Update in AnaesthesiaICPintracranial pressure is the pressure withinCB - PDF document

Update in AnaesthesiaICPintracranial pressure is the pressure withinCBFcerebral blood flow is the flow of bloodCPPcerebral perfusion pressure is the effectiveis 1600ml. The skull is thus a rigid fluid filled box.normal range (5-13 mmHg). If there is an increasethe skull. CSF is forced out into the spinal sac. Thusvolume then causes a rapid increase in ICP. Thisanaesthetic drugs. Successful management of these and O tensions. Poor anaesthetic Teaching pointHigh intracranial pressure (ICP) will causedistortion and pressure on cranial nerves andvital neurological centres. Cerebral perfusionwill be impeded and operating conditions difficultor impossible. Loss of CSF and reduction ofvenous blood volume act to compensa

te forincreases in brain volume. Once thesemechanisms are exhausted, any further increase,however small, will cause a large increase ICP. certain point is reached when a further small changeeither internally, when the temporal lobe is pusheddeath. The rise in ICP may be accelerated becausecompensatory process. However, should free flowto a rapid rise in ICP. In practice, it is imperative to is obtained. This improves venous drainage Fig. 1a.Schematic representation of normalintracranial contents SSAS = spinal subarachnoid space, FM= foramen magnum, AV = arachnoid villi, CP = choroidplexus. Arrows indicate direction of CSF flow, heavy lines the Fig. 1b.Schematic representation of contents of skullFig. 2.Brain volum

e-intracranial pressure relationships: 1-AVOIDHypotonic IV solutionsUSE0.9% Normal SalineAnaesthetic drugs - see next article Update in Anaesthesia19 60402004�Intracranial volume -ICP (mmHg) 3 Update in Anaesthesiathe oxygen saturation of venous blood in the jugular (4) in another study of head-the brain. In other words if compliance is low, thepressure plus one third of the pulse pressure head up tilt, and the blood pressure Teaching pointCerebral perfusion pressure (CPP) = MAP - ICPInadequate CPP (less than 70 mmHg) has beenshown to be a major factor in the poor outcomeof patients with raised ICP. Assessment of CPPis vital and possible either by measurement ofboth ICP and MAP (mean arterial pressure -see

text) or by measuring MAP and making areasonable estimate of ICP. During anaesthesiamust be avoided or treated quickly by volumereplacement or catecholamines whichever is pressure dependent. Thus as arterial pressure rises min in white min in grey matter. There minwill cause an increase or decrease in cerebral arterial arterial pressure (SAP) falls to 80 mmHg. In Teaching pointThere are a number of physiological factorswhich affect or change cerebral blood flow(CBF). Rises in CBF due to hypoxia, hypercapnia(raised blood CO2) and high concentrations ofvolatile agents will cause a rise in ICP once thenormal compensating mechanisms have beenexhausted. Poor anaesthetic technique duringwhich hypoxia, hypercapnia and hypo

tensionoccur will seriously damage the critically illbrain further.Table 2. Physiological causes of raised ICPLow Cerebral Perfusion PressureExaggerated Hypertension Update in Anaesthesia is less than 25 mmHgless available to the tissues. Acute hypocapnictime (5 hours). While hypocapnia is maintained, is returned. When long term Fig 3bVasoconstriction cascadeFig 5Schematic representation between cerebral blood flowFig 3aVasodilatory cascade 0 4 8 12 16 20 24 0 2 4 6 8 10 12 Fig 4Schematic representation between cerebral blood flow Low arterial oxygen tension also hasand raised ICP is clear. In addition, pain from other Update in Anaesthesia tension. only mild noted that over an 11 year pe

riod there1.Durward Q J et al. Cerebral andhypertension. J.Neurosurgery 1983; 59: 938-2.McGraw C P. A cerebral perfusion pressureIntracranial Pressure VII. Edits. Hoff J T &Betz A L. 839-841. Springer-Verlag, Berlin.3.Rosner M J, Rosner S D & Johnson A H.4.Chan K H, Miller J D, Dearden N M, AndrewsP J D & Midgley S. The effects of changes inbrain trauma. J.Neurosurgery 1992; 77: 55-5.Bouma G J, Muizelaar J P, Handoh K &Marmarou A. Blood pressure and intracranialinjury: relationship with cerebral blood flow.6.Ruben B H. Intracranial hypertension inAdvances in Anaesthesia. Edit. Gallagher T7.Jones P W. Hyperventilation in the8.Muizelaar J P et al. Adverse effects of9Gentleman D., Dearden M., Midgeley S. &