Local anaesthesia

Definition

This is where a small area of the body has the pain numbed. It's usually done so that small medical procedures can be performed. For example, often when having a tooth removed, or a cut on the arm stitched, you would use a local anaesthetic to numb it.

This man has had a local in his heart, and the rest of his body removed, completely painlessly!

Physiology

Nerves work through conduction. The nerve impulses are generated through moving charged molecules in and out of nerve cells. Local anaesthetics work by stopping Na+ ions being moved around so easily. In effect it stops the nerves working properly, so pain cannot be transmitted to the brain.

The clever bit about this is that pain and temperature are smaller nerves than touch and motor control. This means they are more strongly affected by the local, and thus you can anaesthetise someone's finger, and they will still be able to move their hand. Obviously, if you chuck loads of local in, the other nerves get affected. That's why after a tooth removal with a local, you feel like you've been punched in the face, and speak like someone drunk.

Kind of moreish...

The original local was... Cocaine. All of them are still based on the same formula, which is why they all end in "ocaine". There are two different types of locals - esters and amine. This is based on their chemical make up. Cocaine, and the early local anaesthetics were esters; which have more side effects, cause allergic reactions more commonly and break down more quickly. The newer ones, such as lidocaine are amines, and cause less problems.

Making it last longer

The amide drugs are often mixed with adrenaline, which is a vasoconstrictor. This means that the drug doesn't "leak" away so much, and stays in the area its injected. This means you don't need to use so much, and it lasts longer. However:

• Local anaesthetic with adrenaline should never be injected into ends of things - such as fingertips, or penises, since the constriction can close off the small blood vessels and cause the finger tip to die. Or the penis to die. And that's quite an expensive lawsuit.

Common drugs

• Lidocaine (lignocaine) - A fast onset, short lasting local. This is the most commonly used now, and has relatively few side effects. If given in overdose, can cause bradycardia, seizures and respiratory depression.
• Bupivocaine - A slow onset, long lasting local. This is widely used for longer procedures, and, like lidocaine, is relatively free of side effect. It has a stronger effect on the heart than lidocaine, and so can cause low blood pressure.
• Procaine (novocaine) - This is an ester agent. No longer used. Cocaine is still (rarely) used as an upper respiratory local anaesthetic, but generally the esters are no longer used, since they have more side effects, and the newer ones are hypoallergenic.