Decompression sickness: a review.
Work in compressed air is a dangerous activity, whether it is carried on under water in a conventional diving suit, in self-contained breathing apparatus or in a diving bell, or in relatively dry conditions in a caisson or an underwater tunnel. Surprisingly large numbers of men have worked at one time or another in compressed air in tunnels or caissons and it is the dangers which arise to the health of these men that are considered here. When a tunnel is driven through water-logged strata or through porous ground under a river, the open end of the tunnel must be sealed off and air pumped in to balance the hydrostatic pressure, should there be any danger of water flooding in and bringing with it unmanageable quantities of silt or sand. A pier of a bridge may be constructed in deep water by means of a caisson (Fr. caisse, a box), which is a compressed air chamber in which a gang of men excavate foundations. In both tunnels and caissons men come and go through an air lock and must be compressed on entering and decompressed at the end of the work period. Although working conditions in these circumstances have much improved over the last 6o years, our understanding of the reactions of the human body to atmospheric pressures greater than the normal and of the pathogenesis of decompression sickness is still insufficient to prevent illness completely. In engineering practice it is customary for working pressure to be expressed as gauge pressure in pounds per square inch (p.s.i.g.) which is the pressure over and above the normal atmospheric pressure of I4-7 p.S.i. (I 03 kg./cm.2). For certain calculations absolute pressure (gauge pressure plus atmospheric pressure) is used, and for clarity it must always be stated whether gauge or absolute pressure is referred to. In naval practice it is usual to think in terms of feet of sea water, each 33 feet of depth corresponding to about one atmosphere or approximately I5 p.s.i. Sometimes the unit of pressure used is the atmosphere (atm.). On the
RADIOLOGY IN DECOMPRESSION SICKNESS: THE CLYDE TUNNEL.
I T H the opening of the first tunnel W under the Clyde in July 1963, and the completion of compressed air work in the second tunnel, it is opportune to review decompression sickness and, in particular, avascular necrosis of bone. Several hundred men have worked for varying periods in compressed air and some patients with bone necrosis will require treatment during the next few years. The twin tunnels are being built at Whiteinch where the river is 400 feet wide; the total length of each tunnel, including sloping approaches is approximately half a mile. Each tunnel lies about 21 feet below the bed of the river, in strata which consist of sandstone and shale overlain by variable glacial and alluvial deposits. When a tunnel is being constructed under a river bed, compressed air is required to prevent water percolating through the surrounding strata. The pressure varies with the types of strata through which the tunnel is passing and in this project it has not exceeded 34+ Ib. per square inch (p.s.i.). The use of compressed air exposes the tunneller to the risk of decompression sickness (‘the bends’, Caisson disease, compressed air illness) and certain precautions must be observed. Air locks are situated at the entrance to each tunnel, through which the men must pass on entering and leaving the tunnel. On entering, the pressure is rapidly raised until the working pressure is reached, but, on leaving, the pressure is reduced slowly to a level at which it is safe for the men to leave the lock. The period of decompression is proportional to the air pressure in the tunnel and the length of exposure to it. Rapid decompression may result in the formation of gas bubbles in the tissues of the body and so give rise to decompression sickness. In spite of all precautions, the condition still occurs and is also found amongst deep sea divers, in men escaping from submarines, in underwater swimmers (Dewey, 1962; Davies, 1962) and in men flying at great heights in non-pressurised aircraft.
A feasibility study of adolescent sex education: medical students as peer educators in Edinburgh schools
Objectives To determine the feasibility of medical students being involved in sex education in secondary schools, as lesson leaders in peer group discussions.