General anesthesia is a marvelous thing, knocking us out and blocking our sense of pain in a matter of seconds before surgery.

But in rare cases, some people are responsive to their surrounds under general anesthesia, yet they cannot remember what happened afterwards.

This is called 'connected consciousness', and now the largest study of its kind to date on the phenomenon suggests that it's more common than first thought, affecting 1 in 10 young adults, and women more than men.

The findings highlight the need to better understand how different people respond to anesthetic drugs, the researchers say. Even after 170 years of use, we still don't have a firm grasp on how general anesthesia works – and now age and sex seem to be another factor in the mix.

"There is an urgent need for further research on the biological differences, particularly sex, that may influence sensitivity to anesthetic medication," says study author Robert Sanders, an anesthetist and neuroscientist at the University of Sydney in Australia.

If the results of the new study can be replicated, it might put us one step closer to understanding who is more likely to experience 'connected consciousness' and how anesthetists can reduce the odds of it happening.

Past estimates had suggested around 5 percent of people going under general anesthesia experienced 'connected consciousness'. But Sanders' team had suspected, based on other research, that it might have been more common in younger people.

The results of the new study suggest that a larger than expected amount of young adults are still responsive under general anesthesia, before surgery begins.

Roughly one in 10 of the 338 young adults in the study, aged between 18 and 40 years, responded to commands asking them to squeeze the researchers' hand once if they understood, and twice if they were in pain while under general anesthesia.

An hour after waking up, participants were asked to recall 16 words that they had heard under anesthesia, to see what they remembered of the experience.

Women were between two to three times more likely than men to experience 'connected consciousness', the study found.

The odds of 'connected consciousness' were also lower if a continuous level of anesthesia was maintained in the minutes after anesthesia was induced and before intubation, the point where a plastic tube is inserted down a person's windpipe to maintain airflow and deliver anesthetic drugs during surgery.

It's important to note that 'connected consciousness' is different to the unintended awareness that an even smaller fraction of people – just 0.1 percent – experience during anesthesia, after which they can recall specific details about the procedure.

'Connected' in this instance refers to parts of the brain still being capable of processing sensations from their environment, half-paying attention but not fully aware.

"Patients expect to be unconscious under anesthesia, and not be in pain, and this demonstrates why research into anesthesia is so important," Sanders says.

Around 13 percent of women in the study responded to commands under anesthesia, compared to only 6 percent of men, even though they received the same weight-adjusted amounts of propofol, a drug used to start and maintain general anesthesia.

"Differences in dosing, if present, were small and do not explain why females experienced connected consciousness more often than males," write the researchers in their paper.

About half of the 37 people who responded to commands also indicated they were in pain, which would have been swiftly rectified by adjusting the dose of anesthetic drugs. One person also clearly recalled the experience of surgery after the procedure ended.

"In our opinion, this is a higher level of consciousness than patients (or their anesthesiologists) anticipate during general anesthesia," Sanders and colleagues write in the paper.

While it may feel like anesthetics knock us out with a slug-punch of drugs that hit before you can count to ten, being in a state of anesthesia only requires a person to be disconnected from their environment, not necessarily involving a full loss of consciousness.

However, that clearly seems to be very fine line for anesthetists to tread, and one which appears to vary greatly from person to person.

At least now, anesthetists might have a better understanding of how maintaining continuous anesthesia in the first few minutes (which is already standard practice in many countries) may help reduce the incidence of 'connected consciousness'.

"It is very important to note that patients did not remember responding to the commands," says Sanders, noting that overall, general anesthetics are very safe.

"It was also reassuring to see that if anesthetic drugs are administered continuously in the time period between induction of anesthesia and intubation, the risk of connected consciousness was greatly reduced," he says.

The study was published in the British Journal of Anaesthesia.