency room Glasgow Coma Scale (GCS), Injury Severity Score, prehospital hypotension, prehospital hypoxia, mechanism of injury, type of injury, side of injury, global average CBF, lobar CBF, number of lobes with CBF below normal, and GOS (discharge, 3 and 6 months). Univariate ordinal regression was performed using these same variables and in combination with principle component analysis (PCA) to determine independent variables for multi-variate ordinal regression. Significant correlation between age, GCS, prehospital hypotension, type of injury, global average CBF, lobar CBF, number of lobes below normal CBF, and GOS was found. Individual lobar CBF was highly correlated with global CBF and the number of lobes below normal CBF. PCA found one principle component among these three CBF variables; therefore, average global CBF and number of lobes with CBF below normal were each chosen as independent variables for multiple ordinal regression, which found age, GCS, and prehospital hypotension, global average CBF, and number of lobes below normal CBF significantly associated with GOS. This study found global average CBF and lobar CBF significantly correlated with GOS at follow-up. There was, however, no individual cerebral lobe that was more predictive than any other, which puts into question the value of calculating lobar CBF versus global CBF in predicting GOS 4).

1)
Astrup J, Siesjö BK, Symon L. Thresholds in cerebral ischemia - the ischemic penumbra. Stroke. 1981 Nov-Dec;12(6):723-5. PubMed PMID: 6272455.
2)
Powers WJ, Grubb RL Jr, Darriet D, Raichle ME. Cerebral blood flow and cerebral metabolic rate of oxygen requirements for cerebral function and viability in humans. J Cereb Blood Flow Metab. 1985 Dec;5(4):600-8. PubMed PMID: 3877067.
3)
Miller C, Armonda R; Participants in the International Multi-disciplinary Consensus Conference on Multimodality Monitoring. Monitoring of cerebral blood flow and ischemia in the critically ill. Neurocrit Care. 2014 Dec;21 Suppl 2:121-8. doi: 10.1007/s12028-014-0021-9. PubMed PMID: 25208667.
4)
Fridley J, Robertson C, Gopinath S. Quantitative lobar cerebral blood flow for outcome prediction after traumatic brain injury. J Neurotrauma. 2015 Jan 15;32(2):75-82. doi: 10.1089/neu.2014.3350. Epub 2014 Nov 25. PubMed PMID: 25019579.
cerebral_blood_flow.txt · Last modified: 2018/04/18 12:17 by administrador