Once placed around the nerve, the duration of a block is determined by two main factors:

• Protein binding
• Rate of removal from the site and subsequent metabolism

Adrenaline is a vasoconstrictor that reduces blood flow at the site where the local anaesthetic is placed.

The addition of 1 in 200 000 adrenaline (Fig 1) can be used to prolong the duration of a block. It can be useful when added to lidocaine. However, because the duration of action of bupivacaine is determined mainly by tissue binding, the addition of adrenaline is not clinically useful.

By restricting lidocaine absorption into the blood stream, the addition of adrenaline allows a higher dose of lidocaine to be used: 7 mg/kg rather than 3 mg/kg. However, since the toxicity associated with bupivacaine is also related to protein binding, adding adrenaline would not allow a higher total dose of bupivacaine to be used.