Cases of serious decompression sickness (DCS) associated with air diving activity in the North Sea remain at an unacceptable high level despite changes in regulations governing air diving practices. This is borne out by a recent study completed by T G Shields et al. entitled Decompression Sickness From Commercial Offshore Air Diving Operations On the UK Continental Shelf During 1982 to 1988.
The aim of this project is to produce a practical and safe decompression procedure from air dives by the introduction of helium and oxygen mixtures during decompression. It is intended that the end product of the project will be a complete set of air diving tables for use by the commercial air diving industry at large, covering the full range of exposure times down to a maximum depth of 50 m.
PHASE I
This part of the trial compared air diving procedures with the new procedure and a total of 78 dives were completed. This trial proved that the "Gas Switch" theory was correct and was safe. But it was found that some bubbles existed for short periods after the divers reached the surface. The significance of this is that it is far safer to have He/O2 bubbles at this stage as they are very unlikely to result in Type Two DCS.
PHASE II Stage I (Onshore)
The aim of this trial was to reduce the number of bubbles found after the dive. The results of this trial were truly remarkable. Twenty divers were used, all of whom had taken part in the earlier series and of these, 19 had no detectable bubbles during or up to 2 hours after the dive. One diver had, for a few minutes, a Grade I bubble after the dive.
It is clear that it has now been established that it is safer to use this procedure to a depth of 40 metres for 60 minutes than an equivalent standard air dive procedure.
PHASE IIStage I (Offshore)
Twenty dives using the second gas switch profile were undertaken at sea in August 1991. The dives were for 60 minutes and at 40 msw. One diver developed knee pain and one diver complained of sore, dry knee joints.
Both were treated successfully. Apart from the fact that the divers found the chamber cold because of the need to constantly flush to contain the oxygen percentage below 24%, there was no obvious factor to account for this problem. The work load was moderate to heavy and in contrast to the NHC series the divers continued to work on deck for the remainder of their shift. Doppler monitoring produced no obvious evidence of bubbles.
PHASE II Stage 2
Studies of the profiles at one, two and three hours have been undertaken in a spontaneous breathing pig model in the University of Trondheim. At two and three hours a small number of bubbles were found briefly after the switch and also a small number after surfacing.
The successful profile run in February lastyear was undertaken, but without the gas switch. This produced massive bubbling but surprisingly no bubbles during the period under pressure.
PHASE II Stage 3
In an attempt to reduce the occurrence of the joint problems, two modifications were made to the original February table and tested in 12 divers at the NHC in September 1991. The 20 msw stop was divided into two stops, one of 15 minutes at 20 msw and one of 30 minutes at 15 msw to reduce the helium uptake. The oxygen stoppage depth was reduced to 5 metres but retaining the 50 minutes period. Of the twelve divers using this schedule, one experienced a shoulder pain and 9 of the 12 produced bubbles. This is a highly significant change from the February schedule.
Judged from this data the profile used in February must be fairly close to an optimum schedule for one hour at 40 metres.
Another revised schedule with an extension of the stoppage time at 5 msw was tried in four divers. One produced a very low level of bubbles post dive.
The new technique being suggested will allow a substantial amount of the excess nitrogen taken up during a dive to be 'washed out' without initiating the major supersaturation associated with decompression and risking bubble formation. This can be undertaken by changing the diver's breathing gas to a helium and oxygen mixture to create a gradient in the lung for gas exchange. Because the helium is taken up more slowly in most tissues of the body than nitrogen, the decompression can be contained during the later stages of the wash out, without incurring a significant risk from the helium absorbed. Finally, oxygen can be used to remove the additional helium. Using this three-phase decompression it is hoped to eliminate the occurrence of intravascular gas bubbles and, therefore, the associated risk of neurological decompression sickness.
Until the introduction of ultrasound, the end point in the formulation of decompression tables was joint pain. This has been shown to be not valid in short duration deep air dives, because the nervous system can be damaged without the warning of joint pain. Doppler ultrasound will be used to monitor for bubbles in the precordial position. Bubble formation in body tissue will also be monitored using the scanning ultrasound techniques developed by the University of Oxford.