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About one in 1000 people who receive general anaesthetic report some level of awareness during surgery.

The phenomenon, known as Anaesthetic Awareness, is triggered when a patient doesn’t get a high enough dose of anaesthesia.

With more than 17,000 Australians and New Zealanders undergoing surgery every day the problem is common.

According to Professor Kate Leslie from the Royal Melbourne Hospital, research suggests 70 percent of people who are aware develop post-traumatic stress disorder.

In the most serious cases, patients can recall entire conversations between doctors when they were meant to be out cold -- and a few even report full sensation while their bodies are paralysed by drugs.

Professor Andrew Davidson from the Royal Melbourne Hospital says levels of awareness can vary from vague memories to the 10 percent who can recall harrowing descriptions of exactly what happened.

About a quarter of extreme cases will lead to long-term post traumatic stress disorder causing things like nightmares, anxiety, depression and, not surprisingly, a fear of going back to the hospital.

A machine that monitors the brain activity of patients during surgery can reduce their risk of waking up under the knife.

The brainwave analysis known as bispectral index (BIS) monitors the depth of anaesthesia.

If the machine isn’t used, anaesthetists rely on a range of signs such as changes in heart rate, blood pressure and temperature.

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