📛In the absence of disease, ICP may rise by 50 mmHg during coughing or sneezing without noticeable neur ologic impairment.
▶️Therefore, it is the interaction of raised ICP with other intracranial pathology which produces the pathologic consequences, as opposed to the rise in ICP per se.
Monitoring of intracranial pressure (ICP)
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📛"Some patients with suspected intracranial hypertension and a decreasing level of consciousness might require invasive ICP monitoring, although its added value beyond clinical or radiological monitoring has not yet been proven"
📛Monitoring methods currently available include ventriculostomy, subarachnoid bolt, epidural sensor, and fiberoptic intraparenchymal monitor; the latter is the most commonly used.
▶️DO YOU KNOW?
📛The major drawback of intraventricular catheters is the rate of infection which is much higher than that observed using intraparenchymal probes.
📛Additionally, interpretation of ICP data after craniectomy is difficult.
Monitoring of CBF
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▶️Normal average CBF in the human is approximately 55 ml/100g (of brain)/min, though values may vary widely across grey and white matter. The ischemic threshold for CBF is approximately 18 ml/100g/min, with 10 ml/100g/ min often considered the threshold for irreversible injury.
📛Laser Doppler flowmetry (LDF) is a parenchymal or surface Doppler probe that measures tissue local CBF in a quantitative manner.
📛Brain tissue oxygen tension (P bt O 2 ) monitoring allows direct measurement of focal tissue oxygen tension in a specific region of the brain. A P bt O 2 level below 10-15 mmHg has generally been the threshold identified at which outcome is worsened
📛 Transcranial Doppler ultrasonography is a useful non-invasive monitor of cerebral hemodynamics, but has been severely disadvantaged by the inability to fix the probe in position.
📛 Jugular venous bulb oximetry is a global hemispheric measure with low sensitivity for detecting regional ischaemia.
🔹The normal SjvO 2 level is approximately 60%
🔹an SjvO 2 of < 50% for greater than 10 min has generally been considered to represent an ischemic desaturation.
🔹High SjvO 2 levels may reflect hyperemia (typically >90%) or an inability of the brain to extract oxygen due to metabolic depression from sedative agents, poor oxygen unloading (e.g. sickle cell disease), or severe brain injury.
📛Near-infrared spectroscopy (NIRS)
🔹measures cerebral regional oxygen saturation by measuring near-infrared light reflected off the chromophobes in the brain, the most important of which are oxyhemoglobin, deoxyhemoglobin, and cytochrome A3.
🔹Its major limitations include the intersubject variability, the variable length of the optical path, the potential contamination from extracranial blood, and most important, the lack of a definable threshold. 🔹Because of the thin scalp and skull in the neonate and infant, NIRS holds promise in this patient population but remains an investigative tool in its present form.
📛Microdialysis catheters, typically inserted in conjunction with an ICP or tissue Po2 monitor, allows sampling of small molecules in the interstitial fluid.
🔹An increasing lactate/pyruvate ratio is sensitive to the onset of ischemia.
🔹High levels of glycerol suggest inadequate energy to maintain cellular integrity and the resultant membrane breakdown.
🔹Excitatory amino acids, such as glutamate, are both a marker for neuronal injury and a factor in its exacerbation.
🔹Currently, the microdialysis catheter is primarily used in two situations: (a) extensive subarachnoid hemorrhage where subsequent vasospasm is likely and (b) traumatic brain injury (TBI)
▶️️At present, none of the methods available is sufficiently reliable or well tested to en able us to influence the clinical management of neurologically i njured patient with absolute certainty
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Reference:
Advanced cerebral monitoring in neurocritical care Nobl Barazangi, J. Claude Hemphill III, eurology India | October-December 2008 | Vol 56 | Issue 4
Intraoperative Neurophysiological Monitoring Second Edition Aage R. Møller
Postoperative management of adult central neurosurgical patients: Systemic and neuro-monitoring David Pfister, Stephan P. Strebel , Basel, Switzerland Luzius A. SteinerBest Practice & Research Clinical Anaesthesiology Vol. 21, No. 4, pp. 449–463, 2007
Textbook of Neuroanaesthesia and Critical Care by Basil F Matta
Handbook of Neuroanesthesia, 4th Edition, James E. Cottrell
