Ascending safely from the depth

Decompression sickness occurs in connection with rapid ascent from deep sea – from high pressure to low pressure – and is caused by formation of gas in the human body. Just as soft drinks contain carbonic acid, blood contains physically dissolved gases. These gases may form bubbles when the pressure drops. Ascending from deep sea is like opening the screw cap of a bottle of carbonated water: The pressure decreases, and bubbles are released.

The more the pressure drops, the larger the bubbles become. The bubbles can develop in different tissues in the body, or in the bloodstream. If a large gas bubble develops in a blood vessel, the bubble may function as a blood clot.

Researchers at NTNU have discovered more of how these bubbles behave and change as divers ascend to the surface. This knowledge will influence the emergency procedures for professional divers and submarine personnel – and may also be used to develop an individually adjusted dive computer.

PIGS SHOW THE WAY – “In theory, the best ascending profile is to rise slowly and gradually, from all depths. But this is not feasible in practice,” says research fellow Andreas Møllerløkken. “The water movements make it too difficult.”

That is why deep or prolonged dives require the divers to make safety stops on their way to the surface to prevent the pressure drop of becoming a shock to the organism. This is normal procedure today.

Møllerløkken and his fellow researcher Christian Gutvik discovered that divers should not only make ordinary safety stops, but actually go a bit further down towards the depth afterwards. In this instance, the ‘divers’ were pigs in pressure chambers. Thanks to these pigs, the researchers were able to map how gas bubbles are formed and change under different conditions.

“We know now what happens with gases in the blood when the pressure changes. And we have seen that this can be influenced by medication,” Møllerløkken explains.

TAILORMADE COMPUTER – The two researchers at the Department of Circulation and Imaging at NTNU are currently developing the inside of a new dive computer. It ’monitors’ the diver in a totally new way and makes him more secure in deep water because it is based on his own physical condition.

A dive computer is usually attached to the wrist and has a depth sensor and a watch. It is programmed to inform the diver of how much time he has left before he must begin ascending to the surface. It also indicates whether safety stops are necessary. However, it is based on theoretical tables of how gases behave in different types of tissues. It makes no consideration to individual factors which strongly influence how gases are absorbed by the blood and the formation of bubbles: the diver's height, weight, body fat percentage, maximum and minimum pulse rates, oxygen uptake, and gender.

The diver feeds the new computer with his personal data. This information, combined with continuous measuring of the pulse, enables the computer to calculate how the body is affected by the dive and tell at any time how the ascent to the surface should be performed.

Measuring the pulse is essential. The heart rate indicates the blood flow in the body, which determines the uptake of gases. For instance, if a person dives with a pulse rate close to maximum pulse over a certain period of time, the gas uptake in the body will be totally different from that of relaxing leisure dives.

The Swiss producer Uwatec wishes to put the new dive computer into production. The features were tested in the Red Sea in January, but some quality assurance remains before the computer is released on the market – probably in 2008.

PINCHING MARGINS – To amateur divers, the time they spends rising to the surface, is wasted. To professional divers, it also represents a loss of money. But if the ascent is to take place without risk, it must not happen too quickly. For this reason, existing safety routines for divers include large safety margins.

The routines are, however, not based on how gas actually behaves in a diver’s body. Research fellows Møllerløkken and Gutvik wish to incorporate the knowledge of bubbles into the safety routines.

“In practice, a bubble-based ascent profile may allow a faster ascent, since it better describes reality. There will no longer be a need for unnecessarily conservative safety margins,” concludes Andreas Møllerløkken.

By Hege J. Tunstad