Don't try this at home, folks: the Lazarus effect is a capricious one. The 13-month-old Canadian baby who came back to life this week, after apparently freezing to death, was lucky. Erika Nordby, who somehow toddled out of the backdoor of a house in the middle of the night in sub-zero conditions - in only a nappy - was "frozen solid" when her mother Leyla found her a few hours later. But Erika had "died" at the right temperature (the air temperature was minus 24C; her body temperature had gone down to 16C). And she was a baby.
The membranes of the cells of a baby's tissue are just that much more flexible, according to Nancy Rothwell, a physiologist at the University of Manchester, and are more likely to survive the formation of ice fragments within them. "But the big thing is being exactly the right temperature, and that was absolutely luck," she says.
Freezing is very, very dangerous: lower the temperature and the blood starts to clot, which can lead to kidney damage (babies once again are much more likely recover). But some other things go your way. Oxygen requirements fall dramatically off as body temperature falls. So lower the temperature in a controlled way - they do it on the operating table every day - and there can be virtually no circulation, and virtually no heartbeat for a period, yet the brain will survive.
People with severe head injuries who have fallen into snow and frozen a bit have survived when others at room temperature have died. This has been so noticeable that doctors have tried chilling head injury victims to enhance their survival. Once again, when it works, it tends to work for the young; the older ones get kidney and cardiovascular problems. But the ones who survive this are more likely to recover from brain injury. At a lower temperature, the oxygen requirement falls 10-fold.
First the fingers and toes go numb, and then the limbs, because the body reacts to keep the vital organs warm: this is the core temperature at the heart of human survival. Oxygen consumption falls, blood thickens, fluids retreat to the centre, the kidneys work overtime: people who are cold become conscious of a need to urinate.
At 31C (well below normal body temperature) hallucinations begin. Nazi doctors experimenting with cold water baths at Dachau established death at a core temperature of about 25C. But there have been dramatic tales of children surviving at far lower temperatures. In 1994, a two-year-old girl in Saskatchewan was found with her limbs frozen solid, her core temperature at 14C. She lived.
The same principle keeps hibernating polar bears alive through the long Arctic night. "They lower their body temperature, the heart rate goes down to say, two a minute, and they manage to survive because they have such low requirements," says Prof Rothwell. The mammalian machinery that keeps bears going as a matter of evolutionary course could also occasionally work its magic for humans caught in the snow.
"Most enzyme activities in the body double with every 10C change. So if you drop by 10C you have halved all chemical reaction, and if you drop 10C more you halve again. Probably if they were around 5C or 8C core, they could survive. There is a lot of luck involved, being at the right temperature, after cooling at the right rate, and then rewarming at the right rate - and the risk was in the rewarming. But it can happen, and it has happened. They were clinically dead, of course, by the criteria we used, but not obviously really dead because they could function again."
In her 1998 televised Royal Institution lectures, Prof Rothwell lowered a PhD student into freezing water to make a point about physiology. She also pointed to Canadian frogs that did, literally, freeze.
"They were frozen solid in the ice, and then they gradually thawed and just woke up. A key thing with the ability to cool - animals or people - is the fluidity of their membranes, so that you don't get fracture. It seems likely that animals that survive very low temperatures - ice fish and so on - have very high levels of polyunsaturated fats in their membrane. They also have antifreeze type things in their blood."
And, she points out, the refrigerator and the ice bucket are now routinely called in to keep life going. "Hearts for transplant are transported in ice, and other organs," she says.
