Download Update 11 - Update in Anaesthesia - WFSA

Update in Anaesthesia 53After tying in the endotracheal tube the blood pressure ismeasured again. It has fallen to 80/55 mmHg. 500mls ofnormal saline (0.9%) is rapidly given, and a 6mg dose ofephedrine is administered IV which restores the bloodpressure to 180/90 mmHg.IV fluid therapyWhen anaesthesia is started, the cardiovascular system isdepressed, particularly the ability to compensate for areduction in blood volume (Cardiovascular PhysiologyUpdate in Anaesthesia 1999;10:2-8). Therefore part ofthe treatment of a fall in blood pressure is a rapid infusionof fluid intravenously.Anaesthesia is now maintained using a further dose offentanyl 150 mcg or iv morphine 10mg slowly, vecuronium10mg and the patient ventilated with oxygen enriched air.He is monitored with a pulse oximeter, an ECG and BP.An infusion of 150mls 20% mannitol (0.25 - 0.5 g/kg) iscommenced, followed by a litre of 0.9% saline, and aurinary catheter inserted.He is transported to the CT scanner which shows a largeextradural haematoma (figure 3) and therefore isimmediately taken to the operating theatre for craniotomy.Figure 3: CT scan of a patient with an acute extraduralhaematomaTeaching PointHypotension following inductionReasons for concern - hypotension will reduceperfusion pressure in the brain. This has two adverseeffects:1 Blood flow through the brain will falldramatically, reducing oxygenation of the brainand cause ischaemia. The brain tolerates thisbadly and will suffer major neurological damage(stroke, paralysis, death).2 The arteries in the brain will sense a reductionin flow and will try to compensate(autoregulation). They will dilate in an attemptto reduce their resistance and thus increase flow.Dilatation will increase their volume and causea further increase in brain volume and intracranialpressure, making the situation worse.Saline vs Colloid vs DextroseThe brain is surrounded by a membrane separating itfrom the vascular space - the blood-brain barrier.This membrane will only allow water to pass throughit. Therefore only fluid with the sameconcentration of sodium as plasma should begiven intravenously. Otherwise, the plasma willbecome more dilute and water will pass from it intothe brain, making the brain swell, and thus increasepressure further.Normal Saline (0.9%) has a similar concentration ofsodium and therefore is the fluid of choice for the brain.Colloid can be given if required to treat hypovolaemiadue by major blood loss.When Dextrose solutions in water (5% Dextrose,Dextrose 4%-Saline 0.18%) are given, the dextroseis metabolised leaving just the water or a very dilutesaline solution. This “dilutes” the blood, reducing theconcentration of sodium in the plasma. The water thenpasses into the brain where the concentration ofsodium is higher. The brain then swells, and intracranialpressure will rise.Vasopressors As explained, the blood pressuremust be raised quickly. Therefore a small dose ofcardiovascular stimulant drug can also be givenintravenously to raise the blood pressure while the fluidis being run in. Suitable drugs include ephedrine, 3-6mg, methoxamine 1-2 mg, or adrenaline 25-100 mcg.