Doctors at St Vincent's Hospital in Sydney said the woman had developed dizziness, nausea and lethargy when she was rushed to Sydney Hospital by ambulance, and eventually transferred to St Vincent's. But after a few hours the situation got even worse when the woman started developing symptoms of a heart attack, said Louis Wang, a doctor at St Vincent's Hospital and the Victor Chang Cardiac Research Institute. "Her ECG [heart monitoring] changed and it was consistent with someone having a heart attack," he said. "She didn't have chest pains, but the ECG changes suggested that not enough oxygen was being supplied to her muscle in an area of her heart." The otherwise healthy 20-year-old had been regularly smoking the hookah, for up to an hour at a time. Dr Wang said when she finally ended up in hospital she had a "horrific" concentration of carbon monoxide in her blood.

"It was 25 per cent, when normal is about 1.5 per cent, although really you shouldn't have any," he said. Dr Wang, who reported the case on Monday in the Medical Journal of Australia, said the design of the hookah pipes made oxygen deprivation a serious risk. "It's a very oxygen-deprived environment. There's not much air going into the whole system, and as a result you are always going to produce a degree of carbon monoxide." He said that as smoking the pipes became increasingly popular many people mistakenly believed they were cleaner or safer than cigarettes. "The residual gas that's still inhaled has a lot of toxins in it, and you can't smell carbon monoxide; it's invisible and odourless but it's definitely there," he said.

Ric Day, the director of clinical pharmacology and toxicology at St Vincent's, said the carbon monoxide was so dangerous because when it entered the body it attached itself to a substance called haemoglobin, which would otherwise be used to carry the oxygen around. So instead of delivering oxygen to the parts of the body that needed it to survive, the haemoglobin delivers potentially deadly carbon monoxide. "You need oxygen for every metabolic process … for working muscles, and the best example of a working muscle is the heart," he said. "In order for energy to be put to the purpose of making muscles contract the process needs oxygen. "Life depends on it – individual cell life, in particular working muscle, but of course the brain as well." Professor Day said that as hookah smoking became more popular it was important doctors were able to recognise the signs of carbon monoxide poisoning, which can be missed using traditional oxygen monitoring techniques. "When people turn up as she did with lethargy, it might not be obvious why that is in a 20-year-old," he said.

Khan Academy video: Hemoglobin moves O2 and CO2 : Learn the two ways that oxygen moves from the lungs to the tissues, and the three ways that carbon dioxide returns from the tissues to the lungs. Luckily, the woman in question had recovered, Dr Wang said. "She made a good enough recovery that she was able to be discharged in a day, but no-one knows what the long-term consequences could be," he said.