Nitrous oxide anaesthetic unsafe, say researchers

Doctors attending a scientific conference in Auckland on Saturday will be told the nitrous oxide gas used in most general anaesthetics is unsafe and should be discontinued.

Preliminary results of research which studied the effects on 2050 patients will be presented to the annual scientific meeting of the Australian and New Zealand College of Anaesthetists.

Australian doctors who carried out the study found nitrous oxide in the anaesthetic mix doubled the rate of serious vomiting and pneumonia after surgery.

It raised the risk of wound infections, the Sydney Morning Herald reported.

Patients who underwent surgery and received the gas were slower to recover and likely to stay in hospital longer if gases used to keep them unconscious included nitrous oxide as a base, rather than oxygen alone or oxygen and air.

The Health Ministry has been clamping down on the use of nitrous oxide for recreational substance abuse as an inhalant.

Nitrous oxide, commonly known as laughing gas, was declared a prescription medicine in 2000.

People have been buying it in small canisters used as a propellant for whipping cream.

It was first used as an anaesthetic in New Zealand in 1874, according to former Auckland anaesthetist Dr Basil Hutchinson, who has written Safety Through Knowledge, a book on the history of anaesthetics.

Anaesthesia director at Alfred Hospital in Melbourne, Professor Paul Myles, said yesterday the announcement in Auckland would really surprise people.

"Nitrous oxide is used in 80 per cent of anaesthetics," he said. "It's the stock in the stew. It's the foundation of anaesthesia and has been that way for 160 years."

Prof Myles led the international study covering 20 hospitals.

He said that because nitrous oxide changed the way the body metabolised vitamin B12 and folate, it might also be causing immune system and heart problems, nerve damage, cancer and birth defects.

Those potential effects, combined with those demonstrated by the new study, spelled "the end of nitrous oxide" in general anaesthesia, he said.

But it could still have a role in securing fast pain relief, such as after injury or while giving birth, or in "simple surgery for young, healthy patients", though he had already stopped using nitrous oxide in his own practice.

Richard Halliwell, an anaesthetist at Sydney's Westmead Hospital said the safety of nitrous oxide had never been fully evaluated before.

"It came into practice before there was rigorous testing of safety or efficacy of drugs and completely escaped current testing standards."

The study indicated hospitals should strongly reconsider its routine use -- particularly in longer procedures and sicker patients because they had less margin to cope with the adverse effects.

College of Anaesthetists' president Michael Cousins said a phase-out of nitrous oxide would be good news for patients.

"If we eliminate nitrous oxide from the equation there will be a lot fewer people feeling very sick," he said.

Anaesthesia had already become much safer and easy to tolerate, and the death rate attributed to it was now less than one in 100,000, said Professor Cousins, head of anaesthesia and pain management at Sydney's Royal North Shore Hospital.

A triennial ANZCA report, released in 2002, surveyed the outcome of 10.3 million operations using anaesthetics between 1997 and 1999, and found deaths related to anaesthesia had fallen by a quarter over the preceding three years.

During the latest period, anaesthesia was wholly responsible for the deaths of 47 patients and partly responsible for the deaths of 83.

- NZPA