Key Points:

Folole Muliaga was seriously ill from heart and lung disease, but her hospital doctors are surprised she died "so soon" after her oxygen machine stopped providing the life-giving gas.

The 44-year-old died about 2 1/2 hours after the mains-powered machine, supplied by the Counties Manukau District Health Board, stopped working about 11am on Tuesday.

Medical experts said yesterday that home-oxygen machines were given only to patients with chronic conditions.

They were not aware that any of the machines had battery back-ups.

"It's not a life-critical thing as a rule," said the health board's chief medical officer, Dr Don Mackie.

"There are things about this case that we don't understand," he said.

"It's a little surprising she died so soon after the machine turned off."

Mrs Muliaga had been treated at Middlemore Hospital for her longstanding illness and was discharged on May 11, Dr Mackie said.

She was suffering from heart and lung disease and her breathing difficulties were related to her obesity. She was given a home oxygen machine because of the severity of her condition.

Asthma and Respiratory Foundation medical director Professor Robin Taylor, the head of Otago University's respiratory research unit, said home oxygen machines were for people with a chronic, mildly reduced level of oxygen in their blood, typically those with chronic obstructive pulmonary disorder.

"You wouldn't use it with acute pneumonia.

"For most patients receiving oxygen therapy the loss of the supply is unlikely to precipitate death as an immediate consequence.

"If you stopped oxygen you would expect patients to feel short of breath, but you would never be expecting them to die on the spot."

Dr Mackie said that following Mrs Muliaga's death, the hospital's home oxygen service would change its practice about advising on power supply security. He said the service had a standardised letter for patients or their families to give to their electricity supplier which expressed the need for a home oxygen machine and stated: "As far as possible this person requires uninterrupted supply".

"We've decided," Dr Mackie said, "that we are going to be more proactive in that and actually offer the letter ... to people who are on the service as part of their training and education on to the treatment, so they have got it there in their hands up front, rather than having to ask us for it."

Dr Mackie said the training for patients and their families included advice on what to do if the machine's power was cut.

They were given "reassurance that this should not be a life-threatening event [although] it can be stressful.

"The first message is sit quietly and keep calm."

They were told to call an ambulance if any severe symptoms developed.

What is it?

A device about the size of a vacuum cleaner. It takes in air, which is about 20 per cent oxygen, and harvests oxygen molecules. Its gas output is about 80-90 per cent oxygen at a rate commonly about 1 to 3 litres per minute. There are two main types, one called a concentrator, which supplies a continuous stream of oxygen. The other is a BIPAP (bi-level positive airway pressure), which lowers the pressure when the patient exhales. This was the kind Folole Muliaga used.

How are they powered?

They plug into mains electricity. Experts the Herald spoke to were unaware of any having a back-up battery supply.

How are they used?

A concentrator supplies the oxygen either to the patient's nostrils through tubes or to the nose and mouth through a tube to a mask; the BIPAP requires a tightly sealed mask.

How long are they used for at a time?

Normally for up to 16 of every 24 hours, typically including overnight.

Who uses them?

Mostly they are prescribed to people with chronic obstructive pulmonary disorder (chronic bronchitis and emphysema). A small number of people who have obesity hypoventilation syndrome are also put on one.

How many people use them?

Rates vary around New Zealand. The Counties Manukau District Health Board, which serves a population of more than 400,000, has 170 adults using them. It has a small number of children on a different kind of home ventilation.

Sources: Counties Manukau District Health Board; Asthma and Respiratory Foundation.