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Title: Evaluation of Cerebral Perfusion Pressure (CPP) and Cerebral blood flow (CBF) in different Head of Bed positions using Transcranial Doppler in Neurosurgical patients
Authors: Nisha B
Issue Date: Dec-2019
Publisher: SCTIMST
Abstract: It is well known that optimal maintenance of cerebral hemodynamics is important to reduce the risk for secondary brain injury in neurosurgical patients. Acute Neurological illness like Traumatic brain injury, Subarachnoid haemorrhage, Intracerebral haemorrhage, Ischemic stroke, Meningitis/Encephalitis and Brain tumours can affect cerebral hemodynamics. Measurements of Intracranial pressure (ICP) and Cerebral perfusion pressure (CPP) are of paramount importance to guide the management in these conditions to avoid secondary brain injury due to inadequate cerebral blood flow. CBF is influenced by multiple factors including systemic arterial pressure, position of the head with respect to heart, venous and CSF drainage, and vascular tone of cerebral vessels. In a normal individual, as the head is raised, homeostatic reflexes maintain the systemic arterial pressure. The higher level of the head above the heart reduces perfusion pressure to the head, but the intracranial pressure is also reduced because of the improved venous drainage. Together with an intact autoregulation response of the cerebral vasculature, the net effect is little change in CBF. However, in patients with impaired autoregulation, a raise in head position may theoretically diminish CBF. (1-4) Different studies demonstrated that cerebral perfusion in patients with tumours is altered in correlation to location, size and histology of the tumour. Some studies refute them and state that autoregulation is intact in these patients with tumours regardless of location and size with a normal level of consciousness. Direct measurement of cerebral perfusion pressure (CPP) as the difference between mean arterial pressure (MAP) and intracranial pressure (lCP) produces a number that does not express the adequacy of brain perfusion. (5) American Association of Neurological Surgeons and the Brain Trauma Foundation (4th ed, 2016) do not include specific recommendations for optimal patient positioning practices after severe brain injury. Current positioning practices in neurocritical care units are largely based on studies in TBI patients that suggest head of bed (HOB) elevation may reduce ICP. There is no consensus on the degree of elevation for best practice. 300 of head elevation are believed to be associated with improvements in ICP and CPP. (6,7) Greater emphasis is focused on maintenance of adequate cerebral perfusion pressure (CPP), but does this ensure sufficient perfusion of the brain is a matter of debate. Perfusion pressure by itself is the propulsion force only; it does not determine the distribution of cerebral blood flow (CBF), ensure adequate collateral circulation, nor account for variations in the venous outflow path. CPP is rather a determinant of cerebral blood flow than a definite number. (8,9) Despite the importance of CPP values, there is huge confusion regarding the measurement of MAP based on the position of transducer level with varying positions of patients adding to the inaccuracies in measurements. Most patients with neurological injury are managed with head elevation and the level of zero calibration is important to measure an accurate CPP. Unfortunately, we find that studies behind the recommended CPP thresholds often do not elaborate on how MAP and CPP were measured. (6, 10-15) A noninvasive method, Transcranial Doppler can be used to measure the blood flow velocities in basal arteries as a surrogate measure of cerebral blood flow. Transcranial colour Doppler (TCCD) ultrasound is a valid measure of blood flow velocity in the major cerebral arteries and is an accepted index of cerebral autoregulation. This has an advantage of providing cerebrovascular imaging with structural flow map of cerebral blood vessels. (10,11) Near-infrared spectroscopy (NIRS), a non-invasive optical technology, is an indirect monitor of cerebral perfusion. Regional cerebral oxygen saturation (rScO2) of the frontal cortex is determined by comparing the specific absorbance patterns of oxygenated and non-oxygenated haemoglobin to near-infrared light. When CBF decreases, tissue oxygen extraction will increase to maintain cerebral metabolism with an eventual decrease in haemoglobin saturation. In the presence of a stable metabolic rate, rScO2 is therefore an indirect measure of CBF and provides information on organ ischemia. (11) However, there is surprisingly little literature, with inconsistent findings on the impact of changes in head positioning on cerebral hemodynamics and associated parameters. Very few studies about the postural influences on cerebral hemodynamics have been done in postoperative setting of neurosurgical patients. We decided to observe these changes in supine HOB 00 , 300 and 600 . We conducted this prospective, observational study to examine the effects of patient’s HOB positioning on NIRS, Cerebral blood flow velocities, Estimated CPP, and MAP after craniotomy for intracranial tumours.
Appears in Collections:Anaesthesia

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