Further direction the viva could take

This topic is not large, and so you may exhaust it fairly quickly. You will be assessed mainly on the core information above, but there a number of directions the viva could take. You may be asked about intra-arterial injection or about indications for, and direct methods of measuring arterial pressure. There is unlikely to be enough time to explore this latter subject in any depth.

Intra-arterial injection: Accidental injection occurs when an artery is wrongly identified as a vein, or when an intra-arterial catheter is mistaken for a venous cannula. Drugs that have been so injected include anaesthetic induction agents, phenytoin, benzodiazepines and antibiotics. In the awake patient severe pain in the hand is a cardinal feature. In the anaesthetised or sedated patient there may be ischaemic colour changes in the distal limb, which because of arterial spasm may be pale, mottled or cyanosed. Thrombosis may follow. The degree of damage depends on the substance injected. Thiopentone causes substantial damage because at body pH it precipitates into crystals, which occlude small arterial vessels and provoke intense vasospasm mediated via local noradrenaline (norepinephrine) release. Propofol, in contrast, seems relatively innocuous. Any such injection should be treated as for the worst-case scenario, because clinical experience of intra-arterial injection of many drugs is limited. Management: After the injection of 500-1000 heparin units to reduce thrombosis risk, warm NaCl 0.9% can be given to dilute the substance. Arterial spasm can be treated with papaverine 40-80 mg, prostacyclin at rate of 1 ^g min-1, tolazoline (which is a noradrenaline antagonist) and phenoxybenzamine (which is an (^-antagonist). Sound though the recommendation may be, these drugs may well not be immediately available, and this advice may be impractical. Dexamethasone 8mg given immediately may reduce arterial oedema. Perfusion can be enhanced by sympatholysis, either by a stellate ganglion block (which is quick to perform) or via a brachial plexus block, using a catheter technique to provide analgesia and a continuous block. Maintenance anticoagulation is recommended for up to 14 days, and hyperbaric oxygen has also been suggested as a means of minimising final ischaemic damage.

Intra-arterial monitoring: See Intra-arterial blood pressure measurement, page 263.

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