Save your breath: it's cold in there: Personal air recycling is one aspect of research by the Navy into survival at sea. Keith Hindell reports

'THIRTY seconds,' the timekeeper warns the research team, before the start of the experiment. Then the volunteer is swung out in a chair above the cold water tank and, exactly on the minute, is lowered into it.

Chris Eastwood, a physiology student, is joining a long line of his peers from Sheffield University who have helped the Institute of Naval Medicine, Portsmouth, to investigate the body's response to cold water. Chris is wired up so that his vital signs are monitored throughout the 45-minute experiment. Every minute the changes in his skin temperature, rectal temperature, blood pressure, and so on, are recorded by students on three sides of the small pool.

Dr Michael Tipton, of the Robens Institute at Surrey University, is supervising the research, while Surgeon Commander Howard Oakley ensures that the experiment is conducted ethically and that the volunteers' health is never put at risk.

But suffer Chris does. The 15C water in the Royal Navy Immersion Facility, at Seafield Park, Hampshire, soon induces uncontrollable shivering. If Chris's core temperature should reach 35C, he will be pulled out of the water regardless of the needs of research. At 33C core temperature, only four degrees below normal, a human begins to lose consciousness, and at 28C risks cardiac arrest.

Chris's deep body temperature is monitored through probes attached to his ear and rectum; he has swallowed a radio pill, which transmits temperature readings as it passes through his body; his heart rate is recorded on an electrocardiogram and his breathing on a pneumotachograph.

The research team is also testing a hypothesis about movement. When a person is dumped into cold water from a boat or a plane, it is best that they keep still to conserve heat. But if they must move, to avoid danger or reach a raft, is it better to swim with arms and legs or legs alone? To test one theory, Chris simulates swimming with his arms by lifting a weight through a pulley. Other volunteers will simulate swimming with legs only, then with all four limbs.

The natural reaction is to swim flat out for about a minute until one becomes used to the temperature, as if indulging in a warming-up exercise. In fact, the swimmer will be cooling down very fast. Dr Tipton says that it is better to sit still and get used to the water; and in rough water, the 'warming-up' method would be foolhardy.

After thousands of hours of work, medical researchers have discovered that the body reacts to cold water by breathing very hard; this reflex gasping can soon develop into uncontrollable breathing, which in rough conditions means you cannot hold your breath and therefore swallow lots of water. This reaction, known as Cold Shock Response, has led Dr Tipton and his colleagues to conclude that the initial period in cold water is the most dangerous; thereafter, the main risk is hypothermia.

In a plane crash at sea or the ditching of a helicopter, the priority is to escape the sinking craft. A high proportion of flight crew and passengers in North Sea helicopter accidents fail to survive, not because of injury but because they cannot hold their breath long enough to escape the semi-submerged fuselage.

In the light of Cold Shock Response, Eric Bramham, managing director of the survival suit manufacturers Shark Sports, and Professor David Eliott, of the Robens Institute, designed an 'air pocket' with a breathing tube attached. The pocket has a 'passive internal distributor', which enables a survivor to breathe from it even when upside down in water.

When immersion is about to occur, you place the tube in your mouth and re-breathe your own breath. Experiments with not only champion swimmers but also ordinary people have shown that this simple system more than triples the time that people can remain submerged without fresh air, thus providing 40 to 50 extra seconds in which to escape or to adjust to the water temperature without breathing uncontrollably.

Medical scientists have known for 40 years that you can extend your time without fresh air by re-breathing exhaled breath, but it was Dr Tipton who made the link with his research on Cold Shock Response.

Back in the tank, after 37 minutes Chris Eastwood's core temperature drops to 35C and he is lifted out of the water and revived gently in a warm bath. Charts of all Chris's vital signs will go into the bank of cold water research, along with those of hundreds of others.

In extensive tests in Nova Scotia, survival instructors and students have made successful underwater escapes in cold water using the Shark Air Pocket. If it really does help to save lives in real accidents, Sheffield University students will be able to claim part of the credit.

(Photograph omitted)