A Tauranga mother of four who has always dreamed of becoming a paramedic has been barred from volunteering on St John ambulances because they have changed their main anaesthetic to one that could kill her.

Lynda Hodge, 37, a supervisor at Specsavers, started training with St John just over a year ago to achieve what she says is "my lifelong dream of becoming a paramedic".

"I just want to help people, that's my whole aim for wanting to be with St John, to help people that need help," she says.

But since July she has been barred from ambulances because St John has changed its main anaesthetic from nitrous oxide to methoxyflurane - a gas that can trigger a potentially fatal reaction called malignant hyperthermia (MH) in families with a rare genetic abnormality.

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Hodge found out that her family carried the gene when her 10-year-old niece suffered a "horrific" MH attack in March after being given an anaesthetic for a simple operation in Tauranga Hospital.

"She had four days in intensive care. It was touch and go, but because of the training that Tauranga Hospital had, they picked up on it straight away," Hodge said.

"Myself and my family - nine of us so far and still counting - have all found out that we have MH and it has been 100 per cent passed on through us.

"My mother has passed it to all of her children, who have also passed it on, and I have passed it to all four of my children, which is very rare."

St John changed its main anaesthetic early this year despite repeated pleas from the national MH testing centre in Palmerston North dating back to 2007, when methoxyflurane was first introduced as one of several anaesthetic options on ambulances.

Centre director Dr Terasa Bulger said the change was driven by St John wanting to save space in its vehicles by removing large nitrous oxide cylinders.

"They used to carry cylinders of nitrous oxide, or laughing gas, for dislocated shoulders and broken legs with bits of bone sticking out and the like. It's the same gas women use in labour," she said.

"But they are quite bulky, and there is an alternative which has been used in the military and in Australia - methoxyflurane. Its brand names are Penthrane or Penthrox, and it's a little green whistle which is much smaller and more compact."

Methoxyflurane was banned by the US Food and Drug Administration in 2005 because of safety concerns and is now made only in Australia, where it is also used by the military and ambulances.

St John medical director Dr Tony Smith said Australian ambulances had used the gas as their sole inhaled painkiller for more than a decade.

"More than a million doses have been administered in Australia without any cases of malignant hyperthermia reported," he said.

"Methoxyflurane is light and can be administered to patients simultaneously, as happened during the Christchurch earthquakes.

"Entonox [nitrous oxide] requires a cylinder and regulator, therefore each ambulance was limited to one patient at a time. Using the same regulator for multiple patients also increases the risk of cross-infection."

He said removing the nitrous oxide also enabled paramedics to use nebulised inhalers for patients with lung conditions, reducing risks involved in giving them oxygen.

Although methoxyflurane is now the sole inhaled painkiller on St John ambulances, Smith said ambulances carried at least seven other non-inhaled painkillers and only used methoxyflurane with patients "who are alert and able to speak to us about their allergies or medical conditions".

Hodge and her children now wear bracelets stating that they carry the MH gene, but she said: "I live in fear that in an emergency the bracelets may be missed and we will suffer the unbelievable tragedy of losing a child."

Bulger said all hospitals that use anaesthetics must hold stocks of dantrolene, an antidote that stops an MH attack if it is given quickly enough. But St John ambulances do not carry dantrolene and Smith said he was not considering stocking it.

"This is because, if we considered the risk high enough to carry the antidote, we would remove methoxyflurane in the interest of patient safety," he said.

However Bulger asked why, in that case, Hodge and others carrying the MH gene were banned from ambulance work and are also banned from joining the armed forces.

"Maybe the risk is small, but if it is zero, why won't they let Lynda Hodge be a St John volunteer?" she asked.

Smith replied: "St John's policy is that methoxyflurane should be withheld from patients known to be genetically predisposed to malignant hyperthermia. Likewise, we have chosen to take the same precaution for our frontline staff who are known to have the same condition."

• Meanwhile, Christchurch baby Chrissa Squire, who suffered severe brain damage from MH after being given an anaesthetic on September 29, has survived the removal of her breathing tube. Her family said they were "taking it day by day".