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The Dept of Anaesthesia & Intensive Care, CUHK
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MannitolOsmotherapy and brain pathology
Non-osmotic actions of mannitolHaemodynamic and antiviscosity effects- decreases the viscosity of blood (not only by decreasing haematocrit, but by decreasing the volume, rigidity, and cohesiveness of RBC membranes thereby decreasing the mechanical resistance to passage through the microvasculature) - also decreases systemic vascular resistance, mild positive inotropic effect on the heart - net effect is an increase in CO and oxygen delivery Free radical scavenging- has been known for some time that mannitol has free radical scavenging properties - in combination with the above this makes it an attractive agent for promoting blood flow in areas of focally compromised perfusion - may also have a role in prevention of no-reflow phenomena - however there are no controlled data supporting the beneficial role of mannitol’s free radical scavenging properties independent of the other well studied actions of mannitol Pharmacokinetics- there is relatively little information on the pharmacokinetics of mannitol infusions - T1/2 elimination is about 30 to 60 mins for doses of 0.25 to 1.5 g/kg body weight - Vd is theoretically the extracellular fluid volume but is limited by rapid renal clearance Uses of mannitol in neurocritical careGeneralOsmotic theory of ICP reduction- although difficult to prove it is reasonable to infer that bolus mannitol at the higher end of the clinically relevant dose range generates substantial blood-brain osmotic gradients and exerts at least some of its ICP lowering effect by direct removal of water from the parenchyma. Haemodynamic theory of ICP reduction- several variants exist, but all emphasise the importance of dynamic changes in the cerebral blood volume (CBV) - osmotherapy is believed to reduce CBV by reducing blood viscosity, increasing CPP, or both. - Muizelaar et al. reported a rapid reduction of the diameter of arterioles and venules on the surface of the brain immediately after a mannitol bolus. Extrapolation has led to the hypothesis that mannitol may induce a decrease in total CBV adequate to explain the reduction in ICP. - other indirect evidence:if the CPP is low at the time of the mannitol bolus the ICP reduction is maximal and vice versa. - shortcomings in the theory:
Diuretic Theory of ICP Reduction- all osmotic agents can produce a variably brisk diuresis that ultimately can result in reduction of the circulating plasma volume and hence CVP - in most circumstances the ICP is virtually identical to the CVP , and it is thus reasonable to assume that at least part of the ICP lowering effect of osmotic agents can be ascribed to their diuretic action. - it is important to note however, the period of ICP reduction precedes the diuretic phase by a substantial period - reduction of CVP probably plays a role in sustaining the effect of osmotherapy rather than producing acute effects on ICP - in support of this: giving a bolus of furosemide will sustain the effect produced by mannitol - on the other hand excessive reduction of circulating volume may negate any beneficial effect of osmotherapy if hyperviscosity is produced or organ perfusion is otherwise compromised. CSF Dynamics and ICP ReductionThe hyperosmolar state and osmotic compensation- mechanisms are complex and not well understood - involves increases in the intracellular electrolytes, amino acids, and so called idiogenic osmoles - idiogenic osmoles represent the generation of osmotically active particles of unknown chemistry or dynamic changes in the osmotic activity of intracellular macromolecules - it is an active process whereby the absolute cellular contents of osmotically active particles or sites are increased. - the elevated osmotic activity serves to counteract the dehydrating influence of hyperosmolar plasma Main clinical significanceIt places limits on the reduction of brain volume that osmotherapy can be expected to achieve - the threshold for osmotic compensation is unknown but has been suggested to occur at ~25 mOsm/kg above normal osmolality osmotic compensation creates the conditions whereby iatrogenic brain oedema may occur if a hyperosmolar state is reversed too quickly eg dilute tube feeding etc - fluid replacement should be carefully carried out with normal saline (still need caution) - need to gradually but purposefully return the plasma osmolality towards normal whilst maintaining haemodynamic stability and renal function. - as a rule of thumb the duration of return to normal osmolality should approximate the duration of the hyperosmolar state Mannitol and midline shift- asymmetric increases in brain volume may develop under a number of conditions
- it may or may not be accompanied by an increase in ICP Do osmotic agents have a role in the circumstance then with a lateralising lesion but no increased ICP? Do osmotic agents worsen midline shift by selectively debulking normal tissue? - the net effects of altered osmotic effectiveness and hydraulic conductivity are difficult to predict a priori - Cascino et al. and Bell et al. have presented neuroimaging data that suggests a preferential reduction in the water content of altered brain regions surrounding neoplasm - animal studies show a similar effect - didn’t measure midline shift directly but can reasonably be inferred because of the enhanced reduction in brain water content in the abnormal hemispheres Haemorrheologic Effects- can be achieved using an infusion - may avoid some of the adverse haemodynamic effects - relatively contraindicated in those with impaired renal function Adverse effectsAcute adverse haemodynamic effects- variable effects on BP following a bolus of mannitol - a slight increase in pulse pressure and MAP is most commonly observed, but transient decreases in BP secondary to decreases in SVR are not uncommon. - the acute vasodilatory effect of mannitol is not well understood and may be due to the following
- acute mannitol induced hypotension is not frequently a serious clinical problem: give slowly over 15 to 30 minutes. - precipitation of acute heart failure is not common and is rarely observed in those with impaired renal function Elevation of ICP- transient usually mild elevations in ICP may parallel the increase in CBV, but sustained or severe increases in ICP are rarely if ever encountered Dehydration and Electrolyte Disturbances- ratio of volume of fluid diuresed to the volume of mannitol administered may be high - osmotic diuretics result in net free water clearance - result is hypernatraemia - other electrolytes occur especially potassium, phosphate and magnesium - cardiac arrhythmias and neuromuscular complications are common Rebound phenomena- Probably multiple mechanisms - definition is any unexpected rise in ICP after the administration of osmotherapy - most widely held explanation is due to penetration of osmotically active particles into the brain, their accumulation creating an osmotic gradient favouring movement of water into the brain, and oedema formation - not commonly reported or encountered clinically - other possible mechanisms
- in the former case, volume depletion sets the stage for hyperviscosity and haemodynamic compromise leading to reactive cerebral hyperaemia - in the latter case, a CNS adjusted to the hyperosmolar state is suddenly exposed to plasma of reduced osmolality © Ross Calcroft September 1999
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©Charles Gomersall,
June, 2013 unless
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