Page last updated at 07:55 GMT, Tuesday, 24 June 2008 08:55 UK

Anaesthetics 'could worsen pain'

Some anaesthetics are irritant chemicals

Some general anaesthetics could actually worsen the pain following surgery, say scientists.

So-called "noxious" anaesthesia drugs - used commonly worldwide - stimulate nerves to cause irritation long after the operation is over.

The US research, published in Proceedings of the National Academy of Sciences journal, could prompt the choice of different drugs.

A UK expert said solving post-surgical pain was a priority for anaesthetists.

The choice of anaesthetic appears to be an important determinant of post-operative pain
Dr George Ahern
Georgetown University Medical Center

Anaesthetists have known for some time that certain drugs, such as the gas isoflurane, while very effective at rendering and keeping patients unconscious, are actually irritant chemicals.

Some already use a painkilling drug to lessen this effect before delivering the anaesthetic itself.

The latest finding, by research staff at Georgetown University Medical Center however, suggests that effects of the irritant is not just short-lived, but lingers on long after both the painkiller and the anaesthetic have worn off.

The drugs act on the same receptors on nerve cells which are activated by contact with other irritants, such as garlic, mustard or chilli.

Mice bred without these receptors were unaffected by the "noxious" anaesthetic gases.

If strongly activated, these can lead not just to an immediate sensation of pain, but also a longer oversensitisation of pain pathways in the nervous system.

In patients, this might mean that the pain they feel after an operation is significantly increased.

Switching drugs

Dr Gerard Ahern, who led the study, said: "It was not really recognised that use of these drugs results in the release of lots of chemicals that recruit immune cells to the nerves, which causes more pain of inflammation.

"The choice of anaesthetic appears to be an important determinant of post-operative pain."

He said that while this effect could be reduced by using other types of anaesthetic, these might not perform as well in other ways.

Professor Ian Power, from the University of Edinburgh, said that post-operative pain remained a serious problem, despite advances in anaesthesia over the decades.

"We are very aware that acute post-operative pain can persist and become chronic and long-lasting, and we have been looking for reasons for that - perhaps this research may provide those.

"If this research were to be validated and proved correct, it would be fairly easy for anaesthetists to move from one type to another."

Professor Richard Langford, a consultant in anaesthesia and pain management at Bart's and The London NHS Trust, said that while the findings were interesting, there was no guarantee a similar effect would be detected in humans undergoing surgery.

"There are a myriad of different factors that combine to produce the experience of pain, including the degree and size of the surgery, and the mood or level of anxiety in the patient."

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